FB

VIN

ON TIMER

High-Side

Driver

0.8V

Q

Vin

LM1771

REGULATION

COMPARATOR

Q

OFF TIMER

GND

HG

LG

Level Shift

and

Shoot

Through

Protection

Low-Side

Driver

SHORT

CIRCUIT

PROTECTION

0.55 V

/Soft-Start

UVLO

UVLO

SD

SD

Q

Q

R

S

Q

Q

R

S

1.2 V

EN

ENABLE

COMPARATOR

LM1771

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SNVS446D – JUNE 2006 – REVISED JANUARY 2016

7 Detailed Description

7.1 Overview

The LM1771 synchronous buck controller has a control scheme that is referred to as adaptive ON-time control.

This topology relies on a fixed switch ON-time to regulate the output voltage. This ON-time is internally set by

EEPROM and is available with three different set-points to allow for different frequency options. The LM1771

automatically adjusts the ON-time during operation inversely with the input voltage (VIN) to maintain a constant

frequency. Therefore the switching frequency during continuous conduction mode is independent of the inductor

and capacitor size unlike hysteretic switchers.

At the beginning of the cycle, the LM1771 turns on the high-side PFET for a fixed duration. This ON-time is

of 150 ns is internally set and cannot be adjusted. This is to prevent false triggering from occurring on the

remains off until the comparator trip-point has been reached. Upon passing this trip-point (set at 0.8 V at the

feedback pin), the PFET turns back on and the process repeats, thus regulating the output.

The NFET control is complementary to the PFET control with the exception of a short dead-time to prevent

shoot-through from occurring.

7.2 Functional Block Diagram

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