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0

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t

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SW

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LM2738

SNVS556C – APRIL 2008 – REVISED JANUARY 2016

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7 Detailed Description

7.1 Overview

The LM2738 is a constant frequency PWM buck regulator device that delivers a 1.5-A load current. The regulator

has a preset switching frequency of either 550 kHz (LM2738Y) or 1.6 MHz (LM2738X). These high frequencies

allow the LM2738 to operate with small surface-mount capacitors and inductors, resulting in DC-DC converters

that require a minimum amount of board space. The LM2738 is internally compensated, so it is simple to use and

requires few external components. The LM2738 uses current-mode control to regulate the output voltage.

The LM2738 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

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

begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW

cycle (D) to maintain a constant output voltage. See Functional Block Diagram and Figure 25.

Figure 25. LM2738 Waveforms of SW Pin Voltage and Inductor Current