Power supply control, how to work?

I have a circuit about power supply control,

please review the circuit as attached picture.

Now, there are some questions need your support,

1) how to work, Q4 & Q3

2) what function is Q4?

3) Can I cancel Q4,and control Q3(MOSFET) with CPU? please review picture 2


thanks very much

1. When the CPU control is high., the +3.3V terminal gets connected to VCC_3.3V.
Q4 turns on pulling the gate of Q3 down which makes Q3 turn on in linear. When the CPU control is 0 or floating., Q4 turns off and R48 pulls the gate to the source turning Q3 bringing +3.3V terminal to ground(through R47)

2. To sink 0.33mA (3.3/10K) current when on and provide open when off.

3. It won't be a good idea as the high level from the CPU might be different form VCC_3.3V. This might cause leakage currents and noise currents.

In *1.jpg, the current flow from left side to the right. Q3 is a switch, Q4 is the control device.

Are you sure Q3 is the right FET device? I think the arrow should be upside-down.

You can use I/O port to control it. But normally high level is on, low level is off. Because we dont' want to waste any energy to shut down anything.

Q3 is the actual switch. Q4 is what you can call a"high-side" driver, because it drives the MOSFET, which is on the "high side", that is, it has no connection to GND.

Since the input is only 3.3V, you can definitely drive it with an IO line on your micro, just remember that when you want to turn it on you need to drive that line LOW. Since R48 and R49 are equal there, the current needed to drive the circuit will be almost the same in either case, so there is no real advantage to using Q4.

You can directly drive the MOSFET with an IO line on the micro as long as the input voltage is not much higher than the micro's supply voltage, because in that case the MOSFET would turn on without being actually driven. So if the input voltage is ever going to be different from 3.3V (actually your micro's supply voltage), then DO use Q4.

spikeyang said:

In *1.jpg, the current flow from left side to the right. Q3 is a switch, Q4 is the control device.

Are you sure Q3 is the right FET device? I think the arrow should be upside-down.

You can use I/O port to control it. But normally high level is on, low level is off. Because we dont' want to waste any energy to shut down anything.

Click to expand...

hi, spikeyang,

the Q3 is P-channel MOSFET, I'm sure.