LDO weirdness.

Using Arduino General Electronics
#1

Riddle me this --

So I have the following two 1.2V fixed output LDO's:

ST LDO 1.2V Fixed Output (Mouser / Datasheet)

NXP LDO 1.2V Fixed Output (Mouser / Datasheet)

The NXP has been EoL'd so I need to swap it out with the ST and here is where the problem starts.

If I follow the datasheet and apply a .1uF CAP from VIN (PIN 3) to Ground and a 10uF cap from OUT (PIN 2) to ground as specified in the ST datasheet page 8.

When I place the ST LDO in the circuit and use my multimeter to measure the output I get 2.64 volts from the ST but when I place the NXP in the same circuit I get the correct 1.2V -- can someone explain why I'm seeing this -- I think I'm going crazy!!!!!

Both are feed 3.3V clean regulated DC.

Thanks!
Danny

#2

and a 10uF cap from OUT (PIN 2)

..try to use multilayer ceramic type 10uF capacitor..

#3

I tried both Tant and Ceramic -- even if I pull the LDO and wire it in the "air" I get 2.64v from 3.3v input -- what's even more mind boggling is that the NXP LDO works as expected with 1.2V -- these should be drop in replacements? I must be doing something wrong, obviously.

#4

You're aware that those two parts have different pinouts?

EDIT: whoops, they don't. Stared at that for a few minutes; I think I'm going nuts.

#5

Chagrin:
You're aware that those two parts have different pinouts?

The SOT-223's don't, and the NXP only lists the SOT-223. The pinout is the standard LM1117 pinout.

Question: Have you included the 120? resistor across the output like the schematic on page 8 of the ST data sheet shows?

#6

The ST 1.2V part has a minimum load current of 10mA, hence the need for the 120 ohm resistor if you can't guarantee that it will always have at least that load. There are several alternative SOT223 parts available without this restriction (and most of them don't need such a large output capacitor). However, the ones I have found all have different pinouts, so you would need to change the PCB layout.

#7

dc42:
The ST 1.2V part has a minimum load current of 10mA, hence the need for the 120 ohm resistor if you can't guarantee that it will always have at least that load. There are several alternative SOT223 parts available without this restriction (and most of them don't need such a large output capacitor). However, the ones I have found all have different pinouts, so you would need to change the PCB layout.

Ah ha! That's it -- missed that detail. The circuit will have at least a 10mA load for sure -- so I should be fine. It startled me to see a no load measure of 2.65V when I was expecting 1.2V.

Thanks!

#8