Guys, if I don't use pin 8 of the SG3525 (softstart function) should I leave it open or grounded?

 

Float. Grounding will sink the ref and prevent starting

 

I once saw a circuit in which there was a resistor with a value of 10K between pin 8 (softstart) and pin 16 (Vref). I spent weeks wondering why the use of this resistor, I could never understand. Does anyone have the idea why they put such a resistor?

 

In looking at the SG3525 from ST, pin 8 has an internal source, while Fairchild KA3525 does not. So I will have reverse my previous post and go with, pull high, as you have indicated you’ve observed. Pulling high will provide for either device to be used. Are there other differences?

 

You realize that you will likely get significant voltage overshoot at the output during startup.
Is that not going to be a problem?

 

In looking at the SG3525 from ST, pin 8 has an internal source, while Fairchild KA3525 does not. So I will have reverse my previous post and go with, pull high, as you have indicated you’ve observed. Pulling high will provide for either device to be used. Are there other differences?

In the KA3525 datasheet, the power supply design is not the same as the SG3525, but in the softstart specification there is a supply of 50uA in both ICs.


I've seen several circuits with a capacitor on the softstart, but I've never seen a circuit in which the IC SG3525 had a 10K resistor for pin 16 (Vref). Only the resistor without any capacitor on pin 8

 

You realize that you will likely get significant voltage overshoot at the output during startup.
Is that not going to be a problem?

I’m thinking output caps would easily absorb a hard start with ramping voltage.

 

I’m thinking output caps would easily absorb a hard start with ramping voltage.

Think again.
The overshoot is due to the output inductor-capacitor resonant surge before the loop feedback can stabilize the voltage.
Adding output capacitance will change the resonant frequency but likely have only a slight effect on the overshoot voltage.
Adding capacitance may also adversely affect the loop stability.

Why do you not want to use the soft-start function to eliminate this?