I am designing a power input selector using the LTC4418 to select an input power source. The designed thresholds and hysteresis are as follows. Both inputs have inrush limiting circuitry on the FET gates due to the overlapping input voltage ranges.
Input 1: 24VDC AC-DC converter
Input 2: 20-36VDC battery
The entire circuit has been modeled in LTSpice. A screenshot is below and the LTSpice simulation file is also attached.
I am getting longer-than-expected transition time between input 2 being disconnected and input 1 being connected to the output when input 2 has a voltage higher than input 1. An example sequence is this. In this example, the extended transition time occurs between steps 7 and 8.
I have narrowed the cause down to the capacitance on node VS1 from capacitor C5. This capacitor value was chosen as 10 * Cs as suggested by the LTC4418 datasheet. The voltage on the VS1 net is 35V when input 1 becomes valid due to the body diode in P-FET M2 allowing Vout to charge it. Due to the selected inrush current limiting circuitry on node G1, G1 has a falling slew rate of about 0.24V/us. Since the voltage at VS1 matches Vout (35.5V) between steps 7 and 8 and VS1 must be discharged to 20.3V before the FETs on input 1 turn on, it takes an additional 65us for input 1 to become connected to the output. This results in a much larger than anticipated droop on Vout and additional inrush current during this transition.
Instead of adding additional capacitance on Vout to support this transition, I feel it is wiser to reduce the amount of time it takes for VS1 to discharge to 20.3V. I see two options and I'm not sure which is the right option.
Have you dealt with this issue before and how did you resolve it? Is one of the above two approaches preferred and an acceptable modification?
I would go with reducing the C5 cap value. If you add a bleeder path, the problem will be that you will always have a current path from your supply through body diode of the MOSFET, through bleeder resistor, through bleeder diode to output which poses a danger of deep deplete you power supply (if it is a battery).