Does My Circuit Contain High-Speed Signals?
My circuit includes the following components:
• PIC16 with internal clock, frequency: 32 MHz
• ST LED2000 LED driver (a buck converter) with a PWM dimming input
• TI BQ24070RHL battery charger
• LED controlled via PWM by the microcontroller.
My question:
Do the following signals qualify as high-speed?
I’m asking this to determine whether I need to ensure a proper return path for them and/or maintain large spacing between them. If not, can I ignore the return path considerations?
Key signals I believe require attention:
- Charger outputs – These are open-drain outputs. These outputs are connected to the microcontroller inputs (with internal PU). I couldn't find rise/fall time specifications in the datasheet.
- PWM outputs from the microcontroller – Base frequency: 200–500 Hz.
One output goes to the LED driver (possibly with or without a pull-down resistor).
Another output drives an LED.
- Push button with a pull-down resistor, connected to a microcontroller input.
1 answer
No, none of your signals sound like "high speed".
However, that doesn't mean you should ignore high frequency noise issues. If you can dedicate one layer to be mostly a ground plane, that would be good. If this is a high volume product optimized for low cost, then you get creative with only two layers. Otherwise, don't waste engineering time trying to squeeze things into only two layers. Use 4 layers minimum, and dedicate one of them to be a ground plane.
Your real sources of noise are not the signals you mentioned, but whatever gets switched by the PWM signal and the output of the switch inside the buck converter.
If the buck switch is perfect, there will be a very high dV/dt when it turns off as the inductor voltage goes as low as it has to to keep the current flowing in the immediate short term. That usually means a fast slew from power to ground. That edge will be a much more significant source of noise than the signals you explicitly mentioned.
Another often overlooked source of noise is the ringing of a switching power supply inductor during the off phase in discontinuous mode. You might be driving the switching power supply at 200 kHz, but when the inductor is off, its resonant frequency with the little parasitic capacitance can easily be several MHz.
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