This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7B4253-Q1: Inrush current breaching overcurrent limit

Part Number: TPS7B4253-Q1

Hi Team,

My customer is using the TPS7B4253-Q1 to power an off-board load. We were just recently provided the load profile:

As you can see, the inrush reaches ~480mA for 150us, which I believe will trip our internal current limit of 301mA (min), 450mA (typ). However, section 7.3.6 of the datasheet mentions that the device has an incorporated soft-start feature to control the initial current through the pass element. I'd like to understand if this soft-start would be able to handle the above inrush current without tripping the overcurrent limit.

Thanks for the help!

-Andy

  • Hi Andy,

    I'm reviewing this question and will reply within 2 business days.

    Thanks,

    Stephen

  • Hi Andy,

    Upon review, I agree that this is likely to trip the current limit of the device.  However I doubt it will be a significant issue for the customer with just 150us duration, if the customer has enough capacitance on Vout to hold up the voltage during this event.  A typical current limit is 450mA as you have mentioned.  A common rule of thumb for current limit bandwidth is about 50us, so for the initial 50us the LDO may try to provide this 480mA of load.  Let's conduct a worst case analysis - the LDO reacts instantly to the load step, and current limit is 301mA.  So for 150us the output will droop as 179mA of load is unable to be sourced by the LDO.  Using the capacitor equation I = C dV/dt and assuming a minimum load capacitance of 10uF (which is the recommended minimum in the datasheet), the worst case droop is 2.685V, which is likely unacceptable.  However if we increase Cout to be 440uF, the droop is just 61mV. 

    If we use a typical current limit setting (450mA) then only 30mA is causing Vout to droop.  In that case, 450mV of droop will occur on Vout.  If Cout is increased to 440uF, using a typical current limit of 450mA, the droop will be just 10mV.

    Thanks,

    Stephen