Advice for rail to rail input comparator

Hello guys !

I'm trying to design the simplest possible hysteretic buck current source : mesure current using a sense resistor, a comparator with hysteresis, and drive the switch accordingly.

For this, I need a comparator with a common mode input voltage range between 0V and 28V, Vcc either +5V or +28V, and not too slow (0.5 us would be OK). And I can't find any.

So, if by any chance you guys have a suggestion...

If your willing to use a quad comparator, the LM339 has a Vcc range of +5V to +36V and the input can be equal to or less than Vcc.

Sure about that ?

Datasheet says : Input common range : 0 to VCC – 2V

That would make Vcc as high as +34V. (Vcc +5 to +36 -2 = 34max) Higher than your +28V.

Can you not use resistive dividers (attenuators) on the comparator input to reduce the input voltage range?

Either the LM339 or 393 should work. However, I am not sure about the switching rate as you mention < 0.5 uS so you may want to check that out. This link is pretty useful as to comparators and setting them up. Something I don't understand is you plan to sense current across a resistor (shunt) which will typically give you a very small mV output proportional to your current. You are going to run that signal into the comparator?

Ron

Actually I'm considering two options for my buck current source :

- hysteretic buck : Sense inductor current with small high-side shunt resistor, use a comparator with hysteresis, drive MOSFET.
The comparator inputs need to accept common mode voltage up to Vcc, because I want it to keep working with as little dropout as possible between input and output (for instance, 10V out, 10.5V-28V in)
The comparator inputs need to accept common mode voltage down to GND, this makes the output fully protected against shorts to ground of BOTH load wires (which is the reason to use high side current sense)

- constant off time : Sense current in the VCC rail of the top MOSFET when it conducts, when it goes over threshold, turn off MOSFET, wait, repeat
The comparator inputs only need to accept common mode voltage up to Vcc, not ground, in this case. So it might be easier to find a suitable one.

Since the voltage I want to put at the input of the comparator comes from a current shunt in the Vcc, I cannot shift it : it is pretty small. My current design uses a MAX4376 current sense amp, which is all right, but I'd like to simplify the design even more. I don't need high precision in the current.