MCP73831 MCP73832 Lipo Charger questions - TVS, Transzorb, RPROG, ESD Protection

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.

Questions


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):

6.1.1.2 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

https://www.powerelectronics.com/content/article/21855733/charging-liion-batteries-for-maximum-run-times:
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:
rProgFormula.png

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

https://www.soselectronic.com/articles/microchip/microchip-with-mcp73831-you-ll-charge-lithium-cells-easily-and-safely-1695:
in respect to thermal stress of [the MCP73831] it´s better to supply it by a voltage close to max output voltage (4.2v).

And

https://www.soselectronic.com/articles/microchip/microchip-with-mcp73831-you-ll-charge-lithium-cells-easily-and-safely-1695:
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?


And finally…

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:


Thank you
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.

rProgFormula.png

Perhaps this explains why the MCP73831 breakout boards from companies like SparkFun and Adafruit don't have a transzorb?

Generally not, because almost all boards in this type of trade are for development only, they are not finished products.

The final product must have all the necessary and / or possible protections, in order to avoid the activation of the guarantee by the customer. But note that these development boards have virtually no guarantee.

Like the development boards that the chip makers themselves produce, the vast majority of boards designed for development are based on the datasheets that the manufacturers make available.

To avoid depreciating the manufacturer's reliability image, they provide information on ESD, maximum supported limits, safe limits, etc.

It is up to the developer to look for the best recipe for joining different components to achieve a reliable final product (although lately electronic devices have been developed with a not very high durability, compared to the old devices, which could last for decades).

So I want to charge my 250mah battery with a supplied current of 250ma.

Care must be taken when choosing the battery charge current, the recharge current equal to (or close to) the battery capacity is considered a fast charge, and only the most sophisticated chargers that do good monitoring (especially the temperature) of the battery have conditions to enable the battery to reach the useful life that the manufacturer has estimated.

The most recommended for low power equipment is to use a slow charge, which is usually between about 10% to 15%.

For example, some portable battery manufacturers (NiCd) cite 11% with a duration of 15 hours, and a shelf charger usually has low current charging values, which are generally compatible with the most used batteries.

That is why it is necessary to know the battery, through the datasheet it is possible to know which is the best way to charge the battery, although it is recommended to recharge with a fast charge, in the datasheet you can some vital observations.

If by chance, it is not possible to find the datasheet of the battery, the most recommended is to use a slow charge, which will make sure that it will not heat the battery, and thus avoid the generation of gases inside the battery.

is there any danger of say, plugging it into a 2.1a (2,100ma) socket?

Yes. That's why developers need to use protection elements, to prevent the circuit from consuming more current than the power supply can provide.

A resettable fuse, PTC, has been used lately. When the current rises too much, the PTC heats up and increases the internal resistance, thus reducing the current.

The choice of the protection element, needs to take into account some parameters, such as the actuation time (slow, fast, ultra fast) and also the peak current that the circuit may have occasionally.

Electrical devices usually have a fuse element with 3x the nominal current, but this is not a rule, the developer needs to study his circuit and measure the best way to protect the internal circuit, and also protect the external circuit, in this case the power supply.

For example, if you consider the power supply to be 2A, and the circuit protection is 1.5A, this may pose a risk to the user, as it is very likely that the user may try to use it on a computer's USB port (PC) , and these PC ports usually provide up to 0.5A, so the 1.5A protection is unlikely to protect, the PC protection is more likely to be acted on earlier.

USB sockets are supposed to supply 5v, but some may be out of spec

Theoretically, the 5V tolerance is 5%, so it should be between 4.75V and 5.25V. (You can find more details in the specifications of the USB standard.)

Regarding the electrical supply recommendation with voltage closer to the final battery voltage, this is due to the fact that the manufacturer may use linear regulation circuits, which means that the voltage will be lowered as if there were a resistor operating there, that is to say that the greater the difference between the input and output of the regulator, the more the chip must heat up.

And since these chips are generally very compact, it is not recommended to allow them to heat up too much, especially to avoid that the temperature reaches the maximum limit that the junction of the active tablet (silicon) is reached.

Some chips even have built-in thermal protection, in order to prevent the junction temperature from being reached, which is certain to irreversibly damage the component.

It may be possible to supply the charger circuit with a current lower than that programmed so that the difference between the input and the output is as small as possible, however, the current balance will be carried out by an external circuit, which could be a power resistor, to withstand peak current.

Or a switched current limiting circuit, which can be found on development boards with a small SHUNT resistor and an LM358 CI, usually has two settings, one for voltage and one for electric current.

Space and weight are at a premium in my project so I need to strip out everything I can

Battery protection can be the difference between something that just stopped working and a sealed coffin. You decide, still want to remove the protection?

Protection generally works under the following conditions:

  • upper voltage reached (4,2V?)
  • lower voltage reached (3,0V?)
  • maximum current reached (1A?)
    (some can monitor the temperature too, usually on a laptop battery)

For illustrative purposes, a pyrotechnic video follows, that you shouldn't try to do that in any way, toxic gases can be produced:

Note: the information was compiled from the internet, please consult an electronic engineer to confirm the data.