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Old 16th April 2012, 10:16 PM   #1
MelB is offline MelB  Canada
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Default LT1083 interesting startup problem

I've never had a linear regulator simply not start into a load. Here are two circuits. Top circuit has a bypass cap (C3) 470uF and the bottom circuit has a bypass cap (C3) of 47uF. The top circuit will NOT start a GM70. It puts out about 2 volts although it is set to put out 19.8V. It will put the 19.8 volts into a 6.8 ohm resistor, but cold the GM70 is only 1.6 ohms. The bottom circuit starts up like a champ. I was just using caps I had lying around. Thought I would share.
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Old 17th April 2012, 12:20 AM   #2
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Quote:
but cold the GM70 is only 1.6 ohms.
Will it start with a 1.6ohm resistor?

I'd guess you're tripping the built-in short circuit protection.
(I'm assuming this reg has it built in,along with thermal,etc protections)
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Old 17th April 2012, 12:53 AM   #3
Arius is offline Arius  United States
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Your issue is with C3. As you stated yourself, it works when C3=47uF and not when C3=470uF. The ADJ cap is not supposed to be any larger than 25uF when R1=120R. With a 120R, your ADJ current is only 100uA.

See the datasheet for more info.
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Old 17th April 2012, 01:27 PM   #4
MelB is offline MelB  Canada
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Ya I read that in the data sheet, but I took that to mean @ 120Hz your Cadj should be at least 25uF. Earlier in the data sheet it says if you use 150uF aluminum on the output it will cover ALL CASES of bypassing the adjustment pin. Should say almost all cases. But starting into a tube filament is probably a special case. You could never run this thing with 1.6 ohms continually at 20V. That would be 12.5 amps. I suppose I got a tad carried away with the 470uF cap, which can only charge as you said with the 100uA adjust current.
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