Brief analysis of lithium battery linear charge management chip BQ2057 and its application

1. Introduction

BQ2057 series is an advanced lithium battery charge management chip produced by TI Company of America. BQ2057 series chip is suitable for single-section (4.1V or 4.2V) or double-section (8.2V or 8.4V) lithium ion (Li-Ion) and lithium polymer ( Li-Pol) battery charging needs, and provides optional packages of MSOP, TSSOP and SOIC according to different applications. The charger's peripheral circuit and its simplicity are very suitable for the compact design of portable electronic products. The BQ2057 dynamically compensates for the internal resistance of the lithium battery pack to reduce charging time. With optional battery temperature monitoring, the battery temperature sensor continuously detects the battery temperature. When the battery temperature exceeds the set range, the BQ2057 turns off the battery. The internal integrated 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 interface with the main controller, with automatic recharging, minimum current to stop charging, low power sleep and other characteristics.

2. Functions and features

2.1 device package and model selection

The BQ2057 series of charging chips are available in a variety of packages and models to meet the design requirements. The package is shown in Figure 2-1. It is available in MSOP, TSSOP and SOIC packages. Its model is shown in Table 2-1. There are four kinds of signals, BQ2057, BQ2057C, BQ2057T and BQ2057W, which are suitable for charging of 4.1V, 4.2V, 8.2V and 8.4V respectively.

The pin functions of the BQ2057 are described as follows:

VCC (Pin 1): Operating power input;

TS (Pin 2): Temperature sensing input for detecting the temperature of the battery pack;

STAT (Pin 3): Charging status output, including: three states of charging, charging completion and temperature failure;

VSS (Pin 4): Input to the operating power supply;

CC (Pin 5): charge control output;

COMP (Pin 6): Charging rate compensation input;

SNS (Pin 7): Charging current sensing input;

BAT (Pin 8): Lithium battery voltage input;

2.2 Charging status flow

The charging of the BQ2057 is divided into three phases: precharge state, constant current charge and constant voltage charge phase.

2.2.1 Precharge phase

After installing the battery and adding power, the BQ2057 first checks the working voltage VCC. When the working voltage is too low, the charger enters the sleep mode. If the working voltage is normal, check whether the battery temperature is within the set range. If not, enter the temperature. Fault mode, otherwise the battery voltage VBAT is detected. When the battery voltage VBAT is lower than the low voltage threshold V(min), the BQ2057 precharges the battery with a constant current IREG 10% current IPRE.

2.2.2 Constant current charging

When the battery is precharged or the battery voltage VBAT is lower than the constant voltage VREG, the BQ2057 enters a constant current charging state, at which time the charging current is monitored by the voltage drop across the external sensing resistor RSNS, which can take the high/low side Connection mode, RSNS is connected between VCC and SNS pins in high-side current detection. RSNS is connected between VSS and SNS pins in low-side current detection.

2.2.3 Constant voltage charging

When the charging voltage reaches constant voltage VREG, it enters constant voltage charging state. BQ2057 monitors the battery pack voltage through VBAT and VSS pins. When the charging current reaches the termination threshold I(TERM), it stops charging. When the battery voltage is lower than the recharging threshold voltage V(RCH), it starts recharging automatically. BQ2057 can not only realize standard voltage charging of 4.1v, 4.2v, 8.2v and 8.4v, but also realize non-standard voltage charging through partial voltage. The method is to use the battery partial voltage value realized by partial voltage resistance as the input of BAT pin.

2.3 Battery temperature monitoring

The BQ2057 continuously monitors the temperature of the battery pack by measuring the voltage between the TS and VSS pins. The commonly used thermistor is used as a temperature sensor and is implemented by a voltage divider resistor, as shown in Figure 3-1. The resistance of the voltage dividing resistor can be calculated according to the parameters. BQ2057 compares this voltage to the internal V(TS1) and V(TS2) threshold voltages to determine if charging is allowed. Since the external partial pressure and the internal limit voltage are all referenced to VCC, the temperature detection circuit is not affected by the fluctuation of the working power supply VCC. When the TS pin is connected to VCC or VSS, the charging function of the BQ2057 can be disabled.

2.4 Charging status indication

The BQ2057 reports the current state of charge through the tri-state pin STAT: charge state high, charge complete low, temperature fault, or sleep state high impedance. When the STAT pin is connected to the single LED or dual LED reverse connection mode, the LED indication of the charging state can be realized, and the STAT port can also be interfaced with the instrument microcontroller, and the microcontroller realizes intelligent instrument management by identifying three states of the STAT port.

3. Typical charger circuit design

The charger circuit designed with BQ2057 is simple and can be widely used in the power management of current portable electronic systems, which makes sense for the compact design of portable electronic products.

The lithium battery charging circuit designed by BQ2057 can realize the charging of one or two lithium batteries. The working power DC+ is selected according to the voltage of the rechargeable lithium battery. The recommended working voltage is 4.5V~18V. The positive voltage of the battery pack is PACK+BAT. The pin and the TS pin detect the voltage division value of the thermistor NTC of the battery pack through the voltage dividing resistor to determine whether the temperature is normal. The BQ2057 can be designed to be charged by a PNP transistor or a P-channel MOSFET, and should be selected when selected. The power consumption should be satisfied when selecting.

4. Conclusion

In our portable electronic instrument, we chose the 7.2V lithium-ion battery pack charging circuit of BQ2057W chip design instrument. The charging circuit is very simple. The whole charging process and status display are realized by BQ2057 alone. The whole power management module is simple and reliable. The charger circuit is valuable for electronic system design involving lithium battery charging requirements.

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