Pico's Mosfet maximum Amperage when partial ly turned on?

[espruino-discuss]

Posted at 2018-10-04 by asez73

Hi,
I'd like to use the mosfet of the pico to control power of a GPS (60 mA at 3.3V, 5V tolerant, own ldo regulator integrated).
So, I got confused somehow from the documentation...
That is, after bridging the jumper B0/Fet, I should use the example provided here we have

digitalWrite(B0,1); // Partially turn on the 'Bat' output (this produces 3.3v on the FET, meaning it has just 1.4v between Gate and Drain)
digitalRead(B0); // turn off the output (also check if USB powered)

a-Does that mean that I will get a 3.3V on VBAT?
b-What would be the max amperage in this case?
c-Would the mosfet get hot?
d-On this Pinout, which pin should I use to connect a load that will be powered through B0 controlled mosfet : VBAT, BAT_IN?

As a separate problem, I need to connect the GPS on a UART AND an I2C (addtionnal sensor here).
e-What would the best pins to use: B6 & B7 for UART, which ones for I2C?

Thank's a lot.

[espruino-discuss]

Posted at 2018-10-04 by @allObjects

Here is the datasheet http://www.onsemi.com/pub/Collateral/FDN358P-D.PDF , where it says in the Absolute Maximum Ratings that you can draw continuously about ***1.5[A] ***... but this is not the only information to be considered. Since the board (safe) design supports a 500[mA] LDO Voltage Regulator, you are with 500[mA] on the safe - cool(er) - side. You may go higher... but the specs do not say a thing about voltages lower than 4.5[V] 200[mOhm]. With raising on-Resistance more heat has to be dissipated...

For running a GPS - 60[mA], you are more then fine...

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Posted at 2018-10-04 by asez73

I hope so!
Thanks!
Still need to,make sure I wont't have random failures: Data sheets don't always give the needed figures or very undirecly:)!

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Posted at 2018-10-04 by @allObjects

Espruino has a lot doc now... the hard part is to find it right away. This may be of interest to you: http://www.espruino.com/Pico+FET+Output

1.5[A] continuously and 5[A] pulsed is a nice number... you now just need the Battery to support deliver it!

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Posted at 2018-10-05 by asez73

Hi,
Thank's but I am looking for what happens at partially turned on, not fully on... :)
That said, you are right, almost everything is written but finding it can be more difficult. Gordon is doing a very important job to document and structure this knowledge. However it is a huge task!

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Posted at 2018-10-05 by @gfwilliams

Hi,

I'll answer below, but I think the real answer is:

The FET is connected to the VCC/VBAT voltage rail, and you can't use it like a voltage regulator - so unless the GPS has its own voltage regulator, using the FET won't be a viable option for you. You'd be better off adding your own to the 3.3v rail.

a-Does that mean that I will get a 3.3V on VBAT?

No, with nothing connected you'll get VCC/VBAT's voltage on BAT_IN.

b-What would be the max amperage in this case?

It totally depends - basically it turns the FET into a resistor. With that little drive voltage the datasheet doesn't specify the resistance, but it should be a few Ohms.

c-Would the mosfet get hot?

If you short BAT_IN to GND then yes, it's possible. It's the same as a resistor in this case.

d-On this Pinout, which pin should I use to connect a load that will be powered through B0 controlled mosfet : VBAT, BAT_IN?

BAT_IN

e-What would the best pins to use: B6 & B7 for UART, which ones for I2C?

I don't think it really matters? It depends if you need any of the other peripherals handy.

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Posted at 2018-10-05 by asez73

Hi,
Thanks again: the gps has its own voltage regulator so I am just using it as is with the mosfet switching it on & off from time to time.
I expect to use a cheap non protected battery design to recharge a smartphone with a usb connector on its output and a micro usb for recharging it from main. This won't happen while the pico is inserted in the battery.