Welcome to battery charge most common issues training for large charge time, which is typically caused by charge current being too low. My name is Jeff [? Faylen. ?] I'm an applications engineer with over 15 years of power supply and battery charge design experience.

First we'll review the battery charging system. The battery charging system consists not only of the battery charger itself, but also its input source and the host processor or microcontroller that is controlling the charger. The input source provided-- it could be either an adapter or a USB port and provides input current and voltage. The control from the host processor or microcontroller can be in the form of I2C or SMBus, commands, reads and writes that change certain registers in the charger, or it could be GPIO pins that change the state of the charger's input pins.

The charger provides a system voltage, usually a minimum voltage to make sure that the system doesn't collapse, as well as current or power to the system, as well as regulating the voltage and providing charge current to the battery. The charger also ensures that the battery isn't charged outside of a certain temperature profile and then provides safety and protection in the form of preventing overvoltage, overcurrent, and over-temperature.

Next we'll review the lithium ion battery charging profile. Assuming we start with a battery that is completely discharged as shown in green on the curve, the battery charger will only provide a small trickle charge or a pre-charge current in order to close the battery protector. Once the battery protector closes, the battery voltage should jump to a low value such as around 2.8 volts. And at that point, the battery charger will begin fast charging. And this whole time between trickle charge, pre-charge, and fast charge is called current regulation phase of the battery charger.

Now some chargers do provide a system output voltage. And during this time when the battery voltage is low, these charges will provide a minimum system to ensure that the system that the battery is intended to power can start up. Once the battery reaches the regulation voltage, or at least the voltage at that pin of the battery charger has reached the regulation voltage, at that point the charge current begins to taper and the battery charger enters voltage regulation mode.

The system voltage typically sits just a few millivolts higher than the battery voltage. And then eventually the charge current will taper completely down to the termination current level and battery charging will stop and the battery charger will have terminated. If for some reason the battery is damaged, then a safety timer will expire to ensure that the charger stops charging a damaged battery.

So unlike a buck converter that simply regulates output voltage, a buck based battery charger regulates more than just output voltage. So as the block diagram in the top left shows, we do have a feedback loop that is monitoring the output voltage or battery regulation voltage, comparing it to a reference and then using that to make sure that the buck converter doesn't try to regulate to a higher voltage. But we also have other feedback loops, including the charge regulation loop to make sure that the charge current going into the battery doesn't exceed a specified voltage.

But battery chargers have other feedback loops, as well, to ensure that the charger doesn't cause other system type problems. For example, the feedback loop shown on this particular slide is monitoring the input current going into the charger. And the purpose of this feedback loop is to ensure that the charger doesn't collapse the input source. So we're assuming that we know the input source's current capability. We set a threshold and then ensure that the charger doesn't pull more than that current out of the input source, thereby collapsing the source.

As you can see from the graph at the bottom left, as the input current increases, we're still able to maintain charge current and provide system current. But if the system current continues to increase, we'll eventually hit the input current limit. And at that point, the charge current that that charger is providing reduces. And then potentially it reduces to the point where the battery is no longer being charged, but is actually having to supplement the system load, because we're sitting at input current regulation. Those loops are called input current limit or Input Current Dynamic Path Management, hence the acronym IIN_DPM.

So in the event that this is causing your charge current to be lower than you expect and therefore a longer charging time, the potential solutions would be to obviously reduce your load on your system, if that's possible. Sometimes that's not possible. The other option would be to increase your power source and/or your IIN_DPM setting. But again, if you know your power source is limited increasing the IIN_DPM setting can cause other problems.

It is possible that the IIN_DPM setting is incorrect. Some of our parts do have BC1.2 D+/D- detection of the input source. And at that particular detection time, the input current limit is set. The expectation is after USB enumeration that that setting will be updated and therefore more accurate representation of what the power source can provide. So that's always good to check to make sure that you have done an USB enumeration and you are setting at the right input current limit setting.

Now some of our parts also have this ILIM pin that clamps the input current limit regardless of what the register setting for IIN_DPM is. This is a safety feature for the parts when they first start up and are in default mode, in other words, before the I2C register is updated. So it's imperative that that particular ILIM pin be disabled after the system is up and able to more correctly set the input current setting.

Just as having too little input current can cause a problem with long charge times, having too low of an input voltage and therefore, a lower input power, can also cause a problem with longer charging times. As the schematic at the top of the slide shows, there is always additional resistance, undesirable resistance, in your charging system. So on the input side that can consist of your wire or cable resistance being too high, the connector between your power supply and the charger having high resistance, and then also PCB board trace resistance being too high. And if that's the case, then even though you might be getting enough current, the voltage drop across that resistance could cause the voltage seen at V best pin to be too low and therefore preventing the charger from running outside of max duty cycle or even be too low and be lower than the battery voltage itself.

So therefore, one of the other loops that we have in a charger is the input voltage loop or VINDPM loop. That particular controller monitors the input voltage against a preset threshold. And then if the input voltage to the charger droops below that particular-- or reaches that particular threshold, the charger scales down the output current to make sure that the input voltage doesn't droop any further.

And as you can see at the graphic at the bottom, assuming we're charging and the system voltage continues to increase and that's of course causing the input voltage or the input current to increase. And the droop across that undesirable resistance causes the voltage seen at the BAT pin to reduce and then finally hit our VINDPM threshold. And at that point the charge current continues to reduce and then finally to the point where the battery is not longer being charged, but must supplement the system load.

So in order to prevent this particular problem, it's recommended to always make sure that you're using the highest quality connector or cable that you can. And then if that's the case, then the next step would be to look at that VINDPM threshold. Can you lower it a little bit more without causing a problem for the power supply or adapter? But you're still going to make sure you don't lower it too much that the battery charger enters 100% duty cycle mode and therefore can't continue to provide enough charge current. And then also you want to make sure that adapter isn't going to be overstressed from that.

A lot of our newer chargers have this feature called input current optimizer, which can be run in order to determine the exact amount of current you can pull from your charger without hitting this VINDPM threshold. And it may be something that is recommended in the event that multiple connectors or cables can be used, third party connectors or cables that could have additional resistance that wasn't initially designed for.

So another reason for having a long charge time is simply having too weak of a power supply to provide both the charge current and the system current that you need. So consider that for a 3 amp charge current of a 4.2 volt lithium ion battery that the input power supply needs to provide at least 15 watts, assuming that the charger can give you about 90% efficiency. So in that case it's desirable maybe to have a high voltage adapter or use a new USB Type C PD port that can provide a higher than 5 volt output and several amps of current, so that the charger can provide higher charge current, as well as the system current.

So just like input loop impedance can cause longer charging times, it's also possible for the output side of the connection from the BAT pin of the charger to the battery itself to have a higher resistance than expected, and therefore cause longer than desired charging times. Essentially, what happens in this particular case is either the wire-- if there's a wire connecting-- or the trace on the PCB, or the connector that connects the battery to the end equipment and then potentially to the charger, or even the battery's own internal impedance can cause the voltage seen at the BAT pin to be higher than the cell voltage. And therefore, the charger thinks that the cell is fully charged, when really the cell is not fully charged. But the charger enters CV mode or Constant Voltage mode too early.

So some solutions to this particular problem would be one, you could potentially increase your termination current, so the current at which you stop charging. And therefore, the charger would at least start charging. But you probably would not have fully charged the cell at that point. So it may not be the best option.

Another option is to use this feature that we call IR compensation that's on some of our later chargers. And this feature actually makes the assumption that you can estimate the voltage drop in real time. So the IR compensation, it artificially raises the battery voltage to compensate for the additional voltage drop in the charging path, at least initially, and then slowly tapers that voltage back down and so that there's no risk of overvoltaging the battery.

Now some of the newest chargers on the market do have a separate high impedance BAT sense pin which you can tie that closer to the battery itself. And therefore, the voltage regulation loop is using that BAT sense pin input to actually determine the actual voltage to regulate against, instead of relying on the BAT pin, which will have current going through it, and therefore the voltage drop.

Another feedback loop that chargers have that a lot of DC to DC buck power supplies don't have is an IC thermal regulation loop. So a lot of ICs will have an IC thermal shutdown. But they might not have an IC thermal regulation loop. And what this particular feedback loop does is monitors the charger IC junction temperature. And if the junction temperature exceeds a maximum operating temperature preset in the device, then the feedback loop reduces the output power, i.e. charge current, that the charger is providing until the junction temperature has reduced to a safe operating level. And then the charger will continue to operate and provide output power, but only enough power that doesn't exceed this junction temperature threshold.

So the only way to prevent this from happening is to improve your PCB layout and therefore the heat sinking for the charger. QFN packages often have an exposed pad that you tie to PCB ground plane with vias to improve heat sinking. Even if the IC doesn't reach its thermal regulation threshold, it is possible that the IC itself gets hot enough that the board or the end equipment case gets too hot and exceeds a thermal budget. So the graphic on the left shows a typical QFN charger and component placement, as well as the vias connecting the exposed pad on the bottom of the QFN package to a ground plane, which is either maybe the bottom of the board and/or hopefully an internal layer as well. The more ground plane that you're attached to, the more heat sinking you have. And therefore the less heat that you will generate.

Careful board layout is critical for proper heat sinking, heat dissipation. Having more space between heat generating components in addition to the charger, is also helpful. If that isn't possible, then potentially you could move to a charger that has external FETs, and therefore place the FETs apart in different spaces across the board to distribute the heat more. But even if that doesn't work, you can use that same methodology but parallel to integrated FET chargers.

This slide shows an example of using parallel chargers where we've taken two chargers of the same family, each having different I2C addresses, which makes it helpful for communicating between the two. But one charger is used for charge and discharge and providing the system output. The other charger, though, is only used to provide additional current during constant current mode.

So as you can see in the configuration of the two block diagram of the chargers of the left, only the BAT pins are connected together. And the system pin on charger two is only connected to the minimum amount of capacitance needed to ensure stability.

As you can see with the scope plot at the top right, charger one continues to provide charge current all the way down until charge termination. Whereas, charger two is forced to terminate early, either by setting the termination charge current level higher or setting the constant voltage regulation point or better relation voltage slightly lower to make sure that it does stop charging. Essentially, this isn't a problem because in constant current mode, you're tying the output of two high impedance current sources together. So therefore, there's no problem with regulation. But once you get to the point of constant voltage regulation, you're tying the output of two voltage regulation sources together. And therefore, they can end up potentially fighting each other. So you want to make sure that you turn off the second charger or the one that's only providing charge current early.

And as you can see from the data in the table provided with a input voltage of 9 volts and on a four layer 2 ounce copper PCB, the single charger providing 3 amps generated an IC topside temperature of 41 degrees C which is well below the thermal junction safe temperature, but could potentially be a problem for a board or inside of a case or a smart phone, for example.

But then using a dual charger with the chargers spread out across the same PCB, we were able to distribute the charge current of 2.5 amps for a total of 4.5 amps with only 1 degree increase-- 1 to 2 degree increase of the IC topside temperature. So therefore, parallel charging is a valid option for getting higher charge current without significantly increasing your board or PCB or case temperature and possibly exceeding a thermal budget.

So in summary, just keep in mind when we're having trouble with your battery charger or debugging your battery charger, almost all of the issues are solvable. Someone's probably already had the issue. These charger typologies have been around for quite some time. And there's a lot of information. When it comes to the problem of long charge or taper or time, just remember to optimize your IIN_DPM and VINDPM settings, potentially use the ICO feature, the Input Current Optimizer feature, if your charger has it, or move from a 5 volt adapter all the way up to a high voltage adapter, in the event that your cable is giving you too much of a voltage drop and therefore the input voltage seen by the charger is too low.

In order to improve heat sinking and also EMI, which didn't really cover this, but good PCB layout is critical to make sure that you have good heat sinking and therefore the IC itself doesn't get too hot and your board doesn't get too hot and your case doesn't get too hot. But also a good PCB layout is imperative for reducing EMI. And in the event that you still have a heating problem, you can move on to parallel chargers.

Again, most of these questions have already been addressed. And you can find a information about these particular issues and others at e2e.ti.com.