[time-nuts] Low Drop Out Voltage Regulator

Hal Murray hmurray at megapathdsl.net
Fri Mar 9 01:19:46 EST 2007


> Can anybody recommend a low drop out voltage regulator that will work
> with a 2 volt input differential, and a output of 22 volts at 2 amps ?

2 V is not low dropout.


> What I have in mind is a 28 volt DC power supply, and 24 volt
> batteries, and a rubidium oscillator that will run at 22 volts. 

Do you want a regulator to charge the batteries or one to run the rubidium 
osc?  If the latter, what's the battery voltage under load near end of 
charge?  You may need a LDO to get 22 out when the 24 has sagged a bit.

Beware that LDOs like to oscillate.  You need some damping in the filter 
caps.  In general, electoylytics work fine but (good, low ESR) tantalums are 
asking for troubles and ceramics guarantee it unless you add a series 
resistor to provide ESR.

The LM317 is the standard adjustable old, low-tech, non-LDO regulator.  It's 
only guaranteed for 1.5A, but the data sheets say 2.2 typ.

Amazing.  I can't find any input-output or dropout specs in TI's data sheet.

ST's has a graph of dropout voltage vs temperature.  2.5V at 1.5A  Same for 
National.


If I was designing something like this, I'd probably make a pass through the 
usual suspects and see what their selector-guide said.  TI, National, ST, 
Linear-Technology...

Plan B is to go to DigiKey and type LDO (or regulator) into their search box 
and narrow things down from there.


Another approach would be to use a switching regulator rather than LDO.   
That's probably easier if you have 48V of batteries so you are always buck 
rather than boost/buck if your 24V batteries sag below 22V.



-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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