This post is about some of the design mistakes which I did in designing my GPS trackers and this mistake is associated with selection for the battery charger chips. I am sharing this so that you will be careful and aware when choosing the battery charger chips for your GSM based applications.
The first charger chip which we are talking about is the BQ24266 battery charger made by Texas instruments.
I had studied the data sheet and found that,
- It can operate from 12V inputs also as well as from USB (5V) input.
- It is having Power Path Management feature which means it can take battery as a power source as well as the USB/12V input. It can power the circuitry either from the charger, whenever the charger is connected and as soon as charger is removed it automatically switches back to the battery source so it can always power the device and it can save the battery power when the AC source is present.
That’s why I selected this chip as it had all these features needed.
The problems :
When we actually design this board I found that it had so many issues compared to what results we have using the other chip (MP2617) today.
Heating issue : This part is having a Thermal pad below it, and in spite of having a thermal pad it was getting too much hot .
Unregulated voltage at the output : Whenever the charger is connected, this device was giving 4.4V at its output. This application is having GSM module on it, and the GSM modules can only run from 3.6V to 4.2V, and anything above 4.2V the GSM module will turn off giving an OVER VOLTAGE WARNING.
That’s the reason why my circuit didn’t work whenever the charger was connected. So I took this up with Texas Instruments people and asked them why is this chip giving higher voltage even though the datasheet says that it’s regulated to 4.2V.
We had a discussion on the TI e2e forum and what they said is, this chip can give output voltage up to certain voltage offset based on what the input voltage is.
You can see the discussion in the above link. Point is, it’s not regulated .
This was the reason I had to change this chip because it was always giving me 4.3V, 4.4V sometimes 4.5V to 4.6V also.
So, Be careful when you are using TI BQ battery chargers because they don’t have internal regulation and there is no option to set the output voltage regulation.
If you are using any of the BQ series charger chipsets in your design for GSM applications, please re-consider your design because this chip will give you an output voltage that’s going to be out of range for your GSM modules, even the TELIT GSM modules also will not work at that output voltage.
The next mistake which I did is the selection of this microchip part MCP73871.
This chip is a linear battery charger chip and it doesn’t have a switching converter and it doesn’t need any inductor for operating.
It has the same features like it is taking up to 5V USB input and give out the 4.2V needed for circuitry and has power path management feature but again it had the same problems like BQ24266.
It gave a voltage at the output same as whatever the battery voltage was when the charging input is not connected.
If battery voltage is 3.7V it will give 3.7V but this was okay when the device is running from the battery.
But as soon as the charger is connected, the device gives out whatever the charger input voltage is. Like, if i connected USB as the charger, then it gave out 5V at the output. It was just bypassing the charger voltage and battery voltage without regulation.
There is no option to set regulated output value when charger is connected.
So, that’s why I had to discuss this with the microchip people and they said that this is not out of spec and the device is designed to give the battery voltage when the battery is present and when the charger is present device output just follows whatever is the charger input voltage.
But, this was not good for my designs and that’s why I switched to this MP2617 monolithic part now. You can also use it in your designs and it will give much better results compare to the any of the TI or microchip parts.
Lets talk a little about this charger chip.
This is the MP2617 switching battery charger.
This battery charger can take up to 12V input and give the regulated 4.2V or 4V to the output circuit and output voltage can be set using resistors.
Even though it’s operating at 12V input it doesn’t get hot.
Another good thing about the this chip is, its costs much lesser and has lesser supporting components required for operating this chip.
The foot print of the part doesn't need thermal pad connection or heat sink. This chip is very efficient and it doesn’t consume much power even though its delivering 2 Amperes of pulse current to the GSM module and doesn’t get even slightly hot.
So, that’s why I am using this chip now in all my designs now.
For visual explanation check the video below :