LM3101 datasheet(8/20 Pages) NSC

8 / 20 page

Functional Description

OscillatorSynchronization Section

The operating frequency is set by a single resistor connect-

ed from the RT pin (pin 1) to ground according to the equa-

tion

FO e 025(RT 20 pF)

kHz X

Inserting a 25 kX for RT sets the oscillator frequency at

500 kHz

The oscillator is capable of synchronizing to an external

source To synchronize the oscillator an external source is

connected to the SYNC pin (pin 4) via a differentiator (see

Figure 1 ) The external source delivers a pulse train to the

differentiator which converts this signal into an AC-coupled

signal The negative-edge of this signal applied to the

SYNC pin will control the oscillator and thus set the operat-

ing frequency The recommended values for RSYNC and

CSYNC are as follows

RSYNC e 1kX (typical)

and

CSYNC RSYNC l 1(8 FSYNC)

F X kHz

To synchronize to a 600 kHz external source and using a

RSYNC of1kX the CSYNC must be

CSYNC l 1(8 RSYNC FSYNC) e 1(8 1kX 600 kHz)

208 pF

220 pF

The oscillator frequency should range from 67% to 90% of

the synchronization frequency In the above example the

oscillator frequency can be between 400 kHz and 540 kHz

TLH11436 – 6

FIGURE 1 Simplified Version of the

Synchronization Circuit

TLH11436 – 7

FIGURE 2 Simplified Version of the Short

Circuit Frequency Shift Circuit

LM3101 Datasheet(PDF) 8 Page - National Semiconductor (TI)