_______________Detailed Description

The MAX783 converts a 5.5V to 30V input to six 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, allow-

ing for small external components. Output current capa-

bility depends on external components, and can exceed

6A on each supply. Two 12V VPP outputs, an internal 5V,

25mA supply (VL) and a 3.3V, 5mA reference voltage

are also generated via linear regulators (Figure 2). Fault-

protection circuitry shuts off the PWM and high-side sup-

ply 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 MAX783 is capable of accepting input voltages

from 5.5V to 30V, but is optimized for the lower end of

this range because the +15V flyback winding controller

is appended to the +3.3V buck supply. This architecture

allows for lower input voltages than are possible with the

MAX782 sister chip, which puts the winding on the +5V

side, while maintaining high +15V load capability.

However, the MAX783’s transformer has a higher turns

ratio (4:1 vs. 2:1), which leads to higher interwinding

capacitance as well as higher switching noise ampli-

tudes at the transformer secondary when the input volt-

age is high. Therefore, the MAX783 standard applica-

tion circuit is optimized with external components for

low-voltage (6-8 cell) designs with maximum input volt-

ages of 20V and less. The MAX783 itself can easily

accept 30V inputs, but expect to see more noise and

higher voltage swings at the transformer secondary

under these conditions. The inductor and filter capacitor

values may also require some adjustment for inputs

greater than 20V; see the

Design Procedure section.

+5V Switch-Mode Supply

The +5V supply is generated by a current-mode PWM

step-down regulator using two N-channel MOSFETs, a

rectifier, plus 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 cir-

cuit that uses a 100nF capacitor connected to BST5.

The +5V supply’s dropout voltage, as configured in

Figure 1, is typically 400mV at 2A. As V+ approaches

5V, the +5V output falls with V+ until the VL regulator

output hits its undervoltage lockout threshold at 4V. At

this point, the +5V supply turns off.

A synchronous rectifier at LX5 keeps efficiency high by

effectively 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.

+3.3V Switch-Mode Supply

The +3.3V output is produced by a current-mode PWM

step-down regulator similar to the +5V supply. The +3.3V

supply uses a transformer primary winding as its induc-

tor; the secondary is used for the 15V VDD supply.

The default switching frequency for both PWM controllers

is 200kHz (with SYNC connected to GND or VL), but

300kHz may be used by connecting SYNC to REF.

+3.3V and +5V PWM Buck Controllers

The two current-mode PWM buck controllers are nearly

identical except for different preset output voltages and

the addition of a flyback winding control loop to the

3.3V side. Each PWM is independent, except both are

synchronized to a master oscillator and share a com-

mon 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 tradi-

tional integrator-type error amplifier and the phase shift

associated with it. They therefore do not require exter-

nal feedback compensation components if you follow

the filter capacitor ESR guidelines in the

Design

Procedure.

The main gain block is an open-loop comparator that

sums four input signals: output voltage error signal,

current-sense signal, slope-compensation ramp, and

precision reference voltage. 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 factor

off, the synchronous rectifier latch is set; 60ns later, the

low-side switch turns on. The low-side switch stays on

until the beginning of the next clock cycle (in continu-

ous mode) or until the inductor current crosses through

Triple-Output Power-Supply Controller

for Notebook Computers

8

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Table 1. Truth Table for VPP Control Pins

D_0

D_1

VPP

0

0

0V

0

1

5V

1

0

12V

1

1

3.3V