LM3478 datasheet(13/21 Pages) NSC

13 / 21 page

Functional Description (Continued)

SHORT-CIRCUIT PROTECTION

When the voltage across the sense resistor (measured on

SEN Pin) exceeds 350mV, short-circuit current limit gets

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,

diode D 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 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

D 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

10135514

FIGURE 6. Frequency Adjust

10135516

FIGURE 7. Shutdown Operation in Frequency Adjust Mode

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