MAX797CSLASHD datasheet(13/32 Pages) MAXIM

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Step-Down Controllers with

Synchronous Rectifier for CPU Power

______________________________________________________________________________________

PWM Controller Blocks:

• Multi-Input PWM Comparator

• Current-Sense Circuit

• PWM Logic Block

• Dual-Mode Internal Feedback Mux

• Gate-Driver Outputs

• Secondary Feedback Comparator

Bias Generator Blocks:

• +5V Linear Regulator

• Automatic Bootstrap Switchover Circuit

• +2.505V Reference

These internal IC blocks aren’t powered directly from

the battery. Instead, a +5V linear regulator steps down

the battery voltage to supply both the IC internal rail (VL

pin) as well as the gate drivers. The synchronous-

switch gate driver is directly powered from +5V VL,

while the high-side-switch gate driver is indirectly pow-

ered from VL via an external diode-capacitor boost cir-

cuit. An automatic bootstrap circuit turns off the +5V

linear regulator and powers the IC from its output volt-

age if the output is above 4.5V.

PWM Controller Block

The heart of the current-mode PWM controller is a

multi-input open-loop comparator that sums three sig-

nals: output voltage error signal with respect to the ref-

erence voltage, current-sense signal, and slope

compensation ramp (Figure 3). The PWM controller is a

direct summing type, lacking a traditional error amplifi-

er and the phase shift associated with it. This direct-

summing configuration approaches the ideal of

cycle-by-cycle control over the output voltage.

Under heavy loads, the controller operates in full PWM

mode. Each pulse from the oscillator sets the main

PWM latch that turns on the high-side switch for a peri-

od determined by the duty factor (approximately

VOUT/VIN). As the high-switch turns off, the synchro-

nous rectifier latch is set. 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 zero (in discontinuous mode). Under

fault conditions where the inductor current exceeds the

100mV current-limit threshold, the high-side latch

resets and the high-side switch turns off.

At light loads (

SKIP = low), the inductor current fails to

exceed the 30mV threshold set by the minimum-current

comparator. When this occurs, the controller goes into

idle mode, skipping most of the oscillator pulses in

order to reduce the switching frequency and cut back

gate-charge losses. The oscillator is effectively gated

off at light loads because the minimum-current com-

parator immediately resets the high-side latch at the

beginning of each cycle, unless the feedback signal

falls below the reference voltage level.

When in PWM mode, the controller operates as a fixed-

frequency current-mode controller where the duty ratio

is set by the input/output voltage ratio. The current-

mode feedback system regulates the peak inductor

current as a function of the output voltage error signal.

Since the average inductor current is nearly the same

as the peak current, the circuit acts as a switch-mode

transconductance amplifier and pushes the second

output LC filter pole, normally found in a duty-factor-

controlled (voltage-mode) PWM, to a higher frequency.

To preserve inner-loop stability and eliminate regenera-

tive inductor current “staircasing,” a slope-compensa-

tion ramp is summed into the main PWM comparator to

reduce the apparent duty factor to less than 50%.

The relative gains of the voltage- and current-sense

inputs are weighted by the values of current sources

that bias three differential input stages in the main PWM

comparator (Figure 4). The relative gain of the voltage

comparator to the current comparator is internally fixed

at K = 2:1. The resulting loop gain (which is relatively

low) determines the 2.5% typical load regulation error.

The low loop-gain value helps reduce output filter

capacitor size and cost by shifting the unity-gain

crossover to a lower frequency.

SHDN SKIP

LOAD

CURRENT

MODE

NAME

DESCRIPTION

Low

Shutdown

All circuit blocks

turned off; supply

current = 1µA typ

High

Low

Low,

<10%

Idle

Pulse-skipping;

supply current =

700µA typ at VIN =

10V; discontinuous

inductor current

High

Low

Medium,

<30%

Idle

Pulse-skipping;

continuous inductor

current

High

Low

High,

>30%

PWM

Constant-frequency

PWM; continuous

inductor current

High

Low Noise*

(PWM)

Constant-frequency

PWM regardless of

load; continuous

inductor current

even at no load

Table 3. Operating-Mode Truth Table

* MAX796/MAX799 have no

SKIP pin and therefore can’t go

into low-noise mode.

X = Don’t Care

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