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MAX786CAI-MAX786EAI-MAX786RCAI-MAX786SCAI
Dual-Output Power-Supply Controller for Notebook Computers
__________________General DescriptionThe MAX786 is a system-engineered power-supply
controller for notebook computers or similar battery-
powered 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-battery-
detection comparators.
High efficiency (95% at 2A; greater than 80% at loads
from 5mA to 3A) is achieved through synchronous recti-
fication and PWM operation at heavy loads, and
Idle ModeTMoperation 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 30µF per ampere of load. Line- and load-
transient responses are terrific, with a high 60kHz unity-
gain crossover frequency allowing output transients to
be corrected within four or five clock cycles. Low sys-
tem 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 synchro-
nizable oscillator for noise-sensitive applications such
as electromagnetic pen-based systems and communi-
cating computers. The MAX786 is a monolithic,
BiCMOS IC available in fine-pitch, surface-mount
SSOP packages.
___________________________ApplicationsNotebook Computers
Portable Data Terminals
Communicating Computers
Pen-Entry Systems
________________________________FeaturesDual PWM Buck Controllers (+3.3V and +5V)Two Precision Comparators or Level Translators95% Efficiency420µA Quiescent Current, 70µA in Standby
(linear regulators alive)25µA Shutdown Current (+5V linear alive)5.5V to 30V Input RangeSmall SSOP PackageFixed Output Voltages:
3.3V (standard)
3.45V (High-Speed Pentium™)
3.6V (PowerPC™)
_________________Ordering Information
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
_____________________Pin ConfigurationIdle Mode is a trademark of Maxim Integrated Products. Pentium is a trademark of Intel Corp. PowerPC is a trademark of IBM Corp.
________Typical Application Diagram
Ordering Information continued at end of data sheet.
MAX786
Dual-Output Power-Supply
Controller for Notebook ComputersStresses 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,= TMINto TMAX, unless otherwise noted.)
ABSOLUTE MAXIMUM RATINGSV+ 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= +70°C)
SSOP (derate 9.52mW/°C above +70°C)....................762mW
Operating Temperature Ranges
MAX786CAI/MAX786_CAI.................................0°C to +70°C
MAX786EAI/MAX786_EAI...............................-40°C to +85°C
Lead Temperature (soldering, 10sec)............................+300°C
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
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. AVCLfor the +3.3V supply is unity gain.
AVCLfor the +5V supply is 1.54.
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0mA, SHDN= ON3 = ON5 = 5V, other digital input levels are 0V or +5V,= TMINto TMAX, unless otherwise noted.)
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
________________________________________________Typical Operating Characteristics(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
_________________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
_________________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
_______________________________________________________________________Pin Description
MAX786
Dual-Output Power-Supply
Controller for Notebook ComputersThe 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 appli-
cations information on the MAX786 not found in this
data sheet.
+3.3V Switch-Mode SupplyThe +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.
_________________Detailed DescriptionThe MAX786 converts a 5.5V to 30V input to four outputs
(Figure 1). It produces two high-power, PWM, switch-
mode 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 gener-
ated via linear regulators, as shown in Figure 2. Fault
protection circuitry shuts off the PWMs when the inter-
nal 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.
Figure 1. MAX786 Application Circuit
MAX786
Dual-Output Power-Supply
Controller for Notebook ComputersA 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 cur-
rent 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.
Figure 2. MAX786 Block Diagram
MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
+5V Switch-Mode SupplyThe +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 undervoltage-
lockout 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 ControllersThe two current-mode PWM controllers are identical
except for different preset output voltages (Figure 3).
Each PWM is independent except for being synchro-
nized to a master oscillator and sharing a common ref-
erence (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 tradi-
tional integrator error amplifier and the phase shift
associated with it. They therefore do not require any
external feedback compensation components if the fil-
ter capacitor ESR guidelines given in the Design
Procedureare 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 oscil-
lator 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, skip-
ping 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.
Soft-Start/SS_ InputsConnecting 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 4µA constant cur-
rent source charges these capacitors up to 4V. The
resulting ramp voltage on the SS_ pins linearly increas-
es 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 10µs.
Soft-start greatly reduces initial in-rush current peaks
and allows start-up time to be programmed externally.
Synchronous RectifiersSynchronous 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 volt-
age 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 syn-
chronous rectifier. The MAX786’s internal break-before-
make timing ensures that shoot-through does not occur.
The Schottky rectifierconducts during the time that nei-
ther MOSFET is on, which improves efficiency by pre-
venting the synchronous-rectifier MOSFET’s lossy body
diode from conducting.
The synchronous rectifier works under all operating condi-
tions, including discontinuous-conduction and idle mode.
Boost Gate-Driver SupplyGate-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 start-
up, the synchronous rectifier (low-side) MOSFET forces
LX_ to 0V and charges the BST_ capacitor to 5V. On the
