TPS92641EVM: Current Behavior

Chuchen Wang

Part Number: TPS92641EVM

Hi,

My customer is evaluating this EVM and has the following question (waveforms are attached):

We used a 30khz pwm signal for dimming and labeled what the duty cycle was set at for each waveform. I'm sending the waveforms again with better labels.

As we lower the duty cycle of the PWM signal, the LED current drifts below the cursor. Is this normal operation?

Thanks,

Chuchen

30kpwm_99duty.tif 30kpwm_77duty.tif 30kpwm_41duty.tif 30kpwm_40duty.tif

over 7 years ago

0 Clinton Jensen over 7 years ago

TI__Mastermind 25500 points

Hello Chuchen,

I am not sure here, I haven't seen this. Are you using SDIM for dimming without a shunt FET? If so I'm not sure exactly how that would work. Does it happen if you use UDIM to PWM dim? What value of COMP capacitor are you using?

Thanks,

Clint

0 Chuchen Wang over 7 years ago in reply to Clinton Jensen

TI__Expert 3470 points

Hi Clint,

I'm checking on your questions, COMP is 1uF. Please see modified EVM schematic attached.

Thanks,

Chuchen

0 Clinton Jensen over 7 years ago in reply to Chuchen Wang

TI__Mastermind 25500 points

Thanks Chuchen,

Please try it using UDIM (TP2 in the above schematic) and see if there is a difference. SDIM is meant to be used with Q3 and it behaves a bit differently as a result.

Regards,

Clint

0 Chuchen Wang over 7 years ago in reply to Clinton Jensen

TI__Expert 3470 points

Hi Clint,

Feedback from customer:

The original waveforms were using the SDIM pin with a shunt FET installed on the demo board.

As you suggested, I tried using the UDIM pin, however, I still have the same problem. Please see the attached waveforms. As I change the duty cycle of the signal going into UDIM, the current drifts.

dutycycle10.tif dutycycle50.tif dutycycle90.tif

0 Clinton Jensen over 7 years ago in reply to Chuchen Wang

TI__Mastermind 25500 points

Hello Chuchen,

I do not understand what is meant by the current "drifts". I do not see any issues with those last pictures, it looks to be working just fine. Could you clarify?

Thanks,

Clint

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TPS92641EVM: Current Behavior