Hi All,

I have a MCP1802 low dropout (LDO) voltage regulator below, with input of 5VDC and output 3.3VDC. I am measuring 1.3V at the input and 1.3V at the output pin. I am not sure what's going on, I am assuming maybe if the shutdown (SHDN) is floating or grounded, would it caused the output pin to measure 1.3V? Any suggestion would be greatly appreciated. See the attached schematics.

Thanks,
WJ

 

What voltage is on the output of the LT3980 with the MCP1802 removed?

 

What voltage is on the output of the LT3980 with the MCP1802 removed?

Hi Ian,

I am measuring the 5V at the L1 and C5

 

Whenever I find a voltage regulator output that has been pulled down to 1.2V or 1.3V the most common cause is an IC connected the wrong way round. It's 1.2V because there are protection diodes and you usually get two forward biassed diodes in series when you connect and IC the wrong way round.
Does it get warm?

 

Whenever I find a voltage regulator output that has been pulled down to 1.2V or 1.3V the most common cause is an IC connected the wrong way round. It's 1.2V because there are protection diodes and you usually get two forward biassed diodes in series when you connect and IC the wrong way round.
Does it get warm?

The zener diode is (D1) gets really warm.

 

A b340 is a Schottky, not a zener. If you have used a zener of <16V it will be warm!

 

A b340 is a Schottky, not a zener. If you have used a zener of <16V it will be warm!

Yes, you are correct. It's a Schottky

 

Yes, you are correct. It's a Schottky

If it's warm there's a lot of current going somewhere!

 

You need to disconnect the two circuits and evaluate the 3.3 volt regulator portion. The regulator may be oscillating, or one of the capacitors may be very leaky, or the connections may be wrong.
OR the device may be defective.

 

Thank you you all for all your suggestions and I appreciate you all. After I checked the all schematic and the PCB and we found that both D2 & D3 were backward causing a reverse bias operation. This could caused excessive current to flow through the diodes and potentially through the MOSFETs See attachment