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Adjustable Voltage Regulator

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ormo

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Hi,

I'm trying to set up an LM337 adjustable negative voltage regulator to give -18V when supplied with ≈-21V at 250mA. Can someone please advise on what resistor combinations I would need to attain this? When I say resistor values I mean according to the diagram in the datasheet, if anyone knows of any other set ups feel free to recommend them.

I had a look on the internet for some calculators for this chip, but they all asked for "Iadj" and I've got no idea what this is :S.

Thanks!
 
Iadj is the current flowing out of pin 2 and can be found on the datasheet, most of the time it's pretty (about 50µA) low so it doesn't make much difference.
Don't you mean -21V?

You can't use the LM337 to get a negative voltage from a positive voltage.
 
Ah, thank you for that explanation. I did put -21V, but I put the ≈ symbol in because it will only be roughly -21V
 
Be careful. The difference between your output voltage and input voltage is ~21-18= ~3 volts. At full current, this voltage should be no less than 2.5 volts, or the regulator will not regulate. See the device data sheet's dropout voltage graph. At full current your ~21 volt input may drop too low to satisfy the 2.5 volt in/out voltage difference requirement.

Also, although, a meter may indicate 21 volts, if there is significant AC ripple on the input voltage, the instantanious input voltage may actually dip well below 21 volts and this will show up as a less than desireable output ripple.
 
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