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TPS92511: Minimum ripple current

na na78
Guru 16770 points
Part Number: TPS92511
Other Parts Discussed in Thread: LM3405A, TPS92513

Hi

We have following design requirement.

VIN:12V

VLED: 2.2V

RFS:3.3K

RIADJ : 4.3K (0.35A as led current)

L : 200uH

These specification will offer Iripple = 0.029App that would be 8% for led current.

Actually, a design with these parameters did not work. (LED current does not reach required 0.35A)

Is there possible that the LED current is not flowed if ripple current is small?

Are there any recommendation of current ripple?

BestRegards

  • 0
    TI__Expert 3910 points
    Hi there,

    The current regulation in this device requires a certain percentage of inductor ripple current for proper function.
    You may place a capacitor in parallel to the LED for smaller LED current ripple.

    Please try 100 uH as the table 3 suggested and see if the current can be correctly regulated.

    Thanks!
  • 0 in reply to Issac Hsu
    Guru 16770 points

    Hi Issac

    Thank you for your reply.
    I'm sorry that I mistook about RFS and VLED. RFS is 33Kohm and VLED is 2.4V on the customer board actually.

    >Please try 100 uH as the table 3 suggested and see if the current can be correctly regulated.
    I believe selention of 100 uH would violate equation (7).

    According to this equation, 1uH/ohm * RFS*RIADJ/10^6 = 142uH which is largr than 100uH.

    I attached waveform of VLX and ILED with following parameters.
    VIN=12V, VLED=2.4V, L=200uH, RFS=33K, RIADJ=4.3K.


    While changing L to 100uH and adding 1uF capacitor in parallel to LED like following circuit, it had not impact.

    (ILED = 350mA does not flow in following circuit, actually)

    What do you think of cause of fail for current reguration?
    Could you please give us your view?

    BestRegards

  • 0 in reply to na na78
    TI__Expert 3910 points
    Hi Na,

    Looks like it's in DCM. TPS92511 cannot regulate accurate current when it's in DCM.
    As you did the calculation and found that 142 uH is minimum, please try using 150 uH for the design.
    The current regulation of this device requires inductor current ripple and working in CCM, so any inductance that is a little bit higher than minimum would be good.

    Thank you!
  • 0 in reply to Issac Hsu
    Guru 16770 points

    Hi Issac

    Thank you for your reply.

    If possible, please show me recommended L with VIN=12V, VLED=2.4V, RFS=33K, RIADJ=4.3K?

    According to my calculation, L=30uH is required to set current ripple as 50% of ILED.
    However, L=30uH is violate equation (7).

    Is it possible to set L =30uH even if (7) is violated?

    As a supplement, LED is SZR05A0A.


    BestRegareds

  • 0 in reply to na na78
    TI__Expert 3910 points
    Hi Na,

    Looking at equation (7), min required L1 > 1e-6 x 33k x 4.3k / 10^6, the number I got from this will be 14.19 uH.
    From equation (6), suggested L1 = (VIN - VLED) * VLED / (2 * Iripple * VIN * fSW), 33k-ohm is 300 kHz, Iripple = 0.175 A,
    My caluclation shows L1 = 18.3 uH.

    As calculated from equation (6) is larger than that calculated from equation (7), L1 cal = 18.3 uH > L1 min at 14.19 uH.
    There is no contradiction.

    I hope the above illustration helps you in the design.
  • 0 in reply to Issac Hsu
    Guru 16770 points
    Hi Issac

    >Looking at equation (7), min required L1 > 1e-6 x 33k x 4.3k / 10^6, the number I got from this will be 14.19 uH.
    I thought it would lead 141.9uH. (1e-6 ≠10^-6 ??)
    Had I been mistaken?

    If 14.19uH is correct as you offered, we can select 18.3uH without concern.



    BestRegards
  • 0 in reply to na na78
    TI__Expert 3910 points
    Hi NA,

    If you look further down on the datasheet, it specifies the operation must be in CCM with another minimum inductor caluclated according to eq (8).

    With eq (8), you will calculate that L1 min at 9.14uH, much lower than that calculated from eq (7).

    I would not follow strictly according to eq (7), rather, I would follow eq (6) and counter-check the number with eq (8).

    As long as the number calculated from eq (6) is larger than eq (8), that's it.

    One question on your application though, do you use analog dimming through injecting current into IADJ pin?

    If not, the above calculations are all valid.
    However, if you do analog dimming, you need to take care of the average current reductions so the inductor ripple current should stay lower in order to maintain CCM operation.

    Thanks!
  • 0 in reply to Issac Hsu
    Guru 16770 points
    Hi Issac

    Thank you for your reply.

    IADJ pin is just connected RIADJ = 4.3Kohm and DIM is opened, not using analog dimming. The circuit is in previous post.

    I ask the customer to use small L like 18.3uH.

    BestRegards
  • 0 in reply to na na78
    Guru 16770 points

    Hi

    The customer changed L to 22uH, but wanted ILED did not flow.

    The following image is ILED and circuit.

    I had questions from the customer.  Could you please answer them?

    1.

    According to eq (5) in datasheet, smaller Fsw is, larger tON is.

    However, tON did not change in this circuit.

    What do you think of cause?

    2.

    Could you please check the circuit constant again whether there is no problem?

    And originally, is TPS92511 suitable to operate with this specification?

    3.

    Can you suggest other driver for SZR05A0A?

    I appreciate if you could reply.

    BestRegards

  • 0 in reply to na na78
    TI__Expert 3910 points
    Hi Na,

    I could not find problem on your customer's schematic.

    Did they try powering the LED with our EVM (by changing the inductance and resistance to suit their need)?

    I suspect there might be layout issue or component codings' different from board and schematic.

    Anyway, TPS92511 is suitable device for LED current is less than 500 mA and LED output voltage is always smaller than input voltage.

    If your customer wants to try more device, please have him/her consider TPS92513 or LM3405A.

    Thank you!
  • 0 in reply to Issac Hsu
    TI__Mastermind 28655 points

    Hello NA,

    I looked at your schematic and values and the waveforms.  The LED current is not getting close to where it needs to be.  The on-time looks very short, 300 nS is about as close as I can see on the waveform.  It seems something is wrong with the board they are using.  Issac mentioned getting the EVM and trying it.  I would run the EVM as it is designed so they can see how it is supposed to operate then modify it to what they need.

    On the board they are using they need to check if there are any issues or perhaps try another board.

    Regards,