The principle and application of battery fuel gauge and charger chip DS2770

frozenviolet

The principle and application of battery fuel gauge and charger chip DS2770 [Copy link]

1. Main features

DS2770 is a battery fuel gauge and lithium/nickel chemical battery charger integrated device produced by Dallas Semiconductor. It can communicate with the main system through the 1-Wire interface to read measurement information such as voltage and temperature, and read and write EEPROM memory at the same time, so it can be widely used in portable devices.

The DS2770 battery management IC can perform a variety of functions. It can charge the battery using a simple current-limited power supply, and can also be used as a high-precision fuel gauge. When connected to the main system through the 1-Wire communication interface, the DS2770 can provide functions such as charging, remaining power estimation, safety management, and non-volatile parameter storage. The main features of the DS2770 are as follows:

●Lithium pulse charging or nickel battery charging (dT/dt charging termination method) can be selected by the user?

●High-precision current measurement with real-time offset correction, optional internal integrated 25mΩ sense resistor

●With 32-byte lockable EEPROM and 16-byte SRAM?

● With Dallas 1-Wire digital communication interface and unique 64-bit ID.

2 Pin Function

DS2770 is a 16-pin TSSO package. Figure 1 shows its pinout. The functions of each pin are as follows:

UV: Battery voltage detection terminal. When the battery voltage is detected to be a low value, this terminal outputs a low level.

CC: Charging control output terminal? Low level is valid;

VCH: charging voltage input terminal;

SNS: Current sensing resistor connection terminal;

VIN: battery voltage detection input;

VDD: chip power supply terminal;

VSS: ground terminal;

DQ: data input/output terminal;

LS1 and LS2: Current detection input terminals.

3. Schematic principle of DS2770

DS2770 uses pulse charging for lithium-ion battery chargers, which is different from linear charging in terms of charging source and charging termination. Pulse chargers require a power supply with a current limit equal to the desired charging rate; when the pulse charging duty cycle decreases to a certain level (such as less than 5% to 10%), pulse charging is terminated. In fact, DS2770 also has an auxiliary charging termination control method, which can terminate charging when the battery temperature exceeds +50℃ or the maximum charging time set by the user is exhausted. When DS2770 is used as a fuel gauge, its minimum resolution is 62.5μA. The EEPROM inside DS2770 allows the system processor to accurately calculate the power.

The internal structure principle of DS2770 is shown in Figure 2. As can be seen from the figure, the device is mainly composed of 1-Wire interface and address, analog-to-digital converter ADC for measuring battery voltage, temperature, current and accumulated current, EEPROM and SRAM, and charging control circuit.

Current measurement can be achieved through the internal 25mΩ sense resistor. IS1 is connected to VSS through a 10kΩ resistor, and IS2 is connected to SNS through a 10kΩ resistor. In this way, the current difference between IS1 and IS2 can be used to indicate whether the battery is being charged (the current difference is positive) or discharged (the current difference is negative).

DS2770 has 256 bytes of access space, of which the lower 32 bytes are command, status and control registers, and the following 32 bytes are user-accessible EEPROM, which can be used to store general information. However, it should be noted that once the EEPROM is written, it will be permanently locked to ensure data integrity. However, the registers and EEPROM can be accessed through the 1-Wire interface.

Figure 3

4 Application circuit of DS2770

When using a pulse charger, a current-limited and voltage-regulated power supply should be used. Since the pulse charger does not regulate the charging current, current limiting is a necessary condition. However, by cleverly using passive components, the DS2770 can also use a current-limited non-regulated power supply. The specific circuit is shown in Figure 3.

In the figure, when the charging power supply is connected between CS and PACK, and DS2770 detects the power supply, it can start charging the battery. In the circuit, Schottky diode D1 is used to prohibit capacitor C1 from discharging through the charging power supply; diode D2 can provide VCH with a margin of about 0.5V higher than VDD, and can also prevent the battery from discharging through the charging power supply; in addition, in order to make the VCH terminal voltage higher than the battery voltage during the entire charging cycle, the capacitance of capacitor C1 should be greater than 1.5μF.