Does My Circuit Contain High-Speed Signals?

Eyal78

New Member
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:
  1. 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.
  2. 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.
  3. Push button with a pull-down resistor, connected to a microcontroller input.

Thanks in advanced for any answer,
Eyal
 
Since it seems :

1) No external high freqs being fed to pins or incoming present
2) No high current loads

You should have a forgiving design. But you have a 32 Mhz clock dumping
charge into a ground return and its supply pin, so good bypassing, low L
trace runs, good caps (combo bulk and ceramic) on power pin always a
good idea.

Caps technologies and their ESR :



OSCON is polymer tant, bulk.

Dont forget how to handle unused GPIO pins, Microchip has ap note on this topic
and or forum threads.


Regards, Dana.
 
Last edited:
Thanks Dana!
What do you think about PWM output (low 500 Hz, but fast edges of the MCU)?
 
C load, if light, and PCB run to its destination short, should not be a
problem. Does output pin have a speed selection ? Eg. its current capability ?
What is its destination ?

I see this in another datasheet :



Note the NSC ap note on layout target towards switchers, but excellent overall ref on PCB layout.
 

Attachments

  • Chapter12.pdf
    1.9 MB · Views: 68
  • Meeting Power Design Challenges 2007 National Semiconductor Seminar.pdf
    1.5 MB · Views: 76
Last edited:
In reference to my original question:
It's the PIC16 MCU output (I think not possible to select slew rate, if this is what you mean), The destination is less than 5 cm away to the LED2000 LED driver. Its load is 2uA.
But isn't it a fast signal anyways, requiring a proper ground as a return path, in proximity to the signal? otherwise crosstalk, EMI and other troubles occur?
 

Many PIC's have a slow slew rate option on the I/O pins, however using it for PWM is a bad idea, as you want the switching device to 'switch' and not work in a linear fashion. Slew limited is usually the default option, so is one of the things you need to disable for most uses - for example leaving it ON severely cripples SPI speed.

But mostly, you're massively over thinking this - just get it built, requirements are really pretty low, just apply a little common sense in your design.
 
The dimming input to LERD2000 is a logic input, looks to be buffed by its specs.

So PWM slew rate option, if it exist on your specific part, eg my post #4
table of GPIO selection for a generic part, would be fine and lower the
EMI generated by design. PWM input slew rate does not affect your
design switching control of the LED2000.
 
Thanks for the files, you made my day
 
Last edited:
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