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A low level will disable the device. One solution would be to use a comparator. When the said power rail is below the threshold level, the comparator open collector/open drain output is low and disables the TPS. Once the power rail is over the threshold, the TPS is enabled. The only question I can't answer since I don't know your system, is how to power the comparator.

Are you implying that you want to enable start up with a controled external ramp?  because that would probably be okay.  Normally, letting EN float will enable the channel that it is controlling, no external power rail required; pulling it low will disable the channel.  If the power rail you are considering for EN/SS is not a controlled ramp, not having a capacitor on EN/Ss will result in the current limit being triggered, so you need a soft start capacitor on the pin anyway.

The system has a 5V rail and 3.3V rail that comes on later.

Since I don't want the TPS40322 to turn on until 3.3V is on, I have two NMOSs (2N7002) configured as follows.  I don't know if VDS of Q19 is low enough to disable the TPS40322 when it is turned on.

To ensure that every device is disabled over the entire temperture range, you need to design for a low level input voltage equal to the minimum value shown in the electrical characteristics table (0.23V).

Add a footprint on the PCB for a capacitor from EN/SS to AGND.  That way, when you trigger overcurrent at start up, you won't have to re-layout your board.

not sure about the overcurrent bit part.  Could you explain a little on that?

Can I simply add a capacitor to my circuit?

At turn on, there will be surge current from charging the output capacitor bank and supplying the load.  If this current trips the overcurrent protection threshold, the part will not start up.  A soft start feature enables the output to ramp up at a controlled rate and this avoids triggering overcurrent.  The soft start time is programmed by a single capacitor on EN/SS, the capacitor is charged up by the softstart current (10uA) that is sourced from the EN/SS pin.  As the capacitor on SS charges up, the duty cycle of the output pulses are linearly increased, when the SS voltage lvel is reached, the device will have reached its steady state operation, no over current issues.  So, using the equation on pg. 17 of the data sheet, for my design I assumed 2ms for soft start time...you can derive the equation for the time it takes for the output capacitors based upon load impedance, capacitance value, power limit threshold, and desired output voltage step change. Based upon a 2ms assumption, 33nF 0603 ceramic capacitor is all it takes.

Thanks for the explanation.

So I can add this cap to my en/ss line (with the circuit) without any issues?

When Q19 is on, cap is discharged and DC/DC is off.

When Q19 is off, SS is enable.

I believe so, yes