We have a situation that I wasn’t anticipating. The older TPS40222 regulator that we were using has an UVLO value of 3.8v on startup. The new LMR10510X value is 2.7v. We are powering our product over a 6 foot USB cable. Power supply input ramp up time is significant. The old regulator would hold off the output until the input reached 3.8v and then buck down immediately to the 3.3v. The new regulator attempts to start up at 2.7v, but can’t regulate so it lets the output follow the input voltage ramp until the output reaches the set point voltage at which point it regulates. This added delay on the LMR10510X output is causing some unpredictable results on our PCBA. Several of the components have different supply Vmin values so the board is coming up in states we haven’t experienced before. See the attached scope picture. Red is the input to the LMR10510X, blue is the output. I’m using 10x probes so multiply the Y axis scales by 10.
Can we use a simple voltage divider (with a filter cap) to drive the EN pin on the LMR10510X so that it won’t attempt to regulate and will hold off the output until the input voltage reaches about 3.8v?
I just tried a simple resistive divider and also a capacitive delay on the EN pin and find that the regulator does not seem to be honoring the 1.8v minimum enable threshold. Is this pin slope sensitive? I don't find any minimum input rise time in the the data sheet. Now that we have a TINA model of the LMR10510X we were able to verify that the regulator will start up at a lower EN voltage than the spec sheet indicates with slow rising signal.