Power Path Controller LT4416

A power design is using the circuit in Fig 2 with some small changes.

Why? Do you mean in series with the E1 input?

I usually do that with a diode from the logic input directly to the rail (cathode to rail) with a pullup resistor for the logic. This does of course assume that there's something on the rail to sink the diode current when the rail power disappears.

I> have trouble understanding the operation of this chip. It is actually fairly complex. I expect E1 to go inactive anyway, just not as soon since the external supply likely has a large cap and will not ramp down to 1.25V so quickly. So is there anything I am missing in how these enable inputs work? Will removing E1 cause any issues with the switchover?

Ok I missed "series" but what is the additional resistor for?

If a diode directly to the rail is not possible for rail present/not present then why not just turn on a small transistor or FET with a base/gate series resistor when the rail is present?

Yeah, that would work. So you are saying wire the diode so the power input can only pull down, not up and the sense input uses a pull up to an approp riate voltage. We can add another pull down in addition to the light volta ge divider resistors in place.

It just seems so simple to use the H1 output since it is already there and obviously gives a good indication of the power status with great output cha racteristics. It just requires the addition of a 1.5K pull down on the E1 input to set the threshold voltage. The two resistors on the E1 input sets the voltage for the indication to just a bit below the two voltages detect ed by the E2- input which controls the switchover from line to battery. Co mpare that to running a 1.4 volt signal between boards when one board is co ntrolling a 10 amp brushed DC motor and has two switching power supplies wi th who knows how much noise. The resulting noise margin is about 0.4 volts .

There's nothing wrong with adding a transistor either, but why bother? I t hink the guy doing the circuit design of the power board thinks it is impor tant to have the indication at the same moment as the power switch circuit activates. It's not at all critical. This is the same guy who added an in put voltage protection chip to protect a chip that has almost the same inpu t voltage range as the protection switch. But it meant adding another set of switch FETs! WTF??? We don't have a requirement for protection against

Yes. Just be careful that the power rail doesn't behave as an open circuit when the power is off (most don't) or the diode won't pull down.