LM3488

LM3488-Q1

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SNVS089M – JULY 2000 – REVISED MARCH 2013

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

The LM3488 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 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 22.

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 150ns and is called the blank-out time.

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Product Folder Links: LM3488 LM3488-Q1