Li-ion charge controller bq2954

    Abstract: bq2954 is a lithium-ion battery charging control chip produced by TI Company in the United States. The article introduces the characteristics, functions and parameters of bq2854, gives recommended working conditions, and finally gives the practical application appliances of the lithium-ion battery charge controller designed with bq2954.

    Keywords: rechargeable lithium-ion battery detection mode pulse width modulation bq2954

1 Overview

bq2954 is a lithium-ion battery charging control chip produced by TI Company in the United States. During the charging process, the chip uses PWM technology to control the charging voltage and charging current, which can effectively reduce system power consumption. Its operating frequency can be adjusted through an external capacitor. In addition, the control chip can also provide effective protection for lithium-ion batteries. Only when the voltage and temperature of the lithium-ion battery meet the set conditions, the chip will enter the fast charging mode. If the battery voltage is lower than the lower limit of the voltage, the bq2954 will automatically switch to trickle charging mode; the bq2954 uses an external thermistor to detect the battery temperature. Its charging process is divided into two stages: the first stage is the constant current charging stage, and the second stage is the voltage control stage. The constant current charging stage can complete 70% of the rechargeable battery capacity, and the remaining 30% is completed in the voltage control stage. Bq2954 has the characteristics of displaying the charging process and charging fault status, thus allowing users to accurately and timely grasp the working status of the charger and lithium-ion battery.

2 Features and pin functions

2.1 Features

The bq2954 lithium-ion battery charging control chip has the following characteristics:

●Adopt PWM technology to control charging current and charging voltage;

●Can detect and program control high-end/low-end current;

●The fast charging process can be controlled by setting the minimum charging current;

●Precharge mode can be used to detect battery failure;

●Charging quality can be ensured by setting temperature and voltage values;

●Can directly drive LED to display charging status.

2.2 Pin function

bq2954 adopts a 16-pin narrow DIP or SOIC package. Its pin arrangement is shown in Figure 1. The functions of each pin are as follows:

●TM (Pin 1): Charging time programming signal input terminal. The maximum charging time can be set through the resistor and capacitor on this pin;

●CHG (pin 2): charging control terminal. This terminal can be used to detect whether the battery is full or removed from the charger. When the battery is full, removed, or fails, the open circuit output inside the chip will show a low level;

●BAT (pin 3): battery voltage signal input terminal;

●VCOMP (pin 4): voltage loop compensation terminal. Stabilize the charging voltage through an external RC network;

●ICOMP (pin 5): current signal compensation terminal. Stabilize the charging current through an external RC network;

●ITERM (pin 6): charging mode selection terminal. Used to set the fast charging mode abort signals IFULL and IMIN;

●SNS (pin 7): charging current detection signal input terminal. The battery current can be detected through the external detection resistor on this pin;

●TS (pin 8): temperature detection signal input terminal. Used to detect the temperature of the battery, and set the upper and lower temperature thresholds through an external resistor divider network;

●TPWM (pin 9): PWM operating frequency setting terminal. The PWM operating frequency is set by an external timing ground capacitor;

●BTST (pin 10): battery detection signal output terminal. When the battery in the charger is removed, this end will be set to high level;

●LCOM (pin 11): LED public output;

●Vss (pin 12): ground terminal;

●Vcc (pin 13): Connect to the 5V bias power supply.

●MOD (pin 14): charging current control signal output terminal. This terminal is a pulse width modulation push-pull output terminal, used to control the size of the charging current. When MOD is low, the charging current will be suspended;

●LED1/CSEL (pin 15): charging status output terminal 1/charging current detection position selection terminal. Used to set the charging current detection terminal to be at the high end or low end of the battery;

●LED2/DSEL (pin 16): charging status output terminal 2/charging detection status display mode selection terminal.

3 Principles and functions of bq2954

The internal functional block diagram of bq2954 is shown in Figure 2.

3.1 Charging algorithm and charging condition detection

The bq2954 uses a two-stage fast charging algorithm. The charging current in the first stage remains constant. When the voltage VBAT on the VBAT pin rises to the built-in threshold VREG, the charging process shifts to the second stage. At this time, the voltage remains constant (VBAT=VREG). When the charging current is lower than the programmed threshold, When the set IMIN threshold is reached, the fast charging process ends and the battery enters the charging completion state.

After the current is inserted into the charger, a new charging cycle can be entered by turning on the current. First, check whether the temperature of the battery to be charged is within the allowed temperature range. If the temperature does not meet the charging requirements, the bq2954 will remain in the S01 state, the "Qualification" state, until the temperature and voltage of the current to be charged meet the requirements. During the charging process, if the battery temperature is too high ("HOT" state), the bq2954 will automatically enter the S04 state, which is the "Done" state, and display the fault status through the LED while interrupting the charging current. If the battery temperature is too low ("COLD" state), charging will be interrupted and automatically transferred to S01 state. Only after the current temperature returns to normal and VBAT is lower than the built-in threshold VRCHG, the circuit will re-enter a new charging cycle. If the temperature and voltage of the inserted battery to be charged meet the requirements, the bq2954 will enter the S02 state, which is the "Conditioning" state. The charging current at this stage will be maintained at ICOND (one-tenth of IMAX). After the delay time tHO, if the detected VBAT value reaches VMIN within the specified time, the bq2954 will enter the fast charge state. If VBAT has not reached VMIN within the specified time tQT, it means that the battery may be in a faulty state. At this time, it automatically enters the charging fault state, and then sets tQT to one quarter of tMOT. Once a charging failure is discovered, a new charging cycle can only be started by reconnecting the power supply and reinserting the battery.

3.2 Charging status

The charging status is displayed through two light-emitting diodes LED1 and LED2. Three different display modes can be set through the DSEL pin. DSEL/LED1 and CSEL/LED2 are dual-function pins: when driving the LED, this pin serves as the output terminal of the drive signal; when used as a programming control pin, it functions as the input terminal of the programming signal. After normal power-up, the bq2954 latches the programming control signals on the DSEL and CSEL pins. When the new programming signal is latched, the LED will turn off for approximately 400ms. When the charging current is lower than IFULL, LED1 and LED2 will indicate that the battery is fully charged. When the charging current is lower than the minimum charging current threshold IMIN, the fast charging process ends. IFULL and IMIN can be set through the ITERM pin. The charging status display settings of different current detection modes are shown in Figure 3.

The charging status display tables under the three conditions of normal fast charging cycle, recharging after fast charging cycle and abnormal charging status are shown in Table 1, Table 2 and Table 3 respectively.

Table 1 Normal fast charging cycle

Table 2 Recharging after a fast charge cycle

Table 3 Abnormal charging status

3.3 Charging status monitoring

a. Voltage and current monitoring

If low-side current monitoring is used, the bq2954 will monitor the voltage difference between the BAT pin and other pins. If high-side current monitoring is used, the bq2954 will monitor the voltage difference between the BAT pin and the Vss pin, which can be obtained through a voltage divider. In order to minimize the impact of the voltage divider on the battery, the resistance value of the voltage divider should be large enough. However, if the resistance value is too high, the noise suppression capability will be reduced. Therefore, the resistance value of the voltage divider should be between 150kΩ and 1MΩ. Select. The calculation formula is as follows.

RB1/RB2=（NVCELL/VREG）-1

Among them: VCELL is the battery charging voltage value set by the manufacturer; N is the total number of batteries to be charged; VREG is 2.05V.

The current sense resistor RSNS is determined by the fast charge current:

RSNS=0.25V/IMAX

In the formula: IMAX is the current value during the constant current charging stage.

In different current detection modes, the current voltage divider settings will also be different. The specific circuit is shown in Figure 4.

b. Battery insertion and removal status monitoring

The bq2954 monitors the insertion and removal status of the battery through VBAT; when VBAT is between the upper voltage limit (VHCO=VREG+0.25V) and the lower voltage limit (VLCO=0.8V), the circuit considers the battery to be in the inserted state. If VRAT exceeds this range, it is assumed that the battery has not been inserted and the bq2954 will transition to battery monitoring technology. When the battery is removed from the charger, VBAT will drop below VLOC. In the battery monitoring state, the BTST pin should be set to high level to activate the lithium-ion battery to be charged in an over-discharge state.

The battery insertion status monitoring time is about 500ms. Since there is a certain delay time tHO (about 1 second), even if VBAT has reached VMIN, the charger will not immediately transfer to the fast charging state. This delay time can prevent misjudgment when a voltage spike occurs on the BAT pin. There is also a time delay during battery removal status monitoring.

c. Temperature monitoring

The temperature is monitored through the voltage difference between the TS pin and the negative electrode of the battery. This voltage is obtained through an NTC (negative temperature coefficient) thermistor and thermistor linearization network. bq2954 golf ball This voltage is compared to the internal threshold voltage to determine whether to enter the fast charge state. These built-in thresholds include:

(1) Temperature "high" state cut-off voltage VTCO: VTCO=0.4Vcc, this voltage corresponds to the maximum temperature value Tco allowed during charging.

(2) Temperature "high" state fault voltage VHTF: VHTF=0.44Vcc. This voltage corresponds to the temperature THTF when charging resumes after the battery temperature exceeds Tco.

(3) Temperature "low" state fault voltage VLTF: VLTF=0.6Vcc, this voltage corresponds to the lowest temperature value TLTF allowed during charging.

Once the temperature exceeds the TLTF~Tco temperature range, the charging process is terminated. Moreover, if the temperature exceeds Tco, the current temperature must drop below THTF when charging is resumed.

The resistors RT1 and RT2 that form the thermistor linearization network are determined by the following formula:

0.6Vcc=V/[1+RT1(RT2+RLTF)/(RT2RLTF)]

0.44=1/[1+RT1(RT2+RHTF)/(RHTFRT2)]

In the formula: RLTF is the resistance value of the thermistor at TLTF; RHTF is the resistance value of the thermistor at HTF;

When using low-side current detection mode, V=Vcc-0.25;

When using high-end current detection mode, V=Vcc;

Tco is determined by the values ​​of RT1 and RT2. It is recommended to use a resistor with an accuracy of 1%.

Inserting a 10kΩ resistor between the TS pin and the negative terminal of the battery or between TS and Vcc can disable the temperature monitoring function.

The low-end and high-end temperature detection settings are shown in Figure 5 and Figure 6 respectively.

3.4 Maximum charging time

The maximum charging time tMTO can be set through the RC network on the TM pin, that is: tMTO=500RC, and the setting range is between 1 hour and 24 hours. It is recommended that C be 0.1μF.

The MTO timer will be reset at the beginning of the fast charging phase. If the MTO timer overflows during the voltage control phase, the fast charge state will be terminated, and the bq2954 will transition to the charge completion state; if the MTO timer overflows during the current control phase, the fast charge state will also be terminated, and the bq2954 will transition to the charging completion state. fault status.

The settings of the RC network and MTO are shown in Figure 7.

3.5 Charging control

bq2954 uses PWM technology to control the charging process to achieve constant current and constant voltage control. The charging current and charging voltage can be monitored through the SNS pin and BAT pin respectively. After comparing the detection signal with the built-in threshold, the device will output a PWM control signal through the MOD pin to maintain a constant current or voltage. The maximum duty cycle of the PWM control signal is 80%.

The voltage on the SNS pin is determined by the resistance of RSNS, and its maximum charging current is:

IMAX=VSNS/RSNS

The switching frequency is determined by the external capacitor CPWM between the TPWM pin and the Vss pin:

fPWM=（1×10 -4）/CPWM

The typical value of the switching frequency is generally 100kHz, and a capacitor of 0.001μF can be selected for CPWM.

3.6 Recharging after fast charging

After the first charging operation is completed, if the battery voltage is still lower than the VRECHG threshold, the fast charging state can be started again. After a delay of about 1 second, a new charging cycle is entered.

3.7 Recommended working conditions

The recommended operating conditions for the bq2954 are listed in Table 4.

Table 4 Recommended working conditions for bq2954

4 Typical applications

The schematic diagram of a high-efficiency lithium-ion battery charger designed using bq2954 is shown in Figure 8. This charger can charge 1 to 4 lithium-ion batteries.