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TPS92515: What is the maximum wire length between EVM and LED lamp?

Kazuya Nakai59
Guru 21595 points
Part Number: TPS92515

 Hello guys,

 One of my customers is evaluating TPS92515 using TPS92515 EVM (TPS92515HVEVM-749) for their new products.

http://www.tij.co.jp/jp/lit/ug/slvuar5/slvuar5.pdf

 In the evaluation, they observed that the output current is unstable (the current change 800mA~1200mA, target is 1000mA) like a oscillation. They use 1meter length wire

(AWG24 or AWG26) between EVM J2 and LED lamp. 

They think this length may be too long  for TPS92515. In other word, this long wire causes the unstable output current.

 Could you give me your answer to the following question.

Q1. What is the maximum wire length between EVM and LED lamp in their test condition?

 Their test condition:

 Vin=24V, Vout=1 LED (Vf=2.0~2.2V), Iout=600mA~1000mA.

Q2. Is there any way to avoid the output current oscillation?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

  • 0
    TI__Mastermind 28655 points
    Hello Kazuya,

    The distance to the LED load will depend on many factors:

    How the connections are routed to the LEDs (big inductance loop or low impedance wiring to the LEDs.
    The output capacitor size and where it is located.
    The switching frequency.
    The size of the wire isn't a big factor in the instability.

    Q1, depends on a lot of factors, impedance to the LEDs, output capacitor size and location, and switching frequency.
    Q2, yes, this just requires knowing what is causing the oscillation and work around what is causing it.

    Do you know what the oscillation frequency is?
  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello Kazuya,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best regards,
  • 0 in reply to Irwin Nederbragt
    Guru 21595 points
    Hello Irwin,

    I'm sorry for my late response. I took long time to get information from the customer.
    New information is the following.

    1. The wire length between EVM and LED is about 1meter. They measured output current when the wire length is 10cm
    but the output current waveform is oscillated.
    2. They connect only one LED to TPS92515 output. Also, LED forward voltage they use is only 1.3V not 2V.
    Can TPS92515 drive the LED which has 1.3V Vf correctly when Vin=24V?

    I am still trying to get the oscillation frequency. I will inform it as soon as I get it.

    If you have any suggestion at this moment, could you please let me know?

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 28655 points
    Hello Kazuya,

    If the output is 1.3V the off-timer may have to be dealt with different such as a pull-up to VCC. The trip threshold for the off-timer isn't much lower than 1.3V at 1.0V. If 1.3 varies the off-time will vary a lot. Try setting the off-time with a single resistor to VCC.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Guru 21595 points
    Hello Irwin,

    Could I ask you a few additional questions as the follows?

    Q1. Is there any application note in case that the pull up resistor is connected to VCC?
    Q2. Could you please tell me how to calculate OFF-Time resistor, ROFF in that case.
    Can Equation 17 on page 23 in the device datasheet be used for ROFF? Then is VLED in the equation need to change VLED to VCC?
    Q3. Is there any way to calculate LED current valiation or current error rate?

    Thank you very much and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 28655 points
    Hello Kazuya,

    Q1, it's very simple, just use VCC instead of Vled for Roff/Coff. The only issue you may run into is average current varying if the Vf of the LED and freewheel diode vary. This can be reduced by having lower ripple (larger value inductor/higher switching frequency)

    Q2, Just use Vcc instead of Vled in equation 23

    Q3, There is to a certain extent. The TPS92515 regulates peak current so the inductor tolerance is included as well as thermal changes. Variances in the LED voltage and freewheel diode when operating at such low voltage as well as other tolerances,

    Best Regards,