Datasheet

V
OUT
+ V
D1
- V
Q
V
IN
- V
Q
1-D
=
UVLO
V
IN
R7
R8
I
UVLO
V
UVLOSEN
-
+
+
-
2
LM3481
SNVS346E –NOVEMBER 2007–REVISED APRIL 2012 www.ti.com
Figure 28. UVLO Pin Resistor Divider
If the UVLO pin function is not desired, select R8 and R7 of equal magnitude greater than 100 kΩ. This will allow
V
IN
to be in control of the UVLO thresholds. The UVLO pin may also be used to implement the enable/disable
function. If a signal pulls the UVLO pin below the 1.43V (typical) threshold, the converter will be disabled.
SHORT CIRCUIT PROTECTION
When the voltage across the sense resistor (measured on the I
SEN
Pin) exceeds 220 mV, short-circuit current
limit gets activated. A comparator inside the LM3481 reduces the switching frequency by a factor of 8 and
maintains this condition until the short is removed.
TYPICAL APPLICATIONS
The LM3481 may be operated in either continuous or discontinuous conduction mode. The following applications
are designed for continuous conduction operation. This mode of operation has higher efficiency and lower EMI
characteristics than the discontinuous mode.
Boost Converter
The most common topology for the LM3481 is the boost or step-up topology. The boost converter converts a low
input voltage into a higher output voltage. The basic configuration for a boost regulator is shown in Figure 29. In
continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regulator
operates in two cycles. In the first cycle of operation, MOSFET Q is turned on and energy is stored in the
inductor. During this cycle, diode D1 is reverse biased and load current is supplied by the output capacitor, C
OUT
.
In the second cycle, MOSFET Q is off and the diode is forward biased. The energy stored in the inductor is
transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The
output voltage is defined as:
(ignoring the voltage drop across the MOSFET and the diode), or
where D is the duty cycle of the switch, V
D1
is the forward voltage drop of the diode, and V
Q
is the drop across
the MOSFET when it is on. The following sections describe selection of components for a boost converter.
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