Hello,
I have this application of dynamically adjustable output voltage with the following specs:
- Vout=16 to 32V, Vin_min=34.155, Vin_nom=34.5, Vin_max=34.845, Iout_max=3.3A, Iout_min=50mA
- minimum ripple voltage
I selected LM25085. I downloaded the Excel calculator and I also formulated my own Excel based on the formulas given in the Datasheet. To validate my Excel, I used the application example in the datasheet (Vin=7 to 42V, Vout=5V, time delay PFET=57ns, etc...) I plugged in the values and both Excel outputted the same Rt=90.9K as expected.
But when I now calculate the L1, there are now disparities in computed values. The example given expects L1=13.5uH which is the same as my own excel outputted, but the Quick Excel Calculator is outputting a different value which is L1=16.2uH minimum. I then tried to just vary the Iout_max and the L1 value also varies. This is unexpected because based on L1 formula (eq.18), it is only dependent on Ton_min, Vin_max, Vout and Iout_min. Am I missing something here?
I went ahead with my computation using my Excel formula and results below:
Vout=16 to 32V, Vin_min=34.155, Vin_nom=34.5, Vin_max=34.845, Iout_max=3.3A, Iout_min=50mA, PFET delay=290ns, Fs=500KHz
Rt=342.8K
Ton_min=1.544uS @ PGATE
Ton_min=1.83uS @SW Node
Then at Vout_min=16V (same Rt), Fs=250.67KHz
Finally, L1 = 52uH @ 32V
L1=345.7uH @ 16V
Using Quick Calculator, L1=7.9uH! @ 32V