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LM3478: Inrush current issue at the start-up

Ishan Panchal
Intellectual 640 points
Part Number: LM3478
Other Parts Discussed in Thread: LM5155, LM51551

Hi There,

I am using LM3478MAX in a Sepic topology for a buck-boot operation. I have followed the Webench design guide. The issue is when I try to power ON the circuit it loads down the bench top DC power supply. I observed that the inrush current is over 2Amps at the start-up and due to this the bench top supply is going into over current mode and shutting down. Please note that there is no load on the 24V output in our application. It will come in picture as and when required. For testing, I tried providing 150mA steady load (resistor) but still its not working.

Can you please let me how can I over come this inrush issue at the bootup ?

Attached is the snapshot of circuit on board and also the webench design. 

Thanks..!!!

7457.WBDesign44.pdf

  • 0
    Intellectual 640 points

    Hi There,

    Please see some snaps from oscilloscope. Names of images gives the info about the waveforms.

    Thanks..!!!

    Boost Converter Debug.zip

  • 0 in reply to Ishan Panchal
    TI__Mastermind 20560 points

    Hi Ishan,

    Thanks for using the e2e forum.
    The LM3478 device has an internal softstart time of 4ms.
    As this feature is implemented internally, it is not possible to modify this softstart with a slower or faster ramp.
    Because of this, it is very difficult to further reduce the inrush current with this device.

    Just for example, newer devices like LM5155 have an external SS pin, which enables the setting of different softstart slopes, as well as external circuits like bipolar transistors to pull low the COMP pin to further reduce inrush currents. (This can been in this app note: https://www.ti.com/lit/an/snvaa44/snvaa44.pdf )

    However, this is not possible for the LM3478 device.
    An alternative solution would be choosing a larger sense resistor to trigger the overcurrent limit and shut down the gate signal this way, but this may lead to hiccup behavior and unsteady ramp-up of the output voltage.

    Depending on how far the design progress already went, you could consider changing to another device with more softstart features like LM5155. A SEPIC EVM is already available for this topology:
    https://www.ti.com/tool/LM5155EVM-SEPIC

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 640 points

    Hi Niklas,

    Thanks for the detailed response..!!!

    The chip is already on the PCB and everything is working fine except this one. But anyways I am ready to re-spin the board to get the required output and operation. 

    I just want to confirm that we will be using this Sepic converter circuitry to power up two solenoid valves rated at 250 ma and 30 ma and these are controlled by a Mosfet. Therefore at the start-up, there won't be any load at the output of LM5155. Would you recommend adding any load resistor at output or its fine ?

    Also I saw that there is LM51551 part which has "Hiccup Mode Overload Protection". Based on my application to run the solenoids, would you recommend LM5155 or LM51551. 

    Please let me know.

    Thanks..!!!

  • +1 in reply to Ishan Panchal
    TI__Mastermind 20560 points

    Hi Ishan,

    Thanks for the update.
    Neither the LM3478, nor the LM5155 should have any problems with light load operation. If the on-time gets too short due to light load, both devices will start to skip pulses to avoid the output from running away. This will lead to larger output voltage ripple, but there is no risk of operation failures or overvoltages at the output.

    The hiccup mode feature of LM51551 is optional. If customer prefers to have additional protection features in case of e.g. output short circuits, this feature is helpful in avoiding overcurrents and device damage.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Intellectual 640 points

    Hi Niklas,

    Thanks for the kind response..!!!

    I have a couple of questions, I am going for a 4.7uf cap on SS pin to get a soft start of 500msec. 

    • Is there any max capacitance limit on SS pin ?
    • Will this affect the RT pin bias resistor ?
    • What is the value of fRT (typical) ?
    • Would it be better to go with the high switching frequency with 500msec of soft start ?

    Please let me know.

    Thanks..!!!