Datasheet

FA/SYNC/SD

LM3481

22 x 10

- 5.74

LM3481

SNVS346E –NOVEMBER 2007–REVISED APRIL 2012

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Figure 23. ΔV

vs R

FREQUENCY ADJUST/SYNCHRONIZATION/SHUTDOWN

The switching frequency of the LM3481 can be adjusted between 100 kHz and 1 MHz using a single external

resistor. This resistor must be connected between the FA/SYNC/SD pin and ground, as shown in Figure 24.

Please refer to the typical performance characteristics to determine the value of the resistor required for a

desired switching frequency.

The following equation can also be used to estimate the frequency adjust resistor.

Where f

is in kHz and R

in kΩ.

The LM3481 can be synchronized to an external clock. The external clock must be connected between the

FA/SYNC/SD pin and ground, as shown in Figure 25. The frequency adjust resistor may remain connected while

synchronizing a signal, therefore if there is a loss of signal, the switching frequency will be set by the frequency

adjust resistor.

It is also necessary to have the width of the synchronization pulse narrower than the duty cycle of the converter

and to have the synchronization pulse width ≥ 300 ns.

The FA/SYNC/SD pin also functions as a shutdown pin. If a high signal (refer to the electrical characteristics

section for definition of high signal) appears on the FA/SYNC/SD pin, the LM3481 stops switching and goes into

a low current mode. The total supply current of the IC reduces to 5 µA, typically, under these conditions.

Figure 26 and Figure 27 shows an implementation of a shutdown function when operating in frequency adjust

mode and synchronization mode respectively. In frequency adjust mode, connecting the FA/SYNC/SD pin to

ground forces the clock to run at a certain frequency. Pulling this pin high shuts down the IC. In frequency adjust

or synchronization mode, a high signal for more than 30 µs shuts down the IC.

Figure 24. Frequency Adjust

Product Folder Links: LM3481