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Original question:

Constant Current LED Driver

TPS92612: Current drop while increasing the cable Length

Tex
Intellectual 300 points
Part Number: TPS92612
Other Parts Discussed in Thread: TPS92620-Q1, LM3409, TPS92692

Tool/software:

Greetings,

I'm facing current drop issue when i increase the length of the cable,

92 LEDs are consuming 1.5A of current with 30cm cable but when i increase the length of the cable huge amount of current drop is there..

for 1 meter all LEDs 1.3A and with 5 meter cable current consumption is 1A.

single LED's forward voltage is 3.2V

Also please tell me till how much length of the cable can TPS92612 drive ?

please give me any solutions, i have be tried multiple types of cables but observed the change only ±10mA difference..

Best regards,

Tex

  • +1
    TI__Expert 8180 points

    Hi Tex,

    It depends on your cable length and line resistance, I am not sure how many LED drivers in your system, but this device is a linear LED driver. you need to ensure enough dropout voltage between SUPPLY and OUT.  you can measure the voltage of  SUPPLY pin, OUT pin and LED anode to see the difference. if you can't change the cable, you can slightly increase the output current setting considering the influence of the cable to ensure the current accuracy on the LED.

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Hi Hailee Xu.. Thanks for your reply,

    My cable length is 5 metres and it's resistance is 1 ohm,

    also i might change the cable length oftenly..

    i'm also having that doubt.. after implementing this circuit, if i change cable length will it send over/less current through the LED ??

    if yes, is there any solution to avoid this ?

    Best regards,

    Tex

  • 0 in reply to Tex
    TI__Expert 8180 points

    Hi Tex.

    Could you share your full schematic? and also the operating condition? If the cable's resistance is 1ohm and the current flow through it is 1.3A, there would be 1.3V drop on the cable, maybe you can increase your supply voltage to see the change.

    Thanks!

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Yes! i do have tried it before.. when the voltage increases the current consumption is also increasing..

    the thing is should i change the voltage every time i change the cable here ???

    regards,

    Tex

  • 0 in reply to Tex
    TI__Expert 8180 points

    Hi Tex,

    Why do you need change the cable many times? is it not a fixed application? please share more details about your application for more analysis.

    It's the behavior of the device according to your operating condition. you need to try to minimize the impact of the cable, like use thicker wires...

    Thanks!

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Yes You are right! this application is not be a fixed application or won't be in fixed location!

     I have shortlisted few Linear constant current driver ICs from TI, but later realised that, the constant current driver sends current continuously! what if any one of the LED is damaged out of 92 LEDs.. the remaining LEDs will take more current and eventually by receiving more current they will also get damaged one by one.. I should be able to prevent that risk.. 

    so, I should be able to control/limit the current consumption!

    Is there any IC which will vary the Voltage when the current consumption is high or low..

    please let me know if such ICs are available..

    Best regards,

    Tex

  • 0 in reply to Tex
    TI__Expert 8180 points

    Hi Tex,

    Yes, if you parallel the output channels of different ICs, the fault can't be detected properly. it is due to your LED string architecture.

    Could you consider the switching LED driver and change the LED architecture? the switching LED driver won't be affected by your output voltage, just keep a constant output current. and from your description, there would be 92LEDs in your systems. how many ICs would you use? and how many LEDs in a LED string? what's your end equipment? you can use more LED in a LED string and controlled by one IC, in this way, the fault can be detected. Please share more application requirement for better recommendation.

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Hi Hailee,

    Current design is in parallel connection.. Yes,i'm ok to change the architecture for the fault detection of an LED..

    I have divided those 92 LEDs into 4 parts to generate a pattern.. First all are connected to a single power supply..at the end it will be connected to 4 different transistors to drive those LEDs.. and those transistor bases are connected to controller..

    Now i have to replace those transistors with fault detection and voltage control..

    LED specs:

    Forward voltage of the LED is 3.2V

    max current consumption is 20mA

    Best regards,

    Tex

  • 0 in reply to Tex
    TI__Expert 8180 points

    Hi Tex,

    Could you draw a diagram of the ICs and LEDs, also indicate the current and voltage requirement, including the requirement of the fault detection and voltage control?

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Hi Hailee,

    I have used NPN transistors to drive those LEDs.. I would appreciate if you have other suggestions to improve my basic circuit..

    Requirements:

    1. 1.6A Current consumption should be constant.

    2. Voltage can vary if i change the cable Length/if any LED is damaged.

    3. If One of the LEDs is damaged other LED shouldn't get more current.

    Best regards,

    Tex

  • +1 in reply to Tex
    TI__Expert 8180 points

    Hi Tex,

    Thanks for your sharing. I have some small questions about your requirements.

    • How many LED drivers are in your application if you need a total output current of 1.6A?
    • What is your input voltage range and what is the Vf of the LED? I notice that you mentioned a series voltage of 73.6V, how many Leds in this LED string? Generally, the Vf of one LED is 2~3V, if you use 23 LEDs in a string, the output voltage exceeds 46V which is higher than the OUT-voltage ABS rating. 

                    

    • Why do you add transistors in the LED strings? for PWM dimming function?

    For linear LED drivers, the input voltage shouldn't exceed the voltage ABS rating, you can consider using 7 LED string/400mA x 13 or 14 LEDs and 7 pcs LED drivers or more LED drivers, TPS92620-Q1 can be an option. Just ensure the input voltage should be always higher than V(dropout) + V(out) + V(line_drouput), then it can meet your three requirements.

    For switching LED drivers, I am not sure about you input voltage, for source topology, you can consider Buck CC driver, like LM3409 or Buck-boost LED driver, like TPS92692; for sink topology, LP886x series could be an option.

    BR,

    Hailee.

  • 0 in reply to Hailee Xu
    Intellectual 300 points

    Hi Hailee,

    Thanks for your reply,

    • For Constant current Driver I request you to suggest an IC which suits for constant current supply
      • single LED Vf = 3.2 to 4.0V 
      • Planning 6 LEDs in a string 38.4V to 48V voltage is required for single string
      • Total current  consumption for all LEDs will be around 2A
      • Finally we should control the brightness
    • Input voltage range would be from 3.2V to 4V.I calculated 73.6V for 23 LEDs If i take only 12 or 6 LEDs that is easier for current balancing..So, now 12 or 6 LEDs will be used in single string.. Please suggest any other ICs for 48V to 50V max voltage..

    Best regards,

    Tex..

  • +1 in reply to Tex
    TI__Expert 4160 points

    Hi Tex,

    Nice to meet you. I am Aaron, product marketing from TI LED driver group.

    I will reach out to you in email and follow-up your technical questions.

    BR, Aaron