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LM25011 RT Calculation

Other Parts Discussed in Thread: LM25011

I'm trying to understand the thought process used to calculate RT on pg. 17 of the LM25011 datasheet. But am somewhat confused where the 152ns and 672ns numbers came from in that example.

Once I understand that thought process, I want to use what I learned to design a system with these operating conditions:

Vin = 20-30V

Vout = 14.2V

Iout (max) = 200mA

Iout (min) = 20mA

I also want to take advantage of the current limit feature and limit Iout to 200mA

  • This will be answered shortly.

  • Hello Xavier,

    The RT resistor equation as used in the page 17 and page 11 of the DS can be basically derived from the TON timer equation ( Eq. 5 in page 11). The TON timer is the fundamental equation which describes in the internal TON timer block inside the IC. As explained in the datasheet, in a constant on-time control scheme, the ON-time is programmed to be inversely proportional to input voltage. That's the basic premise.

    The basic on-timer block in the LM25011 is shown above.

    The product of Ri, Gm, Ci and Vref will yield you the constant (~4x10^-11) in the TON-timer equation. The additional 15ns added in the equation accounts for internal transistor-level design delay.

    Once you have determined your required switching frequency for your application, you can determine the RON as shown in the application example in the DS (using minimum VIN). The approximate values of 152ns and the 672ns can determined by using the TON equation (in page 11 in DS) and the calculated RON resistor (118k). These values are the expected on-time for the respective input voltages and the regulated output.

    Hope this explains what you were looking for.

    Regards,

    Sourav