The DS3231 RTC has SQW/INT pin that is set low when one of the two programmable alarms activate. I'd like to use the alarm to power on the arduino via a MOSFET switch. According to my research, a depletion type N-channel MOSFET is what I need, one that can handle a couple amps at 5V. I am wondering if someone can tell me A) if this is feasible and B) which MOSFET I should look at. Thanks for any tips.
It is not so simple. The alarm output is active low (about 0 V) and that is not enough to turn off any depletion-mode NMOS that I know about. You need a negative voltage with respect to the source for that. One approach would be to invert the signal, which is easily done with an NPN bipolar transistor, and use a logic-level NMOS transistor. Such a circuit would only sink current to ground, but that may be what you want.
To power other circuitry, most people use "high side switches", either PNP junction transistors or P MOSFETs. Assuming you will be running the DS3231 from a 5V power supply, a single logic-level (5V) P MOSFET would probably work as a high side switch, but I haven't tested it. You would need as low on-resistance as possible to support 1 ampere or greater.
Edit: See the following page for some basic information. The high side PMOS circuit of Figure 5, used for reverse-battery protection, is essentially what I'm suggesting. http://www.maximintegrated.com/app-notes/index.mvp/id/636
Been meaning to reply to thank you for this information. I am presently researching the high side PMOS option and will update this thread when I get the components and arrive at a working solution. Cheers.
Dave
dave_dsnj:
The DS3231 RTC has SQW/INT pin that is set low when one of the two programmable alarms activate. I'd like to use the alarm to power on the arduino via a MOSFET switch. According to my research, a depletion type N-channel MOSFET is what I need, one that can handle a couple amps at 5V. I am wondering if someone can tell me A) if this is feasible and B) which MOSFET I should look at. Thanks for any tips.
Power MOSFETs are enhancement-mode.
You can use a p-channel power MOSFET as a high-side switch, if its logic-level.