LP3855-ADJ won't regulate above 5.4V

Thodden Saporetti

Other Parts Discussed in Thread: LP3855-ADJ

I'm using the LP3855-Adj with a 6 Volt input, and trying for an output voltage of 5.6V. The output never goes above 5.4V, though I can adjust it lower. I've try both increasing and decreasing the input voltage. It still regulates to 5.4V with an input voltage of only 5.6V or an input voltage of 6.3V. Below is my schematic, it is a 20k potentiometer, and above ~16k resistance, the output voltage remains the same. The total load is approximately 100mA.

over 9 years ago

0 Donald Jones over 9 years ago

TI__Mastermind 22610 points

Everything suggests a problem with the 20k potentiometer wiper not going above 16k ohms.

0 Thodden Saporetti over 9 years ago in reply to Donald Jones

Prodigy 20 points

I tested the potentiometer's resistance, it does go above 16k, all the way up to 20k. That was one of the first things I checked when I was troubleshooting the circuit

0 Donald Jones over 9 years ago in reply to Thodden Saporetti

TI__Mastermind 22610 points

OK, something is not right. I will need a couple of measurements.

First set the potentiometer's resistance to 18.0k, and set Vin to 6.3V.

Then measure the dc voltages at device pin 3 (Vout) and device pin 4 (ADJ)

Ideally, Vout will go to (1.216V x (1 + (18k/4.7k))) = 5.87V, and ADJ will be at 1.216V.

If Vout stops at 5.4V, then what is Vadj at?

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LP3855-ADJ won't regulate above 5.4V