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I'm using 3 comparators for sensorless back EMF commutation of a BLDC motor. This involves comparing the voltage of each motor phase to a neutral point to detect zero crossings. See Microchip AN970 (PDF) for further details on the application.

Unfortunately, I can't find a comparator that I can run off my wide 6-50V VCC input range. The best I can find in that regard is the LM339 that takes 2-30V, but nothing higher.

That leaves me in a situation where I have to divide the back EMF voltages down to fit whatever supply voltage I run the comparators from. If I run the comparator off a fixed voltage supply (call it VDD) and divide the signals down so that the max signal voltage @ VCC = 50V is within VDD, I'd be worried about attenuating the signal too much when VCC is lower. I guess my first question is, would this signal attenuation actually be a problem? I'd be looking at attenuating it 10:1 to run a comparator off my 5V supply.

Assuming the answer to the above is yes, I've come up with a couple of possible solutions so far (alternatives welcome):

  • Divide the signals down as described above, keeping the fixed VDD as high as possible to reduce how much I have to divide the voltage signals down. I'd prefer to avoid adding a Boost converter off my 5V rail for cost reasons, but a simple charge pump doubler like an LM2767 would at least get me to VDD = 10V and 5:1 attenuation.
  • Generate a VDD supply that is proportional to VCC (probably about VCC/2 for the LM339) and scale the voltage signals by the same amount. This would be preferable, but I'm not sure about is how to best generate a VDD = VCC/2 supply that I can use for the comparators. Can I get away with a resistor divider here? I was considering a step-down charge pump but I didn't have any luck finding a charge pump regulator with a high enough input range. Maybe even just switching VCC through a FET at 50% duty cycle?
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A comparator running at 5V and a voltage divider will work fine, you wont be attenuating the signal in any non-linear fashion so the signal going into the comparators will work for you.

You stated the input voltage will be a range (6 - 50V) and as you said you would use a 10:1 divider, then at 50V your max input would be 5V and at 6V your max value would be .6V. I'm sure any decent comparator will be able to distinguish differences in the 10's of millivolts (conservatively). I looked over the Microchip application note you linked, The circuit shown on page 14 looks great, it provides a virtual center point so you don't need a positive and negative voltage supplies.

Concerning the power supply it seems as though your best bet would be to supply the higher voltage (6 - 50V) then use a buck converter to bring the voltage down to 5V, something like the TI simple switcher designs like http://www.ti.com/product/lm46000 would work (I don't work for TI, I just like their simple switchers). They even have an example schematic.

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  • \$\begingroup\$ I do have a 5V buck regulator already, just nothing else between that and VCC. \$\endgroup\$ – Joe Baker Jun 29 '14 at 18:12
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(1) You could clamp the signals to some maximum voltage.
Input resistor to eg zener diode to ground.
Vzener should be

  • Comfortably below Vdd to allow some variation due to the zeners soft knee.
    Peak voltage will rise slightly with rising Vin > Vzener

  • Comfortably above Vhi_min of your comparator.

You'll probably want Vzener lower than Vin min so you always get clamping.

(2) A differential amplifier or differential comparator can be built that allows Vin and Vcommon mode to be well above Vdd, but no voltages above Vdd are presented to the amplifier.

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