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Ultimate GPS - MOSFET & Pro Trinket 3V
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Ultimate GPS - MOSFET & Pro Trinket 3V

by adam_g on Sat May 12, 2018 4:23 pm

Hi guys,

I have a quick question about controlling power to the the Ultimate GPS Breakout using an N-channel MOSFET.

In the past, I've used the FQP30N06L N-channel logic-level MOSFET to turn power to the GPS on and off when I have the MOSFET's drain pin connected directly to my 4.2-5V battery. However, as I'm now looking to power my circuit from a 7.4-11.1V battery, I'm wondering if it would be possible to power the GPS from the 3V pin of my Pro Trinket 3V. The Ultimate GPS notes that it has a minimum operating voltage of 3.0 V and a current draw of ~25 mA and the Pro Trinket's regulator can output a maximum of 150 mA. I am able to connect the GPS directly to the Trinket's 3V/GND pins and have the GPS work without any issues. However, when I add the MOSFET and connect the circuit as shown below, I end up with my Pro Trinket stuck in a reboot loop as soon as I turn the MOSFET on. I have on hand two different N-channel MOSFETS: the FQP30N06L and the TN0702 and the issue occurs with both components.

I have a feeling this problem is likely due to my lack of knowledge of MOSFETs, and that I'm probably missing something rather obvious that would prevent the circuit from working as such. I'd appreciate if you have any suggestions on what I might be doing wrong!

Cheers,
Adam
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ultimate_gps_mosfet.png
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adam_g
 
Posts: 24
Joined: Sat Feb 18, 2017 11:41 pm

Re: Ultimate GPS - MOSFET & Pro Trinket 3V

by adam_g on Tue May 15, 2018 1:15 pm

Hi there,

Just a quick update. From researching this issue further, it appears that the reset loop problem may be due to the voltage drop experienced by the Pro Trinket when the MOSFET is enabled and power is supplied to the Ultimate GPS. It sounds like the voltage drop may be triggering the ATmega328's brown-out detection and reseting the microcontroller.

The strange thing is, when I replace the Pro Trinket 3V with SparkFun's Pro Mini 3.3V, the circuit operates normally and power is supplied to the GPS as expected. Could this perhaps be explained by the differences between the Pro Trinket's MIC5225 and the Pro Mini's MIC5205 regulator? It looks like the Trinket's 5225 has on average about twice the dropout voltage than that of the Pro Mini's 5205, though I'm unsure if this would be the cause of the problem.

Given that I'd like to continue using the Pro Trinket, I'm wondering how to best tackle this issue. Possible solutions I've seen involve slowing the power on/off cycles of the GPS, adding a capacitor to buffer the spike and/or using a high-side P-MOSFET.

Cheers,
Adam

adam_g
 
Posts: 24
Joined: Sat Feb 18, 2017 11:41 pm

Re: Ultimate GPS - MOSFET & Pro Trinket 3V

by adam_g on Thu Jul 26, 2018 1:33 pm

Hello again,

I'm back with more updates!

So, the problems I was experiencing with reset loops appeared to caused by the MOSFET turning on too quickly for the Pro Trinket 3V to handle. With a 220 Ohm gate resistor, the turn on time was about 50 μs, which was a big enough spike to cause the 3.3 V output of the Pro Trinket to drop to 1.8 V and trigger a reset. Increasing the value of the gate resistor to 47 - 100 kOhm resulted in a slower turn on time of 150 - 250 μs, respectively, and seemed to resolve the issue of the Pro Trinket resetting itself.

Unfortunately, once the reset loop issue was resolved, a new problem presented itself. I replaced my Adafruit Ultimate GPS with the SparkFun Venus GPS, which has a higher current draw of ~75 mA. About 10 seconds after the GPS is turned on, the 3.3 V output of the Pro Trinket will start to experience considerable voltage drops and the red LED will flash. This behaviour occurs with both N-channel (FQP30N06) and P-channel (NDP6020P) logic level MOSFETs, but strangely enough only when I power the Pro Trinket with a 12 V battery. If I use a 7.2 V battery, or the FTDI-friend that provides 5 V, these voltage drops do not occur.

I've included a simplified version of my circuit below, along with the oscilloscope output of the 3.3 V pin on the Pro Trinket when powered from a 12 V battery. I'd appreciate any feedback on what could be happening with the Pro Trinket or if I've made a mistake with my circuit.

Cheers.
Adam

Simplified circuit:
Image

Oscilloscope output:
Image

adam_g
 
Posts: 24
Joined: Sat Feb 18, 2017 11:41 pm

Please be positive and constructive with your questions and comments.