SW

VCON

FB

EN

VIN

GND

OSCILLATOR

RAMP

GENERATOR

OUTPUT SHORT

PROTECTION

THERMAL

SHUTDOWN

DRIVER

CONTROL LOGIC

PWM

COMPARATOR

Ref2

Ref1

NCP

OVERVOLTAGE

DETECTOR

DELAY

ERROR

AMP

OLP

EN

Ref3

LIGHT-LOAD

CHECK COMP

ECO

COMPARATOR

Copyright © 2017, Texas Instruments Incorporated

13

LM3241

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SNOSB38C – JANUARY 2009 – REVISED NOVEMBER 2017

Product Folder Links: LM3241

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Copyright © 2009–2017, Texas Instruments Incorporated

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Circuit Operation

Referring to Figure 29 and the Functional Block Diagram, the LM3241 operates as follows. During the first part of

each switching cycle, the control block in the LM3241 turns on the internal, top-side PFET switch. This allows

current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the

current to a ramp with a slope of around (VIN – VOUT) / L, by storing energy in a magnetic field. During the second

part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns

the bottom-side NFET synchronous rectifier on. In response, the magnetic field of the inductor collapses,

generating a voltage that forces current from ground through the synchronous rectifier to the output filter

capacitor and load. As the stored energy is transferred back into the circuit and depleted, the inductor current

ramps down with a slope around VOUT / L. The output filter capacitor stores charge when the inductor current is

high, and releases it when low, smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch-on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output

voltage is equal to the average voltage at the SW pin.