Power from OBD and protection

[Rajinder1268]

Hi all,
I have a design based around car OBD port.
The current of the design would be 600mA.
I am thinking of using a diode for reverse voltage protection, then a PTC fuse rated at 3A, TVs diode, then my electronics. Most of the 600mA will be from a Bluetooth device @3V3.

I need to power circuits operating from 5V and 3V3.
Is it best to have a 5V linear reg then power a 3V3 linear reg from its output?
Or should they be separate?
Or should I use a buck dcdc for the 5V and then power the 3V3 from this? I am not sure of a good dcdc converter with good efficiency, can anyone recommend one,?


Finally I need to run a CAN interface at 125Kbps. How do I determine the clock frequency (external crystal) for my microcontroller to get the correct bit rate? Is there a simple equation.

Thanks in advance

 

[BradtheRad]

Once you have 5V, you can derive 3.3V via resistive drop through a string of 3 ordinary diodes at 0.6 V each.

 

[KlausST]

Hi,

I'd use a 5V buck, because I don't like the generated heat of a linear regulator of (13.4V - 5V) * 0.6A= about 5W.

Klaus

 

[Rajinder1268]

Hi,

I'd use a 5V buck, because I don't like the generated heat of a linear regulator of (13.4V - 5V) * 0.6A= about 5W.

Klaus

Yes decided on using a buck 12V to 5V, then power a 3V3 linear ldo from the 5V.