Power supply schem question

General project help for Adafruit customers

Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.
Locked
kaluce
 
Posts: 8
Joined: Mon Jan 31, 2011 3:13 pm

Power supply schem question

Post by kaluce »

I'm working on a battery powered device, and I'm trying to work on the lithium charging circuit. I'm using off the shelf sparkfun stuff, a 6ah lithium battery, MAX1555 charging IC, LM1117 voltage limiter, and a 7v wall wart. I'm not sure if this circuit is designed properly though. This is really the first board I'm designing, and I'm really learning as I go. I just don't want to end up frying a $40 battery and/or a $15 uC. The LM1117 is set to 5v output, which should be handled just fine by the MAX1555. VBus is the +5v directly off the USB plug CHG LED should be on if it's charging (from what I understand).

So, does this look correct?

Image

User avatar
westfw
 
Posts: 2008
Joined: Fri Apr 27, 2007 1:01 pm

Re: Power supply schem question

Post by westfw »

C5 should surely not be in series in the VDD to BATT+ circuit.

User avatar
zener
 
Posts: 4567
Joined: Sat Feb 21, 2009 2:38 am

Re: Power supply schem question

Post by zener »

Yes, hook it up like you did C6 and C7...

kaluce
 
Posts: 8
Joined: Mon Jan 31, 2011 3:13 pm

Re: Power supply schem question

Post by kaluce »

Ok, so I altered the circuit and I think I got it, but I'm not sure if you guys meant I should shunt it to ground or not too (wouldn't that create a short circuit?). The target board I've been looking at as a model is a great place to start, but it glosses over power supply design because it expects it to be off board. Is this better?

Image

User avatar
ImaginaryAxis
 
Posts: 267
Joined: Sat Nov 13, 2010 3:16 pm

Re: Power supply schem question

Post by ImaginaryAxis »

C5 and C8 need to be connected from Vbatt to GND, else you are blocking the DC path of the battery.

kaluce
 
Posts: 8
Joined: Mon Jan 31, 2011 3:13 pm

Re: Power supply schem question

Post by kaluce »

sorry for totally dragging this topic out of the briny deep, but I'd shelved the project for awhile, and now I'm just reusing some parts of it in a new one, and I wanted to make sure it was right before copying it

This is the whole circuit from start to finish, Pwr comes in at 7v, gets dropped to regulated 5v, which then goes to the Max1555, goes to the Li-ion battery which gets topped off as needed and into the rest of the circuit.

At least. . . that's how it works in theory. . .

Image

User avatar
Franklin97355
 
Posts: 23910
Joined: Mon Apr 21, 2008 2:33 pm

Re: Power supply schem question

Post by Franklin97355 »

As posted your VBAT is not grounded nor is it connected to the BATT pin of the max1555. Read up on what caps do in respect to DC.

kaluce
 
Posts: 8
Joined: Mon Jan 31, 2011 3:13 pm

Re: Power supply schem question

Post by kaluce »

I guess that's what copying designs from datasheets without understanding what I'm doing gets me. I read up on what I was doing in regards to DC current as instructed, and came up with this. this is connected to ground as specified in the MAX1555 datasheet, and I think now this would let the device charge and use the battery.

Image

User avatar
adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: Power supply schem question

Post by adafruit_support_mike »

NOTE: I'm reading the max1555 datasheet while I write this, so don't assume I'm an expert with that chip.

That said, you *have* solved the problem of capacitors in your DC charging path. C8 will smooth out any ripples in the voltage going into your battery.

At a casual glance, the 1555 looks like a regular battery charger that also has logic to swap between supply sources.. USB or a wall wart. Given that the 1555 will limit the output voltage and current leaving through pin5 (BAT), I don't know if you really need the lm1117 regulating the voltage that goes into pin4 (DC). It certainly won't hurt anything though.

I notice you also have a VDD line leaving the top of the diagram, presumably to some other part of the circuit. At a guess, you want the off-schematic circuit to get power from either the battery or the charging circuit.

You'll probably want some extra hardware in there. It's best to shunt the battery off to its own little world while it's charging, and have a separate path that carries external power to the main circuit.

kaluce
 
Posts: 8
Joined: Mon Jan 31, 2011 3:13 pm

Re: Power supply schem question

Post by kaluce »

The 1555 is actually to smooth out the homebrew PSU I have. Once I finalize the programming, I'll remove that from the board and use a stand alone wall wart which will be smooth enough in voltage that I shouldn't have to worry about it.

I'm supplying 5v@1A from the wart, which should be enough to run the circuit and charge (350mA is required for the charging circuit, and the rest will be available to the circuit it's connecting to)

As for the extra hardware, what specifically would be the parts to add? I've never used rechargeable battery circuits in any of my designs. usually I just rely on the LM117 to regulate power that way. This device is going to be part of a handheld, and using two detachable 3.7v cellphone batteries in parallel as the primary power source (this thing is original 1990s gameboy sized)

User avatar
adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: Power supply schem question

Post by adafruit_support_mike »

Broadly speaking, you want a supervisor circuit or voltage detector watching the wall wart's VCC input and deciding whether to use the battery or not.

Here's a Maxim tech note on the subject:

http://www.maxim-ic.com/app-notes/index.mvp/id/1136

As for specific parts, I haven't used any enough to give you solid advice. You can browse some options by going to Mouser.com, selecting Integrated Circuits, then navigating to Power Management ICs > Supervisory Circuits, and choosing the ones that monitor 1 or 2 voltages.

Locked
Please be positive and constructive with your questions and comments.

Return to “General Project help”