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19-0160; Rev 2; 4/97
KIT ATION ED EVALU INCLUD ION ORMAT INF
Dual-Output Power-Supply Controller for Notebook Computers
________________________________Features
o Dual PWM Buck Controllers (+3.3V and +5V) o Two Precision Comparators or Level Translators o 95% Efficiency o 420A Quiescent Current, 70A in Standby (linear regulators alive) o 25A Shutdown Current (+5V linear alive) o 5.5V to 30V Input Range o Small SSOP Package o Fixed Output Voltages: 3.3V (standard) 3.45V (High-Speed PentiumTM) 3.6V (PowerPCTM)
__________________General Description
The MAX786 is a system-engineered power-supply controller for notebook computers or similar batterypowered equipment. It provides two high-performance step-down (buck) pulse-width modulators (PWMs) for +3.3V and +5V. Other features include dual, low-dropout, micropower linear regulators for CMOS/RTC back-up, and two precision low-batterydetection comparators. High efficiency (95% at 2A; greater than 80% at loads from 5mA to 3A) is achieved through synchronous rectification and PWM operation at heavy loads, and Idle ModeTM operation at light loads. The MAX786 uses physically small components, thanks to high operating frequencies (300kHz/200kHz) and a new current-mode PWM architecture that allows for output filter capacitors as small as 30F per ampere of load. Line- and loadtransient responses are terrific, with a high 60kHz unitygain crossover frequency allowing output transients to be corrected within four or five clock cycles. Low system cost is achieved through a high level of integration and the use of low-cost, external N-channel MOSFETs. Other features include low-noise, fixed-frequency PWM operation at moderate to heavy loads, and a synchronizable oscillator for noise-sensitive applications such as electromagnetic pen-based systems and communicating computers. The MAX786 is a monolithic, BiCMOS IC available in fine-pitch, surface-mount SSOP packages.
MAX786
_________________Ordering Information
PART MAX786CAI MAX786RCAI TEMP. RANGE 0C to +70C 0C to +70C PIN-PACKAGE 28 SSOP 28 SSOP VOUT 3.3V 3.45V
Ordering Information continued at end of data sheet.
_____________________Pin Configuration
TOP VIEW
CS3 1 SS3 2 ON3 3 D1 4 D2 5 VH 6 28 FB3 27 DH3 26 LX3 25 BST3
___________________________Applications
Notebook Computers Portable Data Terminals Communicating Computers Pen-Entry Systems
MAX786
24 DL3 23 V+ 22 VL 21 FB5 20 PGND 19 DL5 18 BST5 17 LX5 16 DH5 15 CS5
________Typical Application Diagram
+3.3V 5.5V TO 30V SHUTDOWN 5V ON/OFF 3.3V ON/OFF SYNC POWER SECTION P MEMORY +5V PERIPHERALS
Q2 7 Q1 8 GND 9 REF 10 SYNC 11 SHDN 12 ON5 13 SS5 14
MAX786
POWER-GOOD
LOW-BATTERY WARNING SUSPEND POWER
SSOP
Idle Mode is a trademark of Maxim Integrated Products. Pentium is a trademark of Intel Corp. PowerPC is a trademark of IBM Corp.
_______________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
Dual-Output Power-Supply Controller for Notebook Computers MAX786
ABSOLUTE MAXIMUM RATINGS
V+ to GND................................................................-0.3V to 36V PGND to GND .......................................................................2V VL to GND ..................................................................-0.3V to 7V BST3, BST5 to GND .................................................-0.3V to 36V LX3 to BST3 ...............................................................-7V to 0.3V LX5 to BST5 ...............................................................-7V to 0.3V Inputs/Outputs to GND (D1, D2, SHDN, ON5, REF, SS5, CS5, FB5, SYNC, CS3,FB3, SS3, ON3) ............-0.3V to (VL + 0.3V) VH to GND ...............................................................-0.3V to 20V Q1, Q2 to GND ............................................-0.3V to (VH + 0.3V) DL3, DL5 to PGND .......................................-0.3V to (VL + 0.3V) DH3 to LX3 ..............................................-0.3V to (BST3 + 0.3V) DH5 to LX5 ..............................................-0.3V to (BST5 + 0.3V) REF, VL Short to GND................................................Momentary REF Current ........................................................................20mA VL Current ..........................................................................50mA Continuous Power Dissipation (TA = +70C) SSOP (derate 9.52mW/C above +70C)....................762mW Operating Temperature Ranges MAX786CAI/MAX786_CAI .................................0C to +70C MAX786EAI/MAX786_EAI ...............................-40C to +85C Lead Temperature (soldering, 10sec) ............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0mA, SHDN = ON3 = ON5 = 5V, other digital input levels are 0V or +5V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER 3.3V AND 5V STEP-DOWN CONTROLLERS Input Supply Range FB5 Output Voltage 0mV < (CS5-FB5) < 70mV, 6V < V + < 30V (includes load and line regulation) 0mV < (CS3-FB3) < 70mV, 6V < V + < 30V (includes load and line regulation) MAX786 MAX786R MAX786S CONDITIONS MIN 5.5 4.80 3.17 3.32 3.46 5.08 3.35 3.50 3.65 2.5 0.03 100 4.0 TYP MAX 30 5.20 3.46 3.60 3.75 UNITS V V
FB3 Output Voltage
V % %/V mV A mA V V V V V mV A A mW A V nA
Load Regulation Either controller (CS_ -FB_ = 0mV to 70mV) Line Regulation Either controller (V+ = 6V to 30V) Current-Limit Voltage CS3-FB3 or CS5-FB5 SS3/SS5 Source Current SS3/SS5 Fault Sink Current INTERNAL REGULATOR AND REFERENCE VL Output Voltage ON5 = ON3 = 0V, 5.5V < V+ < 30V, 0mA < IL < 25mA VL Fault Lockout Voltage Falling edge, hysteresis = 1% VL/FB5 Switchover Voltage Rising edge of FB5, hysteresis = 1% REF Output Voltage No external load (Note 1) REF Fault Lockout Voltage Falling edge REF Load Regulation 0mA < IL < 5mA (Note 2) --- - ---- SHDN = D1 = D2 = ON3 = ON5 = 0V, V+ = 30V V+ Shutdown Current V+ Standby Current D1 = D2 = ON3 = ON5 = 0V, V+ = 30V Quiescent Power Consumption (both PWM controllers on) V+ Off Current COMPARATORS D1, D2 Trip Voltage D1, D2 Input Current D1 = D2 = 0V, FB5 = CS5 = 5.25V, FB3 = CS3 = 3.5V FB5 = CS5 = 5.25V, VL switched over to FB5 Falling edge, hysteresis = 1% D1 = D2 = 0V, 5V
80 2.5 2 4.5 3.6 4.2 3.24 2.4
120 6.5
30 25 70 5.5 30 1.61
5.5 4.2 4.7 3.36 3.2 75 40 120 8.6 60 1.69 100
2
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Dual-Output Power-Supply Controller for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0mA, SHDN = ON3 = ON5 = 5V, other digital input levels are 0V or +5V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Q1, Q2 Source Current Q1, Q2 Sink Current Q1, Q2 Output High Voltage Q1, Q2 Output Low Voltage Quiescent VH Current CONDITIONS VH = 15V, VOUT = 2.5V VH = 15V, VOUT = 2.5V ISOURCE = 5A, VH = 3V ISINK = 20A, VH = 3V VH = 18V, D1 = D2 = 5V, no external load SYNC = 3.3V SYNC = 0V, 5V SYNC High Pulse Width SYNC Low Pulse Width SYNC Rise/Fall Time Oscillator SYNC Range Maximum Duty Cycle Input Low Voltage Input High Voltage Input Current DL3/DL5 Sink/Source Current DH3/DH5 Sink/Source Current DL3/DL5 On-Resistance DH3/DH5 On-Resistance SYNC = 3.3V SYNC = 0V or 5V SHDN, ON3, ON5, SYNC SHDN, ON3, ON5 SYNC SHDN, ON3, ON5 VIN = 0V, 5V VOUT = 2V BST3-LX3 = BST5-LX5 = 4.5V, VOUT = 2V High or low High or low, BST3-LX3 = BST5-LX5 = 4.5V 1 1 7 7 2.4 VL - 0 .5 1 A A A Not tested 240 89 92 92 95 0.8 V V 270 170 200 200 200 350 4 300 200 MIN 12 200 VH - 0 .5 0.4 10 330 230 ns ns ns kHz % TYP 20 500 MAX 30 1000 UNITS A A V V A
MAX786
OSCILLATOR AND INPUTS/OUTPUTS Oscillator Frequency kHz
Note 1: Since the reference uses VL as its supply, its V+ line regulation error is insignificant. Note 2: The main switching outputs track the reference voltage. Loading the reference reduces the main outputs slightly according to the closed-loop gain (AVCL) and the reference voltage load-regulation error. AVCL for the +3.3V supply is unity gain. AVCL for the +5V supply is 1.54.
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3
Dual-Output Power-Supply Controller for Notebook Computers MAX786
________________________________________________Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. +5V OUTPUT CURRENT, 200kHz
100 VIN = 6V VIN = 30V EFFICIENCY (%) VIN = 15V 100
EFFICIENCY vs. +5V OUTPUT CURRENT, 300kHz
100
EFFICIENCY vs. +3.3V OUTPUT CURRENT, 200kHz
VIN = 15V 90 VIN = 6V EFFICIENCY (%) VIN = 15V 80 VIN = 30V 70 +3.3V OFF 60 90 VIN = 6V 80 VIN = 30V
90 EFFICIENCY (%)
80
70 SYNC = 0V, +3.3V OFF 60
70
60
SYNC = 0V, +5V ON
50 1m 10m 100m 1 +5V OUTPUT CURRENT (A) 10
50 1m 10m 100m 1 +5V OUTPUT CURRENT (A) 10
50 1m 10m 100m 1 10 +3.3V OUTPUT CURRENT (A)
EFFICIENCY vs. +3.3V OUTPUT CURRENT, 300kHz
100 QUIESCENT SUPPLY CURRENT (mA) VIN = 15V 90 EFFICIENCY (%) VIN = 6V 80 70 +5V ON 60 VIN = 30V 19
QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
2.5 STANDBY SUPPLY CURRENT (mA)
STANDBY SUPPLY CURRENT vs. SUPPLY VOLTAGE
18 ON3 = ON5 = HIGH 2
2.0
1.5 ON3 = ON5 = 0V 1.0
1
0.5
50 1m 10m 100m 1 10 +3.3V OUTPUT CURRENT (A)
0 0 6 12 18 24 30 SUPPLY VOLTAGE (V)
0 0 6 12 18 SUPPLY VOLTAGE (V) 24 30
SHUTDOWN SUPPLY CURRENT vs. SUPPLY VOLTAGE
500 MINIMUM VIN TO VOUT DIFFERENTIAL (V) SHUTDOWN SUPPLY CURRENT (A) 1.0
MINIMUM VIN TO VOUT DIFFERENTIAL vs. +5V OUTPUT CURRENT
1000 SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY vs. LOAD CURRENT
SYNC = REF (300kHz) ON3 = ON5 = 5V 100 +5V, VIN = 7.5V
400
0.8
300kHz
300 SHDN = 0V 200
0.6
10
+5V, VIN = 30V
0.4
200kHz +5V OUTPUT STILL REGULATING
1
+3.3V, VIN = 7.5V
100
0.2
0 0 6 12 18 24 SUPPLY VOLTAGE (V) 30
0 1m 10m 100m 1 10 +5V OUTPUT CURRENT (A)
0.1 100
1m
10m
100m
1
LOAD CURRENT (A)
4
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Dual-Output Power-Supply Controller for Notebook Computers
_________________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.) IDLE MODE WAVEFORMS PULSE-WIDTH MODULATION MODE WAVEFORMS
MAX786
+5V OUTPUT 50mV/div
LX 10V/div
2V/div
+5V OUTPUT 50mV/div
200s/div ILOAD = 100mA VIN = 10V
500ns/div +5V OUTPUT CURRENT = 1A VIN= 16V
+5V LOAD-TRANSIENT RESPONSE
+3.3V LOAD-TRANSIENT RESPONSE
3A 0A
LOAD CURRENT
3A 0A
LOAD CURRENT
+5V OUTPUT 50mV/div
+3.3V OUTPUT 50mV/div
200s/div VIN = 15V
200s/div VIN = 15V
_______________________________________________________________________________________
5
Dual-Output Power-Supply Controller for Notebook Computers MAX786
_________________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.) +5V LINE-TRANSIENT RESPONSE, RISING +5V LINE-TRANSIENT RESPONSE, FALLING
+5V OUTPUT 50mV/div
+5V OUTPUT 50mV/div
VIN, 10V TO 16V 2V/div
VIN, 16V TO 10V 2V/div 20s/div ILOAD = 2A 20s/div ILOAD = 2A
+3.3V LINE-TRANSIENT RESPONSE, RISING
+3.3V LINE-TRANSIENT RESPONSE, FALLING
+3.3V OUTPUT 50mV/div
+3.3V OUTPUT 50mV/div
VIN, 10V TO 16V 2V/div
VIN, 16V TO 10V 2V/div 20s/div ILOAD = 2A 20s/div ILOAD = 2A
6
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Dual-Output Power-Supply Controller for Notebook Computers
_______________________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 NAME CS3 SS3 ON3 D1 D2 VH Q2 Q1 GND REF FUNCTION Current-sense input for +3.3V; +100mV = current limit level referenced to FB3. Soft-start input for +3.3V. Ramp time to full current limit is 1ms/nF of capacitance to GND. ON/OFF control input disables the +3.3V PWM. Tie directly to VL for automatic start-up. #1 level-translator/comparator noninverting input, threshold = +1.650V. Controls Q1. Tie to GND if unused. #2 level-translator/comparator noninverting input (see D1) External positive supply-voltage input for the level translators/comparators #2 level-translator/comparator output. Sources 20A from VH when D2 is high. Sinks 500A to GND when D2 is low, even with VH = 0V. #1 level translator/comparator output (see Q2) Low-current analog ground 3.3V reference output. Sources up to 5mA for external loads. Bypass to GND with 1F/mA of load or 0.22F minimum. Oscillator control/synchronization input. Connect to VL or GND for 200kHz; connect to REF for 300kHz. For external clock synchronization in the 240kHz to 350kHz range, a high-to-low transition causes a new cycle to start. Shutdown control input, active low. Tie to VL for automatic start-up. The 5V VL supply stays active in shutdown, but all other circuitry is disabled. Do not force SHDN higher than VL + 0.3V. ON/OFF control input disables the +5V PWM supply. Tie to VL for automatic start-up. Soft-start control input for +5V. Ramp time to full current limit is 1ms/nF of capacitance to GND. Current-sense input for +5V; +100mV = current-limit level referenced to FB5. Gate-drive output for the +5V high-side MOSFET Inductor connection for the +5V supply Boost capacitor connection for the +5V supply (0.1F) Gate-drive output for the +5V low-side MOSFET Power ground Feedback and current-sense input for the +5V PWM 5V logic supply voltage for internal circuitry. VL is always on and can source 5mA for external loads. Supply voltage input from battery, 5.5V to 30V Gate-drive output for the +3.3V low-side MOSFET Boost capacitor connection for the +3.3V supply (0.1F) Inductor connection for the +3.3V supply Gate-drive output for the +3.3V high-side MOSFET Feedback and current-sense input for the +5V PWM
MAX786
11
SYNC
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SHDN ON5 SS5 CS5 DH5 LX5 BST5 DL5 PGND FB5 VL V+ DL3 BST3 LX3 DH3 FB3
_______________________________________________________________________________________
7
Dual-Output Power-Supply Controller for Notebook Computers MAX786
_________________Detailed Description
The MAX786 converts a 5.5V to 30V input to four outputs (Figure 1). It produces two high-power, PWM, switchmode supplies, one at +5V and the other at +3.3V. The two supplies operate at either 300kHz or 200kHz, allowing for small external components. Output current capability depends on external components, and can exceed 6A on each supply. An internal 5V, 5mA supply (VL) and a 3.3V, 5mA reference voltage are also generated via linear regulators, as shown in Figure 2. Fault protection circuitry shuts off the PWMs when the internal supplies lose regulation. Two precision voltage comparators are also included. Their output stages permit them to be used as level translators for driving external N-channel MOSFETs in load-switching applications, or for more conventional logic signals. The MAX786 has two close relatives: the MAX782 and the MAX783. The MAX782 and MAX783 each include an extra flyback winding regulator and linear regulators for dual, +12V/programmable PCMCIA VPP outputs. The MAX782/MAX783 data sheet contains extra applications information on the MAX786 not found in this data sheet.
+3.3V Switch-Mode Supply
The +3.3V supply is generated by a current-mode, PWM step-down regulator using two N-channel MOSFETs, a rectifier, and an LC output filter (Figure 1). The gate-drive signal to the high-side MOSFET, which must exceed the battery voltage, is provided by a boost circuit that uses a 100nF capacitor connected to BST3.
INPUT 5.5V TO 30V (NOTE 1)
C1 33F D2A 1N4148 C5 0.1F 25 27 26 LX3 24 V+
C10 33F 23 VL D2B 1N4148 C4 0.1F N2 4.7F 22 +5V AT 5mA
BST3 DH3
BST5 DH5 LX5
18 16 17
+3.3V AT 3A
R1 25m
L1 10H
N1
L2 10H
R2 25m
+5V AT 3A C6 330F
D1 1N5819 C7 C12 150F 150F
N3
DL3
DL5
19
N4
D3 1N5819
(NOTE 2) C9 0.01F +3.3V ON/OFF +5V ON/OFF SHUTDOWN OSC SYNC
1 CS3 28 FB3 2 3 13 12 11 SS3 ON3 ON5 SHDN SYNC
MAX786
CS5 15 21 FB5 14 SS5 6 VH 4 D1 8 Q1 5 D2 7 Q2
(NOTE 2) C8 0.01F COMPARATOR SUPPLY INPUT IN OUT IN OUT COMPARATOR 2 COMPARATOR 1
NOTE 1: INPUT VOLTAGE RANGE 6.5V TO 30V AS SHOWN. SEE LOW-VOLTAGE (6-CELL) OPERATION SECTION FOR DETAILS. NOTE 2: USE SHORT, KELVIN-CONNECTED PC BOARD TRACES PLACED VERY CLOSE TO ONE ANOTHER.
GND 9
REF PGND 10 20 +3.3V AT 5mA C3 1F
Figure 1. MAX786 Application Circuit
8 _______________________________________________________________________________________
Dual-Output Power-Supply Controller for Notebook Computers
A synchronous rectifier at LX3 keeps efficiency high by clamping the voltage across the rectifier diode. Maximum current limit is set by an external low-value sense resistor, which prevents excessive inductor current during start-up or under short-circuit conditions. Programmable soft-start is set by an optional external capacitor; this reduces in-rush surge currents upon start-up and provides adjustable power-up times for power-supply sequencing purposes.
MAX786
V+
+5V LDO LINEAR REGULATOR 5V
P 3.3V PWM CONTROLLER (SEE FIG. 3)
FB3 CS3 BST3 DH3 LX3 DL3 ON SS3
VL
3.3V REF
+3.3V REFERENCE ON
4.5V
SHDN GND PGND 4V FAULT ON3
FB5 2.8V 300kHz/200kHz OSCILLATOR ON STANDBY 5V PWM CONTROLLER (SEE FIG. 3) CS5 BST5 DH5 LX5 DL5 ON SS5
SYNC
ON5
D1
VH Q1 1.65V
D2 Q2 1.65V
Figure 2. MAX786 Block Diagram
_______________________________________________________________________________________
9
Dual-Output Power-Supply Controller for Notebook Computers MAX786
+5V Switch-Mode Supply
The +5V output is produced by a current-mode, PWM step-down regulator, which is nearly identical to the +3.3V supply. The +5V supply's dropout voltage, as configured in Figure 1, is typically 400mV at 2A. As V+ approaches 5V, the +5V output gracefully falls with V+ until the VL regulator output hits its undervoltagelockout threshold at 4V. At this point, the +5V supply turns off. The default frequency for both PWM controllers is 300kHz (with SYNC connected to REF), but 200kHz may be used by connecting SYNC to GND or VL.
+3.3V and +5V PWM Buck Controllers
The two current-mode PWM controllers are identical except for different preset output voltages (Figure 3). Each PWM is independent except for being synchronized to a master oscillator and sharing a common reference (REF) and logic supply (VL). Each PWM can be turned on and off separately via ON3 and ON5. The PWMs are a direct-summing type, lacking a traditional integrator error amplifier and the phase shift associated with it. They therefore do not require any external feedback compensation components if the filter capacitor ESR guidelines given in the Design Procedure are followed. The main gain block is an open-loop comparator that sums four input signals: an output voltage error signal, current-sense signal, slope-compensation ramp, and precision voltage reference. This direct-summing method approaches the ideal of cycle-by-cycle control of the output voltage. Under heavy loads, the controller operates in full PWM mode. Every pulse from the oscillator sets the output latch and turns on the high-side switch for a period determined by the duty cycle (approximately VOUT/VIN). As the high-side switch turns off, the synchronous rectifier latch is set and, 60ns later, the low-side switch turns on (and stays on until the beginning of the next clock cycle, in continuous mode, or until the inductor current crosses through zero, in discontinuous mode). Under fault conditions where the inductor current exceeds the 100mV current-limit threshold, the high-side latch is reset and the high-side switch is turned off. At light loads, the inductor current fails to exceed the 25mV threshold set by the minimum current comparator. When this occurs, the PWM goes into idle mode, skipping most of the oscillator pulses in order to reduce the switching frequency and cut back switching losses. The oscillator is effectively gated off at light loads because the minimum current comparator immediately resets the high-side latch at the beginning of each cycle, unless the FB_ signal falls below the reference voltage level.
10
Soft-Start/SS_ Inputs Connecting capacitors to SS3 and SS5 allows gradual build-up of the +3.3V and +5V supplies after ON3 and ON5 are driven high. When ON3 or ON5 is low, the appropriate SS capacitors are discharged to GND. When ON3 or ON5 is driven high, a 4A constant current source charges these capacitors up to 4V. The resulting ramp voltage on the SS_ pins linearly increases the current-limit comparator setpoint so as to increase the duty cycle to the external power MOSFETs up to the maximum output. With no SS capacitors, the circuit will reach maximum current limit within 10s. Soft-start greatly reduces initial in-rush current peaks and allows start-up time to be programmed externally. Synchronous Rectifiers Synchronous rectification allows for high efficiency by reducing the losses associated with the Schottky rectifiers. When the external power MOSFET N1 (or N2) turns off, energy stored in the inductor causes its terminal voltage to reverse instantly. Current flows in the loop formed by the inductor, Schottky diode, and load -- an action that charges up the filter capacitor. The Schottky diode has a forward voltage of about 0.5V which, although small, represents a significant power loss, degrading efficiency. A synchronous rectifier, N3 (or N4), parallels the diode and is turned on by DL3 (or DL5) shortly after the diode conducts. Since the on resistance (rDS(ON)) of the synchronous rectifier is very low, the losses are reduced.
The synchronous rectifier MOSFET is turned off when the inductor current falls to zero. Cross conduction (or "shoot-through") occurs if the high-side switch turns on at the same time as the synchronous rectifier. The MAX786's internal break-beforemake timing ensures that shoot-through does not occur. The Schottky rectifier conducts during the time that neither MOSFET is on, which improves efficiency by preventing the synchronous-rectifier MOSFET's lossy body diode from conducting. The synchronous rectifier works under all operating conditions, including discontinuous-conduction and idle mode.
Boost Gate-Driver Supply Gate-drive voltage for the high-side N-channel switch is generated with a flying-capacitor boost circuit as shown in Figure 4. The capacitor is alternately charged from the VL supply via the diode and placed in parallel with the high-side MOSFET's gate-source terminals. On startup, the synchronous rectifier (low-side) MOSFET forces LX_ to 0V and charges the BST_ capacitor to 5V. On the
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Dual-Output Power-Supply Controller for Notebook Computers
second half-cycle, the PWM turns on the high-side MOSFET by connecting the capacitor to the MOSFET gate by closing an internal switch between BST_ and DH_. This provides the necessary enhancement voltage to turn on the high-side switch, an action that "boosts" the 5V gate-drive signal above the battery voltage. Ringing seen at the high-side MOSFET gates (DH3 and DH5) in discontinuous-conduction mode (light loads) is a natural operating condition caused by the residual energy in the tank circuit formed by the inductor and stray capacitance at the LX_ nodes. The gate driver negative rail is referred to LX_, so any ringing there is directly coupled to the gate-drive supply.
MAX786
CS_ 1X
60kHz LPF REF, 3.3V (OR INTERNAL 5V REFERENCE) MAIN PWM COMPARATOR
FB_
R Q S LEVEL SHIFT
BST_ DH_ LX_
SLOPE COMP
OSC
25mV
MINIMUM CURRENT (IDLE MODE)
VL 4A CURRENT LIMIT 0mV TO 100mV SS_ 30R SHOOTTHROUGH CONTROL
3.3V ON_ N
1R
SYNCHRONOUS RECTIFIER CONTROL VL R Q S LEVEL SHIFT DL_ PGND
Figure 3. PWM Controller Block Diagram
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11
Dual-Output Power-Supply Controller for Notebook Computers MAX786
Modes of Operation
PWM Mode Under heavy loads--over approximately 25% of full load -- the +3.3V and +5V supplies operate as continuouscurrent PWM supplies (see Typical Operating Characteristics). The duty cycle (%ON) is approximately: %ON = VOUT/VIN Current flows continuously in the inductor: First, it ramps up when the power MOSFET conducts; then, it ramps down during the flyback portion of each cycle as energy is put into the inductor and then discharged into the load. Note that the current flowing into the inductor when it is being charged is also flowing into the load, so the load is continuously receiving current from the inductor. This minimizes output ripple and maximizes inductor use, allowing very small physical and electrical sizes. Output ripple is primarily a function of the filter capacitor (C7 or C6) effective series resistance (ESR) and is typically under 50mV (see the Design Procedure section). Output ripple is worst at light load and maximum input voltage. Idle Mode Under light loads (<25% of full load), efficiency is further enhanced by turning the drive voltage on and off for only a single clock period, skipping most of the clock pulses entirely. Asynchronous switching, seen as "ghosting" on an oscilloscope, is thus a normal operating condition whenever the load current is less than approximately 25% of full load. At certain input voltage and load conditions, a transition region exists where the controller can pass back and forth from idle mode to PWM mode. In this situation, short bursts of pulses occur that make the current waveform look erratic, but do not materially affect the output ripple. Efficiency remains high. can also be driven with an external 240kHz to 350kHz CMOS/TTL source to synchronize the internal oscillator. Normally, 300kHz is used to minimize the inductor and filter capacitor sizes, but 200kHz may be necessary for low input voltages (see Low-Voltage (6-Cell) Operation).
Comparators
Two noninverting comparators can be used as precision voltage comparators or high-side drivers. The supply for these comparators (VH) is brought out and may be connected to any voltage between +3V and +19V irrespective of V+. The noninverting inputs (D1-D2) are high impedance, and the inverting input is internally connected to a 1.650V reference. Each output (Q1-Q2) sources 20A from VH when its input is above 1.650V, and sinks 500A to GND when its input is below 1.650V. The Q1-Q2 outputs can be fixed together in wired-OR configuration since the pull-up current is only 20A. Connecting VH to a logic supply (5V or 3V) allows the comparators to be used as low-battery detectors. For driving N-channel power MOSFETs to turn external loads on and off, VH should be 6V to 12V higher than the load voltage. This enables the MOSFETs to be fully turned on and results in low rDS(ON). The comparators are always active when V+ is above +4V, even when VH is 0V. Thus, Q1-Q2 will sink current to GND even when VH is 0V, but they will only source current from VH when VH is above approximately 1.5V. If Q1 or Q2 is externally pulled above VH, an internal diode conducts, pulling VH a diode drop below the output and powering anything connected to VH. This voltage will also power the other comparator outputs.
BATTERY INPUT
VL
Current Limiting The voltage between CS3 (CS5) and FB3 (FB5) is continuously monitored. An external, low-value shunt resistor is connected between these pins, in series with the inductor, allowing the inductor current to be continuously measured throughout the switching cycle. Whenever this voltage exceeds 100mV, the drive voltage to the external high-side MOSFET is cut off. This protects the MOSFET, the load, and the battery in case of short circuits or temporary load surges. The current-limiting resistors R1 and R2 are typically 25m for 3A load current. Oscillator Frequency; SYNC Input The SYNC input controls the oscillator frequency. Connecting SYNC to GND or to VL selects 200kHz operation; connecting to REF selects 300kHz operation. SYNC
12
VL
BST_
LEVEL TRANSLATOR
DH_
PWM
VL
LX_
DL_
Figure 4. Boost Supply for Gate Drivers
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Dual-Output Power-Supply Controller for Notebook Computers MAX786
Table 1. Surface-Mount Components
(See Figure 1 for Standard Application Circuit.) COMPONENT C1, C10 C2 C3 C4, C5 C6 C7, C12 C8, C9 D2A, D2B D1, D3 L1, L2 N1-N4 R1, R2 SPECIFICATION 33F, 35V tantalum capacitors 4.7F, 6V tantalum capacitor 1F, 20V tantalum capacitor 0.1F, 16V ceramic capacitors 330F, 10V tantalum capacitor 150F, 10V tantalum capacitors 0.01F, 16V ceramic capacitors 1N4148-type dual diodes 1N5819 SMT diodes 10H, 2.65A inductors N-channel MOSFETs (SO-8) 0.025, 1% (SMT) resistors AVX Sprague AVX Sprague AVX Sprague Murata-Erie Sprague Sprague Murata-Erie Central Semiconductor Nihon Sumida Siliconix IRC MANUFACTURER PART NO. TPSE226M035R0100 595D336X0035R TAJB475M016 595D475X0016A TAJA105M025 595D105X0020A2B GRM42-6X7R104K50V 595D337X0010R 595D157X0010D GRM42-6X7R103K50V CMPD2836 EC10QS04 CDR125-100 Si9410DY LR2010-01-R025-F
Table 2. Component Suppliers
COMPANY AVX Central Semiconductor IRC Murata-Erie Nihon Siliconix Sprague Sumida FACTORY FAX [COUNTRY CODE] [1] (803) 626-3123 [1] (516) 435-1824 [1] (512) 992-3377 [1] (814) 238-0490 [81] 3-3494-7414 [1] (408) 970-3950 [1] (603) 224-1430 [81] 3-3607-5144 USA PHONE (803) 946-0690 (800) 282-4975 (516) 435-1110 (512) 992-7900 (814) 237-1431 (805) 867-2555 (408) 988-8000 (603) 224-1961 (847) 956-0666
current. The main switching outputs track the reference voltage. Loading the reference will reduce the main outputs slightly, according to the reference load-regulation error. Both the VL and REF outputs remain active, even when the switching regulators are turned off, to supply memory keep-alive power (see Shutdown Mode section). These linear-regulator outputs can be directly connected to the corresponding step-down regulator outputs (i.e., REF to +3.3V, VL to +5V) to keep the main supplies alive in standby mode. However, to ensure start-up, standby load currents must not exceed 5mA on each supply.
Fault Protection The +3.3V and +5V PWM supplies and the comparators are disabled when either of two faults is present: VL < +4.0V or REF < +2.8V (85% of its nominal value).
Internal VL and REF Supplies
An internal linear regulator produces the 5V used by the internal control circuits. This regulator's output is available on pin VL and can source 5mA for external loads. Bypass VL to GND with 4.7F. To save power, when the +5V switch-mode supply is above 4.5V, the internal linear regulator is turned off and the high-efficiency +5V switch-mode supply output is connected to VL. The internal 3.3V bandgap reference (REF) is powered by the internal 5V VL supply. It can furnish up to 5mA. Bypass REF to GND with 0.22F, plus 1F/mA of load
__________________Design Procedure
Figure 1's schematic and Table 2's component list show values suitable for a 3A, +5V supply and a 3A, +3.3V supply. This circuit operates with input voltages from 6.5V to 30V, and maintains high efficiency with output currents between 5mA and 3A (see the Typical Operating Characteristics). This circuit's components may be changed if the design guidelines described in this section are used -- but before beginning the design, the following information should be firmly established:
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Dual-Output Power-Supply Controller for Notebook Computers
VIN(MAX), the maximum input (battery) voltage. This value should include the worst-case conditions under which the power supply is expected to function, such as no-load (standby) operation when a battery charger is connected but no battery is installed. VIN(MAX) cannot exceed 30V. VIN(MIN), the minimum input (battery) voltage. This value should be taken at the full-load operating current under the lowest battery conditions. If VIN(MIN) is below about 6.5V, the filter capacitance required to maintain good AC load regulation increases, and the current limit for the +5V supply has to be increased for the same load level.
MAX786
Current-Sense Resistors (R1, R2)
The sense resistors must carry the peak current in the inductor, which exceeds the full DC load current. The internal current limiting starts when the voltage across the sense resistors exceeds 100mV nominally, 80mV minimum. Use the minimum value to ensure adequate output current capability: For the +3.3V supply, R1 = 80mV / (1.15 x I OUT ); for the +5V supply, R2 = 80mV/(1.15 x IOUT), assuming that LIR = 0.3. Since the sense resistance values (e.g., R1 = 25m for IOUT = 3A) are similar to a few centimeters of narrow traces on a printed circuit board, trace resistance can contribute significant errors. To prevent this, Kelvin connect the CS_ and FB_ pins to the sense resistors; i.e., use separate traces not carrying any of the inductor or load current, as shown in Figure 5. Run these traces parallel at minimum spacing from one another. The wiring layout for these traces is critical for stable, low-ripple outputs (see the Layout and Grounding section).
Inductor (L1, L2)
Three inductor parameters are required: the inductance value (L), the peak inductor current (ILPEAK), and the coil resistance (RL). The inductance is: (VOUT) (VIN(MAX) - VOUT) L = ------------------------ (VIN(MAX)) (f) (IOUT) (LIR) VOUT = output voltage (3.3V or 5V); VIN(MAX) = maximum input voltage (V); f = switching frequency, normally 300kHz; IOUT = maximum DC load current (A); LIR = ratio of inductor peak-to-peak AC current to average DC load current, typically 0.3. A higher value of LIR allows smaller inductance, but results in higher losses and higher ripple. The highest peak inductor current (ILPEAK) equals the DC load current (IOUT) plus half the peak-to-peak AC inductor current (ILPP). The peak-to-peak AC inductor current is typically chosen as 30% of the maximum DC load current, so the peak inductor current is 1.15 times IOUT. The peak inductor current at full load is given by: (VOUT) (VIN(MAX) - VOUT) ILPEAK = IOUT + --------------------------. (2) (f) (L) (VIN(MAX)) The coil resistance should be as low as possible, preferably in the low milliohms. The coil is effectively in series with the load at all times, so the wire losses alone are approximately: Power loss = (IOUT2) (RL). In general, select a standard inductor that meets the L, ILPEAK, and RL requirements (see Tables 1 and 2). If a standard inductor is unavailable, choose a core with an LI2 parameter greater than (L) (ILPEAK2), and use the largest wire that will fit the core. where:
MOSFET Switches (N1-N4)
The four N-channel power MOSFETs are usually identical and must be "logic-level" FETs; that is, they must be fully on (have low r DS(ON) ) with only 4V gatesource drive voltage. The MOSFET r DS(ON) should ideally be about twice the value of the sense resistor. MOSFETs with even lower r DS(ON) have higher gate capacitance, which increases switching time and transition losses. MOSFETs with low gate-threshold voltage specifications (i.e., maximum VGS(TH) = 2V rather than 3V) are preferred, especially for high-current (5A) applications.
Output Filter Capacitors (C6, C7, C12)
The output filter capacitors determine the loop stability and output ripple voltage. To ensure stability, the minimum capacitance and maximum ESR values are: VREF CF > -------------------------- (VOUT) (RCS) (2) () (GBWP) and, (VOUT) (RCS) ESRCF < ------------ VREF where: CF = output filter capacitance (F); VREF = reference voltage, 3.3V; VOUT = output voltage, 3.3V or 5V; RCS = sense resistor (); GBWP = gain-bandwidth product, 60kHz; ESRCF = output filter capacitor ESR ().
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Dual-Output Power-Supply Controller for Notebook Computers
Be sure to select output capacitors that satisfy both the minimum capacitance and maximum ESR requirements. To achieve the low ESR required, it may be appropriate to use a capacitance value 2 or 3 times larger than the calculated minimum. The output ripple in continuous-current mode is: VOUT(RPL) = ILPP(MAX) x (ESRCF + 1/(2 x x f x CF) ). In idle-mode, the ripple has a capacitive and resistive component: (4) (10-4) (L) VOUT(RPL)(C) = -------------- x (RCS2) (CF) 1 1 ------ + ---------- Volts VOUT VIN - VOUT
Boost Capacitors (C4, C5)
Capacitors C4 and C5 store the boost voltage and provide the supply for the DH3 and DH5 drivers. Use 0.1F and place each within 10mm of the BST_ and LX_ pins.
MAX786
Boost Diodes (D1A, D1B)
Use high-speed signal diodes; e.g., 1N4148 or equivalent.
Bypass Capacitors
Input Filter Capacitors (C1, C10) Use at least 3F/W of output power for the input filter capacitors, C1 and C10. They should have less than 150m ESR, and should be located no further than 10mm from N1 and N2 to prevent ringing. Connect the negative terminals directly to PGND. Do not exceed the surge current ratings of input bypass capacitors.
(
)
(0.02) (ESRCF) VOUT(RPL)(R) = --------------- Volts RCS The total ripple, VOUT(RPL), can be approximated as follows: if VOUT(RPL)(R) < 0.5 VOUT(RPL)(C), then VOUT(RPL) = VOUT(RPL)(C), otherwise, VOUT(RPL) = 0.5 VOUT(RPL)(C) + VOUT(RPL)(R).
Shutdown Mode
Shutdown (SHDN = low) forces both PWMs off and disables the REF output and both comparators (Q1 = Q2 = 0V). Supply current in shutdown mode is typically 25A. The VL supply remains active and can source 25mA for external loads. Note that the VL load capability is higher in shutdown and standby modes than when the PWMs are operating (25mA vs. 5mA). Standby mode is achieved by holding ON3 and ON5 low while SHDN is high. This disables both PWMs, but keeps VL, REF, and the precision comparators alive. Supply current in standby mode is typically 70A.
Diodes D1 and D3
Use 1N5819s or similar Schottky diodes. D1 and D3 conduct only about 3% of the time, so the 1N5819's 1A current rating is conservative. The voltage rating of D1 and D3 must exceed the maximum input supply voltage from the battery. These diodes must be Schottky diodes to prevent the lossy MOSFET body diodes from turning on, and they must be placed physically close to their associated synchronous rectifier MOSFETs.
FAT, HIGH-CURRENT TRACES MAIN CURRENT PATH
Soft-Start Capacitors (C8, C9)
A capacitor connected from GND to either SS pin causes that supply to ramp up slowly. The ramp time to full current limit, tSS, is approximately 1ms for every nF of capacitance on SS_, with a minimum value of 10s. Typical capacitor values are in the 10nF to 100nF range; a 5V rating is sufficient. Because this ramp is applied to the current-limit circuit, the actual time for the output voltage to ramp up depends on the load current and output capacitor value. Using Figure 1's circuit with a 2A load and no SS capacitor, full output voltage is reached about 600s after ON_ is driven high.
KELVIN SENSE TRACES
SENSE RESISTOR
MAX786
Figure 5. Kelvin Connections for the Current-Sense Resistors
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15
Dual-Output Power-Supply Controller for Notebook Computers MAX786
Table 3. EV Kit Power-Supply Controls (SW1)
SWITCH 1 2 3 4 NAME SHDN ON3 ON5 SYNC FUNCTION Enable shutdown mode Enable 3.3V power supply Enable 5.0V power supply Oscillator ON OFF SETTING SETTING Operate 3.3V ON 5V ON 200kHz Shutdown 3.3V OFF 5V OFF 300kHz
Other ways to shut down the MAX786 are suggested in the applications section of the MAX782/MAX783 data sheet.
__________Applications Information
Low-Voltage (6-Cell) Operation
The standard application circuit can be configured to accept input voltages from 5.5V to 12V by changing the oscillator frequency to 200kHz and increasing the +5V filter capacitor to 660F. This allows stable operation at 5V loads up to 2A (the 3.3V side requires no changes and still delivers 3A).
V+ C1 33F 35V R9 1k 6 D2 R10 OPEN 3.3V OUT R1 0.025 L1 10H D1 1N5819 C7 C12 150F 150F 10V 10V N1 C5 0.1F 25 27 26 BST3 DH3 LX3 BST5 DH5 LX5 18 16 17 VH D2 C4 0.1F N2 23 V+ 22 VL C2 4.7F
C10 33F 35V VL (5V)
L2 10H D3 1N5819
R2 0.025
5V OUT C6 330F 10V
N3
24
DL3
DL5
19
N4
1 28 SW1A SW1C 3 SW1B 13
MAX786
CS3 FB3 CS5 FB5 REF ON3 ON5 SYNC
15 21 10 R3 1M 11 8 7 14 C8 0.01F C3 1F 20V VREF (3.3V)
ON3 ON5
N1 - N4 = Si9410DY D2 = BAW56L OR TWO 1N4148s
SW1D
SYNC Q1 Q2
SHDN D1 D2
12 4 5
SHDN D1 D2 SS3 GND PGND 2 9 20
Q1 Q2 SS5
C9 0.01F R6 R5 R8 R7 R4 1M 1M 1M 1M 1M
Figure 6. MAX786 EV Kit Schematic
16 ______________________________________________________________________________________
Dual-Output Power-Supply Controller for Notebook Computers
_________________EV Kit Information
The MAX786 evaluation kit (EV kit) embodies the standard application circuit, with some extra pullup and pull-down resistors needed to set default logic signal levels. The board comes configured to accept battery input voltages between 6.5V and 30V, and provides up to 25W of output power. All functions are con-
MAX786
trolled by standard CMOS/TTL logic levels or DIP switches. The kit can be reconfigured for lower battery voltages by setting the oscillator to 200kHz and increasing the 5V output filter capacitor value. The D1 and D2 comparators can be used as precision voltage detectors by installing resistor dividers at each input.
1.0"
1.0"
Figure 7. MAX786 EV Kit Top Component Layout and Silk Screen, Top View
Figure 8. MAX786 EV Kit Ground Plane (Layers 2 and 3), Top View
1.0"
Figure 9. MAX786 EV Kit Top Layer (Layer 1), Top View
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Dual-Output Power-Supply Controller for Notebook Computers MAX786
1.0"
Figure 10. MAX786 EV Kit, Bottom Component Layout and Silk Screen, Bottom View
1.0"
Figure 11. MAX786 EV Kit, Bottom Layer (Layer 4), Top View
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Dual-Output Power-Supply Controller for Notebook Computers MAX786
______________________Chip Topography
SS3 CS3 FB3 ON3 D1 D2 DH3
LX3 BST3
DL3 VH Q2 Q1 GND V+ 0.181"
VL (4.597mm) FB5 PGND DL5
REF SYNC
BST5 LX5
SHDN ON5
SS5 CS5 0.109"
DH5
(2.769mm)
TRANSISTOR COUNT: 1294 SUBSTRATE CONNECTED TO GND
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19
Dual-Output Power-Supply Controller for Notebook Computers MAX786
__Ordering Information (continued)
PART MAX786SCAI MAX786C/D MAX786EAI MAX786REAI MAX786SEAI EV KIT MAX786EVKIT-SO TEMP. RANGE 0C to +70C 0C to +70C -40C to +85C -40C to +85C -40C to +85C TEMP. RANGE 0C to +70C PIN-PACKAGE 28 SSOP Dice* 28 SSOP 28 SSOP 28 SSOP VOUT 3.6V -- 3.3V 3.45V 3.6V
BOARD TYPE Surface Mount
*Contact factory for dice specifications.
________________________________________________________Package Information
SSOP.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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