I am designing a PCB and am stuck on how best to distribute power. I wonder if people here can help, please?
The board I'm building should accept 24V DC in, which I need to power a Sparkfun stepper driver. Also on the PCB are an Arduino Mega, a USB host shield, and an LCD display. I'd like to drop the voltage down to, say, 9V and feed that into the Arduino and USB host shield. The idea is to power everything with one 24V input and drop the voltage on my PCB, rather than have two DC inputs from two AC/DC adaptors. The latter doesn't seem like the elegant solution.
I've built a test rig with the Arduino and LCD. I tried powering this with a 9V 1A LDO (Digi-Key BA09CC0WFP-E2TR-ND) but it gets way too hot when I feed it 24V, even with a heat sink. So then I looked into board-mount switching supplies. The best I could find is a TDK-Lambda which accepts 18 to 36V and delivers 12V (Digi-Key: 445-2493-ND). 12V is more than I'd want, though, as I find the regulators on the Arduino and host-shield can get rather hot at this voltage. One possibility is to feed the 12V into the 9V LDO and have that power the Arduino and host-shield. This won't produce less heat over all, but at least I can make sure this LDO is soldered to a large ground plane and I could attach an extra heat sink to it also. However, on my test rig with a 12V input the heat sink on the LDO above is almost too hot to touch. So I'm not particularly happy with the TDK-Lambda to 9V LDO solution.
Can anyone think of a better mix of components or a better approach to the problem?
Thanks!
EDIT: I estimate I'd need about 400 mA at 9V. The Arduino plus LCD consume 250 mA, so I'd need a bit extra for the USB host shield plus the USB peripheral it's feeding.
Hmmm... That may work. There are two issues. My prototype system (i.e. not the test rig I mentioned in the original post) runs on two PSUs (24 V for motors and 9V for everything else). When I hook up this system to a PC to upload firmware, I've noticed that it doesn't get enough current to run the USB host shield if the 9V supply isn't on. I'd have to check where the bottleneck is: is it the PC's inability to deliver enough current or the Arduino not delivering the current to the host shield? The other thing is that I'd rather have a through-hole or surface-mount component.
I think it's about time we introduced the schematic for your circuit. Can you draw one with pen & paper and take a photo of it wirh a cell phone and post it?
Even at 12V my existing regulator gets pretty hot at 250 mA. To even think about using an LDO I'd still need a switch on-board PSU to get the voltage. I think I'll go for the switching 24V to 9V unit I list above. It seems perfect as far as I can tell.
