DC-DC Converter ISL98602IRAAZ not outputting voltage

[canadaboy25]

Hey everyone,

I have a T-Con board from an LED TV that is not working.  There is an ISL98602IRAAZ that does not seem to be doing anything.  There is a testpoint labelled 1.2V beside the chip which has no voltage.  I have read that the ISL chip on these boards often fails so I ordered some replacements.  However, after replacing the chip, I still get no output.  I replaced the chip a second time in case one of the cheap Chinese chips I bought was bad.  Still no output.

Making things more difficult, there is no datasheet available for this chip online other than a pinout with short pin names.

I have 12V input on pins 5,30,31,36,37,38
1.22V on pin 25
4.7V on pin 24
4.4V on pin 18

The enable pin has 0V which seems like a problem to me.  I tried tracing the pin out on the board but cannot find where it goes as it is a multilayer board.  I tried putting 3.3V on the enable pin but there was no change.

I'm not sure what half of the pins do which makes troubleshooting the problem quite difficult.

Does anyone have any ideas on things to check for?

Thanks

[amyk]

The 98602 is the "recommended replacement" for the 97622, 97522, 98604, and several others whose full datasheets are available and likely very similar, so I would look to those for inspiration and learning.

[fzabkar]

Did you test the resistance to ground at the 1.2V output? Perhaps there is a shorted load.

[canadaboy25]

The 98602 is the "recommended replacement" for the 97622, 97522, 98604, and several others whose full datasheets are available and likely very similar, so I would look to those for inspiration and learning.

Yes, I noticed that as well.  I have been trying to reference the 98604 datasheet and some of the typical application circuitry looks similar.  However, it is tricky since many of the pin names do not resemble each other.

Did you test the resistance to ground at the 1.2V output? Perhaps there is a shorted load.

Yes, I forgot to mention that I tested the resistance between the 1.2V rail and ground and there is 1K of resistance.

After analyzing the circuitry closer, it appears that there is a 3.3V output at the very top left of the picture I posted.  This output has 75 ohms of resistance to ground.  This seems a bit low to me but I am not sure.  Also, I was able to find that the enable pin of the chip is connected to the 3.3V rail through a resistor.  It seems strange to me that the enable pin would be tied to a voltage rail that is generated by the chip itself.  Perhaps the 3.3V rail is supposed to be a standby rail of the chip and they just tied the enable to it in order to have the chip always active?