Notebook computer battery charging controller bq24700 application-EEWORLD

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1 Introduction

bq24700 is a charging control chip designed for laptop battery packs. The chip has a unique dynamic charging management function that completes charging in the shortest time. The main circuit of the charger is a buck converter. The fixed frequency of the PWM controller is 300kHz, the charging current accuracy is ±4%, and the charging voltage accuracy is ±0.4%, which is particularly suitable for charging lithium-ion battery packs. When charging lithium-ion battery packs, the charging voltage is set with an internal 1.25V±0.5% reference voltage. When charging other battery packs, the charging voltage is set with an external reference voltage. The device can detect the battery pack status in real time and automatically select AC adapter power supply or battery pack power supply. When the battery pack is powered, the bq24700 detects the battery pack power and sends an alarm signal when the power is low.

2 Internal block diagram and pin function

bq24700 uses a 24-pin TSSOP package, and the pin arrangement is shown in Figure 1. The internal block diagram is shown in Figure 2. The functions of each pin are as follows:

Figure 1. Pin arrangement of bq24700
Pin 1 is the AC adapter output status detection terminal. When the voltage of this pin is lower than 1.2V, the bq24700 selects the battery pack to power the system.
Pin 2 is the AC adapter output status indication. When the voltage of pin 1 is higher than 1.2V, this pin outputs a high level.
Pin 3 is the system power selection. When this pin is high, the AC adapter is powered, and when this pin is low, the battery pack is powered.
Pin 4 is the battery pack status detection. When the voltage of this pin is lower than 1.2V, the battery pack is exhausted, and when the voltage of this pin is lower than 1.0V, the bq24700 determines that the battery is not installed.
Pin 5 is the battery pack charging current setting terminal.
Pin 6 is the AC adapter output current setting terminal. When the system current and the battery pack charging current are greater than the set value, the bq24700 enters the dynamic charging mode.
Pin 7 is a 5.0±0.6% reference voltage. This reference voltage is used to power the external circuit, and this pin should be connected to an external 3.3μF capacitor.
Pin 8 is the charging enable terminal.
Pin 9 is the internal reference voltage shutdown terminal.
Pin 10 is the inverting input of the PWM comparator.
Pins 11 and 12 are the inverting and non-inverting inputs of the AC adapter output current detection error amplifier.
Pin 13 is the battery pack charging voltage feedback input.
Pin 14 is the battery pack charging current error amplifier output.
Pins 15 and 16 are the inverting and non-inverting inputs of the battery pack charging current detection error amplifier.
Pin 17 is the signal ground.
Pin 18 is the system voltage detection input.
Pin 19 is the alarm output.
Pin 20 is the external MOSFETS drive voltage setting terminal.
Pin 21 is the PWM drive output.
Pin 22 is the integrated circuit power supply terminal.
Pin 23 is the battery pack power supply selection MOSFET drive output.
Pin 24 is the AC adapter power supply selection MOSFET drive output.

Figure 2. Internal block diagram of bq24700

[page]3 Basic Working Principle

The typical application circuit of bq24700 is shown in Figure 3:

Figure 3. Typical application of bq24700

3.1 Dynamic charging management

Dynamic charging management can make full use of the output current of the AC adapter and complete charging in the shortest time. The battery charging current IBAT is equal to the difference between the adapter output current IADPT and the system current ISYS, that is: IBAT=IADPT-ISYS. When the current required by the system decreases, the bq24700 increases the charging current of the battery pack, as shown in Figure 4. This can keep the output power of the AC adapter stable.

Figure 4 Dynamic charging management

3.2 AC adapter status detection

The AC adapter output voltage is connected to pin 1 after resistor voltage division. When the voltage of this pin is lower than 1.2V, the bq24700 determines that the AC adapter output power is insufficient, the system switches to the battery pack for power supply, and enters the bq24700 sleep mode.
When the AC adapter output voltage is lower than 18V, the detection point is taken at the anode of the AC adapter output diode. When the AC adapter output voltage is greater than 18V, the detection point should be at the cathode of the adapter output diode, so as to ensure that the voltage on pins SRN and SRP is within a safe range after the AC adapter is powered off.

3.3 Battery pack charging

The bq24700 completes the closed-loop control of the charging current by detecting three parameters: battery pack voltage, battery pack current, and AC adapter output current. The resistors connected in series in the AC adapter output circuit and the battery pack charging circuit provide detection data to the two error amplifiers respectively, and the battery pack voltage provides detection data to the third error amplifier after voltage division. The bq24700 charges the battery pack only when the ENABLE pin outputs a high level.
The battery pack charge termination voltage, battery pack charge current and AC adapter output current can be set by discrete devices or by keyboard via DAC interface. The charging circuit of bq24700 is a buck converter with a fixed switching frequency of 300kHz and an external P-channel MOSFET, as shown in Figure 5.

Figure 5 PWM buck converter

[page]3.4 PWM controller and soft start

The outputs of the three error amplifiers of bq24700 are all connected to the COMP terminal, which is compared with the internal 300kHz sawtooth wave to complete the pulse width modulation, and then outputs the driving signal of the corresponding duty cycle through the conversion circuit to drive the external P-channel MOSFET.
In order to ensure that the output voltage of the external MOSFET is within the normal range, the bq24700 has a driving voltage conversion circuit inside. When UCC≤15V, the driving voltage is equal to UCC, and when UCC＞15V, the driving voltage is 0.5UCC.
Soft start can ensure that each functional circuit starts in sequence. When the PWM comparator is turned off, the COMP pin is grounded and the internal 100μA current source is turned off. When the PWM comparator is enabled, the COMP pin is released, and the 100μA current source starts to charge the external capacitor of the COMP pin. As the voltage of the COMP pin rises, the duty cycle of the PWM comparator increases, and the bq24700 achieves soft start.

3.5 Setting the charging termination voltage

The bq24700 sets the charging termination voltage through the BATSET pin. There are two ways to use or not use the internal 1.25V reference voltage (as shown in Figure 6). The battery pack voltage is connected to the error amplifier after voltage division. In order to accurately charge the lithium-ion battery pack, the external voltage divider resistor of BATSET should be very accurate. When the BATSET pin voltage is lower than 0.25V, the internal 1.25V reference voltage is connected to the error amplifier. When the BATSET pin voltage is lower than 0.25V, the 1.25V reference voltage is disconnected from the input of the error amplifier. When the 1.25V reference voltage is not used, the voltage of the BATSET pin cannot be lower than 1.0V.

Figure 6 Charging termination voltage setting

3.6 Battery Pack Charging Current Setting

The bq24700 sets the charging current through the SRSET pin, and the setting voltage can be obtained by dividing the 5V reference voltage. The relationship between the charging current IBAT and the setting voltage USRSET is:

The maximum voltage is set to 2.5V.

3.7 AC adapter output current setting

The AC adapter output current is set through the ACSET pin, and the set voltage can be obtained by dividing the 5V reference voltage. The relationship between the AC adapter output current IADPT and the set voltage UACSET is:

3.8 System Power Supply Selection

When any power supply of the AC adapter or the battery pack fails, the bq24700 can automatically switch to the other power supply to power the system. At the same time, it allows the battery pack to be manually selected to power the system without disconnecting the AC adapter. The switch should use a P-channel MOSFET with low on-resistance to reduce losses and extend the battery pack power supply time. Whether it is manual switching or automatic switching, when the battery pack is powered, the AC adapter stops charging the battery pack. The selection circuit is shown in Figure 7.

Figure 7 System power supply selection
When the adapter output is normal, the ACPRES pin outputs a high level, otherwise it outputs a low level. The AC adapter output voltage is connected to the ACDET pin after voltage division and compared with the internal reference voltage. When the ACDET pin voltage is lower than the internal reference voltage, the bq24700 determines that the AC adapter is powered off regardless of whether the AC adapter still has output voltage. The ACPRES pin flips to a low level, while the ACDRV pin outputs a high level, BATDRV outputs a low level, and ALARM outputs a high level. The bq24700 selects the battery pack to power the system. When the AC adapter output voltage is normal, the ACSEL pin is a high level, and the AC adapter powers the system. If the voltage of this pin is a low level, the battery pack still powers the system. If the switching is unsuccessful, the ALARM pin outputs an alarm signal.
When the AC adapter output voltage is normal, but ACSEL is a low level, the system is powered by the battery pack. When the battery pack is powered, the MOSFET powered by the adapter is turned off. Before the MOSFET powered by the battery pack is turned on, the system first detects the voltage of the VS pin. If the AC adapter powered MOSFET is short-circuited, the voltage of this pin is higher than the voltage of the BATP pin, and the bq24700 will not turn on the MOSFET powered by the battery pack, which can protect the battery pack. If VS is grounded, the protection function is canceled.

3.9 Battery over-discharge detection

The battery pack voltage is connected to the BATDEP pin after voltage division and compared with the internal 1.22V reference voltage. When the BATDEP pin voltage is close to 1.22V, the bq24700 determines that the battery is exhausted and sends an alarm signal. ALARM outputs a high level. If the battery is over-discharged and the AC adapter output voltage is normal, the bq24700 automatically switches to the AC adapter for power supply.

4 Conclusion

The laptop battery charging management solution with bq24700 as the core has complete functions, stable performance and low cost. The charger is used in both laptops and other portable devices.

Reference address：Notebook computer battery charging controller bq24700 application

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