short SG2525 (SG3525)

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sdoom

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Joined 2004
#1
....question


sorry for asking annoying questions. I already searched the forum for that particular info and googled the datasheet but I am not really sure why the amp is not powering up.

Have a little 2 channel Helix here that resists to turn on and stays in protect mode. Outputs are fine so I checked the SG2525 voltages, which is a pain in the a.. because they put it on a small suboard behind other large parts.

Compared to a cheap (but working) amp I had in stock that uses a SG 3525 I find that Pin 9 (comp - input) is 0.05V in the Helix amp where in the working amp it is 1.6V .

As far as I understand the datasheet 0V applied to Pin 9 is what causes the SG2525 not to turn on. Am I right on that voltage level on Pin 9 ?

Additionally , the error amp inputs see the following voltages:

Pin 1 : 2.5V
Pin 2 : 0.9V

Where my working amplifier shows:

Pin 1 : 3.79V
Pin 2 : 3.81V

Does the error amplifier also keep the SG2525 in powerless mode ?

Thanks

Stephan
 
#2
The inputs to the error amp can cause the output of the IC to be shut down. Pin 2 must be higher than pin 1 for the IC to produce output. Is something possibly driving pin 2 to ground?

The voltages from one amp to the other aren't likely to be the same on these pins if the amps aren't identical.
 
#3
Hi Perry,

thanks for the info on the error amp, I thought so that the error amp causes a shutdown. How about the comp-input, would low-level also cause a shutdown of the chip?

I will try to follow the lines that go to the error amp inputs. There is a second sub - board where they put 2 LM339 , everything in SMD ....
 
#4
This is essentially an op-amp. The two error amps are the input to the op-amp and the comp pin is the output terminal. The error inputs determine the voltage on the comp pin. The voltage on the comp pin compared to the instantaneous voltage on the sawtooth waveform determine the pulse width on the output. The voltage on the comp pin must be greater than the instantaneous voltage on the sawtooth waveform for the IC to produce output pulses.
 
#5
Ah ok - now I see, thanks to you I dont have to die beeing stupid 🙂 I just took a look at the block diagram of the 2525 and now that makes sense .

Will try to find out what causes the voltage levels at pin 1 & 2 .

Thanks again for the fast and good explanation !
 
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