The main gain block is an open-loop comparator that

sums four signals: output voltage error signal, current-

sense signal, slope-compensation ramp, and the 3.3V

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 factor (approximately

As the high-side switch turns off, the synchronous recti-

fier 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 continuous-conduction mode) or

until the inductor current reaches zero (in discontinu-

ous-conduction 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, 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 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

Connecting a capacitor from the soft-start pin (SS) to

ground allows a gradual build-up of the 3.3V output

after power is applied or ON is driven high. When ON is

low, the soft-start capacitor is discharged to GND.

When ON is driven high, a 4µA constant current source

charges the capacitor up to 4V. The resulting ramp volt-

age on SS linearly increases the current-limit compara-

tor set-point, increasing the duty cycle to the external

power MOSFETs. With no soft-start capacitor, the full

output current is available within 10µs (see

Applications

Information and Design Procedure section).

Synchronous Rectifier

Synchronous rectification allows for high efficiency by

reducing the losses associated with the Schottky rectifi-

er. Also, the synchronous-rectifier MOSFET is neces-

sary for correct operation of the MAX767’s boost gate-

drive supply.

When the external power MOSFET (N1) turns off, ener-

gy stored in the inductor causes its terminal voltage to

reverse instantly. Current flows in the loop formed by

the inductor (L1), Schottky diode (D2), and the load—

an action that charges up the output filter capacitor

(C2). The Schottky diode has a forward voltage of

about 0.5V which, although small, represents a signifi-

cant power loss and degrades efficiency. The synchro-

nous-rectifier MOSFET parallels the diode and is turned

on by DL shortly after the diode conducts. Since the

low, the losses are reduced. The synchronous-rectifier

MOSFET is turned off when the inductor current falls to

zero.

The MAX767’s internal break-before-make timing

ensures that shoot-through (both external switches

turned on at the same time) 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 mode

and idle-mode.

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

8

_______________________________________________________________________________________

MAX767

SENSE RESISTOR

MAIN CURRENT PATH

FAT, HIGH-CURRENT TRACES

Figure 2. Kelvin Connections for the Current-Sense Resistor

™ Idle-Mode is a trademark of Maxim Integrated Products.