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The other way to implement this would be to control the shutdown pin. If all 3 of the supplies are not valid (e.g. pulled to 3V3), and the chip is still powered, then we can pull the shutdown pin high in order to restart the chip completely and go through the soft start cycle again. With this solution we can add larger capacitors to the board, and should not have any issues with switchover times as the capacitor can store the required energy to hold up during switchover. We should make sure that on initial startup this does not keep the chip in shutdown.
Inrush Current Testing
So I’ve been doing a little more testing with the inrush current required to charge the capacitors. The aux battery is being used as the voltage source, and using the Keysight clamp meter, wrapping the wires around multiple times to get more resolution, and yes, the results below are the actual values (not the value shown on the screen) taking into account the number of turns.
On the DC-DC Input:
Turning On Properly: 1.7A
Stuck in Reset Loop: 0.65A
On the AUX Input:
Note that I have replaced the UVLO time delay cap with a 1.1uF capacitor (2x 2.2uF in series since I didn’t have any 1uF lying around) to increase the delay time to approximately 120mS according to equation 6 in the datasheet.
Turning on Properly: 7.5A
with the capacitor change, the AUX no longer goes into any reset loop.
Holding Artificial UV condition
In this test, I was attempting to see where the inrush current is mainly going - is it to charge the decoupling cap on the input or the bulk capacitors on the output?
DC-DC Input
The top side of R44 (UV threshold) was held LOW and the inrush current was re-rested using the same setup.
We measured about 1.8A.
Once removing the artificial UV condition, the chip started up immediately (the first time). Then if recreating the artificial UV and removing it with the board still powered, then we enter the reset loop again.
Inrush current during restart loop: 0.7A
Remember, however that this is the inrush current measured by the current clamp, and must be sustained for approx. 1ms. Looking at it scope, we can see that the spike happens much quicker, on the order of 10uS.
Aux Input
Holding the UV on Aux, we measure about 1.8A inrush current.
When releasing the forced UV, we measure another 7.5A of inrush current.
Other Testing
Playing with the shutdown pin does allow us to start perfectly when in the restart loop, however, trying to use the valid outputs did not work (possibly due to the resistors values on the gate and the pullup on valid).
Adding the extra capacitance for valid time delay on the AUX does work.