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formula to set adj. voltage reg.. doesn't work?

Hi All,

I'm trying to set the voltage of an adjustable voltage regulator (national semi LP3892

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and I don't understand why the formula isn't helping me spec the right voltage control resistors...

The formula given in the datasheet is:

Vout = Vadj * (1 + R1/R2)

Vadj a constant voltage source from the voltage reg (typ: 1.25v) R2 is supposed to stay under 12k

So I figured R1=46k, R2=10k and that gives Vout = 1.25 * (1 + 4.6) = 7v

HOWEVER

When I actually built the circuit Vout was more like 3.5 - 3.8v.

Rebuilding the circuit with R2=10k, and R1 on a potentiometer I tuned the output to 7v and found R1=17.5k

Am I missing something here? Do I have to factor in more than the published formula tells me?

This is the second adjustable voltage regulator that doesn't seem to work like I expect, and I must be missing something here..

Hopefully someone can help Thanks,

-Andrew

p.s. I'm using 10uf caps between Vin and Gnd as well as Vout and ground.. well over the 1uf min suggested in the datasheet.

Reply to
Andrew
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"Andrew"

** 10uF WHAT caps ???

Not standard, cheap electros I hope.

The maker's data says the bypass caps should need be ceramic - the thing about ceramics is their very low ESR values.

Maybe low ESR tantalums would be OK.

Do you not have a scope to check for oscillation on the output ?

Time to get one if the answer is no.

...... Phil

Reply to
Phil Allison

You have transposed the formula wrong. R1 = (( Vo / Vadg) - 1) R2 This formula works out right every time.

Brian Ellis

Reply to
Brian Ellis

Andrew,

You may want to be sure your capacitors are as high-quality as suggested in the datasheet under Application Hints, Selecting a Capacitor.

Richard

Reply to
Richard Seriani, Sr.

----- Original Message ----- From: "Brian Ellis" Newsgroups: sci.electronics.design Sent: Friday, January 19, 2007 10:51 AM Subject: Re: formula to set adj. voltage reg.. doesn't work?

Brian,

I'm old, and my math skills may be rusty, but I am not seeing how the result of your equation differs from Andrew's.

R1=((Vo/Vadj) - 1)R2

R1/R2 = (Vo/Vadj) - 1

(R1/R2) + 1 = Vo/Vadj

Vo = ((R1/R2)+ 1)Vadj => Andrew had Vout = Vadj * (1 + R1/R2)

Richard

Reply to
Richard Seriani, Sr.

Well the first time I built the circuit (I was populating a custom pcb) I used a 10uF ceramic chip capacitor (1206 size, Y5V) on vin&gnd and a

2.2uf (0603, X5R) on Vout&gnd. Both are Murata multilayer chip ceramic caps rated for 16v.

When I built the second circuit (with a potentiometer, etc) I used cheap aluminum 10uF electrolytics. The main problem is the same though, with R1=46k and R2=10k, the Vout is in the 3-4V range.

Yes I do have a scope.. 150mhz instek dso.

Have others had luck with using the given formula before?

Thanks for the replies,

-Andrew

Phil Allis> "Andrew"

Reply to
Andrew

Hey Brian,

Thanks for the reply but that's the same formula I have listed above, although yours is switched around to solve for R1.

Have you used this before to spec resistors for an adjustable voltage reg? did it work correctly?

Thanks,

-Andrew

Reply to
Andrew

Richard,

Oops! You are right. I guess I am going to have to tune these old eyeballs in a little better.

Brian Ellis

Reply to
Brian Ellis

Andrew,

Yes, I have used this many times before (with no problems). If you don't have a scope, put your volt-meter on AC volts, and measure across the Vin,Vadg and Vout terminals. The AC ripple should never let Vin go below the DC drop out voltage. The Vadg terminal should have very little AC voltage ripple on it (if everything is hooked up right and the input voltage is high enough, is the only thing that I can think of that would make the output voltage be too low).

Brian Ellis

Reply to
Brian Ellis

I'll ask the really stupid questions, now:

  1. What is your input voltage source and value?
  2. Have you checked the resistor values (other than the variable)?
  3. What are you using to measure input and output voltage?
  4. Have you checked all interconnections in your circuit?
  5. What is the value of the adjust voltage at the junction of R1 and R2?
  6. What are you doing with the Enable pin?

Short of a failed regulator (I assume you tried another) your problem may be one of the above.

Good ?Luck.

Richard

Reply to
Richard Seriani, Sr.

"Andrew" = a Top Posting PITA

** You CANNOT use those electros.

** Try switching it on.

DO NOT TOP POST !!!!!!!!!!!

....... Phil

Reply to
Phil Allison

"Richard Seriani, Sr."

** Shame about the silly typos in the para headed "Output Capacitor" .

...... Phil

Reply to
Phil Allison

Ok,

Just wanted to give an update for those that responded -- I was finally able to get it to work by using the printed circuit board I made, and swapping out components.

I think it comes down to A) fried/bad components B) poor connections that went undiagnosed (it's a 3mm leadless package)

On the board I was working on I was able to get it to work as designed by swapping out the voltage reg, and then I had to swap out my PIC too, as that was not responding either.

Glad I can finally get back to the rest of the project

Thanks again,

-Andrew

Richard Seriani, Sr. wrote:

Reply to
Andrew

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