PWM Comparator resets

the RS latch

PWM

Comparator

Blank-Out prevents false

reset

325 ns Blank-Out time

Oscillator Sets

the RS Latch

+

-

92 mV

typ

11

LM3478

www.ti.com

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

Product Folder Links: LM3478

Submit Documentation Feedback

Copyright © 2000–2017, Texas Instruments Incorporated

7 Detailed Description

7.1 Overview

The LM3478 device uses a fixed frequency, Pulse Width Modulated (PWM) current mode control architecture.

The Functional Block Diagram shows the basic functionality. In a typical application circuit, the peak current

through the external MOSFET is sensed through an external sense resistor. The voltage across this resistor is

fed into the ISEN pin. This voltage is fed into the 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 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 switch logic block. 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 20.

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 is about 325 ns and is called the blanking interval and is

specified as minimum on-time in the Electrical Characteristics section. Under extremely light-load or no-load

conditions, the energy delivered to the output capacitor when the external MOSFET in on during the blanking

interval is more than what is delivered to the load. An over-voltage comparator inside the LM3478 prevents the

output voltage from rising under these conditions. The over-voltage comparator senses the feedback (FB pin)

voltage and resets the RS latch. The latch remains in reset state until the output decays to the nominal value.

Figure 20. Basic Operation of the PWM Comparator