sync of SG3525

[Trevorferns]

I want to sync two SG3525, clocked at 75Khz with a separate oscillator running at 85Khz. I plan on using two opto couplers connected to the sync pin of the 3525 with a resistor between the sync pin and +Vcc. Will this work reliably or do i need to use a buffer between the opto and the sync pin. Any info shared on this topic will be appreciated.

[microsim444]

He loo.



The easiest and most reliable way to sync the SG3525 is to use

555 Chip, and supply it from the VREF pins connected (2+16) witch is 5V, and the output of the 555 to the SYNC pin of the SG3525.


And no need for any thing else.

[Trevorferns]

Thanks mate.
But the issue here is that i want to sync the 3525 it to an external 85 Khz signal. As I'd much prefer keeping them isolated, I thought the best way of doing it was through an opto-coupler. Your thoughts on this will be appreciated.

[N-Channel]

Trevor-

The only Application Note I could find is one from Silicon General, PS-7, "Synchronizing the SG1525A PWM". Section 1 of this sheet (Synchronizing One Device to an External Clock) is as follows:

[A. Program the SG1525A oscillator with R(t) and C(t) to free-run at a frequency 10% slower than the external clock frequency.

B Drive the SG1525A SYNC pin terminal (pin 3) with the external clock. Input impedance is 2k. The clock amplitude should be greater than 2 volts and less than 5 volts. Pulse width should be at least 300nS for reliable triggering, but should not exceed the free-running oscillator clock pulse width by more than 200nS.]

Since you want to synch the '3525 at 150kHz, these times should not present a problem. If you were synch'ing it at the 300kHz, you might run into some problems, as the '3525 oscillator's max. frequency is ~400kHz.

EVA is right that there are some things that can't be learned from datasheets alone. I've looked everywhere for anything I can find on synch'ing the SG3525, MC33025, TL494, UC1846, and other popular PWM chips, and this Application Sheet is the best thing, by far, I have seen for the SG3525. I have worked on an "Application Note" of my own to consolidate the synch schemes for all of these chips so I can see them all at-a-glance. If I get some time, I will scan it and post it.

I got this sheet some time ago (before the www), so I have not gone to SG's website to see if it has made it into cyberspace yet.

Hope this helps,

Steve

[Trevorferns]

Steve,

Thanks mate, that will surely help. Am also concerned about using an opto coupler between the two. Any schematic on how it is done reliably ?

Cheers

[N-Channel]

Happy to help out. Do the two '3525s need to be Galvanicaly isolated? If not, then you can sync 'em much easier w/o the opto. I wouldn't try to send a 75kHz signal thru the opto simply because if it's slow speed. I do not have any schematics or reference material on sync'ing thru an opto. If I can somehow scan the PS-7 App Note sheet and post it. I will try.

[Trevorferns]

Thanks, a lot.
Yes the 3525 needs to be galvanic isolated from the sync clock. Hence the thought of using an opto. The speed of the opto is now a new worry that i have !!

[N-Channel]

This might be a stretch, but you might try sending the sync signal through a small pulse transformer, with appropriate driving cktry.