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What is the simplest way to regulate the min DC voltage in a circuit? Is it possible to do with zener diodes?

Desired performance:

Input > 3.3 VDC     Output = Input
Input = 3.3 VDC     Output = Input
Input < 3.3 VDC     Output = 0.
endolith
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PICyourBrain
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3 Answers3

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According to your description, something like this should work:

undervoltage cutoff

The TLV3012 has the reference voltage built-in, so that's 1 6-pin IC, 1 FET, and 2 resistors. Not sure if you consider that "simple". :)

TLV3012 pinout

Similar:

Similar circuit based on this IC

Using a pMOS to switch a load is summarized here.

You could probably use a Zener as the reference for the comparator, too, or maybe rig something up to drive the MOSFET from the Zener more directly, but I don't know how precise that would be.

endolith
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  • That looks good but I really need it to be all passive. I does not need to be very precise. For example, if the voltage cut off was anywhere from 2.6 to 3.2 that would be fine. – PICyourBrain Oct 07 '10 at 19:07
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    It's very unlikely you'll find something passive that does what you want. – Jason S Oct 07 '10 at 19:22
  • Yeah, I can't think of any way to do it without at least one transistor. – endolith Oct 07 '10 at 19:30
  • And if you're worried about current draw in shutdown mode, this op-amp draws 5 µA, and the resistors could be increased to draw even less. – endolith Oct 07 '10 at 19:36
  • I think the main problem with this solution is figuring out how to get the vref. – Kellenjb Oct 07 '10 at 19:45
  • @Kellenjb: Yes this is true, that's why I was thinking a zener. – PICyourBrain Oct 07 '10 at 19:51
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    the chip Endolith mentions has an internal vref generator. – Thomas O Oct 07 '10 at 20:00
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    check out the '1381 voltage trigger' - it's a tiny (TO-92) 3 terminal package that does everything to the left of the PMOS depicted in the 1st schematic – JustJeff Oct 07 '10 at 22:10
  • Hmmm... Looks like it's discontinued, and the datasheet seems to show a window comparator instead, so it will output if the voltage is within a specific range, not just below a threshold. http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+SC+4+BFA7001+MN1380+8+WW – endolith Oct 07 '10 at 23:12
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    It doesn't sound like this circuit is to protect a battery from overdischarge, but if it was, the circuit in the diagram wouldn't work. When the battery gets disconnected from the load, its voltage will rise somewhat as it "recovers". The comparator would then turn the load on again. The effect will be that the load oscillates on and off until the battery is very thoroughly discharged to that voltage level. If left long enough this can destroy the battery even though you were trying to protect it. The circuit needs some kind of hysteresis or latching to prevent this problem. – Matt B. Jul 05 '11 at 23:14
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    @Matt: The question doesn't say anything about batteries, but hysteresis can be added with a few resistors – endolith Jul 06 '11 at 01:56
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If the voltage doesn't have to be quite 3.3V you could use a MAX809 reset circuit. Exists in several voltage versions, closest is 3.08V, custom voltages are possible.
For low power applications (< 20mA) you can use the output directly to power your circuit, otherwise use the output to drive a low \$R_{DS(ON)}\$ MOSFET.

enter image description here

(graph is actually from an MC34064 datasheet, but would look similar for the mAX809)

stevenvh
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2

You can connect a push/pull voltage detector (PMIC supervisor) such as the Microchip MCP112 or TC54 to a logic-level MOSFET, e.g. as described in the TC54 datasheet:

enter image description here

(page 6)

There are some popular thresholds versions of these parts, e.g. MCP112-300, MCP112-315, TC54VC30. So perhaps the nominal 3.15 V tripping point is sufficient for your application. In case it isn't you can look into getting a customized version or use one with V_trip=3V and voltage divide V_In as described in the TC54 datasheet:

enter image description here

(page 6)

For example with V_trip=3V and a target threshold of 3.2V you could chose R1=1780 Ohm and R2=26700 Ohm).

In comparison with the MAX809, the above voltage detectors are more efficient (e.g. I_DD_typ=1µA and I_DD_max<4µA instead of 17µA and 50µA) and implement hysteresis.

maxschlepzig
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