14V Rail - Ahhhh, Nope!

A project log for D-DAQ

automotive parameter & performance monitor & logger

Michael O'BrienMichael O'Brien 06/07/2014 at 08:250 Comments

So I know of LDOs and why they're called such. For some reason I thought there was a bit of magic inside of them that allowed them to output a higher voltage. Maybe this was just me being stupid... Well anyhow, as I'm collecting part info and choosing the resistors and capacitors on mouser, I realized that hey, my LDO's drop out voltage won't allow me to produce 14V with an input of 14.4V. Oh, so that is why I've not ordered my 2nd prototype board yet ;).

Well, all is not lost on my existing board. It looks like I just have to cut a couple traces and jump the 5V output to the ground of the 14V regulator. I'll adjust the voltage divider circuit to output 9V and then I'll get my 14V output. Handy side effect is that I'll be able to still drive D-DAQ with a voltage down to ~9.5-10.5V which is at the low end of a car's voltage when cranking over.

While looking into a solution for this mistake, I came across a nice reference describing some details about LDOs. Though I've talked little about it, heat/thermal dissipation is a big worry of mine. In the current setup, the 5V LDO will be dissipating about 3.1W of power with a 14.4V input. Not a problem at all! :\

Well, thankfully with the current board layout, I have the 3.3V output of the SMPS running right next to the 5V LDO. In light of this new information, I'm switching the 3.3V SMPS over to a 5V output and then changing the 5V LDO to a 3.3V output. This will basically double the efficiency of that LDO (~35% to ~66%) and also drop its power dissipation to ~1W peak. The SMPS efficiency I estimate will be about the same, but maybe better; roughly 93-94%.

Finally, the 14V LDO. Not a problem except the LDO's dropout voltage is ~1.1V. I'm seriously kicking myself because I'm not sure what I missed here. Now, if I got rid of the LEDs, I could drop to a different LDO, but I'd have to use 2 of them in parallel in order to provide enough current for 3 displays when it's a hot day in a car that turns into an oven. This leaves me with an alternate issue: change the a different LDO or make the input voltage greater. I'm favoring the latter because sometimes there are problems outside of our control such as an alternator failure of similar electrical problems that creep up over the life of the vehicle. If the supply voltage drops below ~14.2V but stays above 6V, displays won't turn on, but every other bit will work just fine. Little bit of a problem if you're not familiar with the underlying hardware of D-DAQ when a dark display is indicative of a electrical problem of the car.

For these reasons, I'm going to attempt to bolster the 14V rail by doing 2 things. I'm going to switch over to either a pair of LDOs with a thermal dissipation of ~0.4W @ 50˚C at bare minimum or one that do double that with ease. Also, just before the LDO(s) though, I'm adding a single stage charge pump. I won't get doubled voltage, but I hope to achieve an operating voltage of at least 10V, possibly 9V.

Unfortunately these last two discoveries mean that the first prototype board of D-DAQ will not continue to get stuffed. Once these power supply subsystem changes are implemented on the schematic and board layout I'll again, let it sit for a day or two and have a look over it again and continue finding the components. Ironically, these issues were found as I'm simply going through and generating my BOM and double checking that everything looks sound.