LM25011: Calculation value Ton doesn't equal to the tested value

Hi Mark,

Can you please share all of the application conidtions? Vin, Vout, Fs, Iout? I would like to double-check the following calculations.
40% of the test value, 4.52us, results ~2.7us which is relatively close to the tested 2.89us.

Thanks,
Alejandro

Hi iAlejandro,

Vin is 34V, Vout is from 19.4v to 29.9v. Fs is from 210k to 340k. Iout is from 594 to 679mA. these are all tested value.

I can understanding if Ton is 4.52us for normal mode, it will be 2.7us for current limit mode.

While I can't understand the gap between 6.77us and 4.5us. This 6.77us is according to the formula tON =(4.1 x 10-11 x (RT + 500))/vin + 15 ns = 6.77uS. That is a big gap with tested value 4.5us.

Strange.. Can you also provide a circuit schematic? I double-checked the calculations and they are correct. Below is a summary of recommended component values for the application requirements provided. The Quick Start Calculator tool (link) was used. I would like to review the circuit and make sure it is okay.

Thanks,
Alejandro

Hi Alejandro,

The Rt value in our design is 5.6M. Not 2.115M.

If calculating with Rt = 2.115M, Vin=34V, i can get a Ton = 2.566us.

But in our application, Rt=5.6M, Vin=34V. Calculation result is 6.768us. While my test result is 2.7us. Even it is not at current limit mode, the tested value is 4.52us. This is a big gap.

Hi Mark,

There might be some confusion, I want to make sure we are on the same page. So let's begin by ensuring the following:

• Please provide a schematic for a review. We would like to make sure all is properly connected.
• With an RT of 5.6MΩ it results a fsw of 130kHz, which does not correlate to what you provided of Fs equal to 210kHz to 340kHz (that's one point of interest).
• Can you share waveform of on-time pulses along with output voltage and input voltage (preferably with cursors showing the time scale of pulses).

Thanks!
Alejandro

Hi Alejandro,

Here is a schematic.

Here is 2 scope. 

Green is SW PIN. On-time is 2.89us. Fre=210k.

Red is Vin=34.3v

Blue is Vout=19.4v

Since we are make it works at constant current mode. So the on-time is halved than that not in CC mode.

BR / Mark

Hi Mark,

Let me reach out to the team internally for further inputs. I will get back to you as soon as possible.

Thanks,
Alejandro

Hi Mark,

We have not forgotten about this phenomena. Still waiting to hear back some feedback from rest of team. I will make sure to update you as soon as possible.

Thanks for understanding! Kind regards,
Alejandro