Power sequence solution with limited board real estate

Question

I am designing a power supply using the LTM4639 part. VIN (3.3. V) needs to be sequenced before the bias input (5 V) in order for soft start to work. However there is no fixed timing relationship between both sources in my design and I am running out of board space.

I have considered a DC/DC (3.3 V to 5 V) converter such that it has a common VIN with the LTM4639 and its output is used to power the control circuit of the LTM4639. However such circuit (SOT-23 and several 0603 discrete components) will take up more board space than desired. I am also thinking about a power FET where the gate is tied to 3.3 V (still brushing up on my past transistor knowledge).

Any idea on a simple and small circuit that can sequence a 5 V output after a 3.3 V input? The threshold requirements are not detailed in the datasheet but I don't expect more than a 1 A current demand at the bias circuit.

Answer 1

LTM4639 PGOOD Voltage Low out to Pch FET for high side switched 5Vout then choose "logic level" device with suitable low RdsOn.

Answer 2

This is easy with a NCP301H and a suitable FET (I spec the BSH203 here, but any logic level PMOS with Rds(on) less than 1 ohm at 4.5V Vgs will do), see below:

^{simulate this circuit – Schematic created using CircuitLab}

You should be able to use tiny passives here, too -- 0402 seems eminently feasible, and if your assembly process can handle it, 0201 is worth exploring.