LM2731

www.ti.com

SNVS217G – MAY 2004 – REVISED SEPTEMBER 2015

7 Detailed Description

7.1 Overview

The LM2731 device is a switching converter IC that operates at a fixed frequency (0.6 or 1.6 MHz) for fast

transient response over a wide input voltage range and incorporates pulse-by-pulse current limiting protection.

Because this is current mode control, a 33-m

Ω sense resistor in series with the switch FET is used to provide a

voltage (which is proportional to the FET current) to both the input of the pulse width modulation (PWM)

comparator and the current limit amplifier.

7.1.1 Theory of Operation

At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a

voltage (proportional to this current) is summed with the ramp coming from the ramp generator and then fed into

the input of the PWM comparator. When this voltage exceeds the voltage on the other input (coming from the

Gm amplifier), the latch resets and turns the FET off. Because the signal coming from the Gm amplifier is derived

from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets

the correct peak current through the FET to keep the output voltage in regulation.

Q1 and Q2 along with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation.

The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to

maintain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at

the FB node "multiplied up" by the ratio of the output resistive-divider.

The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches

the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit

input terminates the pulse regardless of the status of the output of the PWM comparator.

7.2 Functional Block Diagram

7.3 Feature Description

The LM2731 is a fixed-frequency boost regulator IC that delivers a minimum 1.8-A peak switch current.

The device provides cycle-by-cycle current limit protection as well as thermal shutdown protection. The device

can also be controlled through the shutdown pin.

Copyright © 2004–2015, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM2731