To recall yesterday’s reasoning, I’m looking for a way to keep the RFM12B from starting up too soon and drawing 0.6 mA before the microcontroller gets a chance to enable it’s ultra-low power sleep mode.
The solutions so far require an extra I/O pin, allowing the microntroller to turn power on and off as needed, with the extra detail that power stays off until told otherwise.
But all I’m really interested in, is to keep that RFM12B from powering up too soon. After that, I never need to power it down again (and lose its settings) – at 0.3 µA, the RFM12B’s sleep mode will be just fine.
One solution is to use a dedicated chip for this, which can reliably send out a trigger when a fixed voltage threshold has been exceeded. That would still need a MOSFET, though, so this increases the cost (and footprint) of such a solution.
The other way would be to create a low-speed RC network, gradually charging a cap until a threshold is reached and turns on the MOSFET switch. Lower cost, no doubt, but in fact not flexible enough in case of a very slow power-on voltage ramp, as in the case of a solar cell charging a supercap or small battery. There is just no way to determine how long the delay needs to be – it might take days for the power rail to reach a usable level!
Yet another option is this little gem (thanks for the initial suggestion, Martyn!):
No I/O pin, no pull-up, nothing!
This trick takes advantage of what was originally considered a drawback of MOSFET switching: the fact that the gate voltage has to reach a certain level before the MOSFET will switch. Assuming that voltage is say 1.5V, then the MOSFET will be turned off as long as the power rail has not yet reached 1.5V, and once it rises above that value, it’ll automatically switch on. Magic!
Does it work? Well, I’m still waiting for some P-MOSFETs to arrive, but I’ve done a little test with an N-MOSFET, connected the other way around and using a 1 kΩ resistor as load. We can look at that combination as a component which has only two pins: a power rail and ground.
If the circuit works as expected, then when applying an increasing voltage, no current will flow until the threshold has been reached, and then it’ll switch on and start drawing current.
As it turns out, this is very easily observable using a Component Tester – like the one built into my scope:
The horizontal scale is the applied voltage (from about -5V to +5V), the vertical scale is the current through that component (from about -5 mA to +5 mA). The straight slanted line is characteristic of a 1 kΩ resistor.
But the interesting bit is that little dent: from under 0V to about 1.5V, the circuit draws no current at all. Once 1.5V or more are applied, the circuit starts conducting, and behaving like a plain 1 kΩ resistor again.
Woohoo, this might actually work: just a single P-MOSFET would be all that’s needed!