I have a PCB that has an electronic power switch for "12V" input (really, 3S or 4S LiPo,) that feeds a buck regulator, that generates 5V, that powers a Teensy 3.6.
The power switch is latched high by a resistive divider on the output of the switched 12V rail.
To be able to turn off when the voltage goes too low, to save the LiPo battery powering the system, there's a voltage divider going to an analog input on the Teensy (for voltage monitoring) There's also a resistor between pin 25 on the Teensy and the power latch input on the power switch, so the teensy can pull this low. Finally, to make sure the voltage divider doesn't overvolt the Teensy pin, there's a 3.3V Zener across the lower leg of the latch-on voltage divider.
When the Teensy is not plugged in, the board works fine; hook up 12V bench power supply, press On switch, power switch turns on, voltage divider latches it high.
When the Teensy is plugged in, the power switch refuses to turn on. The boost regulator doesn't activate, so the Teensy doesn't get power, so it can't keep pin 25 high.
When measuring the Teensy pin 25 to ground when it's not powered, I measure essentially infinite impedance, but somehow, plugging it in to this circuit makes it behave differently.
What can I be missing?
This schematic uses part numbers as values, which makes it a little harder to read, but it's all I have:
Pin 25 is the "PWR_ON" input at the top. The power switch is the IN_ONSW switch at 10 o'clock. The power switch chip is U701. The "must turn on" voltage for INPUT is 2.1 Volts, at 10 microamps of input current.
The power switch is latched high by a resistive divider on the output of the switched 12V rail.
To be able to turn off when the voltage goes too low, to save the LiPo battery powering the system, there's a voltage divider going to an analog input on the Teensy (for voltage monitoring) There's also a resistor between pin 25 on the Teensy and the power latch input on the power switch, so the teensy can pull this low. Finally, to make sure the voltage divider doesn't overvolt the Teensy pin, there's a 3.3V Zener across the lower leg of the latch-on voltage divider.
When the Teensy is not plugged in, the board works fine; hook up 12V bench power supply, press On switch, power switch turns on, voltage divider latches it high.
When the Teensy is plugged in, the power switch refuses to turn on. The boost regulator doesn't activate, so the Teensy doesn't get power, so it can't keep pin 25 high.
When measuring the Teensy pin 25 to ground when it's not powered, I measure essentially infinite impedance, but somehow, plugging it in to this circuit makes it behave differently.
What can I be missing?
This schematic uses part numbers as values, which makes it a little harder to read, but it's all I have:
Pin 25 is the "PWR_ON" input at the top. The power switch is the IN_ONSW switch at 10 o'clock. The power switch chip is U701. The "must turn on" voltage for INPUT is 2.1 Volts, at 10 microamps of input current.