# Solution for how to drive a P-Channel MOSFET when Gate/Source voltage is too low?

I have used this forum to lookup solutions and suggestions from time to time but this is my first ever post! There are many great P and N-Channel MOSFET Questions and Answers in here, but I cannot find a solution to what I am asking. I have needed to change the low range of my Device voltage from 1.5V to 0.6V on a board I have made and have now troubles driving a P-Channel MOSFET used to turn OFF the Low Current measurements in the design by shorting the Shunt Resistor with a P-Channel MOSFET.

The Vgs for the P-Channel MOSFET is no longer enough to fully turn ON the MOSFET when the voltage for the Device is at 0.6V (Vgs = -0.6V).

Please see Schematic Named A for the working solution where the the low range of the Device Voltage is 1.5V.

I may very well have gone down a rabbit trail here with the solutions in Schematics B and C and I am asking for help to either verify my suggested solution (Schematic C) or provide a much better/different/simpler solution that I could not figure out.

Any help would be much appreciated!

Below is the somewhat long explanation of the 3 designs (I am sorry the component IDs are all different between them, copy paste does that automatically):

A - Schematic:

This is the working/current solution used on a board with a Device voltage generated in the range 1.5-5V. Q6 is shorting Shunt resistor R40 when the measured current is over 1mA to avoid a high drop voltage over R40 for higher currents. Q6 can be easily controlled here with N-Channel Q7 from a uProcessor. When Q6 Gate is grounded (via Q7) Vgs will be between -1.5V and -5V, well within the fully ON region of the P-Channel used.

B - Schematic:

This is the new design of the board where the Device voltage generated is in the range 0.6-5V, instead of as above 1.5-5V. Q8 (Same as Q6 above) can no longer be easily turned fully ON as Vgs is not enough (-0.6V). This is an attempt to lower (or increase Vgs) the Gate voltage of Q8 (Same as Q6 above) by instead of referencing Source of Q9 (Same as Q7 above) to GND, I added a negative voltage to Source. This would give a Vgs for Q8 (Same as Q6 above) between -1.6V and -6V. We can now again turn Q8 (Same as Q6 above) fully ON.

The problem is (and there is probably more that I missed) that even if I Float or High-Z the uProcessor Pin controlling Q9 (Same as Q7 above) to turn OFF Q9 (and Q8) the uProcessor pin will be at -1V. Since the uProcessor pin has protection diodes to GND and VCC this will cause the GND diode to conduct (Max is -0.5V).

C - Schematic:

Same as Schematic B but I have added Q2 as a high side switch to try to avoid the problem with the uProcessor pin explained in Schematic B. This way I should be able to control this High Side switch using the uProcessor pin which in turn would turn ON and OFF Q4 (in this schematic. Q7 and Q9 in the other ones).

Does this make sense? There are so many drivers and levels in this design and I may have just went down the wrong way and there is a much simpler solution?

Again, any help would be much appreciated! Take apart my design as much as you want.

EDIT 1 - Better details on what Powers this circuit and requirements:

The schematic is much larger as a whole. The "Device" or "VCC_Device", which is not pictured, is a DC/DC Switcher delivering up to 2.5A that now provides 0.6 to 5V DC out, runs through the measurements shunts and deliver it to the Device Under Test (TARGET_POWER). I may not have explained the Device and the target very well, which is the risk when you try simplify or leave out parts of the schematic.

As pointed out in the comments, I should simplify to the smallest item, which has been done below by tlfong01. The only requirements are that I can turn the MOSFET fully ON using a varied Source Voltage between 0.6V to 5V. The MOSFET should be able to handle at least 2.5A running through it and the Rdson should be kept low (max 40mOhm for max 100mV drop @2.5A) to avoid heat and voltage drop. BTW, it may seem weird that I do not provide Current Measurements for the whole range 0-2.5A, but here is a reason for that (so the suggestion is good to decouple that from the problem statement!).

• Ah, let me see if I can (over) simply your problem. (1) Your "'device" can output 1.5V to 0.6V to turn off a p-type MOSFET. (2) But now the (new) p-type MOSFET will not longer do the job for 0.6V as it used to. (3) Design B tries to do the following workarouond: (a) change the device's old output range of (1.5-5V) to (0.6- 5V). But you then also need to shift down the power source by biasing (shifting down) with a negative voltage reference. / to continue, ... Aug 24, 2020 at 1:57
• (3) Design C basically adds a High side switch to be controlled by a uP/MCU/SBC. I confess I have not at all looked at your scary complicated schematic. Just brainstorming. Aug 24, 2020 at 1:57
• (4) Please refer to the following picture: i.imgur.com/l984sdB.jpg. Do I understand correctly that only the pink dot is the problem area, stuff outside the pink dot is (almost) irrelevant? (5) Of course you can expand the pink dot by adding/changing a High side switch, to be controlled by another uP etc. Aug 24, 2020 at 2:12
• @tlfong01: (4) Correct. The Pink is the actual problem statement. I added some more around it just to give a wider understanding. The schematic is much larger as a whole. The "Device", which is not pictured, is a DC/DC Switcher that now provides 0.6 to 5V DC out, runs through the measurements shunts and deliver it to the Device Under Test. I may not have explained the Device and the target very well, which is the risk when you try simplify or leave out parts of the schematic. But you seem to have a very good understanding of the problem. Aug 24, 2020 at 3:35
• @tlfong01: You might be right. I will cut away the current design, the Current Measurements circuits and just ask the question directly. Maybe that needs to be a new post? Aug 24, 2020 at 3:54

You solved it the right way. In C schematic TPS60400 needs min 1.6V so connect it to 3.3V supply and remove U9 . This way you have better drive for the mosfet.

• Good point. I must have used different voltages during design and lowered it too much. However, I may need to change out Q5 as its Vgs MAX is -8V. With +3.3V I get -3.3V + 5V as a max = Vgs = -8.3V. Thanks for taking the time to look at this design! Sep 26, 2020 at 23:18
• Or add a forward diode on the 3.3V supply to the chip to reduce the voltage to 2.7V.
– Moty
Sep 26, 2020 at 23:29
• Thanks. I will pick this as an answer as it aligns very close or exactly to what I also think is the correct design according to my above schematics and by have you comment on it it makes me feel more ensured that it will work as intended! I will update either way when the boards are produced to provide feedback on good or bad the design worked. Thanks again for the review. Sep 27, 2020 at 2:11

Question

If a P-Channel MOSFET's original Vgs operation range changes from (-1.5 ~ -5V) to (-0.6 ~ -5V), what optimum changes can be made to the rest of the circuit?

Update 2020aug25hkt1315

Part B

After chatting, I have a better understanding the OP's requirements, as summarized below.

User Requirements V0.1

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Part A

The OP suggests two solutions, but more engineering trade offs can be made, to compare and contrast the two solutions, and other alternatives, to arrive an optimum design.

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References

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Appendices

Appendix A - DMP2008UFG characteristics summary

Appendix B - The OP's Circuit Highlighting the power MOSFET Portion

Appendix C - Spec Summary