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I was looking at the design of Adafruit M4 Feather board. The board can charge a battery via USB (and power the board). When USB is disconnected, the battery take over automatically. What I don't understand his how Q3, a P-Channel MOSFET, can be turned on when USB connector is disconnected.

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My understanding is that Vgs must be negative to turn on a PMOS. Normally, in some other circuit, the source is connected to the power and the load to the drain, like the circuit below.

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In the first circuit (Feather M4), when USB is disconnected, Vg = 0V, Vd = VBAT, Vs = ??? How does the whole thing work?

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  • \$\begingroup\$ When Vbus is not connected, there is R9 pull-down that switches Q3 on. \$\endgroup\$ Feb 11 '21 at 4:06
  • \$\begingroup\$ Pulling down it's not enough, Vgs must be negatif (Vg-Vs), my question is what it's the volgate of the MOSFET source, since the source is connected to the load. In the second circuit, the source is connected to Vdd. So when the switch is connected to ground Vg = 0V, Vs = Vdd (5V for example) so Vgs = 0 - 5 = -5V which turn on the PMOS. In the Adafruit circuit, I cannot make sense of what of the volage at the source (Vs). \$\endgroup\$ Feb 11 '21 at 18:00
  • \$\begingroup\$ Pulling down gate makes that p-mosfet open. Btw. that schematic is wrong, that mosfet Q3 is upside down. \$\endgroup\$ Feb 11 '21 at 22:24
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Note: In the M4 Feather schematic the DMG314 part number appears to be incomplete. Searching at both Mouser and Digikey it seems as a DMG3414 is an N-FET and a DMG3515 is a P-FET. The circuit design would extrapolate to that transistor being a P-FET however if that was the case the symbol of the part is oriented wrong (i.e. source and drain need to be interchanged).

However:

If it a properly connected Pchannel MOS FET it turns on when the gate voltage is less than the source voltage by at least the threshold voltage.

In the M4 Feather circuit the source of the PFET is connected to the battery voltage. When the USB cable is disconnected the gate of the PFET is pulled to GND through the 100K ohm resistor. As long as the battery voltage is higher than the PFET threshold voltage then there is a -VBAT being applied across the PFET drain to source which is plenty to turn on the device.

In a way the M4 Feather circuit is almost just like that second circuit that you showed.

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  • \$\begingroup\$ I concur. I think the part number is missing a digit (or more) and the symbol is oriented incorrectly. I would add, that D4 is an MBR120 which is a Schottky diode, yet the symbol is a frequently used Zener diode symbol. (Not my preferred, I prefer "slanted arms".) \$\endgroup\$ Feb 11 '21 at 18:22

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