FB

FA/SYNC/SD

COMP

AGND

PGND

DR

Shutdown Detect

SYNC/Fixed

Frequency detect

Soft-start

+

EA

+

-

+

Slope

Compensation

Thermal

Shutdown

Set/Blankout

Switch

Logic

Switch

Driver

1.275V

Reference

Oscillator

+

220 mV

Level Shifter

6

+

One Shot

Bias

Voltages

6V

Overvoltage

Comparator

Short-circuit

Comparator

Ramp

Adjust

V-I

Converter

I-V

Converter

PWM

Comparator

R

S

Q

UVLO

UVLO

40 é

A

-

-

-

-

LM3481

SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012

www.ti.com

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

The LM3481 uses a fixed frequency, Pulse Width Modulated (PWM), current mode control architecture. In a

typical application circuit, the peak current through the external MOSFET is sensed through an external sense

positive input of the PWM comparator. The output voltage is also sensed through an external feedback resistor

divider network and fed into the error amplifier (EA) negative input (feedback pin, FB). The output of the error

amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM

comparator.

At the start of any switching cycle, the oscillator sets the RS latch using the SET/Blank-out and switch logic

blocks. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns

on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is

reset and the external MOSFET turns off.

The voltage sensed across the sense resistor generally contains spurious noise spikes, as shown in Figure 18.

These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from

resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a

short duration after the latch is set. This duration, called the blank-out time, is typically 250 ns and is specified as

Under extremely light load or no-load conditions, the energy delivered to the output capacitor when the external

MOSFET is on during the blank-out time is more than what is delivered to the load. An over-voltage comparator

inside the LM3481 prevents the output voltage from rising under these conditions by sensing the feedback (FB

pin) voltage and resetting the RS latch. The latch remains in a reset state until the output decays to the nominal

value. Thus the operating frequency decreases at light loads, resulting in excellent efficiency.

8

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