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LM3481 SEPIC Inductors

Bruno Andrade
Intellectual 530 points
Other Parts Discussed in Thread: LM3481, LM5118, LM25118

Hi,

I'm making a SEPIC regulator with LM3481 and I have some questions about minimum load and inductors.

The parameters of the converter:

Vin: 5-30v

Vout: 18v

Load: 3mA to 300mA

I'm trying to find the right inductor values, since I don't want to see any OVP issue this time when the load is 3mA.

The circuit:

Acording to the datasheet on page 22, I found L2 and L1 values as high as 3mH and I found it too high! don't know if it is right.

I'm using a frequency of ~920KHz.

Someone could help me to find the right values of these inductors? Most of the time this circuit will have a load of just 4mA, so the voltage of it must have the ripple as low as possible.

Thanks!

  • 0
    TI__Genius 15810 points

    Hello,

    The inductors would need to be very large to make sure you don't enter discontinuous conduction at light loads to avoid the part going in to the min-ton and OVP mode. I am not surprised to hear that you'd need 3mH. An alternate method to avoid this is to put in a dummy load that will just sit and burn power. This will pull more current and not let the device go in to burst mode. But the efficiency of the system will take a hit. Unfortunately, there is no easy way out of this.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Intellectual 530 points

    Hi Akshay,

    L1 > ((Vinmax - Vq) . (1 - Dmin)) / 2 . Ioutmin . Fs

    L1 > ((30 - 0.1) . (1 - 0.375)) / 2 . 0.003 . 920000

    L1 > 0.0038 = 3.38mH

    L2 > ((Vinmax - Vq) . Dmin) / 2 . Ioutmin . Fs

    L2 > ((30 - 0.1) . 0,375) / 2. 0.003 . 920000

    L2 > 0.00203 > 2.03mH

    Are these inductance values right? Or am I mistaken some calculation?
    Further I will calculate the current values.

    Thanks!

  • 0 in reply to Bruno Andrade
    TI__Genius 15810 points

    Hello Bruno,

    You should be using the minimum value of Vin and the worst case duty cycle at min Vin. Also, if you are using separate inductors, I'd suggest that you make the secondary inductor twice the primary. This helps stabilizing the voltage across the SEPIC cap when you have a transient. By the way, if you want another easy calculation, just visit coilcraft.com and use their SEPIC power inductor calculator tool. They will even suggest actual parts to match your requirements. I hope this helps.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Intellectual 530 points

    Hello Akshay,

    Thanks for the hints. I am also considering using the buck-boost LM25118/LM5118 instead of LM3481, since it needs just one inductor, this helps to save a lot of space on the pcb.

    Beyond the large inductor to guarantee the CCM mode in light loads, Is there any special parameter to consider if I change to 5118?

  • 0 in reply to Bruno Andrade
    TI__Genius 15810 points

    Hello Bruno,

    The LM5118 is also a controller and for 3mA you'll still need a large inductor for forced PWM. Otherwise it should work alright.

    Regards,
    Akshay