LM5141: how to calculate the crossover frequency?

user5859485

Part Number: LM5141

hello :

Now, i have a question about LM5141 of crossover frequency,following detail will show it.

question:

Base on the " how to determine bandwidth form the transient response..."(author is Mr. Sheehan Bob), i test the transient response of LM5141. According to the formula i have differnet result.

Vp=1/(4*fc) equation one

Vp=∆I/(2*3.1415*fc*Cout) equation two.

note: Vin=12V,Vout=5V,∆I=10A(IA(10A)-IB(0A)=10A) ,Vp=80mv, Cout=1.15mF, tp=64us, current slope = 1A/us

LM5141 is peak current mode, but i have diffrent result between equation one and equation two.Then i set the ∆I is 5A ,the result is same as 10A.

Why have diffrent between one and two? which one is ture? and i am intereted in how to derive the formula!

Thank you very much!

following picture refer to article form Bob:

over 6 years ago

0 Robert Blattner over 6 years ago

TI__Expert 6245 points

Hello user5859485,

I have checked the blog named “How to determine bandwidth from the transient-response measurement” on the TI web and found it much easier to understand than the one you found. Note that this artical should show up as in the “Power House” blog.

In the version that I found, Equation 4 is tp=1/(4*fc) rather than Vp=1/(4*fc). This makes more sense since the bandwidth is assumed to be constant, not the response to stimulus. We would expect the timing of the peak response to be constant despite step size.

Equation 6 is the equivalent of equation 2 in the article that you found. It is Vp=deltaI/(e*pi*fc*Cout). Note that e (Euler's number) replaces 2 if a current mode DC/DC is critically damped. This number should scale with the size of the load step.

Note that both of these equations assume that the load step is instant, there is no ESR, and that the duty factor does not saturate (reach 0% or 100%).

Regards, Robert

0 user5859485 over 6 years ago in reply to Robert Blattner

Prodigy 230 points

hi Robert:
Thank you for your reply!
the equation 4 is tp=1/(4*fc), yes,i wrote it wrong. i have test the circuit
i have different result between tp=1/(4*fc) and Vp=deltaI/(e*pi*fc*Cout). fc=3.9KHz(Equation 4 ) fc=12KHz(Equation 6 ), i want to comfirm the difference points, and you said "This number should scale with the size of the load step." how to understand it. i also want to know which formula is ture when i have two diffient result. i am confused.
The worse case is that current from 0 percent to 100percent. so i also want to know whether the crossover frequency is related to load current! form the formula i think that the high deltal current will create the high voltage drop ,thus the result is constant.
thank you very much!