PWM with SG3525

[parkc23]

Hi, could anyone help me with SG3525? I have a PWM circuit with
SG3525 and need to modify the circuit so that I can get ~0.2 Hz cycle
period. This is very slow compared to most of the circuits that I can
get.
Probably, I can change the frequency by changing the capacitor or
resistor at Rt and Ct (pin 5 and 6), but I don't know the equation to
calculate those.
Please help me!!!

 

[CFoley1064]

Subject: PWM with SG3525
From: parkc23@hotmail.com (parkc23)
Date: 8/13/2004 1:01 PM Central Daylight Time
Message-id: <8e31da91.0408131001.2b34383b@posting.google.com

Hi, could anyone help me with SG3525? I have a PWM circuit with
SG3525 and need to modify the circuit so that I can get ~0.2 Hz cycle
period. This is very slow compared to most of the circuits that I can
get.
Probably, I can change the frequency by changing the capacitor or
resistor at Rt and Ct (pin 5 and 6), but I don't know the equation to
calculate those.
Please help me!!!

How do. For questions like this, nothing beats the data sheet:

http://www.st.com/stonline/books/pdf/docs/4286.pdf

It says the approximate frequency is:

1 f = 1/(CT (0.7 RT + 3 RD))

However, the datasheet also says the minimum practical frequency for the SG3525
is about 100 Hz. If you're looking for 0.2 Hz, you'll have to look for another
PWM IC, roll your own PWM with an op amp or comparator, or just use a PIC. The
last solution is probably the preferred one if you need a PWM which has a 5
second cycle time.

Good luck
Chris