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TPS92692: compensation / stability issue

Paul Miller11
Prodigy 225 points
Part Number: TPS92692

I'm having some stability issues with the TPS92692.  My basic design parameters are:

Input voltage: 9-32V

Target switching frequency 500kHz

Output current: 1A

Output voltage: either 4 or 5 LEDs (12-15V)

I'm using the internally generated PWM with the PFET to provide a secondary 50% operating point

I'm also using the IADJ pin to provide a temp sensor feedback

As the system warms, it starts flickering.  I think (although am not sure) that it is because of a capacitance shift in the compensation network (if I use an electronics duster to blow on that area of the PCB, the flickering stops).

I've tried a handful of different compensation configurations and values and haven't been able to solve the issue.  I'm going to back to the compensation section of the datasheet to try to understand how to better approach this...guess-and-check isn't working too well.  I'd really appreciate some guidance.

  • 0
    TI__Mastermind 18890 points
    Hi Paul,

    Is the flickering happening only when you are using the PWM of 50% or does the flicker happens if you have it 100% ON? Does this happens at any input voltage or does it only happens at certain range? You might have done this already buI would tried a larger compensation cap dominant pole like a 0.1uF (type 1 compensation) to see if it the flickering goes away to rule it out. But first we would like to verify your power stage design to make sure that is done correctly. If you can forward us your schematic along with part list to review. You can send it direct to me at tuan.tran@ti.com.

    Mean while if you can have a scope capture of the LED current along with voltages on SS (pin4), /FLT (pin3), IS (pin 18) on one scope capture. Use 20MHZ filter on scope and 200mV/div on IS (pin 18). If it flicker then the LED current will flicker.

    Thanks Tuan
  • 0 in reply to Tuan Tran74
    Prodigy 225 points
    So...initially, when I tried to use compensation components with values either calculated from the datasheet or your TPS92692 calculator, it strobed pretty severely with the 50% duty cycle, but seemed to work normally at 100%. I ended up trying the component values similar to those on the eval board, a 4700pF and a 0.1uF in series with a 0ohm resistor. I had thought it was working, but I think I may have missed the issue; I only see the problem when the circuit has warmed sufficiently (after a good 20-30min). As I have tried different values for the both caps and the resistor in the compensation network I have moved the issue around - some values cause instability for low input voltages, some values cause instability at 50% duty cycle.

    I'll send a schematic and parts list directly. I really appreciate the help.
  • 0 in reply to Paul Miller11
    TI__Mastermind 18890 points
    Hi Paul,

    I am going to close this thread for now and it will open up if you have more questions.

    Thanks Tuan