Opamps and comparators, input max voltage?

Hi

Inspired by Vellemans K8071 led driver, I tried something similar.

As of now, I turned it upside down, as n-channel fets are cheaper and easier to control. This means, that I have a resistor on top, so my measurement for the current is set as VCC -0.3V. My reference is similar, slightly below VCC.

Now, say power is 12V so my Vref is 11.75, but here is the question:

How well do opamps and comparators work with that high input? I tried TL082 and LM2903 (same as LM393 used in K8071) and the LM2903 will never change its output. TL082 seems to work somehow....

WBR Sonnich

Reply to
sonnichjensen
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Check the common mode range of the input.

Reply to
miso

That depends on the specific op-amp design.

You would want an op amp or comparator which supports "rail to rail" inputs... or, at least, inputs which come up to the positive rail.

Quite a few op amps (and comparators I suspect) won't work properly for inputs this close to the rail... many need at least one diode-drop of headroom, and some require more.

Reply to
David Platt

Who the hell could possibly know wtf you're talking about? Dunno the problem but the thinking is so impaired am not surprised to hear you have one.

Reply to
bloggs.fredbloggs.fred

FET amps commonly include the positive rail. LM358, LM393, LM2903, etc. include negative rail. 741, LM833, etc. include neither. TLV2372 includes both (and also has a rail-to-rail output).

The spec you're looking for is common mode input range.

Tim

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Seven Transistor Labs 
Electrical Engineering Consultation 
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Reply to
Tim Williams

Hi all

I found, that using an ordinary opamp, and a PNP darlington makes more sense, so I can keep the measuring and reference at app 1V. A P-channel mosfet is actually more expensive than a PNP Darlington. So basically I am back to what the K8071 is :)

I also found chips like NCP3065 which implements all I need which makes my idea kind of obsolote.

Also MAX16832, HV9910 and HV9925 will do very well.

Reply to
sonnichjensen

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