LM3478 datasheet(12/21 Pages) NSC

12 / 21 page

Functional Description (Continued)

FREQUENCY ADJUST/SHUTDOWN

The switching frequency of LM3478 can be adjusted be-

tween 100kHz and 1MHz using a single external resistor.

This resistor must be connected between FA/SD pin and

ground, as shown in Figure 6. Please refer to the typical

performance characteristics to determine the value of the

resistor required for a desired switching frequency.

The FA/SD pin also functions as a shutdown pin. If a high

signal (refer to the electrical characteristics for definition of

high signal) appears on the FA/SD pin, the LM3478 stops

switching and goes into a low current mode. The total supply

current of the IC reduces to less than 10 µA under these

conditions.

Figure 7 shows implementation of shutdown function when

operating in frequency adjust mode. In frequency adjust

mode, connecting the FA/SD pin to ground forces the clock

to run at a certain frequency. Pulling this pin high shuts down

the IC. In frequency adjust mode, a high signal for more than

30µs shuts down the IC.

10135513

FIGURE 4. Increasing the Slope of the Compensation Ramp

10135551

FIGURE 5.

∆V

SL vs RSL

LM3478 Datenblatt(PDF) 12 Page - National Semiconductor (TI)