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Low Supply Ripple Synchronous Buck Conversion Regulator
POWER MANAGEMENT Description
The SC191 is a synchronous step-down converter with integrated power devices and an integrated front-end LDO regulator to minimize input supply ripple. If supply ripple is not a concern, the front-end LDO regulator can be bypassed externally to maximize efficiency. The internal MOSFET switches provide peak current greater than 550mA to achieve a DC output of at least 330mA over the rated input voltage range, making the SC191 ideal for single-cell Li-ion battery applications as well as fixed 3.3V and 5V fixed input applications. The output is a fixed 1.2V - ideal for low-voltage microprocessors. Other voltage options are available (consult the factory for details). Additional features include internal soft-start to limit inrush current, over-current protection, over-temperature protection, and over-voltage protection. The 1MHz switching frequency allows the use of small surface mount capacitors and inductors, and no other external compensation components are needed. The device is available in a low profile (0.8mm max height) 2.3mm x 2.3mm MLPD 8-lead package, minimizing area without compromising performance.
SC191
Features
Less than 1mV Supply Ripple 2.7V to 5.5V Input Range 330mA Guaranteed Output Current Fixed Frequency 1MHz Operation No Schottky Diode Required Over-Current Protection Over-Voltage Protection Over-Temperature Protection Soft-Start No External Compensation Required MLPD-8, 2.3mm x 2.3mm Package WEEE and RoHS Compliant
Applications
Cell Phones Cordless Phones Notebook and Subnotebook Computers PDAs and Mobile Communicators WLAN Peripherals Wireless Modules 1 Li-Ion or 3 NiMH/NiCd Powered Devices
Patent Pending
Typical Application Circuit
VIN
CIN 4.7F
3 4 2
VIN EN PVIN
SC191
LX VOUT BP
8 5 1 6
L 4.7H
VOUT
COUT CBP 4.7F 4.7F
7
PGND
GND
April 12, 2006
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SC191
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding the specifications below 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.
Parameter Input Voltage Enable Input LX Voltage Output Voltage BP Voltage Thermal Impedance Junction to Ambient1 VOUT Short Circuit to GND Storage Temperature Junction Temperature Peak IR Reflow Temperature ESD Protection Level2
Symbol VVIN, VPVIN VEN VLX VVOUT VBP JA tSC TS TJC TLEAD VESD
Maximum -0.3 to 7.0 -0.3 to 7.0 -1 to VBP + 1 (7V max) -0.3 to 7.0 0.3 to 7.0 135 Continuous -60 to +160 +150 260 2
Units V V V V V C/W s C C C kV
Notes: 1) Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad as per JESD51 standards. 2) Tested according to JEDEC standard JESD22-A114-B.
Electrical Characteristics
Unless otherwise noted: VIN = 3.6V , VEN = VIN , TA = -40C to 85C . Typical values are at TA = 25C.
Parameter Input Voltage Range Input Voltage Ripple Line Regulation Load Regulation P-Channel On Resistance N-Channel On Resistance Start-Up Time Output Voltage Accuracy Front End LDO (FELDO)
Symbol VIN VIN (P-P) VLINEREG VLOADREG RDSP RDSN TSTART VOUT VBP
Conditions
Min 2.7
Typ
Max 5.5
Units V mV
CIN = CBP = 10F IOUT = 150mA 0mA < IOUT < 330mA ILX = 100mA ILX = 100mA IOUT = 150mA IOUT = 150mA IOUT = 150mA, TA = 25C IBP = 110mA(1) -3 -1.5 -0.3
1 0.3 0.002 0.3 0.2 1.25 3 1.5 300 325
%/V %/mA ms % % mV
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SC191
POWER MANAGEMENT Electrical Characteristics (Cont.)
Parameter P-Channel Current Limit N-Channel Current Limit Quiescent Current Shutdown Current LX Leakage Current PMOS LX Leakage Current NMOS Oscillator Frequency UVLO Threshold (Lower) UVLO Hysteresis Thermal Shutdown Thermal Shutdown Hysteresis Logic Input High Logic Input Low Logic Input Current High Logic Input Current Low Symbol ILIM(P) ILIM(N) IQ ISD ILXP ILXN fOSC VUVLO VUVLO-HYS TSD TSD-HYS VIH VIL IIH IIL EN pin EN pin EN pin EN pin -2 -2 0.1 0.1 1.6 0.6 2 2 Switching mode, IOUT = 100A EN tied to GND VIN = 5.5V LX, EN tied to GND VIN = VLX = 5.5V EN tied to GND -20 0.87 2.4 Conditions Min 550 -270 Typ 730 -420 2.5 0.1 0.1 0.1 1.0 2.5 50 145 10 1.12 2.6 1 1 Max 940 -700 Units mA mA mA A A A MHz V mV C C V V A A
Note: 1) FELDO tested at IBP = 110mA. Equivalent to IOUT = 330mA at VOUT= 1.2V.
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SC191
POWER MANAGEMENT Pin Configuration
Ordering Information
DEVICE PACKAGE MLPD-8LD (2.3mm x 2.3mm BODY SIZE) Evaluation Board
TOP VIEW
SC191AWLTRT(1)(2)
8 7 6 5 LX PGND GND VOUT
BP PVIN VIN EN
1 2 3 4
SC191AEVB
Notes: 1) Available in tape and reel only. A reel contains 3,000 devices. 2) Device is WEEE and RoHS compliant.
MLPD-8 2.3mm x 2.3mm 8-lead
Output Voltage Options
PART SC191A VOUT 1.2V*
*Only option currently available - contact Semtech for other voltage options.
Marking Information
91A yw
nnn = Part Number yw = Datecode
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SC191
POWER MANAGEMENT Block Diagram
VIN
3
Voltage References
VREF1 500mV Plimit Amp Current Amp LDO Error Amp
2 PVIN
Front-End LDO
1
BP
OSC and Slope Generator
Control Logic
8 LX
VOUT 5
500mV
PWM Comp
7
PGND
VREF1 Error Amp Nlimit Comp
6 GND EN 4
Pin Descriptions
Pin# 1 2 3 4 5 6 7 8 Pin Name BP PVIN VIN EN VOUT GND PGND LX Pin Function Regulated output, with respect to VIN, of the front-end LDO. A 4.7F decoupling capacitor should be connected to this pin. This output is connected directly to the internal switching MOSFETs. Input power supply for the power devices. Input power supply for the analog functions. Enable (digital input): high input enables the SC191, a low disables and reduces the quiescent current to < 1A. In shutdown LX becomes high impedance. Regulated output voltage and feedback for the SC191. Ground: this pin should be connected directly to PGND on the user's PCB. Power Ground: this pin should be connected directly to GND on the user's PCB and should be the reference point for the input, output and BP capacitor GND connections. These ground paths should be separate and as short as possible to minimize circuit noise. Inductor connection to the switching MOSFETs.
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SC191
POWER MANAGEMENT
Applications Information
SC191 Detailed Description
The SC191 is a step-down, pulse-width-modulated (PWM) DC-DC converter with a Low Dropout (LDO) pre-regulator. The device has an internal synchronous rectifier and does not require a Schottky diode on the LX pin. The device is designed to operate as a buck converter in PWM mode with a fixed-frequency of 1MHz. The SC191 operates as a fixed frequency current mode regulator with the input supply for the switching regulator pre-regulated by a frontend LDO regulator. This technique reduces the supply voltage ripple from 10mV, typically seen from a switching converter, to less than 1mV for the SC191. The supply to the switcher is regulated to the supply voltage minus approximately 300mV. Current feedback for the switching regulator is through the PMOS current path, and it is amplified and summed with the internal slope compensation network and level shifted. The voltage feedback loop is through an internal feedback divider. The on-time is determined by comparing the summed current feedback and the output of the error amplifier. The period is set by the onboard oscillator. Efficiency at moderate to high loads can be improved by shorting the VIN and BP pins together. This bypasses the front-end LDO, reducing voltage drop and improving efficiency at the expense of increasing input supply ripple. Protection Features The SC191 provides the following protection features: * Thermal Shutdown * Current Limit * Over-Voltage Protection * Soft-Start Output Filter The SC191 series of synchronous step-down converters have internal loop compensation. The internal compensation is designed to work with a certain output filter corner frequency defined by the equation: 1 fC = ---------- ------ 2 LxC The internal compensation is optimized to operate with an output filter, L = 4.7H and COUT = 10F. When selecting output filter components the LC product should not vary over a wide range.
Current Limit The part has a number of current limit functions. The front-end LDO regulator has a current limit set at approximately 500mA, which will protect it in the event of a pulsed short circuit. The PMOS and NMOS power devices of the buck switcher stage are also protected by current limit functions. In the case of a short to ground on the output, the part enters frequency foldback mode which causes the switching frequency to divide by a factor determined by the output voltage, which prevents the inductor current from "stair stepping". Over-Voltage Protection Over-voltage protection is provided on the SC191. In the event of an over-voltage on the output, the PWM drive is disabled, tri-stating the LX output and disabling the frontend LDO. Soft-Start The soft-start mode is enabled after every shutdown cycle to limit in-rush current. In conjunction with the frequency foldback this controls the maximum current during startup. The switcher's PMOS current limit is stepped from 25%, to 50%, to 75%, and then 100% of its typical value by a timer driven by the internal oscillator. The oscillator frequency is stepped by 1/8, 1/4, 1/2 and 1 under the control of 4 output voltage thresholds. As soon as the part reaches regulation, soft start mode is disabled.
Thermal Shutdown The thermal shutdown feature protects the device from exceeding a junction temperature of 150C. In thermal shutdown the PWM drive is disabled, the LX output is tristated, and the front-end LDO is disabled. The device will not be enabled again until the temperature reduces by 10C. If during this time the output falls by greater than 60% of its regulation voltage, a soft-start will be invoked.
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SC191
POWER MANAGEMENT Applications Information (Cont.)
Table 1: Output Filter Combinations
L (H) 4.7 4.7 10 COUT (F) 4.7 10 10
Alternatively, the inductor saturation current should be greater than the switch current limit. Final inductor selection will depend on various design considerations such as efficiency, EMI, size and cost (see Table 2 for a list of practical inductor options). CIN Selection To prevent large input voltage ripple caused by discontinuous source input current, a low ESR ceramic capacitor is required. A minimum value of 4.7F should be used for sufficient input voltage filtering, and a 10F capacitor is strongly recommended for optimum input voltage filtering. Input voltage ripple of approximately 1mV can be achieved when CIN = 10F and the front-end LDO regulator is active and pre-regulating the input supply to the switching regulator. COUT Selection A 10F ceramic capacitor is recommended for the output filter capacitor. Output voltage ripple is mainly determined by the filter capacitor in the following equation: VOUT(ESR) = IL(ripple) x ESRCOUT X7R or X5R ceramic dielectric capacitors should be used because of their low ESR and superior temperature and voltage characteristics. Y5V ceramic capacitors should be avoided due to their widely-varying temperature coefficients.
The selection of smaller inductor and capacitor values will move the corner frequency, having an impact on system stability. Due to this issue the practical lower limit for the inductor value is 4.7H. Inductor Selection The value of the inductor should be in the range 4.7H to 10H. The magnitude of the inductor current ripple is dependant on the inductor value and can be determined by the following equation: VOUT VOUT IL = ------ 1 + ------ L x fOSC VIN
(
)
This equation demonstrates the relationship between VIN, VOUT and IL. To minimize conduction losses and maximize efficiency, the inductor should have a low DC resistance. As a minimum requirement, the DC current rating of the inductor should be equal to the maximum load current plus half of the inductor current ripple as described by the following equation: IL IL(PK) = IOUT(MAX) + ---- 2
Table 2: Recommended Inductors
Manufacturer TDK Sumida Taiyo Yuden Coilcraft Coilcraft Part Number LDR655312T-4R7W CDRH3D16LD LMNP04SB100M LPS3015 LP06610-103M Value (H) 4.7 4.7 10 10 10 DCR 0.206 0.073 0.066 0.044 0.41
7
Rated Current (A) (40C rise) 0.9 0.68 0.9 -
ISAT (A) L drop 25% 0.65 0.8
Tolerance ( %) 20 30 20 20 20
Dimensions (LxWxH mm) 6.5x5.3x1.2 4x4x1.8 5x5x2 3x3x1.5 6.9x3.8x1.0
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SC191
POWER MANAGEMENT
Applications Information (Cont.)
Table 3: Recommended Capacitors
Manufacturer Murata Murata TDK Part Number GRM-188R60J475KE19D GRM-188R60G106ME47D C1608X5R0G106M Value (F) 4.7 10 10 Rated Voltage (VDC) 6.3 4 4 Temperature Coefficient X5R X5R X5R Package Size 0603 0603 0603
PCB Layout Considerations Poor layout can degrade the performance of the DC-DC converter and can be a contributory factor in EMI problems, ground bounce and resistive voltage losses. Poor regulation and instability can result. A few simple design rules can be implemented to ensure good layout: 1) Place the inductor and filter capacitors as close to the device as possible and use short, wide traces between the power components. 2) Route the output voltage feedback path away from in-
ductor and LX node to minimize noise and magnetic interference. Use a ground plane to further reduce noise. 3) Maximize ground metal on the component side to improve the return connection and thermal dissipation. Separation between the LX node and GND should be maintained to avoid coupling of switching noise to the ground plane. 4) To further reduce noise interference on sensitive circuit nodes, use a ground plane with several vias connecting all ground planes together.
Suggested Layout
GND CBP VIN SC191 CIN LX
LOUT
COUT VOUT GND
Vias to GND Plane
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SC191
POWER MANAGEMENT Typical Characteristics
Efficiency vs. Load Current (FELDO Active)
100 90 80
Efficiency vs. Load Current (FELDO Bypassed)
100 90 80
Efficiency (%)
60 50 40 30 20 10 0 0.0100 0.1000 1.0000
Efficiency (%)
70
70 60 50 40 30 20 10 0 0.0100 0.1000 1.0000
VIN=3V VIN=3.6V VIN=4.2V
VIN=3V VIN=3.6V VIN=4.2V
IOUT (A)
IOUT (A)
Line Regulation (FELDO Active)
1.21 1.21
Line Regulation (FELDO Bypassed)
IOUT=1mA
1.205
IOUT=1mA
1.205
IOUT=150mA
IOUT=150mA IOUT=330mA
VOUT (V)
1.2
VOUT (V)
IOUT=330mA
1.2
1.195
1.195
1.19 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
1.19 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
VIN (V)
VIN (V)
Load Regulation
1.21
Switching Frequency vs. Temperature
1.01
1
Switching Frequency (MHz)
1.205
0.99
VOUT (V)
0.98
1.2
VIN=4.2V VIN=3.6V VIN=3V
0.97
0.96
1.195 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
VIN=3.6V
0.95 -60 -40 -20 0 20 40 60 80 100 120 140
IOUT (A)
TJ (C)
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SC191
POWER MANAGEMENT
Typical Characteristics (Cont.)
Switching Frequency vs. Input Voltage
1.005 1 4
Quiescent Current vs. Input Voltage
Switching Frequency (MHz)
3.5
Quiescent Current (mA)
0.995 0.99 0.985 0.98 0.975 0.97 0.965 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
3
o
TJ=85oC TJ=25 C
2.5
TJ=-40oC
2
1.5
1 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
VIN (V)
VIN (V)
N-Channel RDSON vs. Temperature
400 350 500 300 400 250 200 150 100 100 50 0 -60 -40 -20 0 20 40 60 80 100 120 140 0 -60 600
P-Channel RDSON vs. Temperature
RDSON (m)
RDSON (m)
300
200
-40
-20
0
20
40
60
80
100
120
140
TJ (C)
TJ (C)
Load Transient Response
VIN=3.6V, VOUT=1.2V IOUT 33 to 330mA
Start-Up
VIN=3.6V, IOUT=300mA
VOUT 100mV/DIV
VOUT 500mV/DIV ENABLE 5V/DIV
200s/Div
(c) 2006 Semtech Corp. 10
200s/Div
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SC191
POWER MANAGEMENT Typical Characteristics (Cont.)
Input Voltage Ripple (FELDO Active)
VIN =3.6V, IOUT=300mA, CBP=10uF, CIN= 10uF VOUT 5mV/DIV
Input Voltage Ripple (FELDO Bypassed)
VIN =3.6V, IOUT=300mA, CBP=10uF, CIN= not fitted
VOUT 5mV/DIV
VLX 2V/DIV
VLX 2V/DIV
400ns/Div Input Voltage Ripple (FELDO Bypassed)
VIN =3.6V, IOUT=300mA, CBP=20uF, CIN= not fitted
400ns/Div
VOUT 5mV/DIV
VLX 2V/DIV
400ns/Div
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SC191
POWER MANAGEMENT
Outline Drawing - MLPD-8
A
D
B
PIN 1 INDICATOR (LASER MARK)
E
DIMENSIONS INCHES MILLIMETERS DIM MIN NOM MAX MIN NOM MAX
A A1 A2 b D E e L L1 N aaa bbb .028 .030 .031 .000 .001 .002 (.008) .008 .010 .012 .087 .091 .094 .087 .091 .094 .020 BSC .012 .016 .020 .016 .020 .024 8 .003 .003 0.70 0.75 0.80 0.00 0.02 0.05 (0.20) 0.20 0.25 0.30 2.20 2.30 2.40 2.20 2.30 2.40 0.50 BSC 0.30 0.40 0.50 0.40 0.50 0.60 8 0.08 0.08
A aaa C A1 1 L1 2 LxN C A2
SEATING PLANE
0.125 CHAMFER N e e/2 bxN bbb CAB
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
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SC191
POWER MANAGEMENT Land Pattern - MLPD-8
DIM
C G Z
C G P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.083) .047 .020 .012 .035 .118 (2.10) 1.20 0.50 0.30 0.90 3.00
Y X P
NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
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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