Using Back To Back LT1085s

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G

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Joined 2002
Hello PPL,

I've got a question for the gallery. Can you use back to back LT1085s (like I've drawn in the digram below) I want to build a good quality 30 volt power supply. It doesn't have to be the best ever. Just a good solid power supply any help is appreciated very much.
 

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I'm not sure there is much point to using a "super regulator" before the output LT part, although it certainly doesn't hurt. I'm not certain LT1085/LT1085 would be any better than LM317/LT1085.

Likewise, D1 and D2 never conduct when the supply is operating, so why use hexfreds here?

Finally, if all things were equal I'd use a larger capacitance after the final regulator to improve low frequency noise rejection.
 

G

Member
Joined 2002
Well I'm just doing my normal "overkill" thing. I can put another 15KuF cap at the output of the supply. The film cap is a 1uF not a 1nF as the diagram says. Would a regular 1n4007 do as a protection diode for the regulators? I'm trying for a supply with a plus or minus .1% output. Will this do it? Thanks for responding.
 
There's a better way to do this

Use the two regulators in a tracking pre-reg configuration. See the LM317 data sheet for details.

It gives a few advantages, better performance and easier adjustment (you only need to adjust V on one reg if you ever need to change Vout).

Set it so most of the dissipation is in the pre-reg, i.e. set the pre-reg to just above the dropout V of the final reg.

Well worth the effort, and uses less components.

Andy.
 
Save you looking

Here's the schematic.

Don't forget to decouple the ADJ terminal of the second reg (IMPORTANT!) and add o/p decoupling as necessary.

I've just noticed you haven't decoupled ADJ in your cct either - this is very important as it reduces noise, o/p impedance and increases line rejection, by 10's of dB.
 

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G

Member
Joined 2002
Hi AWL,

I hope that I've drawn this correctly. BTW can you help me figure out the resistor values in the regulator as I unfortunately have no idea how this works. THe most complicated supply that I have designed to this point is a simple CLC filtered supply. I'm looking for 30 volts at about 300mA.
 

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Gavin

You're not quite right.

The first reg is set to give a voltage just over the dropout V of the second regulator. You have it set to produce 32.5V

e.g. making R2 and R3 equal will give 2.5V, which is probably too much for these low-drop reg's and could be reduced. Check the dropout spec of the reg, and set it to just over max specified V.

The second reg could be set to 30V, but the pot is too large a value, and potentially could be set to give almost 60V (althpough the raw supply would not allow this). A 220R / 5k combo, with a little extra fixed resistance would be better.

I also think you may be sailing too close to the wind here - you do not have a particularly high Vin - if the mains voltage drops (as it does at times of great demand) your regulators may drop out of regulation.

You need to calculate min V for the raw supply, and base all calcs. on this value, and the dropout V of the reg's.

Andy.
 

G

Member
Joined 2002
Hi ALW,

I'm a bit confused . I thought that a regulator was used to drop a certain voltage and maintain it's output voltage(like a really smart resistor). It looks to me now as if it maintains power through itself. What I mean is that if it drops a set voltage then the current thought it increases, maintaining a set power (whatever the load will draw.) Is this correct. If so then I could set the voltage between the regulators to say 5 volts (about four times the dropout voltage) and still get 30 volts out. Hope this doesn't sound too dumb but this is a lesson learned for me.
 
Try this

This may help, the pre-reg is referenced to the regulator o/p not ground, so it only needs to provide a voltage x volts above the final reg o/p.

I've set it to be 1.5V above the reg o/p, i.e. 30V + 1.5V gives 31.5V in to final reg.

The final cap value is way too big, 100uF should be all you need, make it a good one though. The larger the adj decoupling the better most parameters are, but larger values slow startup time.

Andy.
 

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G

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Joined 2002
Thanks ALW. I've decided that the smart thing to do is to go with a bigger transformer. Say 35 - 0 - 35 wired in parrallel. This will give me almost 50v peak. I suppose that I will drop about 10 volts across each regulator. I'll post the results. Thanks again.
 
The value of the final capacitor will be determined by the requirements. 100uF is more than enough for stability and transient performance, but may be insufficient at removing regulator noise. It all depends on what you want.

(I think 15,000uF is excessive though. You have to consider the effect of leakage current and whether that is limiting your noise rejection)
 

G

Member
Joined 2002
I think that this is more workable and should give a good level of performance. The reason for so much filtering is that this is going to be a separate supply that I can use with other components. What do you think?
 

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G

Your resistor values for the first reg are still wrong - take a look at my comments on the diagram I posted. The pre-reg is only set to give 1.5V approx.

You will then only drop 1.5V across the second reg., the rest is dropped by the first.

This means that you have 50V - (Vout-1.5V) across the pre-reg (based on my values).

The first reg will need heatsinking dependant upon current drain.

A>
 
Well let's see here. R3 instead of being connected to ground is connected to the 30v output. This means that the current divider is biased 30v above ground but Rref and Iadj should still be 1.25 and 50uA respectively. That being the case then the values of the resistors should be the same whether R3 is attached to ground or to the junction of Vout and R4. If I'm seeing this wrong then throw me a bone here. Between this and school(no we haven't gotten to regulator IC's yet) I'm seeing funny numbers in my sleep (when I can get some). ;)
 
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