Other Parts Discussed in Thread: CSD18543Q3A
Hello there,
I am designing a buck converter based on LM25145 IC from your offer. Design inputs are as follows:
a) Input voltage range: 6 -42 V DC
b) Output current: 6A
c) Switching frequency: 0.8MHz
d) Allowed inductor ripple: 31% (peak to peak)
e) Inductor value: 2u2
f) M2 (Q2) synchronous transistor): CSD18543Q3A (Rdson at 25 degC and 7V5 is 8.5m Ohm)
According to the datasheet for the controller (http://www.ti.com/lit/ds/symlink/lm25145.pdf, page 24) to set current limit using MOSFET RDSon value of the resistor can be calculated from the following formula:
Rilim = (Iout - Delta IL/2) / Rdson * Rds(on) q2
With my values resistor can be calculated as: ((6-(0.31*6)/2) * 0.0085) / 0.0002 = 215 Ohm
Webench shows resistor value as 1.05k Ohm, so very far from datasheet formula result.
On page 37, in application example, calculation of current limit resistor is done using other formula, which probably looks like this:
Rilim = (Iout + delta IL ) / Rdson *Rdson
For the given example, with Iout = 20A, peak to peak current ripple being set as 30%, with 2m Ohm Rrdson MOSFET resistance and Irdson = 0.2mA current limiting resistor value can calculated as ((20+(0.3*20)) * 0.002) / 0.0002 = 260 Ohm (example has got resistor set to 249 Ohm, which is nearest in resistance range).
The same formula works well for examples on pages 41 and 44. Based on the ‘reverse engineered’ formula value of the current limiting resistor for my design shall be 334 Ohm. It is similar to calculation from the calculating spreadsheet which I found in the product folder, however, there is maximum Irdson taken into account.
Could you clarify please which of the formulas can be used for calculation of current limiting resistor and why they are such big differences between Webench, datasheet and design calculator spreadsheet, please?
Rgds,
Kris
