I have an audio processor board that has a 5V -> 3.3V LDO. For some very strange reason, when I measure the voltage of the output of the LDO, I read 5 instead of 3.3V -- which by the way is a lethal voltage to the chip that's on the board, but for some reason, it doesn't seem to mind it.
And this is the full schematic:
It's a very simple board, just an audio DSP chip and an eeprom, designed for quick prototypes and DIY solutions.
The DSP chip is ADAU1452. For it's "IOVDD" rail, it asks for 3.3V, and absolutely no more than 3.6V. The way this board achieves this voltage is using a typical 1A linear regulator.
The regulator that came shipped with this board was AMS1117, apparently, a very popular choice for Chinese made designs.
When I power the board using a 5V supply, it seems to work just fine. Here's the problem: when I measure the voltage between output pin of the LDO and the ground, I read 5V. Input is 5.1V, output is 5V. How can this be?
The multimeter I am using is Fluke 115, a reputable name brand multimeter, so I know it cannot be lying to me. Why would I possibly be measuring 5V instead of 3.3V like I should?
As a step to debug, I took the original LDO out, soldered jumpers to its pins, and supplied it a 5V input out of the circuit, without any caps. And lo and behold, I read on my multimeter that it was outputting the correct 3.3V! As soon as I put it back into circuit, I read 5 V again...
I thought maybe there is some problem with stability related to input or output caps, so I swapped the part with a brand new pin campatible LDO ADP3338. And again, when I measure, I read 5V at the output.
What am I not understanding here? When I plug my multimeter in this board by connecting the ground probe to the ground lead of the LDO and positive probe to the output lead, why shouldn't I read 3.3V? Is this a measurement fault, or is there some other components in the circuit that can change the voltage of the LDO output? When I look at the schematic, I can't pinpoint a possible cause for the output voltage of the LDO to climb back to 5V. I simply don't understand what's going on.
By the way, ignore that weird, backwards looking Q1. The board came shipped like that. Somebody must have made a blunder when designing the board and later realized oops, gotta put the part backwards. That transistor doesn't seem to be the culprit though.