Looking for a small-form-factor DC/DC solution - we´ve choosen LM25010 device.
Using WEBENCH, we had the following requirements:
VinMin: 28.0V
VinMax: 33.0V
Vout = 5.25V
Iout = 1.0A
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WEBENCH suggested a value of 33.2kOhms for Ron, 22.0uH coil .....
After analyzing the problems seen with our prototypes:
unstable startup, Vin dependent, temperature dependent we came to the conclusion that the value suggested for Ron gives device dependent a switching frequency of more than 1 MHz - which is definitely at the edge.
Using Formula (2) on page 8 of the datasheet with
ron = 33.2k, Vin=30V, Vout = 5.25V, I get 1.2MHz switching frequency.
So this is definitely below the 1MHz specified in the datasheet.
Reducing, VIN the problem disappears / switching frequency lowers.
Is there some problem with the WEBENCH models for these part ?
The suggested Cff with 270pf seems to be another critical part.
Increasing Ron to 68k - we have stable performance @580kHz switching frequency.
rgds.
Wolfgang Gaerber
