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Original question:

Parts damaged during production testing (LX and VOUT pins shorted)

TPS61158: Re: Parts damaged during production testing (LX and VOUT pins shorted)

Andy Calder
Prodigy 30 points
Part Number: TPS61158

Tool/software:

Hi Patrick,

In reference to the thread started by Nicholas Medor "Parts damaged during production testing (LX and VOUT pins shorted)", we were able to determine the root cause of the damaged TPS61158 LED Drivers.

We found that during test, an overvoltage condition occurred where VOUT would approach 32V, exceeding the datasheet's 30V absolute max rating and damaging the part.

This would occur when voltage was applied to the CTRL and VIN pins, before power was applied to the switch node (FTP55 net).

We ran a test to ensure the CTRL pin was grounded prior to power being applied to VIN and switch node (FTP55 net). After power was applied to VIN and FTP55, we applied a PWM signal to the CTRL pin. No voltage spikes were present on VOUT after this sequence.

Our follow up questions for TI are:

1: Does it make sense that applying pulses to the control pin before the switch node is powered will cause voltage spikes on the TPS61158 output?

2: Will driving the control pin low for 3.5ms minimum prior to powering the switch node ensure that no voltage spikes will be generated?  3.5ms is from the TPS61158 "6.5 Electrical Characteristics"

3: Are there any other timing constraints that we should be aware of that could results in damage to the IC?

We look forward to your response. Thank you,

Andy Calder

  • 0
    TI__Mastermind 20515 points

    Hi Andy,

    Please expect some delay on the response since our expert is OoO.

    BR, Jared

  • 0
    TI__Mastermind 34710 points

    Hi Andy,

    Do you have any waveform of VIN, CTRL and FTP55 when power on the IC, I want to know what is the interval time between CTRL, VIN and FTP55?

    If you apply VIN and FTP55 first and then apply CTRL, then the soft-start of the IC will help to prevent voltage spike.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 30 points

    Hi Patrick,
    Here is the errant condition waveform with a voltage spike (21V) occurring on VOUT. Zoomed in waveforms of #1 and #2 below.
    FTP55=Green, CTRL=Blue, VIN=Red, VOUT=Yellow

    #1: 30% BL ON. VIN and CTRL present prior to voltage applied to FTP55. 21.06V spike observed on VOUT.
    FTP55=Green, CTRL=Blue, VIN=Red, VOUT=Yellow

    #2: 100% BL ON. VIN and CTRL present prior to voltage applied to FTP55. 19.75V spike observed on VOUT.
    FTP55=Green, CTRL=Blue, VIN=Red, VOUT=Yellow

    Non-errant condition waveform with VOUT=16.62V. VIN and FTP55 voltage present prior to voltage applied to CTRL pin.
    FTP55=Green, CTRL=Blue, VIN=Red, VOUT=Yellow

    Thanks,
    Andy Calder

  • 0 in reply to Andy Calder
    TI__Mastermind 34710 points

    Hi Andy,

    Thanks, may I also have a look at your schematic?

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Prodigy 30 points

    Hi Patrick,

    Here is a snippet of the schematic along with text from the original post:

    " The tester applies a 1.5kOhm resistive load to the backlight driver IC and drives the backlight driver IC with a 100% duty cycle control signal.  The edited schematic snippet below shows how we've designed the TPS61158 within the product, the voltages at select nets as provided by either the tester or the board itself during test, and how the 1.5kOhm load is applied by the tester.  The 100% duty cycle signal is applied at net BKLEDPWM and is driven to the same voltage level as the VIN voltage (i.e. +3.15V).  Note that the TPS61158 LX pin is labelled as SW by the U12 schematic symbol."

    Regards,
    Andy Calder

  • 0 in reply to Andy Calder
    TI__Mastermind 34710 points

    Hi Andy,

    If you apply VIN and FTP55 before apply CTRL, the soft-start of the IC will help to reduce the voltage spike. If CTRL and VIN are applied before FTP55, the IC will start to boost with a lower boost input voltage (FTP55 needs time to ramp up), so the loop will regulate with a larger error signal and results in a larger voltage spike.

    BR

    Patrick