If you are not familiar with lithium battery chargers I highly recommend you first read part 1 where I explain some of the fundamental concepts. Similar to the MCP , the BQ is a linear battery charger for charging a single lithium cell. One of the extra pins allows you to independently program the pre-charge and charge termination currents separately from the fast charge current. Another additional pin provides a status output indicating there is a sufficient input supply voltage present.
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If you are not familiar with lithium battery chargers I highly recommend you first read part 1 where I explain some of the fundamental concepts. Similar to the MCP , the BQ is a linear battery charger for charging a single lithium cell.
One of the extra pins allows you to independently program the pre-charge and charge termination currents separately from the fast charge current. Another additional pin provides a status output indicating there is a sufficient input supply voltage present. Another pin monitors the battery temperature, and finally a fourth additional pin is a charge current override function for USB applications. With the BQ you can program the charge current between 10 mA and 1, mA.
The charge current is set via resistor tied to the ISET pin. For example, if you have a mAh battery, you want to charge it with a maximum charge current of mA. If you have a mAh battery, on the other hand, then you want to charge it at a maximum current of mA.
If you use a charge current lower than 1C the charging process will take an unnecessarily long time. Since we all want devices that charge as fast as possible you will usually want to charge at the maximum rate allowed by the battery. To quickly review, there are three different charge current levels that you typically need to program for a battery charger :. The MCP uses a single resistor to set the pre-charge current, the fast-charge current, and the charge termination current.
This can be somewhat limiting, so the BQ provides two separate pins for programming the charge currents. One pin sets the fast-charge current and the other pin sets the pre-charge and charge termination currents. When this pin is left floating, the charge current drops to only mA. When the ISET2 pin is pulled low, the programmed charge current is used.
In the original USB specification a device had to request permission from the host via a process called enumeration in order to draw that mA.
Without enumeration the maximum allowable current was only mA. So the USB spec was updated in to allow up to mA without enumeration. This same pin can also be used as an output pin feeding to a microcontroller allowing the microcontroller to monitor the charging process.
Another additional important advantage of the BQ compared to the MCP is that it includes a temperature sense pin. This allows the charger to monitor the battery temperature, and to adjust the charging current as necessary to prevent the battery from getting too hot. We will start by considering a few of the primary things that differentiate this charger from the previous two chargers. As covered in Part 1, linear chargers like the MCP and BQ waste a lot of power, especially if the input voltage is much higher than the output voltage.
This wasted power is dissipated as heat. If the temperature is too high, the charger is forced to reduce the charge current to prevent the overheating of the charger. When this happens the battery will take longer to charge.
Just as with a linear regulator, a linear charger wastes more power when the input voltage is significantly higher than the output voltage.
Side Note: A linear charger is really just a linear regulator with the ability to regulate voltage or current depending on the charging stage , so many of the same fundamental concepts apply to both. The same is also true for switch-mode regulators and switch-mode chargers. For more details on linear and switching regulators see my previous blog on how to pick the right voltage regulators for your project.
A buck regulator takes a higher voltage and steps it down to a lower voltage, while a boost regulator takes a lower voltage and steps it up to a higher voltage. The BQ is a buck switch-mode charger. This type of charger is especially beneficial compared to linear chargers when you have a large voltage differential between your input voltage and output voltage. A buck-switching charger like the BQ is going to waste a lot less power than a linear charger in this application. On the other hand, if the input voltage is only 5V such as with USB chargers then a linear charger probably makes more sense.
Linear chargers are less complex, require fewer components, and are cheaper so only use switch-mode chargers when really necessary. Multi-cell chargers allow you to stack up multiple cells in series to obtain higher output voltages.
For example, instead of just a single 3. You could even stack three cells to get Figure 2 — Two-cell lithium polymer battery pack with a 7. The way around this limitation is to use a boost switching charger, which can take a small input voltage and step it up to a higher output voltage.
This means that the charger can dynamically change the battery charge current based on the amount of available current. For example, say the maximum current your AC adapter can supply is 1 A and your system is pulling mA while you are also trying to recharge the battery. The BQ will then automatically set the battery charge current to mA. In this same example, if the current required by the rest of the system suddenly decreased to only mA, then the DPM feature would allocate up to mA for charging the battery.
Of course, this would only occur if the fast-charge was set for mA or higher. DPM allows the battery to always be charging with the maximum available current.
The less current the system uses, the more current the charger allocates to recharge the battery. This allows you to, manually or automatically, switch the system from being powered from the AC adapter or from the battery.
For example, when powering your product from an AC adapter, if you suddenly unplug it, then the BQ will automatically switch the system to be powered from the battery.
Then, if you plug it back into an AC outlet you can also have it setup to switch back to AC power. Refer to the typical application diagram on page 10 of the datasheet which is also shown below in Figure 3. Note that U1 is not labeled in the diagram but is located in the bottom right corner of the above schematic. U1 and U2 perform the system selector function.
When U1 is turned on the system is powered from the battery, and when U2 is turned on the system is powered directly from the AC adapter. These switches are called break-before-make which means one switch is turned off before the other switch is turned on.
This ensures that both switches are never turned on at the same time which would short the AC adapter voltage directly to the battery. The BQ measures the voltage drop across this resistor to determine the AC adapter current. R13, R15, and C3 all form a low-pass filter so any switching noise is removed from the adapter current sense voltage. This all allows the charger to measure the current being pulled from the AC adapter.
This is important so that the charger knows how to dynamically manage the power DPM , and how much current is available to charge the battery. This is just one pin that ties to the AC adapter voltage through a resistor divider. It allows the charger to know if the AC adapter is present.
You could feed this into an Analog to Digital Converter ADC in your microcontroller to monitor the current charging the battery. VREF pin. The VREF pin outputs a regulated 5 V that can be used as an accurate reference voltage for any of the resistor divider setpoints or for pull-up resistors on any of the open-drain outputs. VS: This pin monitors the system voltage in order to implement the break-before-make function that I mentioned for the system selector function.
BATP: This pin monitors the output voltage on the battery via a resistor divider. This forms the feedback loop which regulates the output voltage of the charger. It is for setting up the alarm if the battery voltage falls below a certain voltage which is set by the ratio of the resistors in the divider.
COMP: Any circuit that has a feedback loop has the potential to become an oscillator if that feedback goes positive. The RC network connected to this pin helps to compensate for this feedback loop to prevent undesired oscillations. Just as with the AC adapter sense resistor, there is a low-pass filter R19, R21, and C8 to filter out any switching noise. VHSP: This is an internal voltage supply pin that generates a voltage that is a fixed number of volts below the AC adapter voltage.
If the AC adapter voltage is above We started with the relatively simple MCP from Microchip. This is a single-cell linear charger with a maximum charge current of mA. This can be a good solution for many USB based charging applications. Next, we reviewed the slightly more advanced BQ from Texas Instruments.
Like the MCP this is also a single-cell linear battery charger. But it has a maximum charging current up to 1A, allows setting the pre-charge and termination currents separately form the fast charge current, and includes a temperature sense function for monitoring the battery temperature. Finally, we reviewed one of my designs in the BQ This is a switch-mode buck charger with the ability to charge multiple cells.
It also includes advanced features such as Dynamic Power Management and a system selector. If you are developing a new electronic product get help from a team of experts and access to in-depth training courses in the Hardware Academy. If I connect two single-cell batteries in parallel to increase Amps, do I need a multicell charger or single-cell charger is enough.
The datasheet of the BQ says it is used to set the AC adapter current limit. This means there are two methods to set the AC adapter current limit. Can you please clarifiy this? R12 and R11 setup a voltage divider for detecting the AC adapter is present. Hope this helps. This is what I already thought. So the Note 1 of the datasheet with additional information about R12 is wrong. I have 4 input devices, but I connect this charger to the power transistor and find a nice cover from the 3 d printer.
I hope you understand. Here are some other things to consider:. Unlike a power bank, the switching here is simply between DC input and battery.
Datasheet Texas Instruments BQ24703
Actual Size. Battery Charge Time. Conditioning and Smart Battery Learn Cycle. Break-Before-Make Transition. Charging Li-Ion Cells.
bq24703 MULTICHEMISTRY BATTERY CHARGER CONTROLLER AND SYSTEM POWER SELECTOR
bq24703 Datasheet PDF
Introduction to Battery Chargers (Part 2 of 2)