Knowledge of circuit design principles of lithium ion battery for wireless handheld devices

   Boost circuit

    The power supply voltage of a single-cell lithium battery is 3.7V, and the working voltage of the RFID reading and writing device is 5V, so a boost circuit is needed for the RFID handset.

    Basic principle of boost circuit

　　 The principle of the commonly used Boost boost circuit is shown in the literature. The working process of the circuit to achieve boost can be divided into two stages: charging process and discharging process. The first stage is the charging process: when the transistor Q1 is turned on, the inductance is charged and the equivalent circuit is obtained. The power supply charges the inductor, and the diode prevents the capacitor from discharging to ground. Since the input is direct current, the current on the inductor first increases linearly with a certain ratio, which is related to the size of the inductor. As the inductor current increases, a lot of energy is stored in the inductor.

    The second stage is the discharge process: when the transistor Q1 is turned off, the inductance discharges, the equivalent circuit. When the transistor Q1 changes from on to off, due to the current retention characteristics of the inductor, the current flowing through the inductor does not instantly become 0, but slowly changes from the value when the charge is completed to 0. The original path has been disconnected, so the inductor can only be discharged through the new circuit, that is, the inductor starts to charge the capacitor, and the voltage across the capacitor rises. At this time, the capacitor voltage can reach a value higher than the input voltage.

　Boost circuit design

　　The boost circuit uses Richtek's RT9266B high-efficiency DC-DC boost chip. RT9266B has the characteristics of low power consumption, low quiescent current, high conversion efficiency, and simple peripheral circuits. The chip has an adaptive PWM control loop, error amplifier, comparator, etc., through an external feedback circuit, the output voltage can be set to any desired amplitude, with high voltage accuracy.

    The boost circuit uses an external 10uH inductor to store energy, uses feedback resistors R1 and R2 to control the output voltage of the boost circuit, and uses the RT9266B internal self-waiting PWM controller to control the on and off of the NMOS transistor to control the output current of the boost circuit . Because the chip has an adaptive PWM controller, it can adapt to a larger range of load changes.

    When this boost circuit is used to boost a 3.7V 2000mAh polymer lithium battery to 5V, the output voltage ripple is only 40mV, and the maximum output current can reach 500mA.

 2 Charging circuit

   2.1 The basic principle of lithium battery charging circuit

    The charging process of lithium batteries can be divided into three stages: pre-charging, constant current charging and constant voltage charging. When the voltage of the lithium battery is lower than the minimum charging voltage, it first enters the pre-charging stage, charging the battery with a small current (usually 10% of the standard current) until the battery voltage reaches the minimum charging voltage. The pre-charging at this stage can prevent the damage caused by charging the lithium battery directly with a large current and constant current after being over-discharged. When the battery voltage is higher than the minimum charging voltage, the charging enters the constant current charging stage. Usually the constant current charging current is taken as 0.5C (C is the capacity of the lithium battery). When the voltage of the lithium battery reaches the standard voltage, it enters the constant voltage charging state, and the charging current is continuously reduced until the current is reduced to about 100mA, and the charging is completed.

   2.2 Design of Lithium Battery Charging Circuit

    The schematic diagram of the lithium battery charging circuit is shown in Figure 3, which is implemented using TI's bq2057. The bq2057 series is an advanced lithium battery charge management chip, suitable for single-cell (4.1V or 4.2V) or double-cell (8.2V or 8.4V) lithium ion and lithium polymer batteries. BQ2057 can dynamically compensate the internal resistance of the lithium battery pack to reduce the charging time; with optional battery temperature monitoring, the battery temperature sensor is used to continuously detect the battery temperature, when the battery temperature exceeds the set range, the BQ2057 shuts down to charge the battery; internal integration The constant voltage constant current device has high/low side current sensing and programmable charging current. The charging status recognition can be realized by the output LED indicator or the interface with the main controller. It has automatic recharging, minimum current termination of charging, and low power. Features such as high power consumption and high voltage accuracy (better than ±1%). The peripheral circuit of the charger designed by this chip is relatively simple, which is very suitable for the compact design needs of portable electronic products.

    The circuit adjusts the frequency of the PWM wave output from the CC terminal through the sense resistor R5 at both ends of the SNS and COMP to control the on and off of the Q1 transistor, so as to realize the control of the maximum charging current.

    The circuit has been actually tested, charging a 3.7V 2000mAh lithium polymer battery, the maximum charging current can reach 810mA, and the battery can be fully charged in 3 hours.

    When the charging circuit is fully charged, the measured battery voltage is 4.12V, which is 0.5V away from the standard voltage of 4.2V. The reason for the error is that during the charging process, the charging current of the lithium battery fluctuates. When the current is lower than a certain threshold at an instant, bq2057 considers that the charging is complete and shuts down the charging circuit.