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LM3478: Inrush current limiting like described in Application Report SNVA742

Part Number: LM3478
Other Parts Discussed in Thread: LM2588, LM5060

Dear sirs,

in in the bringup phase of a design, where we use the LM3478 to provide a stabilized 24V supply.

When we started analyzing the behaviour of the LM3478 we where shocked about the start-up current

which goes down to some mA after that phase.

The inrush-current is nearly 4A at 24V which makes it a problem to use cheap external power-supplies.

Now my question: Is it possible to add an external soft-start circuit as done in the Application Report SNVA742 for the LM2588

(Transistor from the second inductor to the compensation input).

As far as I have seen it, there is also no possibility in WEBENCH Designer to define and simulate such a circuitry - is this true?

Best regards,

Bernhard

  • Hello,

    It is possible to add the external soft start functionality to the LM3478. However this is not available in WEBENCH.

    In a boost converter there are two different types of inrush current.
    1. Initial turn on
    - The output capacitor is at 0V. When input voltage is applied the output capacitor will charge to VIN-Vdiode. This current is inherently uncontrolled.
    -Adding and external soft start circuit will not help with this inrush current. To get around this something like a hotswap (LM5060) would be needed.
    2. Switching Inrush
    - Charging the output capacitor from VIN to VOUT.
    -This can be controlled with the external soft-start circuit.

    Thanks,

    Garrett