TPS25947EVM: TPS25947 – Inrush current question

Hi Gary,

The device is hitting thermal shutdown because the power dissipation across it is causing the thermal shutdown time to be less than charge time. 

TPS25947  uses a thermal shutdown mechanism to protect itself from damage at all times. This behavior is characterized on the EVM and will be available in datasheet with part release.   As you can see, the higher the power dissipation inside the part, the faster is the time to shutdown.

    The device follows two different curves in the startup phase. The curve on the left shows the time taken to thermal shutdown till the VIN-VOUT is higher than ~6V. This is the most stressful part of the device startup as the effect of any load current is amplified by the VIN-VOUT difference, causing the part to hit thermal shutdown faster, as compared to during steady state, where the part can carry much higher currents without hitting thermal shutdown, as shown in the figure on the right.

We can calculate the average power consumption (combination of capacitive and resistive load) and then estimate the time to thermal shutdown using the above curves. As long as the time to charge up the output cap fully is lower than the estimated time to thermal shutdown, the system will startup successfully.  We have also developed a design calculator available on web which tries to model this behavior and predict whether the system will hit thermal shutdown with the given supply voltage and load combination. 

When there is large load present during startup other than capacitive load try reducing the CDVDT cap, load value  to achieve successful startup.

Regards

Kunal Goel