MAX782CSLASHD datasheet(12/32 Pages) MAXIM

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Triple-Output Power-Supply

Controller for Notebook Computers

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+3.3V and +5V PWM Buck Controllers

The two current-mode PWM controllers are identical

except for different preset output voltages and the

addition of a flyback winding control loop to the +5V

side (see Figure 3, +3.3V/+5V PWM Controller Block

Diagram). Each PWM is independent except for being

synchronized to a master oscillator and sharing 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 any

external feedback compensation components if the fil-

ter 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 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 compara-

tor. 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.

A flyback winding controller regulates the +15V VDD

supply in the absence of a load on the main +5V out-

put. If VDD falls below the preset +13V VDD regulation

threshold, a 1µs one-shot is triggered that extends the

on-time of the low-side switch beyond the point where

the inductor current crosses zero (in discontinuous

mode). This causes inductor (primary) current to

reverse, pulling current out of the output filter capacitor

and causing the flyback transformer to operate in the

forward mode. The low impedance presented by the

transformer secondary in forward mode allows the

+15V filter capacitor to be quickly charged again,

bringing VDD into regulation.

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 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 Rectifiers

Synchronous rectification allows for high efficiency by

reducing the losses associated with the Schottky recti-

fiers. Also, the synchronous rectifier MOSFETS are

necessary for correct operation of the MAX782's boost

gate-drive and VDD supplies.

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”) is said to occur

if the high-side switch turns on at the same time as the

synchronous rectifier. The MAX782’s internal break-

before-make 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 condi-

tions, including discontinuous-conduction and idle-mode.

The +5V synchronous rectifier also controls the 15V VDD

voltage (see the

High-Side Supply (VDD) section).

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