Using MOSFET for power saving

[Jatinder Mann]

Hi,

I am wondering what is a good MOSFET to use to cut power off to sensors?

If anyone doesn’t mind providing links to the MOSFETS they use that would be greatly appreciated.

Thanks,

Jatinder Mann

[LIM PHANG MOH]

Hi Jatinder,

It depends on how much current the sensor is sinking. I usually employ a P-MOSFET like DMP2035U-7 (our favorite) to do this. Any MOSFET will do, make sure it is a logic level MOSFET and the maximum VGS is within the limit of your switching. Just make sure you add a very small capacitor in in between the gate and drain to reduce and slow down the in-rush current when turning it on even though the load start-up current is little..

[Jatinder Mann]

Hi Lim Phang Moh,

My sensor (https://media.digikey.com/pdf/Data%20Sheets/Adafruit%20PDFs/4007_Web.pdf) has a quiescent current of about 2.2 mA.

What kind of capacitor do you recommend using? Also would it be necessary to add a resistor when using the capacitor?

Thanks,

Jatinder Mann

[LIM PHANG MOH]

Any 100 nF would do. I went with it because it is common on all my circuits. Tried with smaller ones like 10 pF and 100 pF, makes not much different on turn on time. A pull-up resistor is usually employed so when not driven (during power up), the MOSFET is by default turned off. A current limiting resistor is needed as it work in pairs with the capacitor during turn on time.

[digkarag]

I am facing the same problem. Using atlas scientific pH sensor that consumes 1mA when in sleep mode. The sensor is powered straight from mini ultra pro v3 with 3.3v. Also using i2c protocol to read data.

Is DMP2035U-7 a good solution? How you control it to enable and disable the sensor?
Thanks

[LIM PHANG MOH]

I am facing the same problem. Using atlas scientific pH sensor that consumes 1mA when in sleep mode. The sensor is powered straight from mini ultra pro v3 with 3.3v. Also using i2c protocol to read data.

It’s going to be a problem if you directly use a MOSFET only to control the power. As there’s another I2C device on the bus (EUI-64 chip), when you cut off the power, the SCL & SDA lines will go to ground. The SDA and SCL pins are pulled up to 3.3V. So, you will see a sink of current from 3.3V to ground through the pull-up resistor which means higher sleep current. What you probably need is a I2C bus switch like for example the TCA4311A from TI or anything similar.

[digkarag]

If I initialize the pins as Internal Pull up, I will continue to have a high sleep current?
With a 2n7000 MOSFET I achieve to “cut” the sensor’s ground and have a sleep current of 20uA

[LIM PHANG MOH]

If you are driving something, put it according to it’s appropriate state. Input pull is used when a pin is not connected to anything.

[digkarag]

So for pins that is externally connected with pullups what do you recommend?

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