LM3481 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

Under Voltage Lockout (UVLO) Pin
The UVLO pin provides user programmable enable and shut-
down thresholds. The UVLO pin is compared to an internal
reference of 1.43V (typical), and a resistor divider programs
the enable threshold, V_EN. When the IC is enabled, a 5 μA
current is sourced out of the UVLO pin, which effectively
causes a hysteresis, and the UVLO shutdown threshold,
V_SH, is now lower than the enable threshold. Setting these
thresholds requires two resistors connected from the V_IN pin
to the UVLO pin and from the UVLO pin to GND (see Figure
11). Select the desired enable, V_EN, and UVLO shutdown,
V_SH, threshold voltages and use the following equations to
determine the resistance values:
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 ac-
tivated. A comparator inside the LM3481 reduces the switch-
ing frequency by a factor of 8 and maintains this condition until
the short is removed.
20136596
FIGURE 11. UVLO Pin Resistor Divider
Typical Applications
The LM3481 may be operated in either continuous or discon-
tinuous conduction mode. The following applications are de-
signed 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 12. In continuous
conduction mode (when the inductor current never reaches
zero at steady state), the boost regulator operates in two cy-
cles. 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 de-
termines 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 MOS-
FET when it is on. The following sections describe selection
of components for a boost converter.
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