I did a WebBench design for the TPS40170 Controller with 21 to 52 VDC in and Vout = 5V at 0 to 3.5A. The WebBench design generated a soft-start capacitor (Css) of 3900 pF (connected to the SS pin, pin 5 to GND).
However, according to the design equations in the datasheet, this will give a startup time of approximately 0.35 mS (Tss = Css*0.09, where Css is expressed in nF and Tss is expressed in mSec).
The recommended startup time is 4 mS.
My output capacitor is 220 uF.
The 0.35mS startup time results in a load startup current (assuming a linear ramp) of I = CV/t = 0.00022*5.0/0.00035 = 3.14 amps.
However, if the startup time is set for 4 mS (using a 0.047 uF cap for Css instead of 3900 pF), then the load startup current is I = CV/t = 0.00022*5/0.004 = 275 mA. This is far better than 3.14 amps!
Can someone please verify this for me? I will be happy to share my WebBench design details as well as the design I worked through using the equations in the datasheet.
I can be reached at htaylor@comverge.com
Harold Taylor