LM3481_15 Datasheet, PDF (11/38 Pages) Texas Instruments – LM3481 / -Q1 High-Efficiency Controller for Boost, SEPIC and Flyback DC-DC Converters

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7 Detailed Description
LM3481, LM3481-Q1
SNVS346F – NOVEMBER 2007 – REVISED NOVEMBER 2014
7.1 Overview
The LM3481 device 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 (EA) 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 18.
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, called the blank-out time, is typically 250 ns and is specified as
t_min (on) in the Electrical Characteristics section.
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 overvoltage comparator
inside the LM3481 prevents the output voltage from rising under these conditions by sensing the feedback (FB
pin) voltage and resetting the RS latch. The latch remains in a reset state until the output decays to the nominal
value. Thus the operating frequency decreases at light loads, resulting in excellent efficiency.
Blank-Out prevents false
reset
PWM Comparator resets
the RS latch
V_SL
_
Oscillator Sets
the RS Latch
T_min (on) Blank-Out time
Figure 18. Basic Operation of the PWM Comparator
+
PWM
Comparator
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Product Folder Links: LM3481 LM3481-Q1
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