MPM3650C SoftStart

[Masahiro.A.Koike]

Hi, I’m designing a circuit using the MPM3650C.

Regarding the soft-start function, the relationship between the Vout rise time (tss) and the Css capacitor is defined by equation (1) in the datasheet

It is understood that the internal power IC includes a built-in 22nF Css capacitor, and when the SS pin is left open, the tss can be calculated as:

tss = (0.6V * 22nF) / (0.83 * 6μA) ≈ 2.65ms.

However, after performing a simulation using MPSmart, the result shown in Figure A was obtained when the SS pin was left open, and it appears that the soft-start function is not operating correctly.

On the other hand, the result in Figure B, where an external 22nF Css capacitor was added, shows the Vout rising with a tss equivalent to the calculated value assuming Css = 22nF.

My question is: when calculating tss from equation (1) based on Css, should the Css capacitance used in the calculation exclude the internal 22nF capacitor and only consider the external capacitor connected to the SS pin?

Thank you for your support.

[Krishan.FAE]

It seems that the MPSmart model does not include the internal 22nF capacitor as verified by the simulations that you have made.

[Krishan.FAE]

Also, when calculating with the formula as shown:

There is already some associated tss that you have calculated to be 2.65ms with the 22nF soft start capacitor. To my knowledge, you would need to take this capacitor into account when calculating for soft start. You should be able to verify this with this simulation model and hardware.

Hopefully this provides some insight.

Best,

Krishan

[Masahiro.A.Koike]

Hi Krishan, Thank you for your answer.
I understand that it seems the internal 22uF capacitor is not included.

Thank you for also explaining the equations.

Regards,
Masahiro

[Krishan.FAE]

No problem Masahiro-san,

Because the primary inquiry on this post has been answered, it shall now be closed.

Thank you for continuing to use our modules.

Best,

Krishan