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LM1084: Output Voltage is Not Regulated. What went wrong?

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blackshadow

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guys,

i'm a little bit lost with my very simple adjustable voltage regulator circuit using LM1084.

manual and simulation based analysis shows that the circuit MUST work with proper regulation at any loads.

but in actual circuit, this is not happening :(

AT NO LOAD:

Vin=11.57V (drop from 12V due to supply resistance)
Vout=6.5 (this is as expected per manual and simulations)

WITH LOAD (1.49A)

Vin=10.87V
Vout=4.77V

Per simulation using LT spice (included in the circuit attached), for any load, Vout should be regulated to 6.50V.
the minimum load current is around 5mA which is typical per datasheet.

What went wrong? I'm using a heatsink on top of a fan. I really can't think of any other issue here. I already used 3 LM1084's are are all the same.
 

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Where are you measuring the voltages? Can you show a picture of your circuit? Is there any significant resistance in the wires or connections? There may be something strange about the way the circuit is wired.

Edit: Is R1 connected directly at U1's output pin? Any resistance between U1's output and the R1 connection will cause a significant drop in the output regulated voltage. If you put a small resistance between U1's output and the R1 connection you can see this effect in your simulation.
 
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Where are you measuring the voltages? Can you show a picture of your circuit? Is there any significant resistance in the wires or connections? There may be something strange about the way the circuit is wired.

Edit: Is R1 connected directly at U1's output pin? Any resistance between U1's output and the R1 connection will cause a significant drop in the output regulated voltage. If you put a small resistance between U1's output and the R1 connection you can see this effect in your simulation.

this is a great oversight! i read in the datasheet application circuit that indeed any wire resistance between the Vout pin and the load will cause deregulation issues. i did actually simulated it and i see the deviations.

my circuit is just prototype. the load, the R1/R2 and the Regulator are all on seperate boards connected through wires as long as 1/2 to 1 foot! probably this is causing significant voltage drops.
probably this is the reason why my other regulator circuitry with lower load is working since the resistors and the regulator are on the same board with very close to each other, though the load is still connected by around 1foot wire.

this is my challenge. my load is in a different board so connecting by wire is the only way. do you have any recommendations on how to layout it?
 
I'm not that good at analog, but wouldn't you put the current sense/regulation set (R1 & R2) right out at the load and then long wire it back to the Adjust reference?

I'm in student mode here... <<<)))
 
I'm not that good at analog, but wouldn't you put the current sense/regulation set (R1 & R2) right out at the load and then long wire it back to the Adjust reference?

I'm in student mode here... <<<)))


i can't place my R1/R2 right at the load. the load is an isolated entity.
 
If you have 1 ohm in 1 foot of wire, you need bigger wire. Don't forget to try the cap on the output.

A 10μF output capacitor
is required on these new devices. However, this is included
in most regulator designs.
 
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I'm not that good at analog, but wouldn't you put the current sense/regulation set (R1 & R2) right out at the load and then long wire it back to the Adjust reference?

I'm in student mode here... <<<)))
That would seem logical but it doesn't work for a three-terminal voltage regulator that has no ground reference on the chip. The regulator tries to main a fixed reference voltage (typically 1.2V) between the reference pin and the output pin. If you put the sense reference resistor at the load then any voltage drop due to the load current from the resistance in the wire between load and the regulator output pin will subract from the apparent reference voltage. This will cause the regulator to reduce the voltage even more then the normal voltage drop that this wire resistance would cause by itself. (The reduction is approximately equal to the load voltage drop times the ratio of the output voltage to the reference voltage).

In summary, with a 3-terminal regulator always connect the sense reference resistor as close to the regulator output pin (preferably right on it) as you can.
 
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probably this is the reason why my other regulator circuitry with lower load is working since the resistors and the regulator are on the same board with very close to each other, though the load is still connected by around 1foot wire.

this is my challenge. my load is in a different board so connecting by wire is the only way. do you have any recommendations on how to layout it?
As stated run R1 as close to the regulator output pin as possible. Contrary to that run the common side of R2 as close to the load as possible. Run a large wire between the regulator output and the load.

Doing these things will give you the best regulation possible.

And don't forget the recommended capacitor on the output pin to ground.
 
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