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Original question:

TPS92200: Deviation of max Duty

TPS92200: Criteria for chosing large inductance Value

Yutaka Horie
Prodigy 20 points
Part Number: TPS92200


Tool/software:

Dear TI engineers,

I would like to know how much large inductance I can use for stable operation.
For example, if Vin = 10.8V-13.2V, Vout = 8V to 10V and ILED=500mA, I would like to know the maximum value of inductance for stable operation to suppress LED ripple current.
If I use 33uH, does DC/DC converter will work well?
I know KIND might be little bit small in above case, but I think there is a chance to work well inspite of some undesirable jitter caused by small VFB ripple.
If there is some critical criteria for chosing large inductance, would you kindly let me know it?

Thanks and best regards,
YTK

  • 0
    TI__Intellectual 1860 points

    Hi Yutaka, 

    Glad to help you here!

    If you want to reduce LED current jitter, I recommend to increase COUT and add capacitor across LED. 

    Could you please share your end equipment application? 

    Thanks! 

    Best, 

    John. 

  • 0 in reply to John Shen
    Prodigy 20 points

    Dear John,

    Thanks for your quick reply, I really appreciate it.
    I can understand increasing Cout can make LED current ripple reduced.
    Actually, we are considering increasing the inductance value in order to improve DC/DC efficiency(not only for suppressing LED current ripple).
    So, I would like to know how much large inductance I can chose for stable operation as I described in first comment.

    Thaks and best regards,
    YTK

  • 0 in reply to Yutaka Horie
    TI__Intellectual 1860 points

    Hi Yutaka, 

    Got your point! 

    Considering the peak current mode which is the control mode of TPS92200, the control loop could be stable with any inductor value. 

    But considering internal peak comparison accuracy, we do recommend keep inductor current ripple larger than 0.2A to avoid double pulse. 

    In your application, 33uH is OK enough. 

    Could you please kindly share your end equipment application? 

    Thank you! 

    Best, 

    John! 

  • 0 in reply to John Shen
    Prodigy 20 points

    Dear John,

    Thanks for your reply.
    Actually, in my case, typical value of ripple current would be  50.5mA, when Vin=12.0V, Vout=10V, Fsw=1MHz, L=33uH. This is smaller than 0.2A(would be more smaller considering variation...), but still work well?
    And is the peak current measured by the voltage of FB pin, right?
    If so, how much voltage ripple which is equal to current ripple 0.2A?
    If this IC needs some voltage ripple of VFB pin for stable operation (stable without double pluse), would you kindly let me know about it?

    Thanks and best regards,
    YTK

  • 0 in reply to Yutaka Horie
    TI__Intellectual 1860 points

    Hi Yutaka, 

    1. Inductor current ripple only affect the peak current control loop's peak comparison, which is the HSD FET turning-off point, instead of buck loop stability. 
    2. FB voltage only shows the information of average current, which is filtered by COUT, instead of inductor current information. FB do not need to see any voltage ripple. 

    Inductor value will only affect '1', instead of '2'. Inductor peak-to-peak can be calculated by VIN, VOUT, inductance and switching frequency, as you calculated. If considering efficiency, the inductor current ripple will be around 70mA I think. (Sorry for the last reply. I mis-calculated the ripple current). It may affect '1' and make peak point vary cycle-by-cycle and cause double pulse on inductor current. I will recommend to use a 15uH inductor. 

    Thanks! 

    Best, 

    John. 

  • 0 in reply to John Shen
    Prodigy 20 points

    Dear John,

    Thanks for your quick reply and detail explanation, I got it.
    I will consider to use 15uH for this design.

    Thanks and best regards,
    YTK