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LM76003-Q1: How to reduce the startup time

Sherry Liang
Genius 10255 points
Part Number: LM76003-Q1

Dear team,

My customer wants to use our device to achieve step down function from 24V to 12V, their application is as below,

our device have soft start function, and the soft start time is at least 6ms which is too long for my customer. Do we have any solution to reduce the startup time?

If our device is always on, and the trigger signal connects to the LM76003-Q1's FB pin. Is this solution feasible? Does this solution have any bad impact to our device?

Thanks & Best Regards,

Sherry

  • 0
    TI__Mastermind 25590 points

    Sherry,

    Yes, your solution will work to reduce the start time. Turn on the part and hold FB high (above 1.017V (max FB ref) but less than 4.5V (recommended max operating voltage of FB pin)). Release FB (high-Z) to start the part with a fast start. Ensure this high-Z is high enough impedance to not affect operation of the LM76003-Q1.

    The only issue I see would be potential output overshoot or too much inrush current - essentially just the reasons we implemented soft start to begin with. Other than that, it looks like a good solution.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Genius 10255 points

    Hi Sam,

    Thanks for your reply!

    This device can't remove the soft start function. Is there any special reason? Why do we set a 6ms as the minimum value? I just want to learn more about it, could you please help explain them to me?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Samuel Jaffe
    TI__Genius 10255 points

    Hi Sam,

    1. Css can change the soft start time tss. Can I reduce the time to a value less than 6.3ms by adjusting the capacitance?

    2. About my solution, I have a few questions. The solution is as below,

    Phase 1:  Q1 is on. FB pin is high voltage. At first, because the device is always on, so the VA is high voltage. But the VA will reduce to 0 later

    Phase 2: Q1 is off. VA=0V,  at this time, the device will still start up through soft start, right? If so, the startup time is still 6.3ms.

    Is there any problem about my circuit? If this circuit is wrong, could you please help draw a correct circuit  to me?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 25590 points

    Sherry,

    The soft start feature is set at ~6ms because that's what we see customers typically requiring. The internal soft start means they do not have to buy and place a capacitor on SS. So the feature and spec are designed to address the majority of needs.

    1. You can only add to the soft start time by adding capacitance. You cannot reduce the soft start time by changing the capacitance on SS.

    2. Phase 1: Yes, good.

    2. Phase 2: No, at this time the part has been on for >6.3ms. The internal soft start has completed and the error amplifier is comparing against the full reference. Turning off Q2 will cause FB to fall. Once it reaches the reference voltage, the IC will regulate as quickly as possible to try to keep FB at the reference voltage.

    I see a few possible issues:

    A) D1 blocks current flowing to the FB divider. This means the IC cannot see the output voltage so it cannot regulate the output. This diode will have to be replaced with a short.

    B) Q1 on will cause current to flow to VOUT. Make sure the load on VOUT does not pull FB below the reference. Size R1, R2, and the 100k resistors appropriately.

    -Sam