Circuit Design for Voltage Regulator

Thread Starter

philippom4

Joined Oct 3, 2018
1
Hey everyone, this is my first time posting here so bear with me and please let me know if I break any forum rules in this post.

I am working on a project for one of my classes that needs a lithium ion battery to power the circuit. The digikey part number is 1568-1488-ND. The charge voltage is 4.2V and the charge current is anywhere between 1300mA and 2600mA. I was hoping to use a usb power supply, like the ones that you would use to charge your phone. If my understanding is correct, these power bricks convert 120V AC from the outlet to 5V DC at the usb plug. So I'm hoping to be pointed in the right direction to design a circuit to convert this 5V DC to 4.2V DC with 1.3A to 2.6A at the output. I understand I could use an IC chip but the cost needs to be as low as possible. Thank you.
 

dl324

Joined Mar 30, 2015
9,567
Welcome to AAC!

Li-ion batteries need to be charged properly or they can cause fires. You'd be better off trying to save on cost elsewhere.
 

Audioguru

Joined Dec 20, 2007
11,251
If you are trying to design your own lithium Ion battery charger then read about all the important things you never thought about (www.batteryuniversity.com) or buy a Lithium Ion battery charger IC that does all those things. If you do it correctly and safely then your circuit will probably cost more than the IC.
 

MrAl

Joined Jun 17, 2014
6,940
Hey everyone, this is my first time posting here so bear with me and please let me know if I break any forum rules in this post.

I am working on a project for one of my classes that needs a lithium ion battery to power the circuit. The digikey part number is 1568-1488-ND. The charge voltage is 4.2V and the charge current is anywhere between 1300mA and 2600mA. I was hoping to use a usb power supply, like the ones that you would use to charge your phone. If my understanding is correct, these power bricks convert 120V AC from the outlet to 5V DC at the usb plug. So I'm hoping to be pointed in the right direction to design a circuit to convert this 5V DC to 4.2V DC with 1.3A to 2.6A at the output. I understand I could use an IC chip but the cost needs to be as low as possible. Thank you.
Hi there,

As everyone has pointed out already, charging Li-ion batteries requires a specially designed charger. These kind of chargers have to be able to charge these cells safely and that is very important with batteries that can explode if not done right. So many of the exploding battery problems in the past have occurred while charging Li-ion cells. It's the most dangerous phase of the use of these things.
Also, going from a regulated 5vdc to 4.2vdc requires certain design considerations because of the relatively low overhead voltage of only 0.8vdc. Many dedicated charger chips are made just for that purpose. They actually require a regulated 5vdc as input to keep internal chip power dissipation low.

After all is said and done, you either have to consider a host of different design ideas and practices or you have to buy a dedicated charger IC for the job. If you go with a custom design though you run more of a risk of fire if you dont do something right, but even if you do use a dedicated chip you have to follow the design guidelines for that chip to the letter.

Good luck with it :)

Simplified wall chargers use a regulated wall wart designed with limited current output. The output voltage is set to around 4.2v and the basic current output is limited. There is no timer, no under voltage detection, etc. It's just a cheap low current wall wart that puts out 4.2vdc once the current draw is low enough. The current limit could be as simple as a series resistor and thus the charge time is longer than it has to be because the output impedance is higher than it has to be.
The better chargers have a charger chip made just for charging these kinds of cells.
Some devices have the charger chip on board and so just require a 5vdc regulated wall wart.
 
Last edited:

DickCappels

Joined Aug 21, 2008
6,079
And as dl324 has already pointed out, those cells need to be charged with a specific voltage/current/time profile. You may have read about battery powered appliances bursting into flames here and there. Some of thse were poorly made batteries and some were because of poorly designed (cheap) chargers.

Stepping back a little bit, I have read and believe that you can safely charge an Li-ion cell at a current equal to or less than 1/10th the amp-hour rating. That makes for a very long charging time but that might be another solution.

Regardless of the direction you choose to go, read the datasheet and other relevant literature offered by the cell's manufacturer and follow it carefully.
 
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