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| Nano current inrush switch/low voltage cutout |
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| Howardlong:
I have a target MCU running a simple application that runs just fine on 400nA and 1.8V, except for one thing: to start, it needs at least 17uA to start for about 3ms. It's solar cell based, with a supercap backup, and the solar cell will produce oodles over 17uA in good light, but I also want it to be able to start in relatively low light, say a couple of uA. So I'm looking for circuit suggestions for a super low power switch that only provides current when, say, the input voltage is over 2.0V, so there'll be a brief dip during MCU startup but there'll be plenty of charge in the supercap for this brief time. And, of course, the switch itself needs to be really low current. |
| Marco:
This hackaday project mentions a potential direction for discrete solutions. You can use LEDs as voltage references (together with some small signal BJTs and a MOSFET) which are a little more stable than just using the Vbe of a transistor, as long as you black out the LED any way. It also mentions a couple IC's which should work for you, "XC6135, XC6136 or an Intersil ISL880xx series". |
| Howardlong:
Coincidentally, a BJT and LED was what I was experimenting with yesterday, but my rudimentary circuit was very leaky. Thanks for the link, this looks worth a try. |
| David Hess:
Circuits like power supply controllers which rely on bootstrapping their own supply use under voltage lockout circuits to do exactly what you are describing. The power source charges a reserve like a capacitor and when a voltage trip point is reached, the capacitor is connected to the circuit. It really is not that difficult. A simple implementation might rely on a micropower comparator/reference like the LTC1540 to switch a MOSFET. |
| Howardlong:
--- Quote from: David Hess on March 12, 2019, 02:25:28 am ---It really is not that difficult. --- End quote --- I don't agree when, as in this case, you are measuring power budget in nano amps. Circuits such as potential dividers, voltage references and bias circuits you'd typically use in such designs become very expensive in terms of power requirements. --- Quote ---A simple implementation might rely on a micropower comparator/reference like the LTC1540 to switch a MOSFET. --- End quote --- Thanks, I'll take a look, although it's not fully clear in the DS how it behaves on power up, although there is a brief section on this. |
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