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TPS923655DMTR Thermal Issue: Foldback Activates Well Below 150 °C Setpoint

Thomas Rock
Prodigy 30 points
Part Number: TPS923655DMTREVM

Tool/software:

I’ve designed a backlight driver board using the TPS923655DMTR (see schematic attached) , so far everything on the board works however we noticed that the board quickly triggers the thermal foldback and shuts itself down or flickers the display backlight 5 to 10 seconds after being powered on, we are sure this is a thermal related issue as the IC works perfectly when cooled.

 

We’ve tried high impedance on the TEMP pin  and also set the Foldback to 150C with a 10KΩ resistance, but the IC still seems to shut itself down from overheating however on touch it doesn’t feel that warm certainly no where near the 150C to trigger the thermal foldback.

For reference, the load that we are testing this board and IC on is a 33V, 325mA backlight. Which we wouldn’t have thought would use enough power to require the cooling of the TPS IC.

The circuit design is closely based on the TPS92365xEVM Evaluation board.

 

  • Vias have been placed all over the board to dissipate heat to the GND plane including under the IC itself.
  • Board closely follows the design from the evaluation module.
  • Trace thickness 13TH (0.33mm) to and from the IC, thicker traces have been used through most of the rest of the board design
  • 1Oz (28.35g) Copper weight used
  • The same or larger resistor packages were used in the design compared to the TPS92365xEVM Evaluation board.
  • Used 100V rated capacitors throughout the board.

  • 0
    TI__Mastermind 30551 points

    Dear Sir,

    Pls kindly let me check your question and get back to you soon. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 30 points

    Hi Lucia 

    Do you have any update on this? 

    Thanks 

    Thomas 

  • +1 in reply to Thomas Rock
    TI__Mastermind 30551 points

    HI Thomas,

    Thanks for reaching out!

    (1) Could you pls kindly let me know what is the VIN range, and as you said the LED string forward voltage is 33V/325mA. However, from the schematic, Rsense is 10ohm which corresponds to 20mA. I need the application condition include VIN, VLED and ILED to review the schematic design.

    (2) And what kind of dimming method you are using? Analog dimming or PWM dimming? Do you see this issue when the LED current is full scale or when it's in dimming mode? 

    (3) If it's thermal related, usually when you adjust the thermal protection resistor, you would see some difference of the device behavior. However, looks like it does not. You can also either set it as analog dimming or PWM dimming to lower the LED current to see if it'll still flicker or not.

    (4) If it's not related with thermal shutdown issue, it might also related with the device loop stable. You can try to 1) Increase Ccomp (C11 in your schematic) to 15nF; 2) Change Rdamp (R7 in your schematic) to 10Mohm  3) Change C13 to 1nF/just not populated.

    (5) For OVP resistor design, we also have some suggestion, pls refer to

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 30 points

    Hi Lucia

     

    Thanks for your help, I’ve managed to get the board working.

    Primarily it was C11 being set to 1nF and not 15nF as you mentioned.

    I’ve also set the thermal foldback back, so it triggers at 80C and had no issues with it over heating.

     

    One last thing I was wondering was that on my design I have the option of shorting PWM or ADIM to VCC to give it a constant 5V supply to keep the backlight on constantly. However, the backlight will flicker and turn on and off intermittently sometimes when shorted.

    However, when both LK1 and LK2 are both shorted (so ADIM and PWM are both shorted to VCC) the display backlight stays on constantly without flickering.

     

    The backlight also stays on full brightness when PWM and ADIM are left open when I thought they needed a 5V signal on either pin for the backlight to be on at all?

     

    I was wondering if VDD isn’t suitable for driving the PWM or ADIM pins to keep them constant?

     

    Also I can see the internal LDO is used for soft start but can it also be used to drive the enable pin?

  • 0 in reply to Thomas Rock
    TI__Mastermind 30551 points

    Hi Thomas,

    Let me clarify with below conditions:

    (1) When you short LK1, also short LK2 which means that both ADIM and PWM are shorted to Vcc. You can see the LED stay with full brightness. It can match with the theory.

    (2) When you leave ADIM and PWM floating, you also can find the LED stay with full brightness? Actually, it can't meet with the theory. When ADIM and PWM both floating, there will no switching pulse and no LED current in theory.

    (3) You also mention about below condition. What is the difference between this and condition 1? Looks like both are shorted to Vcc? 

    Thomas Rock said:
    I have the option of shorting PWM or ADIM to VCC to give it a constant 5V supply to keep the backlight on constantly. However, the backlight will flicker and turn on and off intermittently sometimes when shorted.

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 30 points
    Lucia Gao said:
    (1) When you short LK1, also short LK2 which means that both ADIM and PWM are shorted to Vcc. You can see the LED stay with full brightness. It can match with the theory.

    Understood I expected this I was just confirming my expectations.

    Lucia Gao said:
    (2) When you leave ADIM and PWM floating, you also can find the LED stay with full brightness? Actually, it can't meet with the theory. When ADIM and PWM both floating, there will no switching pulse and no LED current in theory.

    I agree with you about this in theory, I wasn’t expecting the display to turn on at all with ADIM and PWM floating, although the display does appear to flicker when they are left open. Although i'd never intend on using the device with ADIM and PWM both floating it's just something i noticed.

    Lucia Gao said:
    You also mention about below condition. What is the difference between this and condition 1? Looks like both are shorted to Vcc? 

    I’m wondering is it possible to just have either PWM or ADIM shorted to VCC (not both at the same time) to keep the display on full brightness or do they both need to be shorted at the same time to keep the display on full brightness?

  • +1 in reply to Thomas Rock
    TI__Mastermind 30551 points

    Dear Thomas,

    If you target to get the LED with full scale brightness (ILED= Vref / Rsense), both PWM and ADIM need to be pulled high (for example, either pulled high to external source or internal Vcc.

    If you just either keep PWM shorted to Vcc but make ADIM floating or keep ADIM shorted to Vcc but make PWM floating, in theory, you can't get full brightness and we don't recommend to LED driver work in this way. In conclusion, we need to configure the LED driver following the suggestion from datasheet 7.3.5.

    Thank you!

    BRs

    Lucia