1A high current lithium battery charger solution

With the increasing functions of today's digital electronic products, the increasing size of LCD screens, and the continuous enhancement of multimedia video functions, the capacity of lithium-ion/polymer batteries on the market has also become larger and larger. At the same time, consumers have expressed their expectations for shortening the charging time of large-capacity batteries. In order to charge these large-capacity batteries more quickly and effectively to meet the growing needs of consumers, Wuxi Xinpeng Microelectronics has launched the high-current lithium-ion battery charging chip AP5056.

AP5056 is a charger circuit that can charge single-cell lithium-ion or lithium-polymer rechargeable batteries with constant current/constant voltage. The device uses a PMOSFET architecture and does not require an external blocking diode when used. The thermal feedback circuit can automatically adjust the charging current so that the chip temperature can be controlled within a safe range when the device has high power consumption or high ambient temperature.

AP5056 requires only a few peripheral components and can adapt to USB power and adapter power, making it very suitable for portable applications. The charging output voltage is 4.2V, and the charging current can be set by an external resistor. In the constant voltage charging stage, when the charging current drops to 1/10 of the set value, AP5056 will terminate the charging cycle.

When the input voltage (AC adapter or USB power) is powered off, the AP5056 automatically enters a low-power sleep mode, where the battery current consumption is less than 2 microamps. Other features include low input voltage latch, chip enable input, automatic recharging, battery temperature monitoring, and status indication.

Charging process: AP5056 has four basic charging modes during the entire battery charging process: trickle charging, constant current charging, constant voltage charging, and charging completion and recharging.

Trickle charge: Before charging begins, AP5056 first checks the input power supply. When the input power supply is greater than the minimum operating voltage or the undervoltage lockout threshold, and the chip enable terminal is connected to a high level, AP5056 starts charging the battery. AP5056 first checks the battery status. If the battery voltage is higher than 3V, the charger enters constant current charging; if the battery voltage is lower than 3V, the charger enters trickle charge mode. The trickle charge current is one tenth of the constant current charge current (taking the constant charge current of 1A as an example, the trickle charge current is 100mA). The trickle charge state continues until the AP5056 chip detects that the battery voltage reaches 3V, and then enters the constant current charging stage.

                           Figure 2: AP5056 package.

Constant current charging: In constant current charging mode, the charging current is determined by the resistor RPROG between the PROG pin and GND. (See "Programmable charging current" below)

IBAT = (VPROG/ RPROG) 1000 (Typical value of VPROG is 1V)

After the AP5056 enters the constant current charging mode, it will continue to charge at the set current value until the battery slowly reaches the voltage regulation point of 4.2V, and then enters constant voltage charging. Constant voltage charging: When the battery voltage slowly approaches 4.2V, the charger gradually switches to constant voltage charging. At this time, the original constant current charging current also slowly decreases, and drops sharply as the battery capacity gets closer to the maximum capacity. Charging completion and recharging: When the charging current is detected to be reduced to 10% of the constant current charging current, the charger stops charging the battery and enters a low-power standby mode. In standby mode, the AP5056 will continue to detect the voltage at the battery end. If the battery voltage drops below 4.05V, the charger will charge the battery again.

Programmable charging current: The charging current of AP5056 is determined by the RPROG resistor connected between the PROG pin and GND. The calculation formula is as follows:

IBAT = (VPROG/ RPROG) 1000 (Typical value of VPROG is 1V)

For example, if the customer needs to get a charging current of 1A, according to the formula

1A = (1/ RPROG) · 1000, solving the equation gives VPROG = 1000Ω, or VPROG = 1KΩ

FIG3 shows the relationship between different power input Vcc and charging current Ibat when RPROG is 1K and 2K. It can be seen that the charging output current does not change much and is only related to the setting value of RPROG.

                                         Figure 3: Power input Vcc VS charging current Ibat.

Typical application circuit

Figure 4 shows a typical application circuit, in which R1 and R2 are determined by the NTC thermistor value. Assuming that the resistance of the thermistor at the lowest operating temperature is RTL, and the resistance at the highest operating temperature is RTH (the data of RTL and RTH can be obtained by checking the battery manufacturer's data or doing experiments), the resistance values ​​of R1 and R2 are:

DC adapter and USB combination solution

When the charger needs to be able to use both the DC adapter and USB charging, the solution shown in Figure 5 can be used. In the solution, if the USB port is used for power supply, the MOS-P gate is grounded, the USB power is conducted to VCC through the MOS-P, and the MOS-N is in the off state. The programming resistor on PROG is 2K, that is, the charging current is set to 500mA, which can prevent the USB interface from being pulled dead by the charger; when a 5V DC adapter is used, the adapter current is added to the VCC pin through the Schottky diode, and the MOS-P is cut off due to the high gate level, which will not affect the USB port. At the same time, the MOS-N is turned on due to the high gate level. At this time, the programming resistor on the PROG pin is equivalent to 1K (two 2K ​​resistors in parallel), that is, the charging current is set to 1A to quickly charge the lithium-ion battery.

                               Figure 5: AC adapter and USB combination solution.

Chip thermal protection function

Taking advantage of the fact that the on-state voltage of the transistor PN junction decreases as the temperature increases, while its change value increases as the temperature increases, the AP5056 is designed with an overheat protection function integrated into the chip. When the internal temperature sensor rises to above about 125°C, the internal thermal protection circuit will automatically reduce the current value of the charging current. As the temperature continues to rise, when the temperature reaches 145°C, the charging current can be completely turned off. This function allows users to safely use the maximum power charging current without worrying about the chip being damaged.

Charging status indicator

AP5056 has two status output indicators, CHRG and STDBY. When the charger is in the charging state, CHRG is set to low level and STDBY outputs high impedance; when the battery is fully charged, CHRG becomes high impedance and STDBY is pulled low.

If the status indication function is not needed, the unused corresponding status output indication pin can be grounded.

Indicator status

Considerations for layout

AP5056 adopts SOP8-PP package, and there is a heat sink at the bottom of the chip to dissipate the heat generated by the chip when it is working. Therefore, in order to achieve a better heat dissipation effect, the copper foil area of ​​the PC board under the heat sink should be as wide as possible and extended to a larger and wider copper foil area outside. The heat sink and the copper foil under the heat sink need to be soldered together to increase thermal conductivity. In addition, using multiple through holes to connect the upper copper foil with the lower copper foil can more effectively expand the heat dissipation area, which is conducive to dissipating heat to the surrounding environment and increasing the heat dissipation effect of the charging chip (as shown in Figure 6).

Figure 6: AP5056 layout diagram.

When the chip heat dissipation is good, AP5056 can provide the circuit with a maximum charging current of 1600MA. In the experiment, at room temperature, the charging current is set to 1600MA, and the IC surface temperature is 60℃ after working continuously for 15 minutes; when it is set to 1000MA, the IC surface temperature is 50℃ after working continuously for 15 minutes.