Application Introduction of DS2770 Battery Monitoring and Charging Controller

1 Overview

The DS2770 battery monitor and charge controller can complete various functions required for battery maintenance. When used with the main system processor, the DS2770 can complete functions such as charging, remaining power estimation, safety monitoring, and permanent data storage. It has a unique ID, a digital temperature detector, an A/D converter to measure battery voltage and current, an integrated current accumulator to control the amount of battery current inflow and outflow, a time-consuming timer, an important data storage device, and a controller that can charge lithium batteries and nickel-metal hydride batteries. Current measurement is completed through a 25mW integrated resistor or an external detection resistor. The accuracy of current, voltage, and temperature can meet the needs of battery charging control and safety. Users can choose to use pulse technology to charge lithium batteries or d T/dt technology to charge nickel-metal hydride batteries. In addition, for greater safety and convenience, there are programmable charging timers and battery low-voltage recovery functions. The processor and DS2770 use a one-wire interface to transmit information. Therefore, the DS2770 only needs four output connections: battery power, charging power, ground, and a one-wire interface. At the same time, the DS2770 also has EEPROM and SRAM memory for battery information storage. EEPROM stores important battery data; SRAM stores temporary data.

2 Internal structure and pin function

The DS2770 uses a 16-pin TSSOP package, and the pinout is shown in Figure 1.

The DS2770 pin functions are as follows:

• Pin 1 (UV): Battery undervoltage detection output. When the battery voltage is lower than the minimum battery voltage threshold (VLB), this pin charges the battery at a small charging rate.
• Pin 2 (CC): Charge control output. When the battery voltage is greater than or equal to VLB, this pin controls the battery charging.
• Pin 3 (VCH): Charging power input. The charging power is connected to this pin, and the DS2770 will make a measurement before deciding whether to charge.
• Pin 4, 5, 6 (SNS): Sense resistor connection. Connect to the negative terminal of the battery pack. When using the internal sense resistor, the sense resistor is connected between VSS and SNS.
• Pin 8 (IS2): Current sense input. Connect the pin to SNS with a 10kW resistor, and connect a 0.1mF capacitor between IS1 and IS2 to complete the low-pass filtering.
• Pin 9 (IS1): Current sense input. Connect the pin to VSS with a 10kW resistor and a 0.1mF capacitor between IS1 and IS2 to complete the low-pass filtering.
• Pin 11, 12, 13 (VSS): Component grounding. Directly connected to the negative terminal of the battery. For external detection resistor method, the detection resistor is connected between VSS and SNS.
• Pin 14 (DQ): Data input/output. Use 1-wire data line. Open drain output driver, the pin is connected to the DATA terminal of the battery pack. The pin has an internal pull-down for detecting power disconnection.
• Pin 15 (VIN): Voltage Sense Input. Monitor the battery voltage through the input pin.
• Pin 16 (VDD): Power input. Used for the input power voltage of DS2770 (2.7～5.5V)
• Pins 7 and 10 (NC): No pins.

3. Functions and Applications

3.1 Power supply mode

The DS2770 has two power modes: active mode and sleep mode. In active mode, the DS2770 continuously measures current, voltage, temperature, and time, and has current flow accumulation and charge control. The host system can use this data. The DS2770 enters sleep mode only when PMOD in the status register is set to 1 and the following conditions occur:

CINI is set to 0 and the DQ line remains low for more than 2 seconds. If charging is in progress, charging stops immediately.

CINI is set to 1, and the DQ line remains low for more than 2 seconds. If charging is in progress, it enters sleep mode after charging is completed.

The DS2770 enters active mode when the following conditions occur:

The DQ line is high.

When CINI is set to 1, the voltage of VCH is greater than VDD.

Once the DS2770 recognizes that DQ is low for more than 2 seconds, it enters sleep mode and it takes 11 seconds for the supply current to drop to ISLEEP. When power is applied to VDD, the DS2770 defaults to active operation mode.

3.2 Charging function

DS2770 can work as a controller supporting charging of lithium batteries and NiMH batteries. The type of battery to be charged is selected by the status register CTYPE (0 for lithium batteries and 1 for NiMH batteries). The charging control of both is completed by the on/off selection of the external DC or current-limited charging power supply. If the battery voltage is lower than VLB and there is a charging power supply, the foot UV drops to a low level. Before fast charging begins, the battery voltage must be restored at a small charging rate. In the application circuit of Figure 2, UV limits the trickle charging current through a 360 W series resistor. The choice of resistor depends on the charging power supply. UV drops to a low level regardless of the state of the battery pack (such as battery temperature and CINI). When the battery voltage reaches VLB, UV rises to a high level. When trickle charging is in progress, CSTAT1 and CSTAT0 of the status register are cleared, and charging is indicated by 0 or 1 values ​​accordingly.

Fast charging can be started in one of the following ways:

Send a start charging command (B5h);

When CINI of the status register is set to 1, there is charging power on the VCH pin.

Note: If VDD is lower than 1.8V, the battery is trickle charged to reach VLB and fast charging begins. After charging begins, the following conditions occur, and fast charging is delayed:

The charging temperature exceeds TCL (0°C) and TCH (40°C);

VCH is less than VDD;

The conversion data is invalid;

The battery voltage is lower than VLB (3.0V).

After the above conditions disappear, the low CC pin starts fast charging. During fast charging, CC remains low and only rises to high level for about 27ms every 55ms when periodically testing whether the charging power supply is disconnected prematurely. As long as the charging power supply is not disconnected and the temperature is within the valid range, charging is carried out in the mode selected by CTYPE. If the charging power supply is disconnected or a stop charging command (BEh) is issued, CC rises to high level and charging is restarted. If the DQ line remains low for more than 2s and CINI is set to 0, charging stops. During fast charging, CTTATI and CSTAT0 of the status register are indicated by 0, 1 respectively; when charging is completed, CSTAT1 and CSTAT0 are indicated by 1, 1. The charging status latch must be cleared. Once charging is completed or failed, the DS2770 can enter sleep mode or remain active.

3.3 Lithium battery charging

The fast charging of lithium batteries is completed in two steps. When the battery voltage is lower than the charging voltage threshold VCV, the battery is charged with a large current by controlling the current of the charging power supply. The CC pin is always kept at a low level to stimulate the pnp or p-channel MOSFET switch. When the battery voltage reaches VCV, pulse charging technology is used. After CC becomes a high level, CC is allowed to maintain a low level for tVCV (875ms). When the battery voltage drops below VCV again, CC becomes a low level again. Since the CC duty factor changes slowly, pulse charging continues. When the battery voltage decays for more than 13.125s, charging stops. The charging decay time includes a low level time of 875ms and a high level of 15 cycles for 14s. The average charging rate is 1/16 of the set charging rate.

During NiMH charging, the battery charge current is controlled via the UV and CC pins. As the battery reaches 3.0V, a transition from trickle charge to fast charge occurs. Fast charging begins when each cell of a three-cell NiMH battery reaches 1V. Although the voltage divider affects the value in the voltage measurement register, it can be used to adjust the transition point for the larger voltage battery. After fast charging begins, the DS2770 uses the most recent temperature measurement to determine the appropriate dT/dt. As shown in Table 1, to avoid erroneous dT/dt detection due to I2R heating, the temperature change rate is invalid for the first five minutes. After five minutes, dT/dt detection is started using an initial change rate in Table 1. The chip then averages the temperature measurement register values ​​to determine the rate of change of temperature rise. Table 1 lists the actual change rate based on the NiMH battery data, which is the instantaneous dT/dt change rate on the load.

[page]3.5 Auxiliary charging device

The chip has two auxiliary charging devices. If the battery exceeds the maximum charging temperature or the charging timer has expired, charging stops. CSTAT1 and CSTAT0 of the status register are both set to 1, indicating that charging is complete. The maximum temperature threshold is TMCT (+50°C). During charging, if the measured temperature exceeds TMCT, charging stops. The maximum charging time can be set in the charging timing register (CTR). The initial value of CTR can be preset at the beginning of fast charging. During fast charging, CTR counts down every 56 seconds. If CTR decreases to zero, charging stops. Since CTR is written, to modify the maximum charging time, the value of CTR can be rewritten at any time during charging. The format of CTR is as follows, and the address is 06.

3.6 Current Measurement

In active mode, the DS2770 continuously measures the current flowing into and out of the battery by measuring the voltage drop across the current sense resistor. The DS2770 has two modes: internal 25m ohm sense resistor and external user-selected sense resistor. In either mode, the DS2770 uses the voltage difference between pin IS1 and pin IS2 (VIS = VIS1-VIS2) as the voltage drop across the sense resistor. A positive VIS value indicates that the battery is charging, while a negative VIS indicates that the battery is discharging. When using an external sense resistor, one end of the resistor must be directly connected to VSS (the negative terminal of the battery) to ensure the correct operation of the current measurement circuit. VIS is measured with 15-bit accuracy, and the measured value in the current register is updated every 3.52 seconds. The current value in the current register is the average value within 3.52 seconds. The following is the current register format, with addresses 0E and 0F.

For the internal sense resistor method, the DS2770 uses amperes as the unit, using a current register with a total value of 0.048A and an accuracy of 62.5uA. When reporting the current value, the DS2770 automatically compensates for the change in internal resistance and temperature effects. For the external sense resistor method, the DS2770 writes the measured VIS voltage into a current register with a total value of 1.2mV and an accuracy of 1.56mA.

3.7 Current Accumulator and Deviation Compensation

By recording the net current flowing into and out of the battery, the current accumulator can estimate the remaining power. When the current flows into the battery, the accumulator increases, and when the current flows out of the battery, the accumulator decreases. The data is stored in the current accumulation register. The format of the current accumulation register is as follows, with addresses 10 and 11.

When using the internal sense resistor, the DS2770 uses a current accumulator with a precision of 250uA and a total value of ±8.19Ah in ampere-hours. When using the external sense resistor, the DS2770 uses a current register with a precision of 6.25uVh and a total value of ±205mVh in voltage-hours. Current measurement and current accumulation are internally compensated for errors caused by component temperature and power supply voltage changes.

Deviation compensation is accurate to LSB at least once per hour. In addition, in order to correct the error caused by the current measurement or current accumulation error caused by the circuit layout, the current deviation register has a user-programmed constant current deviation value. The arbitrarily applied constant current deviation value will make the current measurement inaccurate or cause self-discharge at room temperature. The current deviation compensation value is stored at the 32h and 33h addresses of the EEPROM. Therefore, positive values ​​(0001h to 7fffh) will cause deviations in current measurement and current accumulation during discharge. The following is the format of the current deviation compensation register.

The DS2770 can continuously measure the voltage between VIN and VSS between 0V and 4.992V with an accuracy of 4.88mVh. The measured data is updated every 55ms and stored in the voltage register. The following is the voltage register format. The maximum value of the register is the maximum voltage value. The address is 0C and 0D.

The DS2770 continuously measures the battery humidity temperature over a ±127°C range using an integrated temperature sensor with 0.125°C accuracy. Below is the temperature register format.

The DS2770 has a universal timer with a range of 1024 hours. The timer value is stored in the time register with an accuracy of 56 seconds. When the maximum value is reached, the value rolls to zero and starts timing again from the highest value. In addition, the user can write any value required to the register. The following is the format of the time register, with addresses 02 and 03.

3.9 Application Circuit