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inductor for LP8861

Atsushi Yamauchi
Genius 4810 points


I would like to confirm the inductor for LP8861.

In the datasheet, there is Table 2 for the inductor/capacitor selection.

For this table, did you use the current value? And did you use current ripple 30% of maximum current?

 In the case of small current (ex. 80mA/string), the ripple is big with table 2.

So I would like to confirm wheteher we can use bigger inductance for lower ripple current than it using table 2.

Best regards,

Atsushi Yamauchi

  • 0
    TI__Intellectual 1235 points
    Hi Atsushi,

    Thanks for your inquire. Table 2 are the recommended passive values (Inductance, Cin, and Cout) intended to yield good stability and inductor ripple performance. Accordingly, passive values (Table 2) should be selected based on the operating switching frequency. The recommended selection of inductance/Cout according to Table 2 is focused on good stability. Recommended values of Cin (Table 2) is mainly intended to improve inductor ripple. Different conversion ratios, loads, # of LEDs, and # of strings have been captured on the recommended passive values (Table 2). However, there are limitations depending on the conditions you are targeting.

    For example, when selecting the inductor it should also be checked that the inductor peak current doesn't exceed the minimum switching current limit of the device (1.8A min and 2.0A typ). During transients there is some flexibility (refer to note right after Table 2). This should be checked regardless of what is recommended in Table 2. Usually selecting passive values other than what is recommended in Table 2 in order to lower the current peak current is not recommended. However, there are exceptions and analysis could be done on case basis.
    You said in your case ripple is big. Can you give numerical values about how big? What conditions are you using?

    I hope this helps.

    Regards,

    David
  • 0 in reply to David Diaz Quintana1
    Genius 4810 points

    Dear David-san,

    Thank you for your response.

    Our customer sets the current value 80mA/strings and SW frequency 450kHz.

    And generally the ripple ratio is about 30%.

    In this case, can they use the inductance value of table 2?

    Best regards,

    Atsushi Yamauchi

  • 0 in reply to Atsushi Yamauchi
    TI__Intellectual 1235 points
    Atsushi-san,

    Thank you for the info. Initially it is recommended that you use 33uH based on the SW frequency you are using (assuming inductor peak current doesn’t exceed min current limit). But since you are on the higher side of the frequency range you could be able to use 15uH depending on the other conditions. Could you also provide values you are using for Vin, Vout, # of LEDs, and # of strings? Also, could you provide the configuration (values, type of caps [ceramic vs electrolytic], and voltage rating) you are using for Cin adn Cout? This information will be helpful to understand your case and help to decide what inductor to use.

    Kind Regards,

    David
  • 0 in reply to David Diaz Quintana1
    Genius 4810 points

    Dear David-san,

    Thank you for your help.

    I have confirmed the customer's condition.

    - Vin: 13.5V typ. (6V to 16V)

    - LED: 6 series 3 strings, VLED: typ. 18.3V (16.2V to 20.4V)

              80mA/string

     -Cin: ceramic 10uF * 2 and electrolytic 47uF

     -Cout: ceramic 10uF * 2 and electrolytic 47uF

    Could you please advice the conclusion of the inductance?

    And in this case, could you please give the comment for the ripple current?

    Best regards,

    Atsushi Yamauchi

  • 0 in reply to Atsushi Yamauchi
    TI__Intellectual 1235 points

    Atsushi-san,

    Thank you for providing the additional information. Based on the conditions you are using 33uH and very likely 15uH (I will confirm this last one later) should provide the appropriated performance from stability stand point and keeping coil peak current within current limit capability of the device. Also, looking at your Cin configuration I wouldn't expect to see high ripple. Could you please let me know how large the ripple is (do you have scope shots or measurements)? Also, do you see large ripple using standard EVM or customer PCB?

    Thanks,
    David

  • 0 in reply to David Diaz Quintana1
    Genius 4810 points

    Dear David-san,

    Thank you for your response.

    I will check input capacitor to the customer.

    The customer don't evaluate it, They are calculating the values for parts.

    I have a question.

    About 33uH for this condition, the ripple current is about 0.12A using typical value for input/output voltage.

    In this case, the ripple ratio is about 50%. Are there any problems with the ratio?

    And when they use 15uH, the ratio is bigger than 50% (around 100%). Are there any problems with the ratio?

    I understood they should use 33uH or 15uH for the stability, but I don't know whether there are any problems 

    for these ripple ratio.

    Best regards,

    Atsushi Yamauchi

  • 0 in reply to Atsushi Yamauchi
    Genius 4810 points

    Dear David-san,

    Could you please advice the inductor ripple current for my previous comment?

    Best regards,

    Atsushi Yamauchi

  • 0 in reply to Atsushi Yamauchi
    TI__Intellectual 1235 points
    Atsushi-san,

    Thank you for the additional information. Accordingly, it seems that the ripple current is about 0.12A and that is actually pretty good. I am wondering if your concern comes mainly from the fact you might be approaching ripple current from a Buck point of view. Ripple current is not as impactful in boost converters compared to buck converters (as far as how it effects Vout ripple). Where ripple current matters for a boost converter is that it increases the maximum peak current on top of your average switching current. So in the context of a Boost when it is mentioned 30% to 60% target, it is often meant to be related to current ripple as a percentage of the average inductor current for the maximum conversion ratio condition… which results in the maximum peak inductor current condition. For a buck it is typical to consider ripple current as a percentage of maximum output current, which is basically the switching current, so I am wondering if this is where we may be thinking of different things.

    In a boost converter the average inductor current has more impact on output voltage ripple than the inductor ripple current. Picture images of estimated inductor current and diode output current for your application. The di/dt of the diode is what the output caps see and is largely driven by average inductor current itself. And even though ripple current is higher with VIN=16V case, you actually get larger di/dt for the VIN=7V case due to the higher average current during this condition. So from a output voltage ripple perspective this low VIN case is worst case.

    I hope this helps to resolve your concern.

    Kind Regards,
    David D Quintana
  • 0 in reply to David Diaz Quintana1
    Genius 4810 points
    Dear David-san,

    Thank you for your reply.
    I understood it. I will support this issue to the customer.

    Best regards,
    Atsushi Yamauchi
  • 0 in reply to Atsushi Yamauchi
    TI__Intellectual 1235 points
    Atsushi-san,

    I am glad that answer helped. Could you please send a convenient email account to reach you?

    Thanks,
    David