Battery Charger IC Guide

I The principle of battery charger IC to identify battery insertion

Many existing battery charging solutions use charge management  ICs, which can realize charge current regulation, charge voltage protection, etc. Among them, identifying whether the battery is inserted or not is also one of the basic functions.

For a removable battery product, it is also an important issue to identify whether the battery is inserted or not, so TI's BQ24125 battery charger IC is used as an example to analyze whether the battery is inserted or not.

The following is the logic diagram for the BQ24125 to detect the battery.

Logic diagram of BQ24125 detecting battery

When the voltage on the pins connected to the battery charger IC drops, the reason for the drop is mainly because the battery is discharged with load or the battery is pulled out. When the battery voltage drops to the voltage point for re-charging, the battery charger  IC  first detects whether the battery is inserted or not, and enters the battery detection logic, and then re-charges.

Enabling detection current I_DETECT=400uA, enabling duration period t_DETECT=1s, in order to let the battery input current to the internal IC.  during the enabling period to detect whether the battery pin voltage is greater than V_SHORT, V_SHORT is defined in BQ24125 as follows.

When identifying V_BAT>V_SHORT, it means that the battery is inserted normally and can enter the charging logic judgment, if V_BAT<V_SHORT, it means that the voltage exceeds the minimum voltage that should exist when the battery is inserted, then it enters the next round of judgment.

When V_BAT<V_SHORT, another wake-up current I_WAKE=2mA is enabled, and the duration period of the enable is t_WAKE=0.5s. The wake-up current is to output the charging current to the battery end. The purpose is to avoid the battery over-discharge protection shutdown output resulting in the voltage drop at the battery end. At this time, if no external battery is identified, the output voltage is V_OVP, and V_OVP is defined as follows.

V_OVP is the charging overvoltage protection voltage at the battery end. When the point is exceeded, it will trigger the battery charger  IC  to over-voltage protection. Therefore, if the battery is not inserted, the voltage at the battery end during t_WAKE will be V_OVP, and the battery detection will enter the start cycle detection and start the cycle again from ①. If there is a battery, the battery will recover from the over-discharge protection state, and the voltage will drop to the battery voltage, then the battery will be judged to be present and enter the charging logic.

In view of the above analysis, it can be known that when the battery is not inserted, the voltage at the battery detection end will jump between 0 and V_OVP. When the battery is inserted at the time of detection, it will i.e. resume to the battery voltage, as follows.

battery voltage graph

II What should a good battery charger IC  be like?

• Safety. As a power product use safety is still very important, after all, the charger explosion is possible. So choose the 3C certification battery charger IC.  There are generally no safety problems.

• Power. For battery charger IC power selection, it depends on how much power you need on your side.

• Compatibility. The battery charger IC should not only charge fast but also charge widely. Since it is necessary to charge a wide range, it is necessary to take into account the issue of compatibility.

• Small size. battery charger IC is still used for charging.  Considering its practicality, do not require a lot, but should be easy to carry.

• Cost. As a good battery charger IC, the cost performance must also be taken into account. The higher the power, the higher the price of the charger IC.

Recommended battery charger ICs

• MCP73832T-5ACI/OT  Microchip Technology 

• BQ2057TSN Texas Instruments

• MCP73844-820I/MS Microchip Technology

• UC3906N Texas Instruments

III How to solve the power consumption problem of battery charger IC?

To form the simplest understanding of battery charger IC in your mind, it is most accurate to think of it as a voltage regulator with an adjustable current limiting function. As a voltage regulator, its output voltage is determined by itself, but whether the voltage at its output can be stabilized at the point determined by itself depends on the balance between its current output and load consumption. When the current output capacity is lower than the load consumption capacity, the actual output voltage is determined by the load, for the battery charger IC.  the output voltage at this time is the battery voltage. Only when the battery voltage gradually rises to the output voltage set by the regulator, the battery's ability to draw current gradually decreases, and once the balance between the drawing capacity and output capacity is crossed, the battery draws less and less current, and the output voltage is stabilized at the pre-set point, which is the so-called constant voltage charging state.

In order to comply with the characteristics of Li-ion batteries, the battery charger IC charges the battery at a very low current when the battery voltage is below the pre-charge threshold (approximately 3V). After this threshold is exceeded, charging starts at a high current so that the battery can be fully charged as soon as possible.

The supply voltage into the charging port is usually 5V (nominal). Assuming a charging current of 1A, we can immediately calculate that the maximum power consumption on the linear battery charger IC  will be (5V-3V)x1A=2W. Even if the battery voltage has risen to 4.2V, the battery charger IC  will consume up to (5V-4.2V)x1A=0.8W. For these reasons The battery charger IC will consume as much as (5V-4.2V)x1A=0.8W. Due to these reasons, plus the limitation of heat dissipation, it is normal to feel hot when charging the phone.

To solve the heat problem of linear battery charger  ICs, it is inevitable to introduce battery charger  ICs that work in a switching mode. Another newer approach is direct charging.  which eliminates the voltage conversion link. Since the vast majority of usage environments are where the input voltage is higher than the battery voltage, the switching mode battery charger IC with Buck architecture has become the mainstream of the market with the circuit architecture shown in the figure below.

Buck Circuit Architecture

In essence, it is a Buck regulator with an adjustable current limiting function. However, compared with the common Buck converter and linear battery charger IC.  the Buck mode battery charger IC is much more complex.

The Buck circuit has the effect of current amplification, and its output current is higher than the input current. This has the advantage of higher efficiency, lower heat generation, and faster charging.  but because the input and output currents are not equal, it is not possible to know the input and output currents by measuring only the current at one place on the output, as in the case of a linear battery charger IC.  which must measure both the output and input currents. Only after doing so, the battery charger IC can bring down the input current when the input current exceeds a certain limit so that it does not exceed the limit.

Some external power supplies do not have sufficient power supply capacity, and when the load is heavy, the output voltage will drop. If the battery charger IC does not take this into account and only focuses on meeting the output current needs, the external power supply will be dragged down, so the battery charger IC also needs to monitor the input voltage and reduce the input current demand when the input voltage is below a certain threshold to ensure that the system can still work properly.

The ability to limit the input current to a certain maximum value is called dynamic input current regulation or average input current regulation, and the ability to control the minimum input voltage to a certain threshold is called minimum input voltage regulation. Both of these capabilities are used to protect the input power supply, which is not needed by the battery charger IC itself, but ensures that the system does not go wrong, and is therefore a brand new battery charger IC design must be considered. Engineers should look at this as a priority when selecting a model.

As the Buck can be bucked with high efficiency, the input voltage can be higher, which has the advantage that the input current can be smaller at the same input power, which reduces the requirement for the overcurrent capability of the connected devices, making it possible to transfer more power under the same conditions to meet the needs of fast charging systems. To increase the input voltage it is necessary to improve the voltage withstand capability of the device, which requires the use of a new high-voltage process.

The RT9451, for example, has a maximum operating voltage of 12V (withstands 28V shock) and a charging current of up to 4A, which means it can charge batteries at a very high speed. Due to the increasing number of adjustable parameters, such battery charger ICs are designed to be intelligent, allowing the user to adjust all parameters and control the working process via the I2C interface, while the application design is still very simple from the hardware point of view.

RT9451 application circuit diagram

The RT9451's charging voltage can be adjusted from 3.5V to 4.44V, so it can be used to charge ordinary Li-ion batteries, and it is also feasible to charge Li-iron phosphate ion batteries, so it can be used in a wide range of applications.

Readers who are concerned about the circuit architecture can refocus on the circuit structure of the Buck battery charger IC and you will find that it can be used as a Boost converter, only the direction of its energy transfer has changed.

Buck charging IC circuit structure

Now, the original battery becomes the power provider, and the original input becomes the output, which can be powered by simply connecting to the load, and this is exactly how USB OTG applications require it to be. When used in this way, the components in the circuit do not change, only the operating mode of the battery charger IC changes. The minimum current limit of the switching tube is 4A, ensuring a load capacity of 1.6A or more is not a problem, and thus can meet the needs of many applications.

What are the specific benefits of a battery charger IC for Li-ion batteries that operates in a switching mode compared to a linear charging mode? Take a look at the following comparison chart.

charging curve comparison

Also using the linear battery charger IC and switch mode, the battery charger IC charges a 950mAh Li-ion battery in USB 500mA input current limit mode. The switching mode battery charger IC can charge at a maximum of 600mA, and the charging time from the battery voltage of 3.5V to the start of constant voltage mode is much shorter, thus allowing the battery to be filled up faster, and the heat generated during the charging process will, of course, be much less.