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LP8860-Q1: Allowable power dissipation of OUTx pin

tateo yamashiro
Guru 10890 points
Part Number: LP8860-Q1

Hi,

We got a question from the customer about LP8860-Q1.
Could you help us?

[Question]
How much is the allowable power dissipation of OUTx pin? For example, if the short LED detection is set to 10.6V, is there any possibility to cause damage to the IC under the following condition?

 Vin=12V
 Vf=36V
 If=120mA/ch
 OUTx pin is shorted to the boost output.

Best Regards,
tateo

  • 0
    TI__Expert 4565 points

    Hi Yamashiro-san,

    If OUTx pin is shorted to boost output, OUTx channel will be disabled, and there won't be any power dissipation into LP8860 as LED current is 0mA.

    If LEDs are shorted, and headroom voltage at OUTx is still lower than short detection level 10.6V. If then, LED channel is not disabled, and high power(LED current * headroom voltage) can be dissipated into LP8860. Basically, there is not any hard limit for power dissipation on LP8860, and this will vary with PCB design, and ambient temperature.

    ψJT of LP8860 is 3.2°C/W, so measure top case temperature at max power dissipation condition and calculate junction temp from it. If junction temp is lower than 150°C, device will operate correctly. In most cases, temperature of LP8860 is not an issue as LED short fault threshold is set to cover only 1 LED(or 2) short case, and LED channels can be disabled when actual short happens. Rather, external components such as inductor, diode, SW FET will be more vulnerable to heat accumulation. Especially, SW FET can boil at high SW freq. 

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Guru 10890 points

    Hi, Thank you for your support. We would like to make sure one thing. If LED short is occurred under the following condition, will the short protection be active?

    1. The voltage of 3 strings is between MID_COMP and HIGH_CPMP and the other one is between LOW_COMP and MID_COMP.
    2. OUTx is shorted to FB pin(BOOST output). *Please refer to below figure.

    If the short protection will not be active under this condition, is there any possibility to damage the current sink(OUTx)?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Expert 4565 points

    Hi,

    For case 1, no fault will be triggered as one string headroom is normal and other 3 strings are lower than high comp.

    For case 2, LED short fault will be triggered and if short is hard(voltage slew eate is very fast) , OUTx pin may be damaged. Small cap between OUTx and GND will slow down slew rate and prevent the potential damage. But in usual cases, this cap is not used.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Guru 10890 points

    Hi, Thank you for your reply. Sorry to bother you again. Let me confirm one thing.

    If the LED short condition as shown below figure is occurred in display mode, will the LED short protection be active?

    According to datasheet, in display mode, the trigger of short protection is if one or more outputs voltage is above DRV_LED_FAULT_THR and at least one LED output voltage is between DRV_HEADR and DRV_HEADR + DRV_LED_COMP_HYST. So they think the short protection will not active under the consition that the voltage of 3 strings is between MID_COMP and HIGH_COMP and the other one is over HIGH_COMP(ex. short to Boost output). Is their understanding correct?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Expert 4565 points

    In this case, boost voltage is adjusted to make headroom voltage of at least one  string between low and mid comparator first, then LED short dault can be triggered if highest headroom is still higher than high comparator. If every string headroom is higher than low comparator, no faults can be detected right away.

    Thanks,