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Capacitor values in 317/337 based PSU
Capacitor values in 317/337 based PSU
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Old 12th March 2005, 05:35 AM   #11
peranders is offline peranders  Sweden
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Capacitor values in 317/337 based PSU
If you are a bit lazy you can use my Excel calculation for LM317.

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Old 12th March 2005, 06:08 AM   #12
klitgt is offline klitgt  Denmark
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Originally posted by richie00boy
Check the individual datasheets for the 317T and 337T. The L suffix versions are a little different so don't use those. I believe I have the latest ones and they are May 2003 and November 2001 respectively.

I see 240 ohms for the 317 and 120 ohms for the 337. If you use these values then you simply apply the formula in the datasheets (both are very slightly different) to work out what value of other resistor you need.

On your web site you use 240/3K for the 317 and 120/1,5K for the 337. That is simply doubling the R values for the 317 relative to the 337. The formula will give the same result vor Vout if you use 240/3K at both regulators???
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Old 12th March 2005, 01:47 PM   #13
Mr Evil is offline Mr Evil  United Kingdom
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Originally posted by klitgt
Richie00boy, I understand, but look at the schematic from the National Semiconductor datasheet. Same values for voltage regulating resistors, 120 Ohms both at 317 and 337! AND they are both specified to 1% tolerance.
In that schematic the voltage is variable, so precisely accounting for adjust pin current is a waste of time when it can just as easily be done by turning the pot a little.

As for capacitor values: you need to size C1/2/3/4 so that the input voltage never falls too low (i.e. the lowest part of the ripple must always exceed the output voltage + minimum dropout of the regulator), also taking into account the voltage drop across the resistor (do you really need a CRC filter and a regulator?). For C9/10, personally I wouldn't bother going any larger than 10uF, with probably another 10-100uF more directly on the PCB of whatever circuit is being supplied by the regulator. There is no particular benefit to ripple from increasing the capacitance beyond that (although the noise measurements linked to by Werner are interesting).
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Old 12th March 2005, 05:23 PM   #14
richie00boy is offline richie00boy  United Kingdom
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Capacitor values in 317/337 based PSU
I am following the recommended values for those resistors that is why they are 240 and 120 ohms. The 317 and 337 have different adj pin current demands so by using these values of resistor you create the same offset voltage across them.

MrEvil, the adj pin draws a set current from the potential divider network. You are right that the current flow in this network will change with different resistor/pot settings, but the adj pin current stays constant.

The regulator will work with other resistor values, I'm just trying to optimise things as much as possible That is why I have a CRC filter and a regulator as you seem to find unneeded Doing this enables me to get the best noise and regulation performance.

The reason I have 47uF on the output is because if you add large capacitance here it improves the stability of the regulator. If you only have small capacitance here then large capacitance on the op-amp PCB then the stability and o/p impedance of the regulator is not improved as effectively because of the inductance of the wires/traces to the bigger caps.
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