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Old 5th April 2005, 04:06 PM   #1
bigbulb is offline bigbulb  Canada
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Default High speed comparator

Hi Guys,
I'm designing a self oscillating class d amplifier, with feedback from the mosfet.
Can you guys suggest a good comparator other than LM311, LM319? and I had search for self oscillating class d amp and found thet most are half bridge. may I know how to use full bridge for self oscillating? (2 output from masfet output stage, output from full bridge is float from ground)

regards,
Bigbulb
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Old 5th April 2005, 11:17 PM   #2
sovadk is offline sovadk  Denmark
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Hi I use LMV7239 by National Semiconductors. It's very fast and best of all, it has a push-pull output configuration.
I know that maxim makes a lot of fast comperators, if that can help.

www.maxim-ic.com
www.national.com
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Old 6th April 2005, 01:03 AM   #3
bigbulb is offline bigbulb  Canada
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Thanks sovadk.

I still searching for self osc type of class d in full bridge configuration. I had build the half bridge one and wish to try full bridge. Can't figure out how's the feedback connection, since the speaker output is floating. Please help!
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Old 7th April 2005, 08:42 PM   #4
tiki is offline tiki  Germany
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If you understand German a bit, you may want to give this thread a try.
At least the user Beobachter develops a so called (by himself) SODFA as a full bridge version with pre filter feedback. He posted links to the schematics too.
Regards, Timo
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Old 9th April 2005, 01:11 PM   #5
bigbulb is offline bigbulb  Canada
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Thanks Tiki for the link!
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Old 11th April 2005, 05:45 AM   #6
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You can take it differentially across the load, then use a diff amp to convert to single ended (ugly) or be really slick and keep it fully differential, subtract with differential input, like UCD.

You might want to start with a half bridge though, you'll cook less mosfets getting through the ol learning curve that way.

If you go for the fully differential idea you won't even need to change much of anything to go with a full bridge later on.

I believe Linear Technologies has some insanely fast comparators and offer some of them with differential outputs as well, might want to have a look at their selection.

Cheers
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Old 17th September 2005, 06:37 AM   #7
Kenshin is offline Kenshin  China
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Default LM319 for double channel UcD

LM319 has two compararors per chip. Now use one chip in double channel UcD (or any other self-osc configuation), one comparator for a channel.

Would the parastic coupleing (via inside or via supply railes) mix two switching frequencies and cause the "whistling" problem?

Do I need to use one LM319 chip per channel (and waste half of them)?
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Old 17th September 2005, 07:04 AM   #8
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Hi Kenshin,

I would not even recommend that you put both amplifiers on the same PCB if you intend to use them for audio. I once used three LT1394 comparators on the same board, and noise level was not Hi-Fi. But for motor control (intended application) it was OK. There was no whistling since this was a clocked design.

Best regards,

Jaka Racman
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Old 21st September 2005, 12:24 PM   #9
Kenshin is offline Kenshin  China
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put different amplifiers on different board... would it cause more noise from grounding?
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Old 23rd September 2005, 10:29 PM   #10
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Just view them as two lines interconnecting at a star point beyond the amps themselves.
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