• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 297 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

TPS92638-Q1: What to do with unused outputs

TPS92638-Q1: Unused Output With Resistor Causing False Fault In Slow Increase Voltage

Julio Costa
Prodigy 70 points
Part Number: TPS92638-Q1

Hi Team,

I have a 7 string LED project.

I was informed that "for the unused channel output of TPS92638, connect a proper resistor to GND to avoid any false fault", but, how do I calculate this "proper resistor"?

I tried many values, but, when I have a slow power supply increase, at around 7V, a false fault is flagged.

Thank you in advance,

Julio

  • 0
    TI__Expert 5380 points

    As you know, each PWM inpout controls two ouptuts, so the used channel will be turned together with adjacent channel. 

    The key point is to make sure the output is in the proper votlage range, to avoid triggerring short or open fault. It is bettert to select the resisitor based on the ouitput currrent, you can select the adjacent channel ouptut votlage as reference. For example, 6V output voltage and 30mA output current results in 200ohm.

    Or you can choose a 6V zener or put several diodes to create this dummy load.

  • 0 in reply to Anda Zhang
    Prodigy 70 points

    Hello Anda,

    Thank you for your answer.

    I tried a resistor in a voltage range near to the LED chain and also tried to understand the start-up behavior and find a resistor that leads to a voltage and current that should not cause a failure triggering, but without success. It seems that the VxI characteristic doesn't match.

    As the datasheet doesn't clear explain the start-up behavior, I was hoping to get a better explanation and/or a formula to calculate this resistor.

    The problem with the zener solution is that zener diodes are easily biased with RF currents, which can lead to RF immunity failures, and I'd prefer the resistor solution, instead of several diodes.

    Thank you and best regards,

    Julio

  • 0 in reply to Julio Costa
    TI__Expert 5380 points

    Hi Julio,

    To analysis the root cause, recommend to capture the VIN/PWM/VOUT/FAULT voltage waveform during power up/down to check which fault is triggered. 

    Two points need to be paid attention, one is the output votlage too low, another is the output voltage is too high, these two cases may result in OPEN AND SHORT fault.

  • 0 in reply to Anda Zhang
    Prodigy 70 points

    Hi Anda,

    The TPS92638 is recognizing a pure resistive output as a short circuit in the slow increase power up event and latching down the outputs.

    The solution was to include a series Zener diode. With this, the Zener prevents the false fault in the power supply slow increase and the resistors prevent a fault in the RF immunity test, where the Zener diode can be biased.

    Thanks and regards,

    Julio