In one of my designs i use several LMH7324 comparators with the following power supplies: VCCI = +5V, VCCO=+3.3V and VEE = -5V. Due to the power sequencing the +3V3 starts some time after the +5V and -5V.
This power sequencing was a poor choice... Obviously the LMH7324 chips immediately starts drawing current from the VCCO supply, which goes down to approximatly -1V. I suspect the LMH7324 chips are now "powered" trough a protection diode on the output of the +3.3V regulator. This prevents the +3.3V regulator from starting.
What would be the recommended power sequencing for the LMH7324 is one can not guarantee, that all supply voltages ramp up at the same time?
In my current design i hope to fix this problem with a low drop schottky diode from ground to the +3.3V rail. This should prevent it from going too far into the negative, and allow the +3.3V regulator to start. However, i am worried what the digital outputs of the LMH7324 chips will do while VCCO is powered from ground. The outputs are connected to LVPECL chips with a typical 3x50Ohm Y-termination to ground. The inputs of the LVPECL chips may not receive a negative voltage. Can the outputs of the LMH7324 create a negative voltage in this configuration? Figure 14 in the datasheet shows open emitter outputs, which can not sink current, but i am not sure if that is only for illustration.