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What:

A battery-powered or USB-powered microcontroller project using a diode-OR with feedback to enable the main power supply (a 3.3V LDO).

Issue summary:

One of the requirements is that the device turns on when attached to USB, so I have the following circuit to generate a one-shot when USB is connected. It correctly generates a one-shot pulse when USB is plugged in and starts the LDO by raising PWR_EN signal.

However, when USB is not connected, and the system is operating on battery power only, I have trouble initiating power down (controlled by MCU connected to PWROFF signal):

  • System powers down
  • 3.3V gets disabled
  • Voltage at anode of D11 rises
  • 3.3V gets enabled
  • System turns back on

Looks like a cap charging, but I'm having trouble pinning down the cause other than floating VUSB node -- which is weakly pulled down.

Here's a simulation of the circuit: falstad sim

Question:

What might cause the node to at the drain of FET Q3 to rise when nothing is connected to VUSB?

Red square: USB detection one-shot feeding into 3.3V LDO enable latch Power supply and supply switchover

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1 Answer 1

Reset to default
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I'll bet that reverse leakage current through D11 and/or D2 (50 µA each, worst case) is slowly charging C17.

The fix would be to have a resistor from VUSB to ground whose value is low enough that the voltage on that node doesn't get high enough to enable the LDO. R7 at 100k doesn't seem to be enough, but changing it to 10k ought to do it.

Or, given that the threshold for the LDO's enable is just 1.5 V, switch to low-leakage signal diodes in place of the Schottkys.

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