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I’m reading the specs of ESP32 and I’m trying to find a P-channel MOSFET that I can drive with the GPIO pins. The specs say that ESP32 works from 3.0 V to 3.6 V. Its logical zero is at most 0.1VDD and the logical one is at least 0.8VDD. That means that (at 3.6 V) 0.8*3.6 V=2.88 V is the lowest possible logical one of the GPIO pins.
I found that AO3401 has -0.7 V to -1.3 V gate threshold voltage. That means that for logical one at VDD of 3.0 V (same as the ESP32) it starts to switch from 2.3 V. At 3.6 V starts at 2.9 V.
The problem is that if the ESP32 is driven by 3.6 V it might output 2.88 V for a logical one and that the AO3401 will already begin to switch (from 2.9 V already will be partially switched).
I have looked around the web, in forums, shops for parts, but was unable to find other suitable MOSFET. Ideally one that switches from around -1 V to around -2 V would be nice, but I have not found one. In this case at 3.6 V it will start to switch from 2.6 V (that is lower than 2.88 V). The N-channel IRLZ44N has exactly that: 1 V to 2 V, but its N-channel.
Is there such a MOSFET? Or this 0.02 V difference should not be considered as a problem in practice?
asked 1 hour ago
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- For starters, I think you’ve misunderstood the meaning of gate threshold voltage. The gate threshold voltage is where the MOSFET starts to conduct at all. So the -0.7 to -1.3V range is for where it starts to conduct (typically where it can conduct a few hundred µA). It is not “fully switched on” at -1.3V, but it is guaranteed to have reached the threshold voltage where it starts to conduct at all. For it to be “fully switched on”, you need higher Vgs. Datasheets typically has a graph for
Rds(on), take a look at that. – Klas-Kenny 1 hour ago
- I’m sorry, its a new field for me. But that still does not change the fact that it might start to conduct at -0.7. I have edited the question. – verbessern 1 hour ago
- Please specify your load voltage and current. – Mattman944 36 mins ago
- The output voltages you are referencing are with a non-zero load. Lightly loaded, CMOS outputs drive near the rail. Please post your circuit so there are no mis-understandings. – Mattman944 27 mins ago
- I have not have a particular schema. But for example, it could be another ESP32. As I understand my concern is valid. At certain conditions AO3401 can start conducting even that the ESP32 outputs HIGH by its specs. – verbessern 4 mins ago
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Let’s not mix up some important concepts. The 0.8V logical one is the input voltage. This basically means that any voltage above 0.8V is seen as a logical one, but means nothing for an output logic one. For any modern microcontroller, when you set a pin to output logical one, it will output Vcc with a very slight, negligible drop. So if Vcc is 3.6V, you will apply that voltage on the gate.
When selecting a mosfet, Vgs(th) is misleading. It’s only when the mosfet “starts to wake up”. You need to find a mosfet with a typical resistance rating for your voltage. For example, let’s take “PMV27UPER”. From its datasheet, we see the following:
As you can see, Vgs(th) is rated at a maximum of -0.95V, but at that voltage it will only pass 250 microamps! However, go down to Rds(on) and you will find that it is rated for 3 voltages. 4.5V is higher than what the ESP can do, so you can take the next value of 2.5V
This part tells you that at 2.5V or higher, it will have a maximum drain to source resistance of 45 mohm.
It’s not hard to search for this. When you go to https://www.digikey.com, you can find the following fields. Just select all parts that have a “Max Rds On” which is above Vcc. Here, I assumed Vcc can go as low as 3.0V, then added 0.2V as a buffer. Now all the parts you find can be turned on by the ESP, what remains then is wether the mosfet can pass the current, withstand the voltage, etc.
answered 1 hour ago
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- Thank you for the explanation. I’m actually trying to search by the minimum threshold voltage, to guarantie that the P-channel MOSFET does not conduct for logical one. If the specification states 0.8VDD I guess there is a reason for that. Maybe I can just pull the gate high with a resistor to ensure that its more closer to VDD. – verbessern 2 mins ago