2 MHz

Oscillator

Soft

Start

Ramp

Generator

Thermal

Shutdown

Undervoltage

Lockout

Frequency

Compensation

+

-

Error

Amp

Control Logic

Driver

Current Limit

Comparator

Ref1

PFM Current

Comparator

Ref2

SW

Zero Crossing

Comparator

FB

EN

VIN

PWM Comparator

pfm_low

pfm_hi

GND

Bandgap

+

-

Vcomp

1.0V

Fixed Ver

Adj Ver

+

-

0.5V

+

-

+

-

VREF

LP3905

www.ti.com

SNVS374D – JUNE 2006 – REVISED MAY 2013

Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown

protection.

The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the

input voltage is 3V or higher.

BUCK CONVERTER BLOCK DIAGRAM

Figure 6. Simplified Functional Diagram

CIRCUIT OPERATION

The LP3905 Buck regulators operate as follows. During the first portion of each switching cycle, the control block

in the LP3905 turns on the internal 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 (VIN–VOUT)/L, by

storing energy in a magnetic field.

During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the

input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the

NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of - VOUT/L.

The output filter stores charge when the inductor current is high, and releases it when inductor current is low,

smoothing the voltage across the load.

Copyright © 2006–2013, Texas Instruments Incorporated

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Product Folder Links: LP3905