[sdiy] lm317t diferential input output voltage

[Ullrich Peter]

Hi!

In the Fairchild datasheet you can see that the nominal performance can be reached at 3V difference Vo-Vi (=dropout voltage). In detail you can see that this dropout voltage depends on the output current - see Figure 4 on page 4.

http://www.fairchildsemi.com/ds/LM/LM317.pdf

The OnSemi part seems to be a little bit better - see Figure 10 on page 5:
http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF

The texas part is like the OnSemi - see figure 12 on page 7:
http://www.ti.com.cn/cn/lit/ds/symlink/lm317-n.pdf

So if you don't have a dropout voltage that is in the allowed range, you will get hum or bad regulation.
In this way you could use a pin compatible low drop out voltage regulator. Some types can go down to only 50mV dropout voltage at low currents. But be careful: Most of these LDO-regulators need a minimum output capacitor - otherwise they will not start/work perfectly!

Ciao
Peter

http://www.ullrich.at.tt

________________________________________
Von: synth-diy-bounces at dropmix.xs4all.nl [synth-diy-bounces at dropmix.xs4all.nl]" im Auftrag von "Timothy Daugard [daugard at cox.net]
Gesendet: Sonntag, 23. Februar 2014 23:28
An: dragons; synth-diy at dropmix.xs4all.nl
Betreff: Re: [sdiy] lm317t diferential input output voltage

On 2/23/2014 3:31 PM, dragons wrote:
> Hi,
> I just wondered if anyone knew what the lowest difference between the
> input and output voltage should be on a Lm317T ?
   ( P.S. I did look at the data sheet but couldn't find any figures,
maybe I wasn't looking carefully enough )
>

National data sheet specifies that you have to have a 1.3V reference so
I suspect your input has to be at least 1.3 V higher than the desired
output, but that is just a guess.

Tim Daugard
<Back to lurking, RF, MSP430 and figuring income taxes>

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