input voltage detection circuit

[hsn93]

hello ,

im trying to understand voltage detection circuit here:



so whats the benefit of this "input voltage detection" circuit ? what does it detect exactly ?

as much as i can understand .. the input voltage will close the N-channel fet (Q102) which will short P-channel fet (Q101) to GND which will close it also to deliver (input voltage - voltage drop on Q101) ??
 

[helius]

It works together with the "power multiplexer" to isolate EXT_P5V0 when power is not being supplied, and if it is, prevents power being drawn from VCC_EDBG or VCC_TARGET. You can think of it as a relay switch that closes only as long as a positive voltage is supplied on its input.

[hsn93]

thank you ,

so to understand, this circuit is there because there is 3 power inputs right ?

also, wouldn't the power pass the Q101 because of the diode it has ?



or its ok because if there is voltage anyway the mosfet will be on so the diode doesnt matter ..

[agehall]

Yes, power will pass thru the diode of Q101 when it is applied, but only for a brief period of time. Once the power is applied, Q101 will be turned on via R103 and all power will flow thru the MOSFET itself.

[helius]

No, the body diodes do not pass any current when the MOSFETS are off.
You need to draw a flowchart to see how the circuit operates.

Step 1. Is a positive voltage (w.r.t. ground) available on EXT_P5V0?
If Yes, then: Q100 turns on; Q102 turns on; Q101 turns on; EXT_P5V0 goes to U101./EN, which will disable U101 if it is a logic high (~>2.5V). Stop.
If No, then: Q100 turns off; Q102 turns off; Q101 turns off; R107 pulls U101./EN down to ground, which activates U101. Go to step 2.

Step 2. Is VCC_EDBG_USB_P5V0 above the threshold of U101.VSNS?
If Yes, then U101.IN1 opens and passes to U101.OUT. Stop.
If No, then U101.IN2 opens and passes to U101.OUT. Stop.

For extra credit, describe what happens if EXT_P5V0 is between 0 and 2.5V.

[T3sl4co1l]

No, the body diodes do not pass any current when the MOSFETS are off.

What happens when the left side or right side is in the 0-2V range, indeed!

To OP:

This looks to be a subcircuit which was
a. Poorly implemented following some intention,
b. Poorly intended following its implementation, or
c. Poorly documented / commented following either of the above.

Or something altogether worse, who knows.  In short, it doesn't seem to do what's claimed, and you certainly shouldn't take it at its word.

Tim