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TPS92515: Flicker during low current shunt FET operation.

Tyler Reed
Prodigy 105 points
Part Number: TPS92515
Other Parts Discussed in Thread: TINA-TI,

Hey,

I have observed a flicker/error on the output voltage of the TPS92515 when shunt dimming at the minimal IADJ voltage of 100mV. At higher IADJ voltages and without shunt dimming my circuit works perfectly fine. I have been unable to find a cause to this error. In both real hardware and TINA-Ti simulation this issue persists.

As you can see in both the TINA transient analysis and Oscilloscope scans below the Vout voltage (bottom blue in both scans) does not always match up with my active low Shunt FET PWM.

Any idea what may be causing this error?

I have attached the schematic for this circuit below.

Thanks,

Tyler

6266.TPS92515ShuntFet.TSC

  • 0
    TI__Mastermind 25500 points

    Hello Tyler,

    I have seen similar operation and I believe it is because at such a low output current you are in discontinuous conduction mode. This means the output/inductor current is going to zero every cycle. What happens is when you turn off the shunt FET it could be using a different reference for the off timer depending on if the current is not zero (COFF uses the output voltage) or if it is zero (COFF uses the VCC voltage only). So the initial off time can change cycle to cycle.

    Basically for shunt FET dimming and analog dimming combined you do not want to dim low enough to go into DCM. You need to keep the inductor continuously conducting current and not dropping to zero.

    Regards,

    Clint