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[SOLVED] PMOS and difference amplifier simulation questions

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d123

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Hi,

I'm trying to understand RDSon a little in this circuit and a difference amplifier output voltage. I suspect that such low RDSon is either my bad maths or a very idealized PMOS model as it doesn't seem to look realistic when I try to understand the datasheet curves and relate them to my VGS and load current.

I think 86mV out of diff amp at 1.64A load is:

50mV min OA output
3mV input offset voltage
16mV PMOS RDSon
16mV PMOS RDSon
= 86mV difference amplifier output.

So, presumably RDSon is 10mOhms per PMOS. With a questionable 27mW per Mosfet PD. I substituted the mosfets for 10mOhm resistors and got the same 11.147V and 1.63A output.

Is that right or wrong reasoning, please?

Also, I can't understand why it's the same 86mV (33mV) for a single and for a back-to-back pair. What might I be misunderstanding or is it something to do with the simulation?

FET IDEAL DIODE EXPLORATION 2 BACKTOBACK.JPG

FET IDEAL DIODE EXPLORATION.JPG

- - - Updated - - -

I've tried to upload both datasheets (FDD4141 and LMC6464) several times but the dismal phone cover here is making it impossible, sorry.
 

Quite unlikely that the circuit can measure Vds,on voltage drop correctly, e.g. due to OP output voltage saturation. The error of reasoning is to treat OPout,min and the wanted output signal as additive terms. OPout,min will rather clamp the output signal to a minimal value. The output comes out of saturation if the wanted output signals plus/minus offset errors exceeds the limit.

In a real circuit resistor mismatch will be a major source of additional output offset unless you use very accurate respectively exactly matched resistors.

In simulation, it's however quite easy to measure Vin-Vout directly. Even if your objective is evaluation of differential amplifier, you should know the real input signal for comparison.
 
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    d123

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Hi,

Thanks.

Quickly, diff amp just in simulation to try to measure PMOS voltage drop and estimate RDSon from there. Had diff amp schematic from other circuit with gain of 25 where resistor matching and diff measurement output voltage precision are not vital. It's the PMOS I'm interested in in this simulation but the diff amp result had me puzzled. Am doing sim as am looking for Schottky substitute with lower Vforward and nowhere near as much leakage and saw 'ideal diode' using MOS, seems a suitable substitute as a switch rather than a passive part for blocking reverse voltages on a circuit with three different supply voltages.

I understand that besides doing the maths, replacing a transistor switch with a resistor and obtaining the same Vout and Iout is a valid way of getting an idea of simulated, idealized RDSon?

And, sorry for my ignorance here, I read a little about OA saturation before replying and I still don't understand that part of your answer. Could you explain it in other terms, please? Is sensing 12V - 11V on a 12V supply the reason for the incorrect measurement (OA datasheet says input can go beyond supply rails but isn't fully rail-to-rail out) or the 33mV difference the reason or something else?

Thanks.
 

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