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TPS92691-Q1: TPS92691 EVM PWM dimming problems

Part Number: TPS92691-Q1

Hi, I found a few problems when I tested the TPS92691-001 sepic EVM.

1.The maximum Iout=300mA at PWM dimming mode, while the guidebook shows the maximum Iout=500mA. Is there a limit to the range of Iout at PWM dimming mode? And what influence the limit?

I used an electronic load at CU (constant Uout) mode, and the Uout=7.5V to simulate LEDs.

2.The L2 inductance continued to make noises when using the PWM dimming. I input a PWM signal VPP=3V~5V, f=100hz~1KHZ according to the guide book, but the noises always existed. When the duty cycle was constant,  reduced the PWM frequency and the noises reduced, too. And changing Uout didn't influence the noises.

thanks a lot

  • Hello Iris,

    1.  The limit for the max output current is determined by the power stage design.  When we talk about 300mA we are talking about the LED current during the PWM dimming being 300mA and not the average.

    2.  The phenomenon of acoustic noise in a known phenomenon with inductor and capacitor in the audible range.  There will always be some acoustic noise depending on the inductor and capacitor being used since the PWM frequency is in the audible range.

    Thanks Tuan

  • Hi, you mean my maximum Iout 300mA during the PWM dimming is peak current? Or you mean the maximum Iout 300 mA depends on every design and is not stable?  I haven't understood your explain yet. By the way, the picture shows maximum Iout=500mA under 13 LEDs ,right?

  • Hi Iris,

    Thank you for clarifying. We will get back to you about this within 24 hours.

    Best Regards,


  • Iris,

    Maximum Iout 300mA is during PWM dimming is the peak current during the pulsed current.  So for 10% and 100Hz dimming at 300mA then the average current is only 30mA because of the duty cycle of the dimming with 300mA pulses.  That the fundamental of PWM dimming and average current. 

    Figure 18 is just a normalize figure to show that if you PWM at the PWM input pin with 700mV at the IADJ pin then you would get 500mA PWM'ing current based on the PWM duty cycle.  the load is 13 LED's.  For example, if you PWM 50% on the PWM pin then you get 50% of pulsing current on the LED's. 

    The evaluation board should be stable if you use it in the recommended range of Table 1 in the evaluation document.  The current range that you can control via the IADJ pin is 100mA to 1.7A and the max output power is 25W.

    Thanks Tuan