LM317 with pass transistor

[Chris]

Hi,
I'm looking at a circuit in the TI datasheet for the LM317 voltage
regulator. Fig 14 on page 12 shows a circuit with a 2n2905 PNP
transistor and a TIP73 pass transistor. I am wondering if I can put
in multiples of the TIP73 parallel to get higher current.

I would like to balance the system so that the overcurrent limit of
the LM317 works to shut down the whole thing if the complete system is
overcurrent.

Can anyone give me some idea about how they arrived at the value of 22
ohms for the input side of the LM317 and 5k ohms for the base of the
2n2905?

Also, I have a pile of 2n3055 transistors, I would probably use those
instead of the TIP73 pass transistor. I would also put in some .1 ohm
emitter resistors on each of the pass transistors, like in other pass
transistor setups I've seen.

My goal is to get to about 25 or 30 amps of capacity.

 

[Charles]

Hi,
I'm looking at a circuit in the TI datasheet for the LM317 voltage
regulator. Fig 14 on page 12 shows a circuit with a 2n2905 PNP
transistor and a TIP73 pass transistor. I am wondering if I can put
in multiples of the TIP73 parallel to get higher current.

To a point.

I would like to balance the system so that the overcurrent limit of
the LM317 works to shut down the whole thing if the complete system is
overcurrent.

The IC has internal current limiting and you will have to use an external
current sense resistor and shunt transistor to remove drive to the PNPs.

Can anyone give me some idea about how they arrived at the value of 22
ohms for the input side of the LM317 and 5k ohms for the base of the
2n2905?

Sure. Vbe is 0.7 volts at turn on and when the 22 ohm resistor produces
that drop, the PNP current boost transistor is on.

Also, I have a pile of 2n3055 transistors, I would probably use those
instead of the TIP73 pass transistor. I would also put in some .1 ohm
emitter resistors on each of the pass transistors, like in other pass
transistor setups I've seen.

NPNs don't fit the circuit you posted about.

My goal is to get to about 25 or 30 amps of capacity.

http://books.google.com/books?id=bk...X&oi=book_result&ct=result&resnum=2#PPA310,M1

 

[Chris]

Sorry, I left out the voltage information. In the end I just
want a 13.8 volt output with 20-25 amps. So I don't have
to preserve the wide range of output voltages available with
the LM317.

All of the three-legged regulators have these
neat features of overcurrent protection, etc. I was just
wondering how to preserve those while handling much more
current.

Thanks to both Charles and Dave M. for lots of good links.
I will dive into that info.

Chris