Datasheet

FB
FA/SYNC/SD
COMP
I
SEN
AGND
PGND
DR
V
IN
Shutdown Detect
SYNC/Fixed
Frequency detect
Soft-start
+
EA
+
-
+
Slope
Compensation
Thermal
Shutdown
Set/Blankout
Switch
Logic
Switch
Driver
1.275V
Reference
Oscillator
+
220 mV
Level Shifter
6
+
One Shot
Bias
Voltages
6V
Overvoltage
Comparator
Short-circuit
Comparator
Ramp
Adjust
V-I
Converter
I-V
Converter
PWM
Comparator
R S
Q
V
CC
UVLO
UVLO
40 éA
V
fb
+ V
ovp
-
-
-
-
LM3481
SNVS346E –NOVEMBER 2007–REVISED APRIL 2012 www.ti.com
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
The LM3481 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 over-voltage 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.
8 Submit Documentation Feedback Copyright © 2007–2012, Texas Instruments Incorporated
Product Folder Links: LM3481