0

0

VIN

VD

TON

t

t

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IL

IPK

SW

Voltage

LM2736

SNVS316H – SEPTEMBER 2004 – REVISED DECEMBER 2014

www.ti.com

7 Detailed Description

7.1 Overview

The LM2736 device is a constant frequency PWM buck regulator IC that delivers a 750 mA load current. The

regulator has a preset switching frequency of either 550 kHz (LM2736Y) or 1.6 MHz (LM2736X). These high

frequencies allow the LM2736 device to operate with small surface mount capacitors and inductors, resulting in

DC/DC converters that require a minimum amount of board space. The LM2736 device is internally

compensated, so it is simple to use, and requires few external components. The LM2736 device uses current-

mode control to regulate the output voltage.

The following operating description of the LM2736 device will refer to the Simplified Block Diagram (Functional

Block Diagram) and to the waveforms in Figure 11. The LM2736 device supplies a regulated output voltage by

switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle

begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the

output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (VSW)

swings up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the

current-sense amplifier, which generates an output proportional to the switch current. The sense signal is

summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional

to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the

output switch turns off until the next switching cycle begins. During the switch off-time, inductor current

discharges through Schottky diode D1, which forces the SW pin to swing below ground by the forward voltage

(VD) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage.

Figure 11. LM2736 Waveforms of SW Pin Voltage and Inductor Current

8

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