LTM8052 soft start time

Hello Michael,

The voltage across the soft-start capacitor by the end of the soft-start process is 3.3V typical.

Unfortunately, there is no fixed formula or equation to calculate the actual Vout soft-start ramp time of LTM8052 in the datasheet.

You could download a free version of LTspice software from the company website and the LTspice demo board circuit of LTM8052_DC1866A,

do a quick simulation of the demo board LTspice circuit. You can tweak the values of the soft-start capacitor, probe the soft-start pin voltage and

Vout voltage waveforms to see the soft-start ramp time of Vout.

The LTspice model of this part can accurately estimate Vout soft-start ramp time. I already bench verified the LTspice model 's

estimated Vout soft-start ramp time with several values of the soft-start capacitor, the result between bench test vs. simulation is very

accurate. Therefore you can use LTspice demo board circuit as a quick reference for your design with confidence.

Below is the web link to download LTspice software and LTspice demo board circuit:

http://www.analog.com/en/design-center/evaluation-hardware-and-software/lt-spice-demo-circuits.html

Thanks,

Best Regards,

Gina.

Hi Gina,

thank you for your reply. Sorry, it is only now that I noticed it. 

If the capacitor is charged to 3.3V by 11 uA current, isn't it straightforward to calculate the soft-start time? 

Tss = (3.3V/11uA)*C = 0.3 MOhm * C, or numerically Tss [sec] = 0.3*C [uF].

Thus, a 0.1 uF capacitor results in 30 ms soft-start time, 1 uF - in 300 ms, and so on. Correct?

Hello Michael,

C*deltaV=I*deltaT equation is always correct to calculate the soft-start ramp time of SS pin voltage with a known value of Css, Iss and Vss. However, unlike other conventional voltage regulators, the LTM8052 also utilizes the SS function to control other internal parameters of the part. Therefore, if you run the LTspice model of the part or get a demo board and probe SS pin and Vout pin of the part with a known value of soft start cap (Css) during start up test, you will see Vout soft-start ramp time is indirectly related to SS pin ramp time.

Thanks,

Best Regards,

Gina.

Hello Gina,

thank you very much for the explanations.

Best regards,

Michael