LM3478MM National Semiconductor, LM3478MM Datasheet - Page 13

• 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 Description
SHORT-CIRCUIT PROTECTION
When the voltage across the sense resistor (measured on
I
activated. A comparator inside LM3478 reduces the switch-
ing frequency by a factor of 5 and maintains this condition till
the short is removed.
Typical Applications
The LM3478 may be operated in either continuous or dis-
continuous 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 LM3478 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 8. 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,
SEN
Pin) exceeds 350mV, short-circuit current limit gets
FIGURE 7. Shutdown Operation in Frequency Adjust Mode
(Continued)
FIGURE 6. Frequency Adjust
13
diode D is reverse biased and load current is supplied by the
output capacitor, C
In the second cycle, MOSFET Q is off and the diode is
forward biased. The energy stored in the inductor is trans-
ferred to the load and output capacitor. The ratio of these two
cycles determines the output voltage. The output voltage is
defined as:
(ignoring the drop across the MOSFET and the diode), or
where D is the duty cycle of the switch, V
voltage drop of the diode, and V
MOSFET when it is on. The following sections describe
selection of components for a boost converter
OUT
.
10135514
10135516
Q
is the drop across the
D
is the forward
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