Sunday, January 21, 2007

CAN Bus designs - Servo and Power Supplies

Over the last month I've been working on the hardware for my CAN enabled smart RC servo board. Due to space constraints, I've decided to make it a double board.

Common components are two 6 pin 0.1" headers with 0.6" spacing. The CPU board contains the CPU, CAN transciever, and a few power conditioniong components. I am currently trying to avoid needing a LDO onboard. A four pin (two power two signal) header provides access to the network. The power board contains the H bridge and related sensors (current, voltage, temperature). I'm still trying to decide where to stick the analog potentiometer. One pin header is 6 analog signals. The other header is the four PWM lines and a ground and 5V power supply to provide power to the sensors. There's also a pair of separate unregulated power lines to provide motor power.

To make the LDO-less idea feasible, I need an efficient and low noise power system available. While I have some good sized capacitors on the digital supply input pins, I may need to take a different approach if this doesn't work well. This is why I decided to work with a split board design instead of attempting to beat together a monoboard design, at least until I have the design proven out.

So, to power this system, I'm looking at a DC-DC converter. I've been checking out Microchip's SMPS hardware. I'd love to create a dsPIC based design, but I think that'll take more work than I want to right now, plus I need to do a lot of safety design then. So, I'm planning a SMPS based around a stand-alone TI chip, the TPS5430, with a supervisory and control PIC riding along side. The idea here is to provide a garaunteed 2A 5V supply with a 10mV output ripple. I hope to make each power module talk to the rest to allow for some load balancing, but I'm afraid of some sort of interference. 22AWG wire is rated for 2A continuous power, roughly. The supply can do 3A with a 4A burst. I'm thinking about making the CAN passthrough on the board have one side powered. I'm considering making some of my less space constrained designs having an input voltage monitor so I can feed information back to the monitoring PIC to allow it to adjust the 5V output to provide sufficient power to the entire CAN string. I just don't know enough yet.

It's actually quite funny that I've been finding the dsPICs to be equivalent cost to similarly equipped 18F chips. While I haven't been able to verify, given that I'm aiming for near-constant operation for a lot of these devices, I hope that running a much lower clocked dsPIC will pull less power than the 18F for moderate-math applications.

1 comment:

David said...

Have you considered this SMPS:
Black regulator: Very cheap high-performance switchmode +5v regulator.

If you substitute a power transistor (for the cheap switching transistor), is that the only change needed to run it at 2 A ?