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SC414/SC424
6A Integrated FET Regulator with 5V LDO
POWER MANAGEMENT Features
Input voltage -- 3V to 28V Internal power MOSFETs -- 6A Integrated bootstrap switch Smart power-save protection Integrated 5V, 150mA LDO with bypass capability TC compensated RDS(ON) sensed current limit Pseudo-fixed frequency adaptive on-time control Designed for use with ceramic capacitors Programmable VIN UVLO threshold Independent enable for switcher and LDO Selectable ultrasonic power-save (SC414) Selectable power-save (SC424) Internal soft-start and soft-shutdown at output Internal reference -- 1% tolerance Over-voltage and under-voltage fault protection Power good output SmartDriveTM Lead-free 4x4mm, 28 Pin MLPQ package Fully WEEE and RoHS compliant, and halogen free
Description
The SC414/SC424 is a stand-alone synchronous buck regulated power supply. It features integrated power MOSFETs, a bootstrap switch, and a 5V LDO in a space-saving MLPQ4x4mm 28-pin package. The device is highly efficient and uses minimal PCB area. It uses pseudo-fixed frequency adaptive on-time operation to provide fast transient response. The SC414/SC424 supports using standard capacitor types such as electrolytic or special polymer, in addition to ceramic, at switching frequencies up to 1MHz. The programmable frequency, synchronous operation, and selectable power-save provide high efficiency operation over a wide load range. Additional features include cycle-by-cycle current limit, soft-start, under and over-voltage protection, programmable over-current protection, soft shutdown, and selectable power-save. The device also provides separate enable inputs for the PWM controller and LDO as well as a power good output for the PWM controller. The input voltage can range from 3V to 28V. The wide input voltage range, programmable frequency, and 5V LDO make the device extremely flexible and easy to use in a broad range of applications. It can be used for single cell or multi-cell battery systems in addition to traditional DC power supply applications. The 5V LDO or an external 3.3V to 5V supply can be used to provide the bias voltage for the SC414/SC424. When the SC414/SC424 is used as a 5V output switching regulator, the 5V LDO can be used as an initial bias supply for the device. Once the switch regulator output is in the switch over range, the LDO will be bypassed by the switcher output for optimum efficiency.
Applications
Notebook, desktop, tablet, and server computers Networking and telecommunication equipment Printers, DSL, and STB applications Embedded applications Power supply modules Point of load power supplies
September 11, 2009
(c) 2009 Semtech Corporation
1
SC414/SC424
Typical Application Circuit
ENABLE LDO ENABLE PSAVE
V5V RTON 154K RILIM 9.09K RPGOOD 10K
28
ENL
27
TON
26
AGND
25
EN/PSV
24
LXS
23
ILIM
22
PGOOD
PGOOD AGND LX LX PGND PAD 1 21 20 19 18 17 16 15 PAD 3
PAD 2 V5V FB 1 2 C1 1F C2 1F 3 VOUT 4 5 6 C3 100nF 7
VIN FB V5V AGND VOUT VIN VLDO BST
SC414/SC424
PGND PGND PGND LX
LXBST
PGND
PGND
VIN
VIN
VIN
VIN
LX
CBST 1F VIN +12V RGND 0 CIN 10F
8
9
10
11
12
13
14
CIN 10F
L1 1.5H +
VOUT VOUT
FB
RFB1 10K
COUT 220F
C4 10nF
1V @ 6A, 250kHz
RFB2 30K
Key Components
Component CIN COUT L1 Value 10F/25V 220F/15m/6.3V 1.5H/9A Manufacturer Murata Panasonic Vishay Part Number GRM32DR71E106KA12L EEFUE0J221R IHLP2525CZER1R5M01 Web www.murata.com www.panasonic.com www.vishay.com
All other small signal components (resistors and capacitors) are standard SMT devices.
2
SC414/SC424
Pin Configuration Ordering Information
Device
PGOOD EN/PSV
Package MLPQ-28 4x4 MLPQ-28 4x4
Evaluation Board Evaluation Board
SC414MLTRT(1)(2) SC424MLTRT(1)(2) SC414EVB
21 20
AGND
TON
28
27
26
25 Top View
24
ILIM
23
ENL
LXS
22
FB V5V AGND VOUT VIN VLDO BST
1 2 3 4 5 6 7 8 9 10
LX LX PGND PGND PGND PGND LX
SC424EVB
AGND PAD 1 LX PAD 3
19 18 17
Notes: 1) Available in tape and reel only. A reel contains 3000 devices. 2) Lead-free packaging only. Device is WEEE and RoHS compliant, and halogen free.
VIN PAD 2
16 15 11 12 13 14
LXBST
PGND
VIN
VIN
VIN
SC414 and SC424 MLPQ-28; 4x4, 28 LEAD
Marking Information
SC414 yyww xxxxx xxxxx
yyww = Date Code xxxxx = Semtech Lot Number
PGND
VIN
SC424 yyww xxxxx xxxxx
yyww = Date Code xxxxx = Semtech Lot Number
3
SC414/SC424
Absolute Maximum Ratings
LX to PGND (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +30 LX to PGND (V) (transient -- 100ns max.) . . . . . . -2 to +30 VIN to PGND (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +30 EN/PSV, PGOOD, ILIM, to GND (V) . . . . . . -0.3 to +(V5V + 0.3) VOUT, VLDO, FB, to GND (V) . . . . . . . . . . . -0.3 to +(V5V + 0.3) V5V to PGND (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6 TON to PGND (V) . . . . . . . . . . . . . . . . . . . . . -0.3 to +(V5V - 1.5) ENL (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to VIN BST to LX (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0 BST to PGND (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +35 AGND to PGND (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +0.3 ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Recommended Operating Conditions
Input Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0 to 28 V5V to PGND (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0 to 5.5 VOUT to PGND (V) . . . . . . . . . . . . . . . . . . . . . . . . . . 0.75 to 5.5
Thermal Information
Storage Temperature (C) . . . . . . . . . . . . . . . . . . . . -60 to +150 Maximum Junction Temperature (C) . . . . . . . . . . . . . . . . 150 Operating Junction Temperature (C) . . . . . . . .-40 to +125 Thermal resistance, junction to ambient (2) (C/W) High-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Low-side MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 PWM controller and LDO thermal resistance . . . . . 40 Peak IR Reflow Temperature (C) . . . . . . . . . . . . . . . . . . . . 260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES: (1) Tested according to JEDEC standard JESD22-A114. (2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless specified: VIN =12V, TA = +25C for Typical, -40 to +85 C for Min. and Max., TJ < 125C, V5V = +5V, Typical Application Circuit
Parameter
Input Supplies VIN UVLO Threshold(1) (not available for V5V < 4.5V) VIN UVLO Hysteresis
Conditions
Min
Typ
Max
Units
Sensed at ENL pin, rising edge Sensed at ENL pin, falling edge EN/PSV = High Measured at V5V pin, rising edge
2.40 2.235
2.60 2.40 0.2
2.95 V 2.565 V 3.0 V 2.90 V 20 A
2.50 2.40
2.9 2.7 0.2
V5V UVLO Threshold Measured at V5V pin, falling edge V5V UVLO Hysteresis ENL, EN/PSV = 0V, VIN = 28V VIN Supply Current Standby mode; ENL=V5V, EN/PSV = 0V 130
8.5
4
SC414/SC424
Electrical Characteristics (continued)
Parameter
Input Supplies (continued) ENL, EN/PSV = 0V , V5V = 5V ENL, EN/PSV = 0V , V5V = 3V SC414, EN/PSV = V5V, no load (fSW = 25kHz), VFB > 750mV(2) SC424, EN/PSV = V5V, no load, VFB > 750mV(2) V5V = 5V, fSW = 250kHz, EN/PSV = floating, no load
(2)
Conditions
Min
Typ
Max
Units
3 2 1 0.4 4 2.5 0.744 0.7425 0.750
7 A
V5V Supply Current
mA
V5V = 3V, fSW = 250kHz, EN/PSV = floating, no load(2) Static VIN and load, 0 to +85 C, V5V = 3V or 5V FB Comparator Threshold Static VIN and load, -40 to +85 C, V5V = 3V or 5V Continuous mode operation Frequency Range Minimum fSW , (SC414 only), EN/PSV = V5V, no load Bootstrap Switch Resistance Timing Continuous mode operation, VIN = 15V, VOUT = 3V, RTON = 300k V5V < 4.5V(3) Minimum On-Time (2) V5V = 5V Minimum Off-Time (2) V5V = 3V Soft-Start Soft-Start Ramp Time (2) Analog Inputs/Outputs VOUT Input Resistance Current Sense Zero-Crossing Detector Threshold LX - PGND, V5V = 3V or 5V -3 1350
0.756 0.7575 1000
V V
kHz 25 10
1500
1650
ns
On-Time
80 320
ns
ns 390
1.7
ms
500
k
0
+3
mV
5
SC414/SC424
Electrical Characteristics (continued)
Parameter
Power Good Upper limit, VFB > internal 750mV reference Power Good Threshold Lower limit, VFB < internal 750mV reference Start-Up Delay Time (Time between EN going high and PGOOD going high) Fault (noise immunity) Delay Time(2) Leakage Power Good On-Resistance Fault Protection Valley Current Limit Valley Current Limit ILIM Source Current ILIM Comparator Offset Output Under-Voltage Fault Smart Power-save Protection Threshold (2) Over-Voltage Protection Threshold Over-Voltage Fault Delay(2) Over-Temperature Shutdown(2) Logic Inputs/Outputs Logic Input High Voltage Logic Input Low Voltage EN/PSV Input for PSAVE Operation (2) EN/PSV Input for Forced Continuous Operation (2) EN/PSV Input for Disabling Switcher (2) EN/PSV Input Bias Current ENL Input Bias Current FB Input Bias Current EN/PSV= V5V or AGND VIN = 28V FB = V5V or AGND -1 ENL ENL % of V5V % of V5V 45 1V 0 -10 11 1 0.4 100 42 0.4 +10 18 +1 V V % % V A A A 10C hysteresis With respect to AGND VFB with respect to internal 750mV reference, 8 consecutive clock cycles VFB with respect to internal 750mV reference VFB with respect to internal 750mV reference -8 V5V = 5V, RILIM = 5k V5V = 3V, RILIM = 5k 3 4 3.4 8 0 -25 +10 +20 5 150 +8 5 A A A mV % % % s C 10 V5V = 3V V5V = 5V -10 2 ms 4 5 1 s A % +20 %
Conditions
Min
Typ
Max
Units
6
SC414/SC424
Electrical Characteristics (continued)
Parameter
Linear Regulator -- LDO (not available for V5V < 5V) VLDO Accuracy VLDO load = 10mA Start-up and foldback, VIN = 12V LDO Current Limit Operating current limit, VIN = 12V VLDO to VOUT Switch-over Threshold (4) VLDO to VOUT Non-switch-over Threshold (4) VLDO to VOUT Switch-over Resistance LDO Drop Out Voltage (5) VOUT = +5V From VIN to VVLDO, VVLDO = +5V, IVLDO = 100mA 135 -140 -450 2 1.2 200 +140 +450 mV mV V 4.9 5.0 85 mA 5.1 V
Conditions
Min
Typ
Max
Units
Notes: (1) VIN UVLO is programmable using a resistor divider from VIN to ENL to AGND. The ENL voltage is compared to an internal reference. (2) Guaranteed by design. (3) For V5V less than 4.5V, the On-Time may be limited by the V5V supply voltage and by VIN. See the TON Limitations and V5V Supply Voltage section in the applications Information. (4) The switch-over threshold is the maximum voltage differential between the VLDO and VOUT pins which ensures that VLDO will internally switch-over to VOUT. The non-switch-over threshold is the minimum voltage differential between the VLDO and VOUT pins which ensures that VLDO will not switch-over to VOUT. (5) The LDO drop out voltage is the voltage at which the LDO output drops 2% below the nominal regulation point. Thermal resistance, junction to ambient -- guaranteed by design (C/W)
7
SC414/SC424
Typical Characteristics
Characteristics in this section are based on using the Typical Application Circuit on page 2 (SC414/SC424).
Efficiency vs. Load -- Forced Continuous Mode
100
VOUT vs. Load -- Forced Continuous Mode
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
1.05
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
80
Output Voltage (V)
1.03
Efficiency (%)
60
1.01
40
0.99
20
0.97
0
0.001
0.01
0.1 IOUT (A)
1
10
0.95 0.001
0.01
0.1 IOUT (A)
1
10
Efficiency vs. Load -- Powersave Mode (SC414)
100
Efficiency vs. Load -- Powersave Mode (SC414)
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
100
Externally biased, VIN = 12V, VOUT = 1V
80
3V Bias
80
5V Bias
Efficiency (%)
60
Efficiency (%)
0.01 0.1 IOUT (A) 1 10
60
40
40
20
20
0 0.001
0 0.001
0.01
0.1 IOUT (A)
1
10
Efficiency vs. Load -- Powersave Mode (SC424)
100
Efficiency vs. Load -- Powersave Mode (SC424)
100
Externally biased, VIN = 12V, VOUT = 1V
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
80 3V Bias
Efficiency (%)
80
5V Bias
Efficiency (%)
60
60
40
40
20
20
0 0.001
0.01
0.1 IOUT (A)
1
10
0 0.001
0.01
0.1 IOUT (A)
1
10
8
SC414/SC424
Typical Characteristics (continued)
Characteristics in this section are based on using the Typical Application Circuit on page 2 (SC414/SC424).
Frequency vs. Load -- Forced Continuous Mode
400
VRIPPLE vs. Load -- Forced Continuous Mode
100
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
350
80
Frequency (kHz)
300
VRIPPLE (mVP_P)
0.01 0.1 IOUT (A) 1 10
60
250
40
200
150
20
100 0.001
0 0.001
0.01
0.1 IOUT (A)
1
10
VOUT vs. Load -- Powersave Mode (SC414)
1.05
VOUT vs. Load -- Powersave Mode (SC424)
1.05
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
1.03
1.03
VOUT (V)
1.01
VOUT(V)
1.01
0.99
0.99
0.97
0.97
0.95 0.001
0.01
0.1 IOUT (A)
1
10
0.95 0.001
0.01
0.1 IOUT (A)
1
10
VOUT vs. Line -- Forced Continuous Mode
1.05
VOUT vs. Line -- Forced Continuous Mode
100
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
Internally biased at VLDO = 5V, VIN = 12V, VOUT = 1V
1.03
80
Output Voltage (V)
0A 1.01
1.5A
VRIPPLE (mVP_P)
60
3A
6A
0.99
3A
6A
40
0.97
20
1.5A
0A
0.95
0
5
7
9
11 13 15 Input Voltage (V)
17
19
21
5
7
9
11 13 15 Input Voltage (V)
17
19
21
9
SC414/SC424
Typical Characteristics (continued)
Characteristics in this section are based on using the Typical Application Circuit on page 2 (SC414/SC424).
Ultrasonic Powersave Mode -- No Load (SC414)
VIN = 12V, VOUT = 1V, IOUT = 0A, VLDO = V5V = EN/PSV= ENL = 5V
Powersave Mode -- No Load (SC424)
VIN = 12V, VOUT = 1V, IOUT = 0A, VLDO = V5V = EN/PSV= ENL = 5V
(50mV/div) (50mV/div)
(5V/div) (5V/div)
Time (10s/div)
Time (10ms/div)
Self-Biased Start-Up -- Power Good True
VIN = 0V to 12V step, VOUT = 1V, IOUT = 0A, VLDO = V5V = EN/PSV= ENL = 5V
Forced Continuous Mode -- No Load
VIN = 12V, VOUT = 1V, IOUT = 0A, VLDO = V5V = ENL = 5V, EN/PSV= float
(500mV/div) (50mV/div)
(2V/div) (10V/div) (5V/div) (5V/div) Time (10ms/div) Time (2s/div)
Enabled Loaded Output -- Power Good True
VIN = 12V, VOUT = 1V, IOUT = 1A, VLDO = V5V = ENL = 5V. EN/PSV= 5V
Output Over-current Response -- Normal Operation
VIN = 12V, VOUT = 1V, VLDO = V5V = ENL = 5V, EN/PSV= floating; IOUT ramped to trip point (500mV/div)
(500mV/div)
(5A/div) (5V/div)
(10V/div)
(5V/div)
(5V/div) Time (1ms/div) Time (100s/div)
10
SC414/SC424
Typical Characteristics (continued)
Characteristics in this section are based on using the Typical Application Circuit on page 2 (SC414/SC424).
Transient Response -- Load Rising (SC414)
VIN = 12V, VOUT = 1V, IOUT = 200mA to 6A, VLDO = V5V = EN/PSV= ENL = 5V
Transient Response -- Load Falling (SC414)
VIN = 12V, VOUT = 1V, IOUT = 6A to 0A, VLDO = V5V = EN/PSV= ENL = 5V
(50mV/div)
(50mV/div)
(5A/div) (5A/div)
(5V/div)
(5V/div)
Time (10s/div)
Time (10s/div)
Transient Response -- Load Rising (SC424)
VIN = 12V, VOUT = 1V, IOUT = 0A to 6A, VLDO = V5V = EN/PSV= ENL = 5V
Transient Response -- Load Falling (SC424)
VIN = 12V, VOUT = 1V, IOUT = 6A to 0A, VLDO = V5V = EN/PSV= ENL = 5V
(50mV/div)
(50mV/div)
(10V/div)
(10V/div)
(5A/div) (5A/div) (5V/div)
(5V/div)
Time (10s/div)
Time (10s/div)
Shorted Output Response -- Normal Operation
VIN = 12V, VOUT = 1V, IOUT = 0A, VLDO = V5V = EN/PSV= ENL = 5V (500mV/div)
Shorted Output Response -- Power-UP Operation
VIN = 12V, VOUT = 1V, IOUT = 0A, VLDO = V5V = EN/PSV= ENL = 5V
(500mV/div)
(5A/div) (5A/div)
(10V/div) (10V/div) (5V/div) (5V/div) Time (40s/div) Time (1ms/div)
11
SC414/SC424
Pin Descriptions
Pin #
1
Pin Name
FB
Pin Function
Feedback input for switching regulator used to program the output voltage -- connect to an external resistor divider from VOUT to AGND. Bias input for internal analog circuits and gate drives -- connect to external 3V or 5V supply or bias connection to VLDO. Analog ground Switcher output voltage sense pin, and also the input to the internal switch-over between VOUT and VLDO. Input supply voltage 5V LDO output Bootstrap pin -- connect a capacitor from BST to LXBST to develop the floating supply for the high-side gate drive. LX Boost -- connect to the BST capacitor. Switching (phase) node Power ground Open-drain power good indicator -- high impedance indicates power is good. An external pull-up resistor is required. Current limit sense pin -- used to program the current limit by connecting a resistor from ILIM to LXS. LX sense -- connects to RILIM resistor Enable/power save input for the switching regulator -- connect to AGND to disable the switching regulator. Float to operate in forced continuous mode (power save disabled). For SC414, connect to V5V to operate with ultrasonic power save mode enabled. For SC424, connect to V5V to operate with power save mode enabled with no minimum frequency. On-time programming input -- set the on-time by connecting through a resistor to AGND Enable input for the LDO -- connect ENL to AGND to disable the LDO. Drive with logic to +3V for logic control, or program the VIN UVLO with a resistor divider between VIN, ENL, and AGND.
2 3, 26, PAD 1 4 5, 8-11 PAD 2 6 7 12 15,20, 21, PAD 3 13, 14, 16-19 22 23 24
V5V AGND VOUT VIN VLDO BST LXBST LX PGND PGOOD ILIM LXS
25
EN/PSV
27 28
TON ENL
12
SC414/SC424
Block Diagram
V5V 2 V5V V5V Bootstrap Switch AGND D Reference Control & Status DL Soft Start Hi-side MOSFET B FB 1 FB Comparator DL Zero Cross Detector VOUT 4 Bypass Comparator Valley Current Limit A VLDO 6 Y B VLDO Switchover MUX LDO VIN 3 ILIM C PGND 24 Lo-side MOSFET LXS On- time Generator Gate Drive Control V5V 12 LX LXBST 7 BST PGOOD 22 EN/PSV 25 A VIN VIN
TON
27
28 ENL A = connected to pins 5, 8-11, PAD 2 B = connected to pins 15, 20, 21, PAD 3 C = connected to pins 13, 14, 16-19 D = connect to pins 3, 26, PAD 1
13
SC414/SC424
Applications Information
Synchronous Buck Converter
The SC414/SC424 is a step down synchronous DC-DC buck converter with integrated power MOSFETs and a 5V LDO. The device is capable of 6A operation at very high efficiency. A space saving 4x4 (mm) 28-pin package is used. The programmable operating frequency range of 200kHz to 1MHz enables optimizing the configuration for PCB area and efficiency. The buck controller uses a pseudo-fixed frequency adaptive on-time control. This control method allows fast transient response which permits the use of smaller output capacitors. The adaptive on-time is determined by an internal oneshot timer. When the one-shot is triggered by the output ripple, the device sends a single on-time pulse to the highside MOSFET. The pulse period is determined by VOUT and VIN. The period is proportional to output voltage and inversely proportional to input voltage. With this adaptive on-time configuration, the device automatically anticipates the on-time needed to regulate VOUT for the present VIN condition and at the selected frequency. The advantages of adaptive on-time control are:
* * * * *
Input Voltage Requirements
The SC414/SC424 requires two input supplies for normal operation: VIN and V5V. VIN operates over the wide range from 3V to 28V. V5V requires a 3.3 or 5V supply input that can be an external source or the internal LDO configured to supply 5V. If the LDO is enabled, V5V voltage must be > 5V.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC414/SC424 is pseudo-fixed frequency, adaptive on-time, as shown in Figure 1. The ripple voltage generated at the output capacitor (ESR) is used as a PWM ramp signal. This ripple is used to trigger the on-time of the controller.
VIN TON VLX CIN
Predictable operating frequency compared to other variable frequency methods. Reduced component count by eliminating the error amplifier and compensation components. Reduced component count by removing the need to sense and control inductor current. Fast transient response -- the response time is controlled by a fast comparator instead of a typically slow error amplifier. Reduced output capacitance due to fast transient response
One-Shot Timer and Operating Frequency
One-shot timer operation is shown in Figure 2. The FB Comparator output goes high when VFB is less than the internal 750mV reference. This feeds into the gate drive and turns on the high-side MOSFET, and starts the oneshot timer. The one-shot timer uses an internal comparator and a capacitor. One comparator input is connected to VOUT, the other input is connected to the capacitor. When the on-time begins, the internal capacitor charges from zero volts through a current which is proportional to VIN. When the capacitor voltage reaches VOUT, the on-time is completed and the high-side MOSFET turns off.
FB Comparator
+
Q1 VLX
VFB
FB Threshold VOUT
L Q2 ESR
FB + COUT
FB 750mV
Gate Drives DH
VIN Q1 VLX Q2 VOUT FB +
L ESR COUT
VOUT VIN RTON
One-Shot Timer
DL
Figure 1 -- PWM Control Method, VOUT Ripple
On-time = K x RTON x (VOUT/VIN)
Figure 2 -- On-Time Generation
14
SC414/SC424
Applications Information (continued)
This method automatically produces an on-time that is proportional to VOUT and inversely proportional to VIN. Under steady-state conditions, the switching frequency can be determined from the on-time by the following equation.
fSW VOUT TON VIN
RTON
1 25pF fSW
400
(V5V 1.6V) 10 VOUT
Note that when VIN > (V5V - 1.6V) x 10 , the actual on-time is fixed and does not vary with VIN. When operating in this condition, the switching frequency will vary inversely with VIN rather than approximating a fixed frequency.
The SC414/SC424 uses an external resistor to set the ontime which indirectly sets the frequency. The on-time can be programmed to provide an operating frequency from 200kHz to 1MHz using a resistor between the TON pin and ground. The resistor value is selected by the following equation.
RTON 1 25pF fSW 400 VIN VOUT
VOUT Voltage Selection
The switcher output voltage is regulated by comparing VOUT as seen through a resistor divider at the FB pin to the internal 750mV reference voltage (see Figure 3).
VOUT R1 R2 To FB pin
The maximum RTON value allowed is shown by the following equation.
RTON _ MAX VIN _ MIN 10 1.5 A
Figure 3 -- Output Voltage Selection Note that this control method regulates the valley of the output ripple voltage, not the DC value. The DC output voltage VOUT is offset by the output ripple according to the following equation.
VOUT 0.75 1 R1 R2 VRIPPLE 2
Immediately after the on-time, the DL (drive signal for the low side FET) output drives high to turn on the low-side MOSFET. DL has a minimum high time of ~320ns, after which DL continues to stay high until one of the following occurs:
* *
VFB falls below the 750mV reference The Zero Cross Detector senses that the voltage on the LX node is below ground. Power Save is activated when a zero crossing is detected.
When a large capacitor is placed in parallel with R1 (C TOP) VOUT is shown by the following equation.
VOUT 0.75 1 R1 R2 VRIPPLE 2 1 (R1 CTOP )2 1 R 2 R1 CTOP R 2 R1
2
TON limitations and V5V Supply Voltage
For V5V below 4.5V, the TON accuracy may be limited by the input voltage. The original RTON equation is accurate if VIN satisfies the below relation over the entire VIN range: VIN < (V5V - 1.6V) x 10 If VIN exceeds (V5V - 1.6V) x 10, for all or part of the VIN range, the RTON equation is not accurate. In all cases where VIN > (V5V - 1.6V) x 10, the RTON equation must be modified as follows.
Where is the angular switching frequency.
Enable and Power-save Inputs
The EN/PSV and ENL inputs are used to enable or disable the switching regulator and the LDO. When EN/PSV is low (grounded), the switching regulator is off and in its lowest power state. When off, the output of the switching regulator soft-discharges the output into a 10 internal resistor via the VOUT pin. When EN/PSV is allowed to float, the pin voltage will float to 33% of the voltage at V5V. The switching regulator turns on with power-save disabled and all switching is in forced continuous mode. For V5V < 4.5V, it
15
SC414/SC424
Applications Information (continued)
is recommended to force 33% of the V5V voltage on the EN/PSV pin to operate in forced continuous mode. When EN/PSV is high (above 45% of the voltage at V5V) for SC414, the switching regulator turns on with ultrasonic power-save enabled. The SC414 ultrasonic power-save operation maintains a minimum switching frequency of 25kH z, for applications with stringent audio requirements. When EN/PSV is high (above 45% of the voltage at V5V) for SC424, the switching regulator turns on with powersave enabled. The SC424 power-save operation is designed to maximize efficiency at light loads with no minimum frequency limits. This makes the SC424 an excellent choice for portable and battery-operated systems. The ENL input is used to control the internal LDO. This input provides a second function by acting as a VIN ULVO sensor for the switching regulator. When ENL is low (grounded), the LDO is off. When ENL is a logic high but below the VIN UVLO threshold (2.6V typical), then the LDO is on and the switcher is off. When ENL is above the VIN UVLO threshold, the LDO is enabled and the switcher is also enabled if the EN/PSV pin is not grounded.
FB Ripple Voltage (VFB) FB threshold (750mV)
Inductor Current
DC Load Current
On-time (TON) DH
DH on-time is triggered when VFB reaches the FB Threshold.
DL
DL drives high when on-time is completed. DL remains high until VFB falls to the FB threshold.
Figure 4 -- Forced Continuous Mode Operation
Ultrasonic Power-save Operation (SC414)
The SC414 provides ultrasonic power-save operation at light loads, with the minimum operating frequency fixed at slightly under 25kHz. This is accomplished by using an internal timer that monitors the time between consecutive high-side gate pulses. If the time exceeds 40s, DL drives high to turn the low-side MOSFET on. This draws current from VOUT through the inductor, forcing both VOUT and VFB to fall. When VFB drops to the 750mV threshold, the next DH (the drive signal for the high side FET) on-time is triggered. After the on-time is completed the high-side MOSFET is turned off and the low-side MOSFET turns on. The low-side MOSFET remains on until the inductor current ramps down to zero, at which point the low-side MOSFET is turned off.
Forced Continuous Mode Operation
The SC414/SC424 operates the switcher in Forced Continuous Mode (FCM) by floating the EN/PSV pin (see Figure 4). In this mode of operation, the MOSFETs are turned on alternately to each other with a short dead time between them to avoid cross conduction. This feature results in uniform frequency across the full load range with the trade-off being poor efficiency at light loads due to the high-frequency switching of the MOSFETs. For V5V < 4.5V, it is recommended to force 33% of the V5V voltage on the EN/PSV pin to operate in forced continuous mode.
16
SC414/SC424
Applications Information (continued)
Because the on-times are forced to occur at intervals no greater than 40s, the frequency will not fall far below 25kHz. Figure 5 shows ultrasonic power-save operation.
minimum fSW ~ 25kHz FB Ripple Voltage (VFB) FB threshold (750mV) Inductor Current (0A)
DH Inductor Current Zero (0A) FB Ripple Voltage (VFB) Dead time varies according to load FB threshold (750mV)
On-time (TON) DH On-time is triggered when VFB reaches the FB Threshold.
On-time (TON) DH 40s time-out DL
DH On-time is triggered when VFB reaches the FB Threshold
DL
DL drives high when on-time is completed. DL remains high until inductor current reaches zero.
Figure 6 -- Power-save Operation
After the 40sec time-out, DL drives high if VFB has not reached the FB threshold.
Smart Power-save Protection
Active loads may leak current from a higher voltage into the switcher output. Under light load conditions with power-save enabled, this can force VOUT to slowly rise and reach the over-voltage threshold, resulting in a hard shutdown. Smart power-save prevents this condition. When the FB voltage exceeds 10% above nominal (exceeds 825mV), the device immediately disables power-save, and DL drives high to turn on the low-side MOSFET. This draws current from VOUT through the inductor and causes VOUT to fall. When VFB drops back to the 750mV trip point, a normal TON switching cycle begins. This method prevents a hard OVP shutdown and also cycles energy from VOUT back to VIN. It also minimizes operating power by avoiding forced conduction mode operation. Figure 7 shows typical waveforms for the Smart Power-save feature.
Figure 5 -- Ultrasonic Power-save Operation
Power-save Mode Operation (SC424)
The SC424 provides power-save operation at light loads with no minimum operating frequency. With power-save enabled, the internal zero crossing comparator monitors the inductor current via the voltage across the low-side MOSFET during the off-time. If the inductor current falls to zero for 8 consecutive switching cycles, the controller enters power-save operation. It will turn off the low-side MOSFET on each subsequent cycle provided that the current crosses zero. At this time both MOSFETs remain off until VFB drops to the 750mV threshold. Because the MOSFETs are off, the load is supplied by the output capacitor. If the inductor current does not reach zero on any switching cycle, the controller immediately exits powersave and returns to forced continuous mode. Figure 6 shows power-save operation at light loads.
SmartDriveTM
For each DH pulse, the DH driver initially turns on the high-side MOSFET at a slower speed, allowing a softer, smooth turn-off of the low-side diode. Once the DH node is high and the LX voltage has risen 1V above PGND, the SmartDrive circuit automatically drives the high-side MOSFET on at a rapid rate. This technique reduces ringing while maintaining high efficiency and also avoids the need for snubbers or series resistors in the gate drive.
17
SC414/SC424
Applications Information (continued)
VOUT drifts up to due to leakage current flowing into COUT Smart Power Save Threshold (825mV) FB threshold DH and DL off High-side Drive (DH) Single DH on-time pulse after DL turn-off Low-side Drive (DL) DL turns on when Smart PSAVE threshold is reached DL turns off when FB threshold is reached Normal DL pulse after DH on-time pulse
Inductor Current
VOUT discharges via inductor and low-side MOSFET Normal VOUT ripple
IPEAK ILOAD ILIM
Time
Figure 8 -- Valley Current Limit Setting the valley current limit to 6A results in a peak inductor current of 6A plus peak ripple current. In this situation, the average (load) current through the inductor is 6A plus one-half the peak-to-peak ripple current. The internal 8A current source is temperature compensated at 4100ppm in order to provide tracking with the RDSON. The RILIM value is calculated by the following equation. RILIM = 1250 x ILIM x [0.088 x (5V - V5V) + 1] When selecting a value for RILIM do not exceed the absolute maximum voltage value for the ILIM pin. Note that because the low-side MOSFET with low RDSON is used for current sensing, the PCB layout, solder connections, and PCB connection to the LX node must be done carefully to obtain good results. RILIM should be connected directly to LXS (pin 24).
Figure 7 -- Smart Power-save
Current Limit Protection
The device features programmable current limiting, which is accomplished by using the RDSON of the lower MOSFET for current sensing. The current limit is set by RILIM resistor. The RILIM resistor connects from the ILIM pin to the LXS pin which is also the drain of the low-side MOSFET. When the low-side MOSFET is on, an internal ~8A current flows from the ILIM pin and through the RILIM resistor, creating a voltage drop across the resistor. While the low-side MOSFET is on, the inductor current flows through it and creates a voltage across the RDSON. The voltage across the MOSFET is negative with respect to ground. If this MOSFET voltage drop exceeds the voltage across RILIM, the voltage at the ILIM pin will be negative and current limit will activate. The current limit then keeps the low-side MOSFET on and will not allow another high-side on-time, until the current in the low-side MOSFET reduces enough to bring the ILIM voltage to zero. This method regulates the inductor valley current at the level shown by ILIM in Figure 8.
Soft-Start of PWM Regulator
Soft-start is achieved in the PWM regulator by using an internal voltage ramp as the reference for the FB comparator. The voltage ramp is generated using an internal charge pump which drives the reference from zero to 750mV in ~1.8mV increments, using an internal ~500kHz oscillator. When the ramp voltage reaches 750mV, the ramp is ignored and the FB comparator switches over to a fixed 750mV threshold. During soft-start the output voltage tracks the internal ramp, which limits the start-up inrush current and provides a controlled soft-start profile for a wide range of applications. Typical soft-start ramp time is 1.7ms.
18
SC414/SC424
Applications Information (continued)
During soft-start the regulator turns off the low-side MOSFET on any cycle if the inductor current falls to zero. This prevents negative inductor current, allowing the device to start into a pre-biased output. This soft start operation is implemented even if FCM is selected. FCM operation is allowed only after PGOOD is high. then begins a soft-start cycle. The PWM will shut off if V5V falls below 2.7V.
LDO Regulator
The device features an integrated LDO regulator with a fixed output voltage of 5V. There is also an enable pin (ENL) for the LDO that provides independent control. The LDO voltage can also be used to provide the bias voltage for the switching regulator. A minimum capacitance of 1F referenced to AGND is normally required at the output of the LDO for stability. If the LDO is providing bias power to the device, then a minimum 0.1F capacitor referenced to AGND is required, along with a minimum 1F capacitor referenced to PGND to filter the gate drive pulses. Refer to the layout guidelines section.
Power Good Output
The power good (PGOOD) output is an open-drain output which requires a pull-up resistor. When the output voltage is 10% below the nominal voltage, PGOOD is pulled low. It is held low until the output voltage returns to the nominal voltage. PGOOD is held low during start-up and will not be allowed to transition high until soft-start is completed (when VFB reaches 750mV) and typically 4ms has passed. PGOOD will transition low if the VFB pin exceeds +20% of nominal, which is also the over-voltage shutdown threshold (900mV). PGOOD also pulls low if the EN/PSV pin is low when V5V is present.
LDO Start-up
Before start-up, the LDO checks the status of the following signals to ensure proper operation can be maintained. 1. ENL pin 2. VLDO output 3. VIN input voltage When the ENL pin is high, the LDO will begin start-up, see Figure 10. During the initial phase, when the LDO output voltage is near zero, the LDO initiates a current-limited start-up (typically 85mA) to charge the output capacitor. When VLDO has reached 90% of the final value, the LDO current limit is increased to ~200mA and the LDO output is quickly driven to the nominal value by the internal LDO regulator.
VVLDO Final 90% of VVLDO Final Voltage regulating with ~200mA current limit
Output Over-Voltage Protection
Over-Voltage Protection (OVP) becomes active as soon as the device is enabled. The threshold is set at 750mV + 20% (900mV). When VFB exceeds the OVP threshold, DL latches high and the low-side MOSFET is turned on. DL remains high and the controller remains off, until the EN/PSV input is toggled or V5V is cycled. There is a 5s delay built into the OVP detector to prevent false transitions. PGOOD is also low after an OVP event.
Output Under-Voltage Protection
When VFB falls to 75% of its nominal voltage (falls to 562.5mV) for eight consecutive clock cycles, the switcher is shut off and the DH and DL drives are pulled low to turn off the MOSFETs. The controller stays off until EN/PSV is toggled or V5V is cycled.
V5V UVLO, and POR
Under-Voltage Lock-Out (UVLO) circuitry inhibits switching and tri-states the DH/DL drivers until V5V rises above 2.9V. An internal Power-On Reset (POR) occurs when V5V exceeds 2.9V, which resets the fault latch and soft-start counter to begin the soft-start cycle. The SC414/SC424
Constant current startup
Figure 10 -- LDO Start-Up
19
SC414/SC424
Applications Information (continued)
LDO Switch-Over Operation
The SC414/SC424 includes a switch-over function for the LDO. The switch-over function is designed to increase efficiency by using the more efficient DC-DC converter to power the LDO output, avoiding the less efficient LDO regulator when possible. The switch-over function connects the VLDO pin directly to the VOUT pin using an internal switch. When the switch-over is complete the LDO is turned off, which results in a power savings and maximizes efficiency. If the LDO output is used to bias the SC414/SC424, then after switch-over the device is selfpowered from the switching regulator with the LDO turned off. The switch-over logic waits for 32 switching cycles before it starts the switch-over. There are two methods that determine the switch-over of VLDO to VOUT. In the first method, the LDO is already in regulation and the DC-DC converter is later enabled. As soon as the PGOOD output goes high, the 32 cycle counter is started. The voltages at the VLDO and VOUT pins are then compared; if the two voltages are within 300mV (typically) of each other, within 32 cylces, the VLDO pin connects to the VOUT pin using an internal switch, and the LDO is turned off. In the second method, the DC-DC converter is already running and the LDO is enabled. In this case the 32 cycles are started as soon as the LDO reaches 90% of its final value. At this time, the VLDO and VOUT pins are compared, and if within 300mV (typically) the switch-over occurs and the LDO is turned off.
Switch-over MOSFET Parasitic Diodes
The switch-over MOSFET contains parasitic diodes that are inherent to its construction, as shown in Figure 11.
Switchover control VLDO Switchover MOSFET VOUT
Parasitic diode V5V
Parasitic diode
Figure 11-- Switch-over MOSFET Parasitic Diodes There are some important design rules that must be followed to prevent forward bias of these diodes. The following two conditions need to be satisfied in order for the parasitic diodes to stay off.
* *
V5V VLDO V5V VOUT
If either VLDO or VOUT is higher than V5V, then the respective diode will turn on and the SC414/SC424 operating current will flow through this diode. This has the potential of damaging the device.
ENL pin and VIN UVLO
The ENL pin also acts as the switcher under-voltage lockout for the VIN supply. The VIN UVLO voltage is programmable via a resistor divider at the VIN, ENL, and AGND pins. ENL is the enable/disable signal for the LDO. In order to implement the VIN UVLO there is also a timing requirement that needs to be satisfied. If the ENL pin transitions low within 2 switching cycles and is < 1V, then the LDO will turn off but the switcher remains on. If ENL goes below the VIN UVLO threshold and stays above 1V, then the switcher will turn off but the LDO remains on. The VIN UVLO function has a typical threshold of 2.6V on the VIN rising edge. The falling edge threshold is 2.4V.
Switch-over Limitations on VOUT and VLDO
Because the internal switch-over circuit always compares the VOUT and VLDO pins at start-up, there are voltage limitations on permissible combinations of these pins. Consider the situation where VOUT is programmed to 4.7V. After start-up, the device would connect VOUT to VLDO and disable the LDO, since the two voltage are within the 300mV switch-over window. To avoid unwanted switchover, the minimum difference between the voltages for VOUT and VLDO should be 500mV.
20
SC414/SC424
Applications Information (continued)
Note that it is possible to operate the switcher with the LDO disabled, but the ENL pin must be below the logic low threshold (0.4V maximum). The table below summarizes the function of the ENL and EN pins, with respect to the rising edge of ENL. EN low high low high low high ENL low, < 0.4V low, < 0.4V high, < 2.6V high, < 2.6V high, > 2.6V high, > 2.6V LDO status off off on on on on Switcher status off on off off off on signal. If PGOOD is high, then the switcher is already running and the LDO will run through the start-up cycle without affecting the switcher. If PGOOD is low, then the LDO will not allow any PWM switching until the LDO output has reached 90% of it's final value.
Using the On-chip LDO to Bias the SC414/SC424
The following steps must be followed when using the internal LDO to bias the device.
* * *
Figure 12 below shows the ENL voltage thresholds and their effect on LDO and Switcher operation.
ENL voltage LDO on Switcher on if EN = high 2.6V 2.4V
VIN UVLO hysteresis
Connect V5V to VLDO before enabling the LDO. Any external load on VLDO should not exceed 40mA until the LDO voltage has reached 90% of final value. Do not connect the EN pin directly to the V5V or any other supply voltage if Vout is greater than or equal to 4.5V
ENL low threshold (min 0.4V) AGND
LDO on Switcher off by VIN UVLO
Many applications connect the EN pin to V5V and control the on/off of the LDO and PWM simultaneously with the ENL pin. This allows one signal to control both the bias and power output of the SC414. When VOUT > 4.5V this configuration can cause problems due to the parasitic diodes in the LDO switchover circuitry. After the Vout > 4.5V PWM output is up and running the switchover diodes can hold up V5V > UVLO even if the ENL pin is grounded, turning off the LDO. Operating in this way can potentially damage the part.
LDO off Switcher on if EN = high
Design Procedure
When designing a switch mode supply the input voltage range, load current, switching frequency, and inductor ripple current must be specified. The maximum input voltage (VINMAX) is the highest specified input voltage. The minimum input voltage ( VINMIN) is determined by the lowest input voltage after evaluating the voltage drops due to connectors, fuses, switches, and PCB traces. The following parameters define the design.
Figure 12 -- ENL Thresholds
ENL Logic Control of PWM Operation
When the ENL input is driven above 2.6V, it is impossible to determine if the LDO output is going to be used to power the device or not. In self-powered operation where the LDO will power the device, it is necessary during the LDO start-up to hold the PWM switching off until the LDO has reached 90% of the final value. This prevents overloading the current-limited LDO output during the LDO start-up. However, if the switcher was previously operating (with EN/PSV high but ENL at ground, and V5V supplied externally), then it is undesirable to shut down the switcher. To prevent this, when the ENL input is above 2.6V (above the VIN UVLO threshold), the internal logic checks the PGOOD
* * * *
Nominal output voltage (VOUT ) Static or DC output tolerance Transient response Maximum load current (IOUT )
21
SC414/SC424
Applications Information (continued)
There are two values of load current to evaluate -- continuous load current and peak load current. Continuous load current relates to thermal stresses which drive the selection of the inductor and input capacitors. Peak load current determines instantaneous component stresses and filtering requirements such as inductor saturation, output capacitors, and design of the current limit circuit. The following values are used in this design. efficient. However, larger inductance translates directly into larger packages and higher cost. Cost, size, output ripple, and efficiency are all used in the selection process. The ripple current will also set the boundary for powersave operation. The switching will typically enter powersave mode when the load current decreases to 1/2 of the ripple current. For example, if ripple current is 4A then Power-save operation will typically start for loads less than 2A. If ripple current is set at 40% of maximum load current, then power-save will start for loads less than 20% of maximum current. The inductor value is typically selected to provide a ripple current that is between 25% to 50% of the maximum load current. This provides an optimal trade-off between cost, efficiency, and transient performance. During the DH on-time, voltage across the inductor is (VIN - VOUT ). The equation for determining inductance is shown next.
L ( VIN VOUT ) TON IRIPPLE
* * * *
VIN = 12V + 10% VOUT = 1V + 4% fSW = 250kHz Load = 6A maximum
Frequency Selection Selection of the switching frequency requires making a trade-off between the size and cost of the external filter components (inductor and output capacitor) and the power conversion efficiency. The desired switching frequency is 250kHz which results from using components selected for optimum size and cost . A resistor (RTON) is used to program the on-time (indirectly setting the frequency) using the following equation.
RTON 1 25pF fSW 400 VIN VOUT
Example
In this example, the inductor ripple current is set equal to 50% of the maximum load current. Therefore ripple current will be 50% x 6A or 3A. To find the minimum inductance needed, use the VIN and TON values that correspond to VINMAX.
L (13.2V 1V ) 318ns 3A 1.26 H
To select RTON, use the maximum value for VIN, and for TON use the value associated with maximum VIN.
T ON V OUT V INMAX f SW
TON = 303 ns at 13.2VIN, 1VOUT, 250kHz Substituting for RTON results in the following solution. RTON = 130.9k, use RTON = 130k Inductor Selection In order to determine the inductance, the ripple current must first be defined. Low inductor values result in smaller size but create higher ripple current which can reduce efficiency. Higher inductor values will reduce the ripple current/voltage and for a given DC resistance are more
A slightly larger value of 1.5H is selected. This will decrease the maximum IRIPPLE to 2.53A. Note that the inductor must be rated for the maximum DC load current plus 1/2 of the ripple current. The ripple current under minimum VIN conditions is also checked using the following equations.
TON _ VINMIN 25pF R TON VOUT VINMIN 10ns 311ns
22
SC414/SC424
Applications Information (continued)
IRIPPLE ( VIN VOUT ) TON L
Assuming a peak voltage VPEAK of 1.150 (100mV rise upon load release), and a 6A load release, the required capacitance is shown by the next equation.
2.03 A
IRIPPLE _ VINMIN
(10.8 1V ) 311ns 1. 5 H
1 .5 H 6 A COUTMIN 1.05 V
1 2.53 A 2
2
2
Capacitor Selection The output capacitors are chosen based on required ESR and capacitance. The maximum ESR requirement is controlled by the output ripple requirement and the DC tolerance. The output voltage has a DC value that is equal to the valley of the output ripple plus 1/2 of the peak-to-peak ripple. Change in the output ripple voltage will lead to a change in DC voltage at the output. The design goal is for the output voltage regulation to be 4% under static conditions. The internal 750mV reference tolerance is 1%. Assuming a 1% tolerance from the FB resistor divider, this allows 2% tolerance due to VOUT ripple. Since this 2% error comes from 1/2 of the ripple voltage, the allowable ripple is 4%, or 40mV for a 1V output. The maximum ripple current of 2.53A creates a ripple voltage across the ESR. The maximum ESR value allowed is shown by the following equations.
ESRMAX VRIPPLE IRIPPLEMAX 40mV 2.53 A
1V
2
COUTMIN = 772F If the load release is relatively slow, the output capacitance can be reduced. At heavy loads during normal switching, when the FB pin is above the 750mV reference, the DL output is high and the low-side MOSFET is on. During this time, the voltage across the inductor is approximately -VOUT. This causes a down-slope or falling di/dt in the inductor. If the load di/dt is not much faster than the -di/dt in the inductor, then the inductor current will tend to track the falling load current. This will reduce the excess inductive energy that must be absorbed by the output capacitor, therefore a smaller capacitance can be used. The following can be used to calculate the needed capacitance for a given dILOAD/dt. Peak inductor current is shown by the next equation. ILPK = IMAX + 1/2 x IRIPPLEMAX ILPK = 6A + 1/2 x 2.53A = 7.26A
Rate of change of Load Current dlLOAD dt
ESRMAX = 15.8 m The output capacitance is chosen to meet transient requirements. A worst-case load release, from maximum load to no load at the exact moment when inductor current is at the peak, determines the required capacitance. If the load release is instantaneous (load changes from maximum to zero in < 1s), the output capacitor must absorb all the inductor's stored energy. This will cause a peak voltage on the capacitor according to the following equation.
L IOUT COUTMIN 1 IRIPPLEMAX 2
2 2
IMAX = maximum load release = 6A
L COUT ILPK
ILPK VOUT 2 VPK
IMAX dlLOAD VOUT
dt
Example
dlLOAD dt 1.25 A 1s
VPEAK
VOUT
2
23
SC414/SC424
Applications Information (continued)
This causes the output current to move from 6A to 0A in 4.8s, giving the minimum output capacitance requirement shown in the following equation.
CTOP
1 .5 H COUT 7.26 A
7.26 A 6A 1s 1V 1.25 A 2 1.05 V 1V
VOUT
R1 R2
To FB pin
COUT
443 F
Figure 13 -- Capacitor Coupling to FB Pin ESR loop instability is caused by insufficient ESR. The details of this stability issue are discussed in the ESR Requirements section. The best method for checking stability is to apply a zero-to-full load transient and observe the output voltage ripple envelope for overshoot and ringing. Ringing for more than one cycle after the initial step is an indication that the ESR should be increased. One simple way to solve this problem is to add trace resistance in the high current output path. A side effect of adding trace resistance is a decrease in load regulation.
Note that COUT is much smaller in this example, 443F compared to 772F based on a worst-case load release. To meet the two design criteria of minimum 443F and maximum 15m ESR, select two capacitors rated at 220F and 15m ESR or less. It is recommended that an additional small capacitor be placed in parallel with COUT in order to filter high frequency switching noise.
Stability Considerations
Unstable operation is possible with adaptive on-time controllers, and usually takes the form of double-pulsing or ESR loop instability. Double-pulsing occurs due to switching noise seen at the FB input or because the FB ripple voltage is too low. This causes the FB comparator to trigger prematurely after the minimum off-time has expired. In extreme cases the noise can cause three or more successive on-times. Doublepulsing will result in higher ripple voltage at the output, but in most applications it will not affect operation. This form of instability can usually be avoided by providing the FB pin with a smooth, clean ripple signal that is at least 10mVp-p, which may dictate the need to increase the ESR of the output capacitors. It is also imperative to provide a proper PCB layout as discussed in the Layout Guidelines section. Another way to eliminate doubling-pulsing is to add a small (~ 10pF) capacitor across the upper feedback resistor, as shown in Figure 13. This capacitor should be left unpopulated unless it can be confirmed that doublepulsing exists. Adding the CTOP capacitor will couple more ripple into FB to help eliminate the problem. An optional connection on the PCB should be available for this capacitor.
ESR Requirements
A minimum ESR is required for two reasons. One reason is to generate enough output ripple voltage to provide 10mVp-p at the FB pin (after the resistor divider) to avoid double-pulsing. The second reason is to prevent instability due to insufficient ESR. The on-time control regulates the valley of the output ripple voltage. This ripple voltage is the sum of the two voltages. One is the ripple generated by the ESR, the other is the ripple due to capacitive charging and discharging during the switching cycle. For most applications, the total output ripple voltage is dominated by the output capacitors, typically SP or POSCAP devices. For stability the ESR zero of the output capacitor should be lower than approximately one-third the switching frequency. The formula for minimum ESR is shown by the following equation.
ESR MIN 3 C OUT
2
f sw
24
SC414/SC424
Applications Information (continued)
Using Ceramic Output Capacitors
When applications use ceramic output capacitors, the ESR is normally too small to meet the previously stated ESR criteria. In these applications it is necessary to add a small virtual ESR network composed of two capacitors and one resistor, as shown in Figure 14. This network creates a ramp voltage across CL, analogous to the ramp voltage generated across the ESR of a standard capacitor. This ramp is then capacitively coupled into the FB pin via capacitor CC.
L Highside RL CC FB pin CL R1 COUT R2
comparator offset is trimmed so that under static conditions it trips when the feedback pin is 750mV, 1%. The on-time pulse from the SC414/SC424 in the design example is calculated to give a pseudo-fixed frequency of 250kHz. Some frequency variation with line and load is expected. This variation changes the output ripple voltage. Because constant on-time converters regulate to the valley of the output ripple, 1/2 of the output ripple appears as a DC regulation error. For example, if the output ripple is 50mV with VIN = 6 volts, then the measured DC output will be 25mV above the comparator trip point. If the ripple increases to 80mV with VIN = 25V, then the measured DC output will be 40mV above the comparator trip. The best way to minimize this effect is to minimize the output ripple. To compensate for valley regulation, it may be desirable to use passive droop. Take the feedback directly from the output side of the inductor and place a small amount of trace resistance between the inductor and output capacitor. This trace resistance should be optimized so that at full load the output droops to near the lower regulation limit. Passive droop minimizes the required output capacitance because the voltage excursions due to load steps are reduced as seen at the load. The use of 1% feedback resistors may result in up to an additional 1% error. If tighter DC accuracy is required, resistors with lower tolerances should be used. The output inductor value may change with current. This will change the output ripple and therefore will have a minor effect on the DC output voltage. The output ESR also affects the output ripple and thus has a minor effect on the DC output voltage.
Lowside
Figure 14 -- Virtual ESR Ramp Current
Output Voltage Dropout
The output voltage adjustable range for continuous-conduction operation is limited by the fixed 320ns (typical) minimum off-time. When working with low input voltages, the duty-factor limit must be calculated using worstcase values for on and off times. The duty-factor limitation is shown by the next equation.
DUTY TON(MIN) TON(MIN) TOFF(MIN)
Switching Frequency Variations
The switching frequency will vary depending on line and load conditions. The line variations are a result of fixed propagation delays in the on-time one-shot, as well as unavoidable delays in the external MOSFET switching. As VIN increases, these factors make the actual DH on-time slightly longer than the ideal on-time. The net effect is that frequency tends to falls slightly with increasing input voltage.
The inductor resistance and MOSFET on-state voltage drops must be included when performing worst-case dropout duty-factor calculations.
System DC Accuracy -- VOUT Controller
Three factors affect VOUT accuracy: the trip point of the FB error comparator, the ripple voltage variation with line and load, and the external resistor tolerance. The error
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SC414/SC424
Applications Information (continued)
The switching frequency also varies with load current as a result of the power losses in the MOSFETs and the inductor. For a conventional PWM constant-frequency converter, as load increases the duty cycle also increases slightly to compensate for IR and switching losses in the MOSFETs and inductor. A constant on-time converter must also compensate for the same losses by increasing the effective duty cycle (more time is spent drawing energy from VIN as losses increase). The on-time is essentially constant for a given VOUT and VIN combination, to offset the losses the off-time will tend to reduce slightly as load increases. The net effect is that switching frequency increases slightly with increasing load. lator with a maximum current of 6A. The total PCB area is approximately 20 x 25 mm. Critical Layout Guidelines The following critical layout guidelines must be followed to ensure proper performance of the device.
* * * * * * *
IC Decoupling capacitors PGND plane AGND island FB, VOUT, and other analog control signals BST, ILIM, and LX Capacitors and Current Loops
PCB Layout Guidelines
The optimum layout for the SC414/SC424 is shown in Figure 15. This layout shows an integrated FET buck regu-
IC Decoupling Capacitors A 0.1 F capacitor must be located as close as possible to the IC and directly connected to pins 2 (V5V) and 3 (AGND).
AGND plane on inner layer
RES_GND -- AGND connects to PGND close to SC414/SC424
RILIM
Pin 1 marking SC414/SC424 with vias for LX, AGND, VIN
All components shown Top Side
RFB1 CFF
RFB2
CIN
PGND
VIN plane on inner or bottom layer
CBST
COUT
VOUT Plane on Top layer PGND on inner or bottom layer LX plane on inner or bottom layer
Note: This figure is not to scale
L
Figure 15 -- PCB Layout
26
SC414/SC424
Applications Information (continued)
* * * *
All other decoupling capacitors must be located as close as possible to the IC.
*
PGND Plane PGND requires its own copper plane with no other signal traces routed on it. Copper planes, multiple vias, and wide traces are needed to connect PGND to input capacitors, output capacitors, and the PGND pins on the IC. The PGND copper area between the input capacitors, output capacitors, and PGND pins must be as small as and as compact as possible to reduce the area of the PCB that is exposed to noise due to current flow on this node. Connect PGND to AGND with a short trace or 0 resistor. This connection should be as close to the IC as possible.
*
RILIM should be close to the IC and connected to LXS with a Kelvin trace to pin 24 on the IC. This will be a sufficient connection and will prevent the need to connect the resistor further into the LX plane. The feedback components for the switcher need to be as close to the FB pin of the IC as possible to reduce the possibility of noise corrupting these analog signals.
*
AGND Island AGND should have its own island of copper with no other signal traces routed on this layer that connects the AGND pins and pad of the IC to the analog control components. All of the components for the analog control circuitry should be located so that the connections to AGND are done by wide copper traces or vias down to AGND. Connect PGND to AGND with a short trace or 0 resistor. This connection should be as close to the IC as possible.
* * *
BST, ILIM and LX LX and BST are very noisy nodes and must be carefully routed to minimized the PCB area that is exposed to these signals. The connections for the boost capacitor between the IC and LX must be short and directly connected to the LXBST (pin 12). The connections for the current limit resistor between the ILIM pin and LX must be as short as possible and directly connected to pin 24 (LXS). The LX node between the IC and the inductor should be wide enough to handle the inductor current and short enough to eliminate the possibility of LX noise corrupting other signals. Multiple vias should be used to provide a good connection to LX between the IC and the inductor.
* * * * *
FB, VOUT, and Other Analog Control Signals The connection from the V OUT power to the analog control circuitry must be routed from the output capacitors and located on a quiet layer. The traces between VOUT and the analog control circuitry (VOUT, and FB pins) must be short and routed away from noise sources, such as BST, LX, VIN, and PGND between the input capacitors, output capacitors, and the IC. ILIM and TON nodes must be as short as possible to ensure the best accuracy in current limit and on time.
* *
Capacitors and Current Loops The current loops between the input capacitors, the IC, the inductor, and the output capacitors must be as close as possible to each other to reduce IR drop across the copper. All bypass and output capacitors must be connected as close as possible to the respective pin on the IC.
* *
*
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SC414/SC424
Outline Drawing -- MLPQ-4x4-28
A D B DIM A A1 A2 b D D1 E E1 e L N aaa bbb SEATING PLANE C 2.58 1.29 D1 e LxN DIMENSIONS MILLIMETERS MIN 0.80 0.00 0.17 3.90 0.96 3.90 2.48 NOM MAX 1.00 0.05 -
PIN 1 INDICATOR (LASER MARK)
E
A1 A aaa C
A2
(0.20) 0.23 0.29 4.00 4.10 1.06 1.16 4.00 4.10 2.58 2.68 0.45 BSC 0.30 0.40 0.50 28 0.08 0.10
0.730.10
1.29 E1
1.450.10
2 1 N
E/2
R 0.20 PIN 1 IDENTIFICATION
bxN D1 D/2 bbb C A B
NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
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SC414/SC424
Land Pattern -- MLPQ-4x4-28
1.29 K X DIM C G H2 (C) H1 Y P K 2.58 NOTES: 1. 2. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 3. SQUARE PACKAGE-DIMENSIONS APPLY IN BOTH X AND Y DIRECTIONS. 1.29 H G Z H H1 H2 K P X Y Z DIMENSIONS MILLIMETERS (3.95) 3.20 2.58 0.73 1.45 1.06 0.45 0.30 0.75 4.70
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com
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