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LMR14006: Selection of L

na na78
Guru 16770 points
Part Number: LMR14006

Hi

Our customer has following requirement. 

Device : 14006X (Fsw=1.1MHz)

Vin: 13V-30V

Vout:12V

Iout:0.1A

We are considering following way to select inductance.   Could you please review it?

1.

Supposing Iout=0.6A with Vin(max)=30V, Vout=12V and Kind=0.3.

2.

As a result, L=36uH can be obtained.

3.

If L=36uH, ripple current would be 0.18A.

4.

If ripple current is 0.18A, the device should operate in CCM even if Iout is required 0.1A.

5.

However, if Vin = 13V ripple current would be 23mA. 

I concern for small ripple current and phase margin because of selecting 36uH.

Is this way reasonable for selecting inductance?

If it is not proper L,  can you provide proper L?

BestRegards

  • 0
    TI__Mastermind 27045 points
    Hi,

    Is actual load current 0.1A? why you calculate inductor with Iout=0.6A? 68uH is suggested if load current round 100mA.
  • 0 in reply to Andy Chen
    Guru 16770 points
    Hi Andy

    Thank you for your reply.

    >Is actual load current 0.1A? why you calculate inductor with Iout=0.6A?
    Actual load is 0.1A. The reason why I calculate L with 0.6A is L_min is obtained from equation (1) in datasheet.

    Lo(min) = (Vin_max - Vout)/(Iout * Kind) * Vout/(Vin_max * Fsw) = 218uH.
    If Iout = 0.1A is assumed, L_min is 218uH. It is too large inductance.
    We want to select smaller one.

    You suggested 68uH but I could not see the way to get this L value.
    Could you please show us how to calculate 68uH for 0.1A load current?


    BestRegards
  • 0 in reply to na na78
    Guru 16770 points
    I appreciate if you could reply.


    BestRegards
  • 0 in reply to na na78
    TI__Mastermind 27045 points
    Hi

    Current mode control require proper inductor ripple current for stable operation, 100mA peak-peak is normally the minimum ripple current for most case. so Kind=1, then calculated inductor is 65uH.
  • 0 in reply to Andy Chen
    Guru 16770 points
    Hi Andy

    Thank you for your reply.

    >Current mode control require proper inductor ripple current for stable operation, 100mA peak-peak is normally the minimum ripple current for most case.

    Certainly, 100mApp current ripple could be obtained with Vin = 30V.
    However, ripple would be ~12mApp when Vin = 13V and L=65uH.
    Is this current ripple supported by device?

    BestRegards
  • 0 in reply to na na78
    TI__Mastermind 27045 points
    Hi

    Chip will works on low drop-out mode for Vin=13V to 12Vout, actual Fsw is below 1.1MHz. the minimum ripple current is about 20mA at Vin=14V, this is OK for stable operation.