LM51561: Way to disable current limiting

Hello Dice-K,

Thanks for reaching out to us via e2e.

The CS pin is not only used to detect an overcurrent in case of an error. It provides an essential feedback for the control mechanism.
The LM51561 uses the peak current mode control scheme, so it has an inner cycle-by-cycle current control loop plus a slower outer voltage control loop.
It cannot work properly if the current control loop is disabled.

Best regards,
Harry

Hi Harry,
Thank you for your reply.

Are the following ideas correct?
・There is no way to disable current limiting.
・Reducing the sense resistor RS raises the threshold, making it less likely that the current will be limited.

Best regards,
Dice-K

Hello Dice-K,

Your wording (e.g. less likely) still sounds as if the current limit was some kind of exceptional event, which might sometimes happen, but usually not.

This is not the case.
The current feedback is used at every cycle to determine how long the FET has to stay on during this cycle.
It is the fundamental inner control loop of this part.

Therefore, the sense resistor has to be calculated properly to configure the maximum value of the (triangular) current ripple that will flow through the inductor.
This depends on your input- and output-parameters, the chosen inductor, etc.

Calculations are shown in this document:
https://www.ti.com/lit/an/snva941/snva941.pdf?ts=1659424858468&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5156H

I hope this helps to clarify your questions.

Best regards,
Harry

Hi Harry,
Thank you for your reply.

I am reading the below document for flyback.
https://www.ti.com/jp/lit/an/snva866/snva866.pdf

I think that RS is calculated by equation (11).

I would like to confirm whether it is okay to reduce RS in order to give some margin to the current limit.

Best regards,
Dice-K