New Linear Charging Solution for Lithium Ion Battery

Lithium ion batteries are widely used in modern portable electronic products because of their high energy density, excellent charge and discharge performance and no pollution.

Compared with other types of batteries, lithium-ion batteries not only have excellent performance, but also put forward higher requirements for chargers. These requirements are mainly reflected in the control of charging process and lithium battery protection, such as large charging current, high-precision charging voltage, phased charging mode and perfect protection circuit.

Chip introduction

Se9018 is a linear charging chip of lithium-ion battery in constant current / constant voltage mode. It adopts internal PMOSFET architecture and integrates anti backcharge circuit without external isolation diode.

The preset charging voltage of the chip is 4.2V, and the accuracy is ± 1.5%. The charging current can be set through the external resistance, and the maximum continuous charging current can reach 1a. When the junction temperature of the chip is higher than 140 ℃ due to high working power, high ambient temperature or poor heat dissipation performance of PCB, the internal thermal feedback circuit will automatically reduce the charging current and control the chip temperature within a safe range. In order to maintain the high-efficiency working state of the chip, measures should be taken to minimize the chip working power and chip temperature, such as connecting a small resistance in series at the input end (reducing the input voltage), increasing the area of PCB heat dissipation copper foil, making the chip heat sink fully contact with PCB copper foil, etc.

Figure 1 se9018 pin bitmap

Figure 2 Schematic diagram of se9018

Se9018 has an internal integrated battery temperature monitoring circuit. When the battery temperature exceeds the normal range (too high or too low), the chip will automatically stop the charging process to prevent the battery from being damaged due to too high or too low temperature.

Battery temperature monitoring is realized by judging the temp terminal voltage (vtemp), which is provided by a resistance voltage divider network including NTC thermistor inside the battery.

When vtemp is at 45% & times; VCC and 80% & tips; Between VCC, the chip judges that the battery temperature is within the normal range; When vtemp 45% & tips; VCC or vtemp 80% & tips; During VCC, the chip judges that the battery temperature is too high or too low; When the temp terminal is grounded, the battery temperature monitoring function is disabled.

Se9018 includes two open drain state indication output terminals CHRG and stdby. When the circuit is in the charging state, CHRG terminal is set to low level and stdby terminal is in the high resistance state; When the battery is fully charged, CHRG terminal changes to high resistance state and stdby terminal is set to low level. When the battery temperature monitoring function is used normally, if the chip is not connected to the battery or the battery temperature exceeds the normal range, CHRG terminal and stdby terminal are in high resistance state; When the battery temperature monitoring function is disabled, if the chip is not connected to the battery, the stdby terminal is low, and the CHRG terminal outputs a pulse signal.

Other functions of se9018 include manual shutdown, undervoltage locking, automatic recharging, etc.

A typical lithium ion battery charging circuit based on se9018 is shown in Figure 3. CE terminal is at high level, and se9018 works normally.

Figure 3 se9018 typical application circuit

1. Setting of charging current

The charging current IBAT during constant current charging is set by the resistance rprog between porg terminal and GND terminal. The relationship between IBAT and rprog resistance is as follows:

Formula 1

For example, if you want to obtain a constant charging current of 1a, rprog = 1200 Ω can be obtained according to Formula 1.

2. Battery temperature monitoring circuit setting

The setting of battery temperature monitoring circuit is mainly to set R1 and R2. Assuming that the resistance of NTC thermistor at the lowest working temperature is RTL and the resistance at the highest working temperature is rth (the data of RTL and rth can be found in the relevant battery manual or through experiments), the resistance values of R1 and R2 are respectively:

Formula 2

Formula 3

In practical application, R2 can be removed if only high temperature protection is required and low temperature protection is not required. At this time, the resistance value of R1 is:

Formula 4

3. Manual shutdown setting

During charging, se9018 can be placed in shutdown state by setting CE end to low level or removing rprog (PROG end floating) at any time. At this time, the battery leakage current drops below 2ua and the input current drops below 70ua.

4. Undervoltage locking status

If the input voltage Vcc is lower than the undervoltage locking threshold or the difference between VCC and battery voltage Vbat is less than 120mV, se9018 is in undervoltage locking state.

When the chip is in shutdown state or undervoltage locking state, CHRG terminal and stdby terminal are in high resistance state.

5. Normal charging cycle

When each input terminal and battery of se9018 are in normal state, the charging circuit enters the normal charging cycle, which includes four basic working modes: trickle charging, constant current charging, constant voltage charging, charging end and recharging.

If the battery voltage Vbat is lower than 2.9v, the charging circuit enters the trickle charging mode. At this time, the charging current is one tenth of the constant current charging current (if the constant current charging current is set to 1a, the trickle charging current is 100mA), The trickle charging state will be maintained until the battery voltage Vbat reaches 2.9v. The trickle charging mode is mainly to avoid damage to the internal structure of the battery caused by high current impact when the battery voltage is too low.

When the battery voltage is higher than 2.9v but less than the preset full charge voltage of 4.2V, the charging circuit is in the constant current charging mode. As described above, the charging current is determined by rprog.

When the battery voltage reaches 4.2V, the charging circuit enters the constant voltage charging mode. At this time, the bat terminal voltage is maintained at 4.2V and the charging current gradually decreases. The main function of this process is to reduce the influence of battery internal resistance on charge saturation voltage and make the battery charge more fully.

When the charging current decreases to 1 / 10 of the constant current charging current, the charging circuit stops charging the battery and enters a low-power standby state. In standby mode, se9018 will continuously monitor the battery voltage. If the battery voltage drops below 4.05v, the charging circuit will charge the battery again.

6. Indicator Status

Table 1

7. Circuit compatible with USB power supply and adapter power supply

At the same time, the se9018 chip can realize the charging circuit suitable for USB power supply and adapter power supply. The circuit diagram is shown in Figure 4.

Figure 4 USB and adapter scheme

When the USB power supply is used, the PMOS and NMOS gates are pulled down to the low potential, the PMOS is turned on, the USB power supply supplies power to the se9018, and the Schottky diode prevents leakage from the USB end to the adapter end. NMOS is off, RP1 is disconnected, rprog = 2.4k Ω, and the constant current charging current is 500mA

When the 5V adapter is used for power supply, the PMOS and NMOS gate are at high potential, and the PMOS is cut off to prevent leakage from the adapter end to the USB end. The 5V voltage of the adapter supplies power to the se9018 through the Schottky diode. NMOS is turned on and RP1 is connected into the circuit. At this time, rprog is RP1 in parallel with 2.4k Ω resistance. By setting RP1, a constant current charging current greater than 500mA can be realized.

conclusion

This paper discusses the linear charging solution of intelligent high current lithium-ion battery. The se9018 chip has the characteristics of fast charging speed, strong battery protection function and less number of peripheral components. Moreover, the chip is also suitable for USB power supply and adapter power supply. It is a practical intelligent high current lithium-ion battery charging chip.

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