Tool/software: WEBENCH® Design Tools
hi,
i attached here the next link:
https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=7937D62F4F500AE5
the issue is there, please your help.
thanks,
itzik
Hi Itzik,
Like John has mentioned, the internal compensation limits the Cout range of this device. Webench generates the recommended output capacitance based on the datasheet equation:
Assume the following conditions based on the datasheet and your application:
deltaVout= 5% of Vout = 0.25V
Istep = 9.8A
ESR= 3 mOhms
Fsw= 350kHz
Then Cout > (9.8) / ((0.25 - 9.8*0.003)*(350000/5)) = 634uF. This is the effective output capacitance with DC bias into consideration and the reason why Webench recommends 3 x 270uF.
Regards,
Jimmy
Hi Itzik,
Webench requested the capacitors have a Irms rating of 5.8A because of the worst case ripple current condition.
Equation 14 shown below:
The ripple current out of the inductor must be within the Irms limits of the downstream output capacitor.
Your application uses the following conditions:
Vin = 28V
Vout = 5V
D = 5/28
L = 2.2uH
Fsw = 350kHz
deltaIL = (28-5)*(5/28) / (2.2e-6)*(3500e3) = 5.33A
This is the reason why the tool chose the capacitor with Irms rating of 5.8A.
Regards,
Jimmy
