LTC3811 power stage transconductance - exponential behaviour

Hello Tom,

The power stage transconductance vs sense resistor value curve is equal to the slope of the current sense threshold vs COMP voltage curve (page 9 of the D/S, first plot, top row) divided by the value of the current sense resistor. Therefore, the curve should be of the form A/x as you stated, not an exponential curve. I do not have an explanation for the shape of your curve. You can send me the simulation to my email address for further analysis if you wish.

Best regards,

Mike

Hi Mike,

Thank you, that would be very helpful. What's you email address so I can send you the simulation?

Thanks,

Tom

Hello Tom,

My email address is:

Michael.shriver@analog.com

Thank you,

Mike

Hi Mike,

I think I've tracked down the issue. I was including some series inductance in my sense resistor in an attempt to make the simulation more realistic (and model the behavior of the sense filter properly). Turns out LTSpice gives all inductors a 1mR ESR by default. So my 1mR sense resistors were actually becoming 2mR, 2mR was becoming 3mR etc. When I manually set the ESR to 0R I get the expected response.

Thanks,

Tom

Hi Mike,

I think I've tracked down the issue. I was including some series inductance in my sense resistor in an attempt to make the simulation more realistic (and model the behavior of the sense filter properly). Turns out LTSpice gives all inductors a 1mR ESR by default. So my 1mR sense resistors were actually becoming 2mR, 2mR was becoming 3mR etc. When I manually set the ESR to 0R I get the expected response.

Thanks,

Tom

Hello Tom,

Thank you for letting me know. Your explanation makes sense. Overlooking the 1mOhm default DCR is easy to do when using a discrete inductor to model the sense resistor's ESL. It has happened before.

Best regards,

Mike