I have a device that uses an usb connector to talk to a computor. Lookng for a method on charging a 6 Volt lithium battery set. Obviously I need a switch power supply that can change a 5 V 1.5 A source to a 7 volt with enough current to charge the battery in a reasonable time. I can find power supplies on the digikey web page that will do this but at 50 mA which would take for ever to charge. Is there a method or a joule thief power supply part that can do this at a good rate.
Are you looking to get 5V @ 1.5A out of a normal USB port on a computer? Typically, USB's max output is 2.5W (500mA) and that target is realized only with device/host negotiation (external chargers designed for cell phones often can supply that much). Also, there are some major issues with just throwing a voltage on lithium rechargeable; they die a fiery death if not done properly. Is the output of this boost converter going into an appropriate charger (constant current then constant voltage) that balances the cells if needed? If this is for a hobby project I'd highly recommend just buying an off the shelf charger for your battery. If this is for a production unit, it might warrant a bit more research before deciding what option to go with.
I had no idea you could pop a battery if you did not have a constant current and voltage source on it. Well now I know the external charger not being an option how do I get there with only a 5 V source from a PC. 500mA should be plenty of current for this application but I cann't seem to find a solution to manage the low voltage (5 V). The microchip MCP73213 seems to be designed for what I need except I need a 9 V input. Back to the orginal question is there a switch power supply that can ramp up the 5 V to a 9 V + source. I'd use a 3.7 V battery if I did not have too many 5 V devices to interface to. Looks like I'd need at least 130 mA of current.
Lets get your requirements striaght. You say that you need 9V input, did you mean output, as in you need to use 9V for a specific reason? Are you still trying to pull off of USB? Is there some other outside source?
Please restate your systems power requirements in a clearer form.
Why not use something like VWRBS2-D5-S9-SIP. dc-dc converter, 4.5-9VDC input, 9VDC output, 222mA maximum output
You will of course need to limit the current and reduce the voltage to meet your charging requirements, but that's the easy part.
Sorry I did mean input. So it looks like a 5 V 500mA input to a step up.
to step the voltage over 9 V
MCP73213 to charge the duel lithium batteries
Albiet the effeciency is bad but I can charge the device anywhere with a usb cable. Not a bad trade. Just wondering how much of the 500mA I get from the usb cable before I layout the board.
I do need a voltage higher than 5 plus the dropout voltage of a linear reg to run other devices. The lithium batteries supplies the 6 V to the circuitry in noncharging mode.
If all you are trying to do is power 5V components and charge a lithium battery off you should be able to get away with a single cell lithium technology battery and two chips(charger w/OVP and boost converter w/ over discharge protection). There's likely no need for a linear regulator when a switcher can just generate the 5V you need and offer over and under voltage protection (both critical in this case) for the Li battery. If the linear regulator is off in an untouchable part of the design or there are other needs like noise or ripple problems that the linear regulator can help solve then there may be a case for it.
Are you sure you have a 6V set of batteries (I'll assume set of 2). Most Li based product are nominally around 3.7V (7.4nom or ~8.4 charging), and most chargers are designed around that range of voltages and specific type of battery (there are many types of Li batteries). Designing incorrectly with Li rechargeables can have extreme consequences, especially if going into a product. You may want to consider a chemistry such as NiMH that has it's own trade-offs, but doesn't have the same issues if done incorrectly. If working with lithiums, quite a more research and design requirements need to go in the process and talking to your lithium battery supplier for recommendations might be warranted. Here's another approachable article on some of the pitfalls of using Li technology. It is geared more toward hobbyists, but the information is relevant all the same. There are lots of references designs out there (more on than just in that link) for this and similar applications, and even premade boards that include both the http://www.google.com/search?q=MCP73831T+and+TPS61200.
Unfortunatly it is a 3V battery I am locked into the socket by another party. The lithium battery is one sold at digikey Br-2/3agn. If there is a single battery that is 3.7 V that can be used to generate a 3.3 V, 1.7 V and 5V, I have to get the other party to concede but not impossiable. This does sound like a better solution. The linear is not required so a switcher will work just would like to avoid. I did not see any thing on the MCP73213 that said 6 V or 6.6V and was planning on asking microchip when I decided on the final solution for the battery charger. Threre is an LCD that could draw allot of current on this board but I think I could get away with a lithium battery but I am not sure.
Possibly the best solution (for charging a 6 V battery(Chargeable ones only) is to possibly use a doubler and try a zener to clip it to what evr level required and try chrging. Please note that you the chrging current is limited at source(USB PORT) to 500 ma, ideally you should not draw more than 250 ma, internal circuit capacities could be different with different computers/devices, That's why!
The Constant Current and Constant Voltage specs of a Li Rechargeable Battery serve as limits not to exceed. Charging the Battery under those limits are safe.
If you use USB as the power source, you're unlikely to exceed the Constant Current limit. Just observe the Constant Voltage limit.
Measuring the voltage of a battery doesn't comsume power through the current-sense resistor like measuring the charging current, and is more easily accurate.
For only 2 cells, and within a limited number of charge/recharge cycles, cell balancing is not necessary if you start out with matched cells.
Also, at the low charging current provided by USB, over-temperature protection might not be necessary.
If your moniker AllieCat suggest your cheap modus operandi, you can cut lots of corners to save money and save power waste (in order to pump charge into the battery faster) when designing the battery charger for USB only.