Hi @anishkgt,
You have the basic idea right for the ALD810019. However, for the circuit with added transistor and resistors, the current gain is much more dramatic. With that circuit, you can move up to a 2.7V threshold SABmosfet. I didn’t find a schematic with specific values, but by using what was given in the datasheet plus examining the image of the board, I have an idea of what is used.
Here is the schematic from the datasheet for one capacitor with the optional resistors removed for clarity:
In this circuit, M1 is used to turn on Q1. Rx2 pulls the gate voltage of Q1 high, thereby turning it off when M1 is not conducting. As M1 begins to conduct, current is drawn through Rx2 and the voltage at the gate of Q1 begins to drop, causing it to begin to conduct. By scaling Rx2 appropriately, there won’t be much of a voltage drop at the gate of Q1 until 1uA is drawn through Rx2. This occurs when the voltage across the capacitor reaches the Vth value of M1. As Q1 begins to turn on, the current will rapidly increase through Q1 and Rp1. Rp1 is scaled so as to allow around 100mA to conduct when C1 is a little above the threshold voltage, when Q1 is fully enhanced and only dropping hundredths of a volt.
From the image on our website, Rp1 is a 24 Ohm 1W resistor and it looks like Rx2 is a 1.8M Ohm resistor. I could not identify Q1, but I think the BSS215P you mention would probably work fine, as it has a relatively low Vgs-th and reasonably low Rds-on. Something like the DMG2301LK-7 is another option with a few additional benefits, as it has a lower Rds-on and can handle more power, which gives it a bit more design margin. You may have to play a round a little with the value of Rx2 to get the optimal turn-on characteristic, but 1.8M Ohm is probably a good starting point.