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LM53603-Q1: The device can't start when the load is up to 1.4A.

Part Number: LM53603-Q1

Dear all

When I test the load capacity of the device, there is a question about it, as below, my design is according to EVM, Vout set 5V.

1. When my load is 1.4A, then Vin power on, but there is not any output, Why ? the below waveform is SW and Vin pins(blue is Vin, yellow is SW)

2. When Vin power on, then put on my load 1.4A, the device is normal , the output could be 5V.

3. When my load is below 1.2A, the device is normal, the output could be 5V.

  • Hi Went,

    Thanks for your enquiry. Please forward the schematic an layout for review.

  • Dear Tim

    Thanks a lot. Below is my sch and PCB, please check it.

  • Dear Tim

    Is there any reply ?

    It is very urgency thing for my customer, please help priority to do it. Thanks a lot.

  • Went,

    Note that if an electronic load is connected to the output, it may draw a transient current pulse during startup that may engage current limit. Can you zoom in to see the SW node waveform, Vin and Vout (e.g. 2us/div). Also, I recommend probing the inductor current.

  • Dear Tim
    1. Have you check the EVM ? Is it the same phenomenon ?
    2. In your opinion, it cause by current limit, does it mean the current of my inductor is too small ? but I use 4A current of inductor , could you share me how much current of inductor is fit to LM53603-Q1 ?
  • Dear Tim

    I had test start by load of 2Ω cement resistance, it can start in normal.

    But in my board, I had test start with 4G module, it cann't start at sometimes.

    I want to check below two questions:

    1. If my load with capacitance is too heavy, it will be not start at sometimes, is it normal ? Like my test results ?

    2. Is there any measure that can avoid to start fail ?

    3. In the datasheet, as below picture, Fs=2.1MHz,64 clock,64*1/2.1M = 30us,that means the time of overcurrent more than 30us, the device cann't start ?

  • Hello Went,

    Tim has asked me to help since I am more familiar with this device.

    It is true that if the part is run in current limit for more than 30 – 60 us, the part will repeatedly try to restart. The reason that the time I list is longer than your calculation is that typically frequency is reduced in current limit. If the part is running in current limit, the part should repeatedly try to restart, see figure 10 and 11 in the datasheet. Also note that the 30-60 us current limit duration starts after the preprogrammed soft start time, TSS in the EC table, during each startup attempt. If the part is not trying to restart, something else is happening.

    Regards, Robert
  • Dear Robert

    I see, thanks a lot.