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Adjustable Voltage REgulator (LM1084): Not Regulated @ different Loads

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blackshadow

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i have a problem with my adjustable voltage regulator (see attached circuit).

i have a load that needs 5.5V and around 2.4A.

im using an adjustable voltage regulator LM1084.

without load,

vin=12V
vo=5.5V

with load,

vin=11.72
vo=4.368
I_load=1.87A


my expectation is that with or without load, the Vo=5.5V.
any idea why this happened?
also note that the Voltage Supply input changed from 12V to 11.7..
 

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Your voltage supply has some resistance. Right click on it. Maybe R=.1 ohm
What is the "IN" voltage to your LM1084? I think the MOSFET is dropping the voltage.
 
Your voltage supply has some resistance. Right click on it. Maybe R=.1 ohm
What is the "IN" voltage to your LM1084? I think the MOSFET is dropping the voltage.

the mosfet have very small Rds(on) around 50milliohms.

at no load condition,
Vsupply=12V
Vin=11.946
Vo=5.5

with load,

Vsupply=11.72V
Vin=10.92V
Vo=4.37 @ 1.87A
 
the mosfet have very small Rds(on) around 50milliohms.

at no load condition,
Vsupply=12V
Vin=11.946
Vo=5.5

with load,

Vsupply=11.72V
Vin=10.92V
Vo=4.37 @ 1.87A


i put a supply resistance of R=0.1ohm and the Vsupply indeed drops down to 11.7V.

on the MOSFET, even with high Vgs, it seems Vds at saturation increases with current flowing through it.

however, the only remaining problem is in the Vo. it should be at least maintained to 5.5V even with the load.

Note: Drop-out Voltage is only 1.3V
 
It says in the datasheet "Power dissipation is kept in a safe range by current limiting circuitry. Refer to Overload Recovery in Application Notes."

I don't know how simulator could "simulate" this. Are you having trouble with the simulator or a real circuit? Anyway, the regulator is dissipating ~13W of power.
 
Last edited:
It says in the datasheet "Power dissipation is kept in a safe range by current limiting circuitry. Refer to Overload Recovery in Application Notes."

I don't know how simulator could "simulate" this. Are you having trouble with the simulator or a real circuit? Anyway, the regulator is dissipating ~13W of power.

this is a real circuit. i just used a supply resistance of R=0.1ohm to simulate tha actual 11.72V drop at the power supply.
but other than that, those values are from the real circuit.

i already tried lower current load so as not to overload it but i find it not regulating anymore.. probably a damaged one?
 
In your circuit diagram the MOSFET only has 3v between gate and source, it would barely turn on. Is that intended?
 
Is the regulator mounted on a heatsink?
 
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