Customer has designed TPS73533DRV (3.3V fixed voltage LDO) on a board and had CM do build. All boards tested good.
They ask:
In our most recent build at same CM, we have a problem where the output voltage is incorrect, it gets 5V instead of 3.3V.
After a bit of testing, we have come to the strange conclusion that these may be "wrong" parts, they are not fixed 3.3V. However, this seems "impossible" as the adjustable voltage version is not supposed to come in this package.
* Fixed voltage version should work with only input voltage, so it is really dummy proof. There is no voltage setting resistor. However, both parts on all the boards we have tested to date are wrong, outputting 5V.
* If I remove the capacitor on the NR pin, it should still work (which we proved on a working board), yet it does not on these new boards, it floats around until I ground that node. At which point it pops up close to the input voltage.
* If I put a resistor divider network on that pin to set the voltage as per the datasheet ... it actually works, the voltage is as expected.
Regarding the output, if I hang a resistor network off the NR/FB pin, it actually "works". R1 = 20k, R2 = 10k gives me ~3.65V as per Fig.21 of datasheet. On the "good" device, if I measure the NR pin, I get ~1.16V (it's a crappy measurement), which I'm guessing should be correct as it should be close to the bandgap voltage (Fig.1 on the datasheet). On the "bad" devices, it is closer to ~0V or actually seems to be floating around.
It doesn't seem to be an isolated failure, it's the same on both parts on the board and on every board we've tested so far.
UPDATE - We just had our manufacturer swap two boards (1 from a previous batch that is working and one of the new "bad" boards). Short summary is that the good board with "bad" parts is bad, we get 5V out. The "bad" board with the good part looks like it is okay now. We will have them re-work more boards with a different batch of the TPS73533DRV just to make sure.
Is there any way that the wrong parts could've been packaged? Or any other explanation for this strange behavior?
