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Original question:

LMZM33606: Soft-Start time change options

LMZM33606: Soft-Start time change options

senthil kumar
Prodigy 30 points
Part Number: LMZM33606

Hi,

I'm using 30V to 5V conversion with LMZM33606.

I used SS pin connected to three different option.

                  Option 1: 5mSec,

                  Option 2: 11mSec and

                 Option 3: 16.5mSec.

when I select the capacitor to achieve 11mSec. I observed 8.5mSec as a SS. I measure the capacitor before assembly 22nF and after assembly it has become 17.7nF

with the formula 17.7nF = ~8.5mSec. which is matching with observed SS.

Can you please clarify

1. that are the possibility to reduce the capacitor value.

2. Do we have any internal capacitor which is reducing the capacitor value

  • 0
    TI__Mastermind 47060 points

    Hi,

    Looking at the functional block diagram for this device on Figure 7.2, it looks like there is a 2uA current source that charge current. It doesn't look like there are any internal soft-start capacitor here. Addition of an external soft-start cap will slow down the slew rate since the 2uA current source will need to charge up this cap. 

    Are you measuring the capacitance of 17.7nF on the board or after the Css capacitor has been removed off the board? If it is on board then perhaps you are reading a combination of the component and parasitic PCB capacitance that is lowering the cap value. 

    Additionally when MLCC capacitors experience DC bias they will slightly derate to a lower effective capacitance.

    Regards,

    Jimmy