Design circuit and finished product of new high performance lithium battery charger

One-piece inductor

Foreword

Li+ lithium batteries are widely used in high-end portable products such as GSM/CDMA and digital cameras, camcorders and PDAs due to their small size, light weight and high energy density. They all need to build a high-performance lithium-ion battery. Charger, to ensure that Li+ battery avoids over-charging, over-discharge and other damages during use, which leads to the requirement that the lithium battery charger has strict and perfect protection circuit to truly realize various safety protections. characteristic. To this end, the new DS2770 and DS2720 chips can be used to design a high-performance lithium battery charger combination with charge control, power control, battery counting, battery protection, timing and battery identification. It can be used to replace the existing lithium battery protection / charging control circuit - the charger on the market. The following describes the functions and features of the high performance lithium battery charger combination design.

1 charging combination circuit --- the composition of the charger

1.1 As shown in Figure 1. The entire combined circuit consists of a DS2770, a charge controller/fuel gauge, a DS2720 battery protector, and a DS2415 real-time clock (RTC). They all share a ground (Vss or Gnd), power supply ( Vdd) and communication line (DQ or DATA). All capacitors (from C1 to C10) and resistors (from R1 to R12) are used to filter interfering signals and protect ESD.

The charger's load (ie, the master) is powered by the PACK+ and PACK- pins, and the charger's data communication with the host system is via the standard l-Wire interface (labeled DATA). The PS of the DS2720 chip The pin is connected to the on/off control of the main system and is used as the enable input of the charger (active low). The pin Charge·source in Figure 1 can be connected to the charging power supply, and the charging current can be rated according to the battery. The charging conditions are limited and the charging voltage is up to 15 V.

The entire charger consumes less than 100μA of typical current in the operating mode, while the typical current consumption is less than 20μA when it is at rest (ie, the lithium battery is not charging).

1.2 Introduction to the functions and features of the DS2770 chip

The DS277 is a new chip that integrates a battery gauge and a Li-Ion or Ni-based battery charge controller. The block diagram is shown in Figure 2. It also includes an optional 25mΩ test set for charging current measurements. The built-in measurement circuit detects voltage and temperature values ​​as a criterion for termination of charge and a criterion for a safe charging environment. All measurements are stored in 16-byte SRAM memory and its 40-byte EEPROM is reserved for the user. All information exchange with the main system is achieved through its standard l-Wire communication interface. The chip is low-power, consumes only 80μA in operating state, and consumes 0.5μA static current.

Its pin function description:
UV: When the battery voltage is detected as a low value, the output is active low.
CC: Charge control output, its active low.
Vch: Charging voltage input.
SnS: current measuring resistor connection terminal.
Vdd and Vss: chip power terminal
Vss: Ground
DATA: data input / output
LS1 and LS2: Current detection input.
Vin: Battery voltage detection input

2 About the protection characteristics of the charging combination circuit

See the middle of Figure 1. Apply two external N-channel MOSFET protection tubes (IRF840) VP3 and VP4 and DS2720 chips to protect the cell Li+ or lithium polymer battery. It can protect the battery from overcharging and over-discharging. Safety features such as consumption, excessive discharge current, and excessive temperature damage. The DS2720 features a small current charging function to recover a deeply discharged battery. The main system software can also detect the cause of the battery failure through the DQ pin of the DS2720 and DS2770 chip and report it to the user from the main system.

It should be noted that the MOSFET is connected to the high end of the charging combination circuit between the charging power supply and the positive terminal of the Li+ battery, as shown in Figure 1. To ensure that its data is in a protective fault or when the charging combination circuit is in sleep mode. When not lost, it is best to power the DS2770 and DS2415 directly from a Li+ or lithium polymer battery. Otherwise, the data will be lost when the MOSFET is turned off. The DS2720 chip is low-power, consumes only 12.5μA in operating state, and consumes 1.5μA in static current.

Its DS2720 has its pin function:
Positive input of the PLS battery pack
PS: System (power) switch speculates input, active low.
DQ: Data input/output.
CP: Power storage capacity.
CC: Charging control input
DC: Discharge control output.

3 About charging and control

One of the features of the DS2770 is to charge the battery with a simple current-limited power supply. By controlling external PNP transistors (VP1 (FMMT718) and VP2 (Model 4403), the DS2770 can deliver Li+ or Li-polymer based batteries at a constant current. The group is charged until the voltage rises to the factory's undefined 4.1V or 4.2V limit. It then fills the battery in a pulsed charge.

The DS2770 also provides an auxiliary charge termination control that stops charging when the battery temperature exceeds +50 ° C or exceeds the user-set maximum charge time. To start charging, just connect a current-limiting power supply (up to 15V) to the chargesou rce.

4 About the charge count

The DS2770 can also be used as a high-precision fuel gauge. Current measurement is achieved by an internal 25mΩ sense resistor (see Figure 2) with a minimum resolution of 62.5μA and a dynamic range of up to ±2A. In GSM/CDMA applications, the DS2770 easily tracks the discharge current. Its internal auto-compensation maintains measurement accuracy over the entire operating range of the chip and enables real-time measurement of accumulated current, voltage and temperature. In addition, the battery characteristic data stored in the EEPROM of the DS2770 enables the main system processor to accurately calculate the power while consuming only a small amount of system resources. And because the DS2770 is powered directly by the battery, the battery count information is not lost when the battery pack is removed or due to a protective faulty power supply failure.

5 real time clock RTC

The DS2415 provides an RTC with an accuracy of 2 minutes/month for the main system. It requires an external crystal of 32.768kHz/6pF to be connected to the X1 and X2 pins of the DS2415. Since the DS2415 is powered directly from the battery, this is a structure that is not available in other charger circuits. By placing the DS2415 clock on the inside of the MOSFET, it can provide a high-precision guaranteed clock for the main system, and even keep the correct time information when the main system power is lost, eliminating the need to add a super capacitor or button battery in the main system. The trouble of being a backup power source. DS2415 pin function:

Vbat charging voltage input pin 2.5v.-5.5v.

Other similar to the above is also indicated.

6 Saving battery pack information

The DS2770 contains 40 bytes of EEPROM for user access, while the DS2720 adds an additional 8 bytes. Battery pack manufacturers can use these spaces to save relevant battery pack information, such as battery chemistry type, assembly date, battery-specific information for battery counting, etc. Once written to the EEPROM, it will be permanently locked, even when the main system power is lost and ES D Data integrity is guaranteed at the time of the event. In addition, each chip has a unique 64-bit serial number for easy identification by the host system or charger.

Conclusion

According to this scheme, the high-performance lithium battery charging combination road-charger is characterized by simple operation, low cost, excellent performance, safety and reliability. It should be said that it has a higher performance-price ratio.