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Thread: Potential noise from multiple PWM signals?

  1. #1

    Default Potential noise from multiple PWM signals?

    I'm building a circuit to power an RGBW MC-E where I'll have control over the brightness of each emitter. The PWM signals will be coming from four 555 timers (one for each color) and the output pin of each timer will be connected to a separate enable pin of an On Semi CAT4101 linear LED driver IC. The circuit will be powered by four AA batteries.

    I plan to include the 0.1 uF filtering capacitor between Vin and GND as it's shown in the datasheet, however I'm still worried the multiple PWM signals could cause a lot of noise, and possibly a visible flicker in the output of the LED's. Is there anything I should consider before populating the PCB? Maybe a capacitor in parallel with each LED? Any help would be appreciated.

  2. #2

    Default Re: Potential noise from multiple PWM signals?

    hi,

    no cap on the led! you will bridge the pwm function and the brightness controll will not work probertly.
    wich frequency do you plan to youse with the 555? if you drive your driver ic below 100khz i see no problem with the pcb layout. do it with short lines. you can also freewire the 555 or take a breadboard, still no problem.

  3. #3
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    Default Re: Potential noise from multiple PWM signals?

    Audible noise usually only results from coils/inductors. Especially if the circuit is unstable and their are parametric oscillations below the switching frequency. Make sure you have plenty of bypass capacitors and short wiring to a ground plane.

    You won't get any visible flickering provided you switch greater than 200 Hz.

    A capacitor across the LEDs shouldn't be necessary - it'll just smooth out the current flow through the LED - but at the cost of higher losses due higher peak current in the driver and losses in the capacitor internal resistance.

  4. #4

    Default Re: Potential noise from multiple PWM signals?

    Quote Originally Posted by mash.m View Post
    hi,

    no cap on the led! you will bridge the pwm function and the brightness controll will not work probertly.
    wich frequency do you plan to youse with the 555? if you drive your driver ic below 100khz i see no problem with the pcb layout. do it with short lines. you can also freewire the 555 or take a breadboard, still no problem.
    Thank you. Now that I think about it more, I can see how a parallel cap would interfere with dimming.

    This is the first time I've tried to control this particular driver IC with a PWM signal. The frequency will be between 180 and 325 hertz with a 50% duty cycle.

    Quote Originally Posted by MikeAusC View Post
    Audible noise usually only results from coils/inductors. Especially if the circuit is unstable and their are parametric oscillations below the switching frequency. Make sure you have plenty of bypass capacitors and short wiring to a ground plane.

    You won't get any visible flickering provided you switch greater than 200 Hz.

    A capacitor across the LEDs shouldn't be necessary - it'll just smooth out the current flow through the LED - but at the cost of higher losses due higher peak current in the driver and losses in the capacitor internal resistance.
    Sorry, "noise" probably wasn't the word I should have used. I'm worried about the four separate PWM signals interfering with each other and causing some sort of "ripple" throughout the circuit, which in turn effects the dimming control of the four separate LED's. This is a relatively small circuit with a foot print of about 100 mm x 100 mm.

    Thanks for your help. It appears I was getting concerned over nothing.

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