Re: PWM Control IC Problem (SG3525)



Steve <nobody@xxxxxxxxxx> wrote in
news:Xns9AD0CDEF8F97Fnobodynobodycom@xxxxxxxxxxxxxx:

alphamnemonic <shmotmail@xxxxxxxxx> wrote in
news:ba29f199-331f-48c1-ab96-28c3a81d42d8
@i18g2000prn.googlegroups.com:

On Jul 2, 6:20 pm, Steve <nob...@xxxxxxxxxx> wrote:
s...@xxxxxxxxxxxxxx wrote in news:16ea6110-be19-4a21-aecc-
10780259c...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:





On Jul 2, 3:35 pm, alphamnemonic <shmotm...@xxxxxxxxx> wrote:
Hi Everybody

This chip normally puts out square pulses. In the power supply I
am working on (http://www.laserfaq.org/sam/150rcsch.pdf) it is
putting out a sawtooth wave. I am testing it in 'standby mode',
in which Q4 is off and R33 is in control of the duty cycle.
Replacing the IC1 has no effect. Does anyone have any ideas?

Where are you seeing sawtooths?  Pin 13?

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That is my question. Why do you have output A and output B (pin 11
and 14
)
grounded?

Also, pin 13 is VC in for the transistor collectors. Why do you have
this connected as an output?- Hide quoted text -

- Show quoted text -

Yes the sawtooth is on pin 13. Pin 13 is being used as an output and
pins 11 and 14 are grounded because the chip is being used in single-
ended supply mode (see
http://www.datasheetcatalog.org/datasheet/motorola/SG3525AN.pdf p.6
for an example of this configuration). I can see why you find that
strange--I did too!


I'd avoid the funky setup you have. In my opinion I'd connect pin 13
and 15 together and use either output A or B.

I'm not sure what Q4 is doing, I've never seen it used this way, but
you must know more than me since you designed this elaborte circuit.


Keep us posted at what you do to resolve the problem.


I have been looking at the circuit and the datasheet.

I'm willing to bet the schematic is drawn wrong. It doesn't make any
sense to have pin 13 driving the base of a transistor.
.



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