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TPS92512HV STARTUP ISSUE

WhiteHorse
Genius 4430 points
Other Parts Discussed in Thread: TPS92512HV

E2e,

 

We have a customer that has designed in the TI TPS92512HV and have been experiencing some odd start up behavior that they would like to prevent if possible. 

 

 If PWM is high (5V) when the IC is turned on the unit pounds the LED with an initial current of 2A. It will then immediately recover and find its set current but this equates to a flash of light at start up, as well as stress to the LED. 

 

 Items to note. 

 

 1) Comp pin cap is .01uF this was necessary to prevent a turn on hiccup the unit did when PWM was brought from 0 to 1%

 2) Switching frequency is set to 300K this also prevented some plashing we were experiencing due to possible min or max duty cycle issues. 

 3) output capacitance is 3uF and output inductor is 33uH

 4) Max current is set to 1.44A

 

 This flashing means they cannot use this part in analog applications.

 

How can this condition be eliminated?

 

Thanks for your help.

 

Regards,

John Wiemeyer

  • 0
    TI__Expert 3910 points
    Hi John,

    Your question has been re-directed to our application engineer in US. Please wait for his reply later today.

    And please remember to click on "Verify Answer" button once our application engineer answered your question.
    Thank you very much!
  • 0
    TI__Mastermind 25500 points
    Could you give me the input voltage and output LED stack voltage when this happens? I have tried this under quite a few conditions and cannot re-create it. We also didn't see it during silicon evaluation, but that doesn't mean it can't happen. One thought is that the abs max rating on the PDIM pin is 5V so if that isn't tightly regulated you could cause damage or at least leakage that could cause issues. Have you tried resistor dividing this signal down so you are not running at abs max? I would replace the part just in case if you do try so that you are sure it isn't damaged.
  • 0 in reply to Clinton Jensen
    Genius 4430 points

    Hi Issac,

    The customer was able to create this symptom on multiple samples, and when they hold the PWM pin low through a power cycle everything works great. Their say their 5V is very tightly regulated as it runs their processor that is driving this pwm PIN.

    Their LED Vf is 21V @ 1.4A and the incoming supply volts is 45V. They saw this issue when they had the UVLO set to 42V, and their supply ran out of power and dropped below 42V, and the unit began an oscillating flash cycle. It was connected to a current meter, and they noticed the current was shooting higher than the set point. They then connected the unit to a fixed bench supply and power cycled with the meter set on max hold, and saw peak currents as high as 3A.

    They have currently updated the software to ensure a proper power sequence, but this is still a concern and they would like resolved. They don't have a processor in every unit they design, so this is not always an option.

    Thanks for your help.

    Regards,
    John WIemeyer

  • 0 in reply to WhiteHorse
    TI__Mastermind 25500 points
    I still don't get that behavior, but understood. I would have some concern too. Is the current meter a DMM or a current probe? If it is a DMM type in-line meter that may be the issue. If you must test transients, including startup, with a DMM in series (never recommended) you need to make sure it is on manual setting. Otherwise they dynamically switch the internal resistance at a certain current level and can cause all kinds of weird things to happen.