I’m working on a design that for the most part is working … with a few issues. I’m hoping there are some experts here that may be able to suggest some changes to address them. Let’s start with the attached schematic.
On the left hand side is the charging circuit which uses a MAX8903 to charge a single attached Li-Ion Power Cell using either USB or DC-in to do that. It also provides power-out through the SYS pins and I have a main power cut-off on there. This allows me to cut off power to the rest of the unit, while leaving the battery attached to the MAX8903 so that I can charge it regardless of the unit being turned off or on. This is a must.
Moving on, you’ll notice the battery is also feeding two LED strips through some Sziklai pairs. The reason for that is so that the LED strips can take full advantage of the current capacity of the power cell as they will pull upwards of 10A (and yes, the battery can supply that with no problem.) And they also don’t need any more than the 3.7V (nominal) that the battery will provide.
Next, after the main on/off switch is the boost circuit to drive the actual controller (which is an Atmel 32U4.) The circuit, based on a LM2621, will supply 5V/1A to the controller. The controller will trigger the Sziklai pairs when the strips need to be turned on or off.
As I said before, this all works for the most part … but here are the issues:
a) Because the main on/off switch is after the charging circuit, that leaves battery voltage on some parts of the circuit. What I’d like to figure out is whether there is a way to cut power off from the circuit, as in completely isolate the battery power, cut it off completely. Then, when I plug in either USB or DC in power, or flip the main power switch on, it then reconnect the battery to the MAX8903 to actually be charged or used (notice that it needs to be connected to the MAX8903 to provide power through SYS out pins.) While I could easily use a DTDP switch to cut of the battery, that would require turning the unit back ON when I want to recharge the battery, and frankly I like the idea that I don’t need to do that right now. So I’d like to keep the battery from being connected to a permanent toggle switch like that.
b) I’m getting what I can only refer to as a flyback whenever I turn the unit on or off, and it’s enough to cause some of the LEDs to flash on each time. Obviously that’s bad and I need to figure out how to stop that. I’m assuming it’s because of the way they are wired to VCC, straight from the battery. I don’t know if solving item (a) above will fix this, but I need to address it. I just don’t have a clue how.
c) While the Sziklai pairs work as far as shutting off VCC to the strips, they don’t work well when the software calls for turning them on or off. What I mean with that is, the LED strips are addressable, specifically LPD8806 based strips. When I turn on/off the Sziklai pairs, it scrambles the strips. For example, if the last command sent was to turn off all the LEDs and then the Sziklai pairs are turned off, the next time I turn those Sziklai pairs back on, the strip will come on with random LEDs lit up, random colors, just completely random. The reason I want to constantly be turning the strips on or off is to save on battery. The LPD8806s have what we tend to call ‘ghost power consumption’, where even when all the LEDs are turned off, the IC is still eating up current. So by cutting that off, I will save on battery consumption. The actual unit has 48 of those ICs in it and over time, that ghost consumption starts to add up. There are times where the LEDs are all turned off, but the controller itself is on, waiting for an interrupt to fire to turn the LEDs back on. So when they’re off, I want to cut off VCC to the strips and save on battery. Right now I’m turning on the Sziklai pairs and leaving them on till I shut the unit off. While that works, I’m draining the battery faster that way.
So there you have it, the issues I would like to tackle and fix, if at all possible. I have one other item that I would like to address, but that’s a wishlist item and I feel the above need to be addressed before moving on. So let the ideas/suggestions/questions begin …