LM3478MM National Semiconductor, LM3478MM Datasheet - Page 9

• Manufacturer Part Number: LM3478MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3478MM

• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Features: High Efficiency
• Package / Case: 8-MSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Functional Block Diagram
Functional Description
The LM3478 uses a fixed frequency, Pulse Width Modulated
(PWM), current mode control architecture. In a typical appli-
cation circuit, the peak current through the external MOS-
FET is sensed through an external sense resistor. The volt-
age across this resistor is fed into the I
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 ex-
ceeds the negative input, the RS latch is reset and the
external MOSFET turns off.
SEN
pin. This voltage
9
The voltage sensed across the sense resistor generally
contains 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 LM3478
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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