LM3488_15 Datasheet, PDF (11/37 Pages) Texas Instruments – LM3488/-Q1 High-Efficiency, Low-Side, N-Channel Controller for Switching Regulators

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7 Detailed Description
LM3488, LM3488-Q1
SNVS089N – JULY 2000 – REVISED DECEMBER 2014
7.1 Overview
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
resistor. The voltage across this resistor is fed into the I_SEN pin. This voltage is then level shifted and 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 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 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 150ns and is called the blank-out time.
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 LM3488 prevents the output voltage from rising under these conditions. The over-voltage comparator
senses the feedback (FB pin) voltage and resets the RS latch under these conditions. The latch remains in reset
state till the output decays to the nominal value.
Figure 20. Basic Operation of the PWM Comparator
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