Hi kicad1978,
I would fully agree with Vicente refarding to EMC (take the inspiration from our designs TWR-KL43 etc).
Be very carefull with placing correct components on that part, especially on internal voltage regulator (see also Mark's Space: USB VBUS over voltage issue.). Our VREG is very sensitive on overvoltage stress, the max. VREGIN is 5.5V. Any short voltage pulses over that value (even 6V with dedicated energy) can lead to damage voltage regulator. Hence, it is striclty required to add TVS diodes, I would also prefer capacitor on VREGIN to be tantal or electrolytic (with high ESR which will damp oscilation, or serial ceramic capacitor + resistances etc.). The oscilation are deeply dependent on VREGIN capacitance and the length of USB cable. Shorter cable (lower inductance) can produce higher frequency oscilations which at the end can lead into higher voltage levels at very begining of USB cable plug-in. The higher ESR (or external serial R) can damp these oscilation down, However, high speed TVS can also significantly help to reduce/absorb these voltage peaks. Also some power switch placed between USB power and VREGIN can help isolate/delayed each other diring power on.
The final behaviour of the board will of course depends on many factors: PCB layout, component selection (R, C, TVS), quality of component assembling, working conditions, application behaviour (if there is a direct higher power consumption at the very begining) etc.
I guess that your questions came out from the view of reducing the total cost. I personaly do not like such approach because it comes out from assumption of "ideal world", no EMC, no parasitic componenst etc. However, I fully understand these trend :-) .
These are some of my suggestion:
Q1:
- in some cases it can help.
Q2:
- yes, place it close to VOUT33
Q3:
- I would prefer to have it there. The value can slightly depends also on application requirements. Sometimes it is necessrary to detect power lost. In such case the low voltage detection can detect the event and make some special required actions e.g. store some data into the flash which at the end will require some energy to be stored in external capacity. But this is most probably not your case.
Q4:
- yes, I prefer tantalum, or electrolytic or serial ceramic with resistance (R = ~ESR of tantal resistance). Place it close to VREGIN pin (the inductance created by the PCB path from VREGIN to USB connector will be added to inductance of USB cable). Higher inductance L will dicrease the resonant oscilation frequency (L + C on VREGIN) which at some point can help to reduce voltage transients.
Q5:
- depends on your application. If your use ADC measurement and VREFH as reference for in your application then I prefer ti have it there. SAR ADC on Kinetis devices works as redistributive charging principle SAR, it means there are capacitor banks inside which need to be properly charged/discharged during aproximation steps of converison. These can require some special current peak during converison. These capacitance can help for that. Place it close to VREFH then.
NOTEs:
- I strictly recommend to add TVS diodes to USB power and data (make inspiration from our designs)
- add ferite beads to USB connector case and GND
- if ADC used and higher accurancy required then I recommend to isolate VDD and VDDA from higher frequency components (adding ferite beads)
- power switch on VREGIN can also significantly help, and in some cases it can be even cheaper solution.
regards
R.
