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Options for switching a 12V load with a 1.8V logic signal

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eddie1:
I'm switching a (small, microamps to few milliamps at absolute most) 12V load using a 1.8V logic-level MCU pin. Low power consumption is part of the challenge. The small output current requirements mean I can use a small MOSFET that is capable of being driven directly from an MCU or similar source with relatively low drive capability.

A p-MOSFET directly taking the logic-level signal won't work because the logic high (even if it was 5V) wouldn't be enough to turn the MOSFET off.

There are several approaches I've looked at:

1. Use an off-the-shelf load switch. This is one of the easiest ways, but I spent quite a bit of time going through Digi-Key and reading data sheets and couldn't find a suitable one with low (like under 1 uA) quiescent current. The Diodes AP22850 almost gets there (5 nA typical at 10V), but its max recommended voltage is 11V (12V absolute max).

2. Use an off-the-shelf gate driver. Same problem as above, plus high active current because most gate drivers are designed to switch larger MOSFETs with high gate drive requirements.

3. Make the output open drain (actually, keep it push-pull but add an n-MOSFET to protect the non-12V-tolerant MCU pin, which will also have the effect of inverting it and making it OD), then use a p-MOSFET with a gate pull-up to 12V. Also very simple and good for prototyping, but the issue again is the current draw of the pull-up resistor when the output signal is grounded.

4. Use a 12V-capable comparator as a level shifter. The TI TLV3701 (and others in that family) are the only 12V-capable parts I've found with a supply current rating in the nanoamp range -- 560 nA typical. The downside is it's pretty slow (almost 40 us response time at 50 mV overdrive, 7 us rise time, 9 us fall time). It's still an option, pending further testing to see what current draw is at different switching frequencies.

5. Use a 12V-capable level shifter IC (e.g. Nexperia HEF4104BT, TI CD4504B series, ON MC14504B) to drive a p-MOSFET. This is looking like a good option, if not slightly complex. Two layers of conversion are required (1.8V to 3.3V or 5V, then 3.3V or 5V to 12V) because all of the 12V level shifters are old designs and do not support 1.8V logic. The Nexperia part is the most efficient of the parts I've tested so far, coming in at ~508 nA @ 12V to shift a 1 kHz signal, up to ~16.10 uA to shift a 32.768 kHz signal. However, this ignores any current draw from the 5V supply as I do not have the equipment to measure both at the same time.

6. Use a PNP since the base can safely be left floating to turn it off. The issue with that is the base current.

7. Use a PNP to drive a p-MOSFET. The PNP would only have to drive the MOSFET gate charge current, not the load itself. This would allow the use of a much larger base resistor. However, that introduces the issue of slow rise and fall times. Because of that, it seems like there's a practical higher limit to the PNP base resistor value. The base resistor is usually pretty low (~1k?) in schematics I see around, and looking at Digi-Key for pre-biased BJTs, those max out at 200k (and only a few parts at that; the selection is much wider at 100k).

Building on the last option, here's an approach I thought of. I'm interested in feedback to know if this is crazy or not.

A PNP driving a p-MOSFET basically needs to supply the gate charge, ideally as fast as possible, and then the output voltage needs to be maintained but current draw is just about zero (realistically maybe a few nanoamps in leakage current?). The gate charge is minimal for any MOSFET designed for microamp/single-digit milliamp power levels.

Can a very high base resistor (say 100M, maybe more) safely be used?

If the answer is "yes," that still leaves the issue of rise and fall times. The drive circuit is being controlled by an MCU though, and that provides a lot of flexibility. The obvious approach is to use two MCU pins. Both connect to the base (with a buffer in between to protect the MCU pin from seeing 12V), one with a high (like 100M mentioned above) series resistor and one with a much lower value. The lower-value pin would be pulsed at turn-on time, quickly charging the p-MOSFET gate, after which the high-value pin would keep the PNP on, basically holding the gate at 12V and supplying the p-MOSFET's leakage current.

That's basically tuning the PNP drive circuit for the actual "burst then almost zero" load profile it will see. It seems like a waste to use a low-value resistor with the associated high base current draw if all it's going to be supplying is the leakage current of a MOSFET.

If such a high-value base resistor won't work, here's a related approach:

Use a low-value base resistor and drive the PNP from only one MCU pin. Add a small ceramic capacitor to the p-MOSFET gate, between the gate and ground. When the p-MOSFET gate needs to go high, pulse the PNP -- turn it on for a very short time (potentially a few hundred nanoseconds). The capacitor at the gate would effectively latch the gate at 12V, until the p-MOSFET gets switched on (i.e. grounded) again (or until the capacitor discharges, but that shouldn't be a concern).

The capacitor would need to be sized to be as low value as possible (as it needs to be charged and discharged every time the p-MOSFET gate changes) while high enough that, with leakage currents, it would never drop too low.

The PNP is only on for a very short time at each state change, minimizing the base current draw.

The EFM32's LESENSE peripheral can be (ab)used as a pulse generator to do this.

Thanks!

james_s:
You can use a Darlington transistor or make your own out of a pair of transistors, these can have huge gains, the base current can be extremely small. If you want to high side switch you can use a NPN or N channel mosfet to switch a PNP or P channel mosfet.

T3sl4co1l:
Heck, old school BSS138 will get close, and if you need better, RUM001L02 is rated for that.

If you need high side switching, use N into P and some resistors to set gate/base voltage.

Tim

EEEnthusiast:
Why are you looking for a high side switch when the low side NMOSFET can do the job easily?
The BSS138 is a good choice here. Its Vgst is well below 1.8V and with 12V, the leakage should be within 100nA at room temp.

eddie1:
Thanks all!

This is for that switching regulator project/experiment, so it has to be high side. The 12V feeds an inductor and has to be switched on and off. I've prototyped it out using the N+P with pull-up method, but I'd like to find another solution that doesn't use pull-up (or pull-down) resistors with their associated current draw. AFAIK, any method using MOSFETs is going to require a level-shifted control input; a 1.8V logic high, or even 5V, won't turn a p-MOSFET off if the load is 12V. (At least, I can't find any with a sufficiently high Vgs(th) for that.)

The Darlington transistor is an option I hadn't considered. Can a BJT (whether PNP Darlington or just a single PNP) properly be run with extremely high base resistors, like in the megaohms area, even 100k+? (Provided the load current is small enough and gain high enough, of course.) With such high resistors, would there be a risk of random environmental factors (noise, etc.) causing problems? With sufficiently high current gain it'd be simpler to power the load directly from the BJT rather than using the BJT to level shift the control input and drive a p-MOSFET that powers the load.

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