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LM317T Adj. Voltage Regulator

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jjimenez101

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Hello all,

I have a LM317T Adjustable Voltage regulator, does anyone know how to set it up so that I get an output of 6V @ at least 1.5 Amps with an input of 7.2 Volts??

Thanks in advance.
 

audioguru

Well-Known Member
Most Helpful Member
The LM317 needs an input voltage at least 2.5V more than its output voltage for it to work properly. It cannot have an output of 6V with only 7.2V input.
Its datasheet calls it "dropout voltage" and a graph shows it already dropped out (not regulating) when its input is 2.25V more than its output voltage.

7.2V is an odd voltage. Is it from a Ni-Cad or Ni-MH rechargable battery? Its voltage drops to about 6V so an LM317 would be able to regulate a max voltage of only 3.5V.
 

Tarsil

New Member
jjimenez101 said:
Hello all,

I have a LM317T Adjustable Voltage regulator, does anyone know how to set it up so that I get an output of 6V @ at least 1.5 Amps with an input of 7.2 Volts??

Thanks in advance.
If u want to obtain regulated 6V from 7.2V u might want to use a "low dropout regulator". Google it. U'l find some discrete pojects and some IC's like LM2937 (u can only use that one with an external tran at 1.5A or more).
 
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Tarsil

New Member
t.o. said:
Perhaps use a DC-DC converter to bring up the voltage prior LM317.
Yup! Or ...if u don't mind some riple:D use directly a switch mode regulator for obtaining 6V. Al tough the low dropout is a cheaper solution.
 

t.o.

New Member
As you mentioned about switch mode, I encountered some difficulties on L4975A from ST. I put 35V as the input and tried to regulate to 12V. However, the output seems to be missing some steps. The Osc is about 5us and the output is 15us. I don't know the reason. I've compared the pins with the working ones and only the outputs behaves like that. To my horror, I changed the new device and it behaved the same.
 

Roff

Well-Known Member
LT1764A is an LDO regulator with only 340mV dropout at 3 Amps. It's not a jellybean part that you can get from a surplus house, but it will work. I found this several months ago for a more demanding application. There are probably other LDOs that will work.
 

audioguru

Well-Known Member
Most Helpful Member
I don't think his source voltage will remain at 7.2V.
I think it is a rechargable battery whose voltage will drop to 6V. Then even a low dropout regulator won't work.
 

Roff

Well-Known Member
audioguru said:
I don't think his source voltage will remain at 7.2V.
I think it is a rechargable battery whose voltage will drop to 6V. Then even a low dropout regulator won't work.
Then I guess he needs a negative dropout regulator. :D
That would be a switcher, as has been suggested.
 

Hero999

Banned
This will give a dropput in the 50mV range (even less if you use a good enough MOSFET), so it will work even if your battery drops to 6V.
 

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Roff

Well-Known Member
Hero999 said:
This will give a dropput in the 50mV range (even less if you use a good enough MOSFET), so it will work even if your battery drops to 6V.
Neat idea. Shouldn't the MOSFET be pointing the other way (source to output)? Also, I believe a logic-level MOSFET and a rail-to-rail op amp would be needed for operation down to 6 volts. The voltage doubler will only yield about (6+ 6-1.4)=10.6V, which is the maximum gate voltage, even with a rail-to-rail op amp.
A P-channel MOSFET would eliminate the need for the 555 voltage doubler. You would have to reverse the op amp inputs, and provide frequency compensation to prevent oscillations. This might not be practical. I haven't tried it.
 
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Hero999

Banned
I was adapted from a circuit which used a p-channel MOSFET but I'd forgotten to swap the source and drain which I've now rectified. An n-channel MOSFET would be better as it would give a lower dropout because the on resistance is lower. You could maximise the voltage doubler output by using a 7555/TS555 and schottky diodes or use a specialised charge pump IC.
 

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OutToLunch

New Member
there's no need for a better charge pump - a typical 30V N-Channel FET will be fully ON at 4.5Vgs. It will have a slightly lower Rds(on) at 10Vgs but not so much better to justify the extra overhead of a more complex charge pump.
 
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