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Old 2nd September 2004, 06:48 PM   #1
kafka is offline kafka  Poland
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Default OPA548 and shutdown

I can't force my OPA548 to shutdown mode. Is's working in inverting mode with gain = 6. As a reference it has tl431 +2.5V. It works as a -15V power supply. But I can't shut it down. I even soldered E/S pin to negative rail and there is still voltage on the output. I've tried with several chips... What's wrong?
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Old 5th September 2004, 09:39 AM   #2
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No expirience? No one would help me?
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Old 5th September 2004, 10:29 AM   #3
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I am not quite sure why you need the regulated reference. As long as the ref is grounded and e/s logic zero with respect to it the shutdown will work.
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Old 5th September 2004, 10:37 AM   #4
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I, too, am confused. Could you post a schematic?
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Old 5th September 2004, 12:08 PM   #5
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A mistake. It works in noninverting mode. The circut works. I have -15 on the output, but I can't shut it down even then I soldered pin 7 to -20.
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Old 5th September 2004, 02:56 PM   #6
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I wonder if the problem is not the single negative supply. From the datasheet it seems that polarity should not make a difference but it's still nice to see what's inside the chip.

Are you measuring the output under load?
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Old 5th September 2004, 04:16 PM   #7
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No I haven't. But I know I can deliver enough current to make the whole system work. Even a 24V relay .
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Old 5th September 2004, 05:07 PM   #8
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Have you tried just using the 3 resistor regulator as shown in the datasheet? Just to see if the chip works with the negative rail.

Have you built a positive rail version that works as it is supposed to?
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Old 5th September 2004, 05:17 PM   #9
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Positive version works excellant. Chip works with negative rail 'cos at the output I have -15V, as I wanted.
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Old 5th September 2004, 05:26 PM   #10
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Hmmm.

Well, If you have two seperate secondaries on your transformer you could just use two +15 versions on dual bridges, that would get you working.

Other than that, drop a line to TI's tech support. When I had a query a couple of years ago they were very helpful.
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