LT1083/84/85 linear regulator family: what's the output voltage range? - diyAudio
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Old 25th July 2007, 03:57 AM   #1
jarthel is offline jarthel  Australia
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Default LT1083/84/85 linear regulator family: what's the output voltage range?

I have the datasheet in front of me. BUT I cannot find the output voltage range for these chips. All I see is input-output voltage differential which is 30V.

I look at the net and no luck there.

Thank you.
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Old 25th July 2007, 04:11 AM   #2
gootee is offline gootee  United States
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So, as long as the i/o differential is <= 30v, and the i/o differential is > the max dropout voltage, which is around 1.5V IIRC, then they'll work.

As it is with all three-terminal regulators that I am familiar with, strictly speaking, there is no other limitation of the absolute output voltage range. For example, if you wanted to float the whole circuit at around 2000 volts, then the input and output voltages could be at around 2000 volts, as long as V_in - V_out <= 30V, always.

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Old 25th July 2007, 10:39 AM   #3
jarthel is offline jarthel  Australia
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I somewhat believe that is wrong (i.e. no limit on output voltage).

I have looked at the datasheet of lm317 and lt1086. LT1086 (also from linear) has a max input voltage of 30V. so output voltage range is definitely 30V - dropout voltage.

LM317 is around 30+ V and the HV version I think goes up to 40+ (fairchild's equivalent is 50+ V)
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Old 25th July 2007, 12:02 PM   #4
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Tom is right. If you put in 100V and set it for output of 80V you're OK. But that means you either need an adjustable reg like the 317 series, or use a non-adjustable and then put the ref voltage on the "ground" pin (which then is no longer at ground, of course).

Check the data sheet for the LM317 for applications examples for floating regs.

If you use a non-adjustable with the ref pin grounded, yes then you are stuck within Vin max.

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Old 25th July 2007, 11:27 PM   #5
jarthel is offline jarthel  Australia
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Quote:
Originally posted by janneman
Tom is right. If you put in 100V and set it for output of 80V you're OK. But that means you either need an adjustable reg like the 317 series, or use a non-adjustable and then put the ref voltage on the "ground" pin (which then is no longer at ground, of course).

Check the data sheet for the LM317 for applications examples for floating regs.

If you use a non-adjustable with the ref pin grounded, yes then you are stuck within Vin max.

Jan Didden
I'm just confused that the datasheet would specify an output voltage range and you're saying there's really no limit?

in this datasheet (http://cache.national.com/ds/LM/LM117.pdf - 1st sentence), it is clearly said the max output voltage is 37V.

sorry to be a pain but I'm really confused.

thank you.
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Old 25th July 2007, 11:48 PM   #6
BWRX is offline BWRX  United States
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It says "The LM117 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range."

That does not imply that the maximum output voltage is 37V. You can float the regulator at a much higher voltage as long as the input to output differential is under 40V and it can handle the power dissipation.
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Old 26th July 2007, 02:53 PM   #7
jarthel is offline jarthel  Australia
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thank you all for the help
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