LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 9

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

Functional Block Diagram
Functional Description
The LM3488 uses a fixed frequency, Pulse Width Modulated
(PWM), current mode control architecture. In a typical appli-
cation 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
level shifted and fed into the positive input of the PWM com-
parator. The output voltage is also sensed through an external
feedback resistor divider network and fed into the error am-
plifier negative input (feedback pin, FB). The output of the
error amplifier (COMP pin) is added to the slope compensa-
tion ramp and fed into the negative input of the PWM com-
parator.
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 MOS-
FET) 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.
SEN
pin. This voltage is then
9
The voltage sensed across the sense resistor generally con-
tains spurious noise spikes, as shown in Figure 1. 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 condi-
tions. 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.
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