I’ve searched the forums and read the datasheets but there are still questions regarding the MCP73831 Lipo Charger that I haven’t been able to find answers to…
Apologies for the lengthy post but I figured it made sense to keep all the questions in one post to make it easier for anybody else using the MCP73831 to find this information.
To give you some background…
I posted the full schematic for my project in another thread as I didn’t want to spam the forums but in hindsight I don’t think it was a good idea. Instead, I’m going to try and focus on one part of the schematic at a time. This part deals with charging and I’m reaching out to the community to see if you can help with some things which I’ve failed to understand?
My project is powered by a single cell 250mah lipo battery (typical 3.7v, 4.2v at max charge). To charge the battery, I have a micro USB socket onboard and the charging is being controlled by a Microchip MCP73831.
Here’s my schematic for the charging part of the circuit.
My first questions relate to ESD protection...
Question 1 - Is this a suitable transzorb?
From Page 18 of the [url=http://[datasheet](https://cdn.sparkfun.com/assets/learn_tutorials/6/9/5/MCP738312.pdf):
188.8.131.52 Input Overvoltage Protection (IOVP)
Input overvoltage protection must be used when the input power source is hot-pluggable. This includes USB cables and Wall-type power supplies. When the supplies are connected/disconnected from the system, large voltage transients are created which may damage the system circuitry. These transients should be snubbed out. A transzorb connected from the V+ input supply connector to the 0V ground reference will snub the transients.
Looking at the document revision history in the Appendix, you can see that the above extract was one of the last things to be added to the datasheet. Perhaps this explains why the MCP73831 breakout boards from companies like SparkFun and Adafruit don’t have a transzorb?
This is a question I've done lots of research into but haven't been able to find a definitive answer to.
I’ve found this transzorb (PESD5V0V1BB) but I’d like somebody with more experience than me to confirm if this is a suitable solution?
Question 2 - Human Body Model ESD Protection?
Page 3 of MCP73831 datasheet also has this to say about ESD Protection.
ESD protection on all pins:
Human Body Model (1.5 kOhm in Series with 100 pF)……..4 kV
What does this mean?
My interpretation of this is that the MCP738312 is succeptable to damage from static generated by the human body, and this is how you should protect from it. Could somebody please elaborate? Is this what the datasheet means? Where in the circuit should those components go?
My next questions relate to current/amps...
Question 3 - is a 4k Resistor on RPROG correct for a 250mah battery?
I read that
The preferred fast charge current for Lithium-Ion cells is at the 1C rate. For example, a 1000 mAh battery pack has a preferred fast charge current of 1000 Ma.
So I want to charge my 250mah battery with a supplied current of 250ma.
According to page 15 of the datasheet:
Fast charge current regulation can be scaled by placing a programming resistor (RPROG) from the PROG input to VSS. The program resistor and the charge current are calculated using the following equation:
The preconditioning trickle charge current and the charge termination current are ratiometric to the fast charge current based on the selected device options.
So my understanding is that I need a 4k resistor (1000/250=4) on the PROG input. Is that correct?
Question 4 - is it safe to use a 2.1a USB socket?
So, my charging circuit is drawing 250ma approx (right?).
All USB sockets are capable of delivering this, but is there any danger of say, plugging it into a 2.1a (2,100ma) socket? I think the answer to this is that the device will only draw the current it needs so plugging into a 2.1a socket is safe, but I thought I best check just in case.
Question 5 - Do I need to/ How would I limit supply voltage to 4.2v?
USB sockets are supposed to supply 5v, but some may be out of spec. I read that
in respect to thermal stress of [the MCP73831] it´s better to supply it by a voltage close to max output voltage (4.2v).
The chip can be easily supplied by a standard 5V voltage, but in cases of increased risk of overheating […] a common Si diode in series can be helpful. This will decrease supply voltage in 0.6-0.7V.
I understand from this that the author is suggesting you should limit the voltage to the MCP73831 to the max voltage of the battery it will be charging (in this case 4.2v). The reason being that it will help prevent the MCP73831 from overheating. Do you think this is necessary, and if so, could you elaborate on the authors suggested solution (Si diode in series)?
Question 6 - What do the different model options mean?
Page 25 of the datasheet shows this table for different options of MCP73831 that are available.
For example, the MCP73831-2ACI/OT has the AC option.
What do the different options mean? Or more specifically, which one is right from my project?
Question 7 - Can I remove the circuitry from the battery?
Apologies if this is a stupid question, but if you don't ask then you don't know...
Space and weight are at a premium in my project so I need to strip out everything I can. Given that my schematic has protective circuitry, can I remove the protective circuitry from the LIPO battery? Like the one seen in this photograph:
As I say, I appreciate this is a long post so thank you for having reached this far. Hopefully putting all of the questions in one place like this will help others to find this information in the future. Thank you in advance for any time taken to contribute to the thread.