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Old 28th January 2005, 05:58 PM   #21
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Old 25th February 2005, 10:11 AM   #22
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Question cap choice

Hi,

I'm coming around to build the psu with the design I post in this thread and I can't decide on the caps. The schematic has 2 electro caps before the regulator and 1 after for each rail. I'm thinking of 2 x 2200uF before and a single 2200uF after. All Panasonic FCs. The odd arrangement is because these are the largest values FCs I can get here. The PSU is for a OPA2134 /BUF634 pre amp and head phone amp. Am I putting the right amount of capacitance? Is there any better choice of caps for this?

Cheers,
KK
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Old 25th February 2005, 12:32 PM   #23
AndrewT is offline AndrewT  Scotland
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Hi,
you have a cap on the adj pin!
Check the datasheet, you will have to consider a diode across each cap to prevent reverse voltage across the reg and again when using large values of output caps a diode across the reg is usually (always) advised. Check also on max output cap.
I recommend swapping some electrolytic to Film cap on the output.
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Old 26th February 2005, 04:52 AM   #24
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Hi Andrew,
Yes, I have followed the data sheet and put a 10uF cap to bypass the adj. pin. I see you are refering to the 'Protection Diodes' section of the data sheet about the need for a diode across the reg when high value output capacitor is used. I agree and will put that in. When you say replacing the electrolytic cap with film for output capacitor, what values are you suggesting?The data sheet says at least 150uF for the output cap. This seems too high for a film cap. The schamatic has a 0.1 uF MP cap after the electrolytic. Would it help?

Cheers,
KK
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Old 26th February 2005, 05:41 PM   #25
AndrewT is offline AndrewT  Scotland
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Hi,
I am not suggesting replacing the electrolytic.
Just swapping part of its value for a better Hi freq cap.
Try 1% upto maybe 10% if its not too pricy. Or you could try Lo ESR / hi freq cap for some or all of electrolytic.
e.g. 100uF electro // 47uF LoESR // 3u3F film // 100nF ceramic.
A thought just occurred.- some low volts drop regs need a hi ESR at the output to control oscillation. Just ignore my rantings if you are in that category.
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Old 1st March 2005, 09:13 AM   #26
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Lightbulb cap value

Hi,

I asked LT about the ESR for the output Cap and received this reply:

"Increasing the C out will improve stability & transient response. I would avoid using too high and too low ESR values."

Then I checked the ESR of a 2,200uF Panasonic FC cap and got a value of 0.02 ohm at 60 Hz, which I think is OK. So I'll start with this after the regulator:

2,200uF FC//0.1uF metalized polyprop//0.1uF ceramic

and use my ears to decide.

Cheers,
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Old 1st March 2005, 12:11 PM   #27
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before selecting the output capacitor you should read the following from National Semiconductor -- "Linear Regulators: Theory of Operation and Optimization" (AN1148) and Engineering Note: "Capacitors are key to Voltage Regulator Design" --
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Old 1st March 2005, 02:30 PM   #28
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Quote:
2,200uF FC//0.1uF metalized polyprop//0.1uF ceramic
don't even think about it.......


start with this...

Quote:
National Semiconductor -- "Linear Regulators: Theory of Operation and Optimization" (AN1148) and Engineering Note: "Capacitors are key to Voltage Regulator Design"
or....just use a 470uF of whatever brand.
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