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Old 8th July 2012, 03:34 PM   #1
Baldin is offline Baldin  Denmark
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Default Complementary output from comparator

Hi All

Need some help and good ideas here

Having problems getting my amps to start up with no signal on input.
Thinking that I can make an inner loop, making the modulator start oscillating at a higher frequency before the output stage is ready (charging Cpump).

Anyway to do this I need a complementary output from the comparator.

I'm using a MAX913 (but could be any other fast comp), which gives something like 0V and 3.5V on the output. I need something like +-2.5V to +-5V (important thing is that is swings around 0)

Attached is a suggestion (works in sim), but it must be possible to make a solution which is fast and has a lower component count.

Any good ideas??

Thanks in advance and kind regards Baldin
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Old 8th July 2012, 09:16 PM   #2
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Quote:
Originally Posted by Baldin View Post
Hi All

Need some help and good ideas here

Having problems getting my amps to start up with no signal on input.
Thinking that I can make an inner loop, making the modulator start oscillating at a higher frequency before the output stage is ready (charging Cpump).

Anyway to do this I need a complementary output from the comparator.

I'm using a MAX913 (but could be any other fast comp), which gives something like 0V and 3.5V on the output. I need something like +-2.5V to +-5V (important thing is that is swings around 0)

Attached is a suggestion (works in sim), but it must be possible to make a solution which is fast and has a lower component count.

Any good ideas??

Thanks in advance and kind regards Baldin
Hey Baldin,
As I know MAX913 already has a complementary output. I do not know your circuit but there are several ways to make an amp oscillating without input signal. For eg. a soft start circuit with a basic transistor+ resistor+Cap. Why you do not use a basic charge pump(with a 555 or similar tech.) for H side? Or a series resistor with a fast recovery D.

BW
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Old 9th July 2012, 08:09 AM   #3
Baldin is offline Baldin  Denmark
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Ok, complementary is maybe the wrong word; what I need is an output which is capable of swing between plus and minus (not just zero and plus).

I'm using IRS2110, and I have actually tried a lot of different methods, but non working well in all cases (combination of output connected to load, and signal or not on input).

What did you mean by series resistor and D ... from Vcc to Cpump?

Kind regards Baldin
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Old 9th July 2012, 01:17 PM   #4
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LT1711, AD8561
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Old 9th July 2012, 06:00 PM   #5
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Correct me if i am wrong. Combining the -Ve and GND can serve the single supply operation in these comparators as per datasheet, which inturn can be powered from dual supplies to get swing from + to - rail.
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Old 9th July 2012, 06:35 PM   #6
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exactly!
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Old 9th July 2012, 07:47 PM   #7
Baldin is offline Baldin  Denmark
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I actually tried LT1711 a long time ago, in such configuration, (not to solve this problem) and it never became my friend. It draws a lot of power and is to my ind quite sensitive to PSU and layout. AD8561 seems better in this respect.
Thanks for the tip
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Old 10th July 2012, 07:14 PM   #8
Baldin is offline Baldin  Denmark
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Actually just using 2 resistors in series from output to -5V does the trick ... at least for this purpose, which was to make a point swinging around GND/zero to be used for feedback .... some times the most simple way, is the best
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