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TPS61165 LED Driver Audible Noise

Mark Williams
Intellectual 510 points
Other Parts Discussed in Thread: TPS61165

Hi,

I am using the TPS61165 as a constant current boost driver for a TFT panel backlight. Part of the reason I chose this part was for the 'analog dimming' to eliminate audible noise from the inductor/caps.

I have noticed under certain combinations of input voltage and dimming level that I hear audible noise from the converter circuit.

Further investigation has led me to the cause being that the converter hits the minimum on time of 40ns and this results in a pulse-skipping mode causing 7kHz audible ripple.

The circuit input range is 10 to 15V and the output is 60mA (max) which results in an LED voltage of around 19V. We dim the LED down to 10% in our 'power save' mode.

My schematic is below:

It seems with these component values the converter is barely having to switch to maintain the boost voltage and current, when dimming this requirement is less. Therefore even at 100% PWM the converter switching duty cycle is just 16% maximum. Dimming puts the converter in pulse skipping mode as the switch minimum on time is not short enough to run continuously and maintain the desired current.

I have tried changing the inductor to 10uH (the datasheet recommends 10 or 22uH) and this improved the audible noise accross the range. At low LED PWM the pulse skipping still occurs:

(Yellow trace switch node, green trace LED current)

At 60mA the duty is only 16%:

I then tried this with a 2.2uF output capacitor but the PWM was the same and the ripple on the LED current got marginally worse.

Is it safe for me to try a lower inductance value than the recommended 10 to 22uH? I presume this is recommended for devices with larger input to output step and higher current? In my application a lower inductance would probably keep the PWM constant (no audible noise) but I am concerned about loop stability.

Mark

  • 0
    TI__Intellectual 1055 points

    Mark,

    Are you using ceramic output caps?  If so I think the audible noise is due to the capacitor type.  Ceramic caps are prone to audible noise because of their piezoelectric effects.  Using caps with a lower voltage rating, or smaller value caps should reduce this effect.  For example, I would expect replacing the 4.7uF output cap with 2x 2.2uF caps would reduce the amplitude of the acoustic noise.  Additionally, if it's possible in your application, we've found that sandwiching the caps (caps on alternate sides of the pcb, same x/y position) helps to cancel the mechanical expansion/contraction caused by the caps.  Another option is to switch to electrolytic output caps, this would reduce the audible noise as well.

    As you mentioned, reducing the inductor size is another option to stabilized the switching frequency and move out of the audible range.  However, if the inductor is less than 10uH, the loop stability is a concern.  The integrated 'slope compensation may not be adequate.  The loop stability should be carefully evaluated in your application.

    Feel free to contact me or respond to this thread if you have any additional questions or concerns on this.

    Thanks,

    Nisha