My question boils down to - what is the behaviour of a P-Channel MOSFET when the gate is left floating? Does it conduct (Drain to Source) with minimal resistance in this case?
I'm looking at this circuit:
https://www.olimex.com/Products/Duino/AVR/OLIMEXINO-328/resources/OLIMEXINO-328-schematic.pdf
Which uses this P-Channel MOSFET:
http://www.mouser.com/ds/2/200/irlml6402-50379.pdf
If you look at the middle left where it says "Battery Charger", I believe that the circuit is using the FET to switch the input to the MCP1700T step down regulator between the LIPO_BAT and +5V.
I.e.:
(1) when +5v is present (on the Gate), Gate to Source is Positive, so the FET turns off.
(2) when the Gate is floating, Gate to Source is ... (negative?!?, so the FET is on?)
It's the second scenario that I find hard to fathom, so wanted to check before I incorporate something similar in my own circuit. Many thanks.
I'm looking at this circuit:
https://www.olimex.com/Products/Duino/AVR/OLIMEXINO-328/resources/OLIMEXINO-328-schematic.pdf
Which uses this P-Channel MOSFET:
http://www.mouser.com/ds/2/200/irlml6402-50379.pdf
If you look at the middle left where it says "Battery Charger", I believe that the circuit is using the FET to switch the input to the MCP1700T step down regulator between the LIPO_BAT and +5V.
I.e.:
(1) when +5v is present (on the Gate), Gate to Source is Positive, so the FET turns off.
(2) when the Gate is floating, Gate to Source is ... (negative?!?, so the FET is on?)
It's the second scenario that I find hard to fathom, so wanted to check before I incorporate something similar in my own circuit. Many thanks.