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Old 15th March 2012, 11:50 PM   #1
PauloPT is offline PauloPT  Portugal
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Default Capacitance after regulator

Hi,

I want to build a rgulated PSU for a 100W / 8 ohm power amplifier. I'll use 2 paralleled LT1083 chips per rail. Before the regulator I have about 20mF of capacitance. Should put another bank of capacitance after the regulator?
Thanks.

Kind regards,
Paulo.
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Last edited by PauloPT; 16th March 2012 at 02:29 PM.
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Old 18th March 2012, 01:11 AM   #2
gootee is offline gootee  United States
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What does the regulator datasheet say about the cap at its output? You don't need a lot, there.

How many centimeters between regulator output and power output devices' rail connections?

At each power output device's rail connections is where some capacitance could do the most good, for decoupling (to reduce rail voltage disturbance caused by trying to supply fast transient currents through rail inductances), and for accurate transient response. Probably use at the very least a few hundred uF there, as close to device rail connections as possible, and more uF would be better. Also use some small-size caps, for high-frequency bypassing, right AT each power device's rail connections, for HF stability.
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Old 18th March 2012, 01:59 AM   #3
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You tend not to need a huge amount after the regulator.
A 150uF is quoted in the spec. I would also add a 100nf.
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Old 18th March 2012, 10:12 PM   #4
PauloPT is offline PauloPT  Portugal
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So there is no need for big caps after the regulator That was my first ideia too but I got confused by this post Regulated Power Supply for power amp
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Old 19th March 2012, 06:57 AM   #5
gootee is offline gootee  United States
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Quote:
Originally Posted by PauloPT View Post
So there is no need for big caps after the regulator That was my first ideia too but I got confused by this post Regulated Power Supply for power amp
Well, maybe not RIGHT after the regulator, but closer to the point of load there usually IS a need for big (or at least big-enough) caps. You can definitely get transient distortion and other problems if you don't have big-enough decoupling caps, or if they're not close-enough to the point of load (usually the output power devices' power supply connections; as close to each device as possible).

Now that you pointed out that other thread, I would have to consider doing what Eva said. He does PSUs professionally and is almost always correct, in my estimation.
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Old 19th March 2012, 07:12 AM   #6
qusp is offline qusp  Australia
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the LT like just about all LDOs doesnt need or like large capacitance, depending on the load you may want largish local decoupling, but depending on the distance to the load i would watch the use of ceramic bypass
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Old 19th March 2012, 07:33 AM   #7
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Regulation typically improves power supply stages as a result of use of negative feedback typically across a pass transistor(s). Pass transistors are integral with most band gap regulators. However capacitors are still needed to satisfy the regulators requirements, and it is best to look up application notes for specific products. see page 10 of the attached application data.

LT1083 - 7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators - Linear Technology

You also need to define if LT 1083 can be paralleled, and page 14 of the attached application data provides precisely how to go about that.

In your toolbox of pSU ideas keep in mind the use of capacitance multipliers, a beneficial attribute using the transistors hfe to multiply capacitance.

see: Capacitance Multiplier Power Supply Filter

and for any given power supply 1000uf for every amp of current is a good rule so as to not to damage rectifiers due to charge and discharge.

Cheers / Chris
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Old 19th March 2012, 03:01 PM   #8
sesebe is offline sesebe  Romania
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Default Voltage??

Short question: What is the voltage in input of regulator and on output?
The lowdropout regulators have a very low input voltage limit. For 100W/8Ω you need from 45V to over 50V. The LT1083 has only 20V maximum voltage input (permanent power).

You can not use this IC to stabilise the voltage (or maybe I'm wrong?)

Last edited by sesebe; 19th March 2012 at 03:06 PM.
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Old 19th March 2012, 03:17 PM   #9
AndrewT is offline AndrewT  Scotland
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Why would you want/need to stabilise the supply voltage to your Power Amplifier?
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Old 19th March 2012, 03:19 PM   #10
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Quote:
Originally Posted by sesebe View Post
Short question: What is the voltage in input of regulator and on output?
The lowdropout regulators have a very low input voltage limit. For 100W/8Ω you need from 45V to over 50V. The LT1083 has only 20V maximum voltage input (permanent power).

You can not use this IC to stabilise the voltage (or maybe I'm wrong?)
The 20V max is the max input-output differential. And it is 30V

jan didden
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