Design and implementation of a lithium-ion intelligent charger

Publisher:ShiningSmileLatest update time:2011-11-11 Source: chinaaet Reading articles on mobile phones Scan QR code
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1 Introduction

Lithium-ion batteries have higher energy density and advantages such as small size and light weight compared with other batteries, but they have higher requirements for protection circuits. During the use of batteries, overcharging and over-discharging must be strictly avoided. Usually, lithium-ion batteries are charged in a constant current-constant voltage mode. To ensure safe charging, the voltage of the rechargeable battery is generally detected to determine whether the battery is full. In addition to voltage detection, other auxiliary methods are also required as backup measures to prevent overcharging, such as detecting battery temperature and limiting charging time. In addition, since lithium-ion batteries will also cause battery damage when they are overcharged, it is generally necessary to detect whether the battery is chargeable before charging. Usually, when charging lithium-ion batteries quickly, it is necessary to ensure that the voltage of each battery is higher than 2.5V, the temperature is higher than 2.5℃ and lower than 50℃, which requires the charger to have a pre-charging process. From this point of view, although lithium-ion batteries have higher performance indicators, they have higher requirements for the protection measures of the charger. If discrete components are used to form a lithium-ion battery charger, the circuit will be very complicated and the design time will be long. Maxim's latest MAX1757 can be used to build a 1-cell to 3-cell lithium-ion battery charger. This chip has a built-in 14V power switch, which simplifies the design of multi-cell lithium-ion battery chargers. Some medium-power, high-end portable products (such as digital cameras, camcorders, etc.) are generally powered by 2 to 3 lithium-ion batteries, and the MAX1757 is very suitable for battery chargers. [2][5]

2 Introduction to chip pin functions and charger working principle

2.1 Brief introduction to chip pin functions

MAX1757 uses a 28-pin SSOP chip package. The specific function of each pin is not introduced in detail. Only the pin functions used in this article are introduced here. VL pin: chip power input terminal. 5.4V is output from the DCIN terminal through the linear regulator, and VL is connected to a ceramic capacitor of 2.2μF or more to the ground; ISTTIN pin: input current limit adjustment terminal. Connect a resistor divider between VREF and GND to adjust; ISTTOUT pin: battery charging current adjustment terminal. Connect a resistor divider between VREF and GND to adjust; REF pin: 4.2V reference voltage output terminal. This terminal is connected to a ceramic bypass capacitor of 1μF or more to the ground; GND pin: analog ground; VADJ pin: voltage adjustment terminal. Connect a resistor divider between VREF and GND to adjust the battery charging voltage, and the adjustment range is 4.2V±5%; PGND pin: power ground. The current flows from the source of the low-side FET switch to PGND; LX pin: the external inductor terminal of the power supply; BST pin: the internal high-side MOSFET drain bias; DCIN pin: the power input terminal. Its input is the VL-enhanced regulated power supply, and this terminal is connected to a bypass capacitor of more than 0.1μF. [1]

2.2 Working Principle of Charger

Figure 1 shows the internal circuit of MAX1757, including input current detector, voltage detector, charging current detector, timer, temperature detector and main controller. The input current regulation circuit is used to limit the total input current of the power supply, including system load current and charging current. When the input current is detected to be greater than the set current limit threshold, the input current can be limited by reducing the battery charging current. Because the power supply current varies widely when the system is working, if the charger does not have the input current detection function, the input power supply (wall adapter or other DC power supply) must be able to provide the sum of the maximum load current and the maximum charging current, which will increase the cost and size of the power supply. The input current limiting function can reduce the charger's requirements for the DC power supply and simplify the design of the input power supply. The voltage detection circuit can regulate and monitor the battery voltage and charging current respectively with the current detection circuit. The voltage detection accuracy of MAX1757 is ±0.8%. The voltage detection and current detection results are sent to the main controller, which drives the internal high-side MOSFET to turn on or off to control the charging current or limit the battery voltage. The timer and temperature detector provide additional protection for battery charging. Since the charging efficiency cannot reach 100%, the charging time limit should have a margin. The temperature sensor should be installed close to the battery, and the temperature sensor can be a thermistor with a negative temperature coefficient, with a resistance of 10kΩ at +25℃ (Philips, Cornerstone Sensor Company, and Fenwall Electronics Company can provide appropriate products to meet user needs). MAX1757 detects battery temperature at a frequency of 1.2Hz.

Figure 1: Internal working principle of the charger

3 Charging process curve

MAX1757 has built-in charging state control, and Figure 2 is a charging process curve. The charging process is divided into pre-charging, fast charging, full charging and top-off charging states.

Figure 2 Charging process curve

3.1 Pre-charge status

After installing the battery, connect the input DC power supply. When the charger detects that the input voltage is greater than the battery voltage, it resets the timer and enters the pre-charge process. During this period, the charger charges the battery at 1/10 of the fast charge current to restore the battery voltage and temperature to normal. The pre-charge time is determined by the external capacitor of timer 1. If the battery voltage reaches above 2.5V within the specified charging time and the battery temperature is normal (higher than 2.5℃ and lower than 50℃), the charging enters the fast charge process; if the battery voltage is lower than 2.5V, it is considered that the battery is not rechargeable and the charger displays a battery failure.

3.2 Fast charging status

The fast charging process is also called constant current charging. At this time, the charger charges the battery with a constant current ICHG. According to the charging rate recommended by the battery manufacturer, most lithium-ion batteries use a 1C charging rate (indicating the speed of battery charging, that is, the charging and discharging currents are usually expressed as multiples of the battery's rated capacity C, called the charging rate). It takes about 1 hour to fully charge the battery. During constant current charging, the battery voltage will rise slowly. Once the battery voltage reaches the set termination voltage (usually 4.1V or 4.2V), the constant current charging is terminated, the charging current decreases rapidly, and the charging enters the full charging process.

3.3 Full charge and top cut-off charging status

During the full charge process, the charging current gradually decays until the charging rate drops below C/10 (the default setting is that the current decreases to 330mA) or the full charge time expires, and then the top cut-off charging is turned on; during the top cut-off charging, the charger replenishes the battery with a very small charging current. Since the charger has a charging current passing through the battery internal resistance when detecting whether the battery voltage has reached the termination voltage, although the charging current gradually decreases during the full charge and top cut-off charging process, reducing the impact of the battery internal resistance and other series resistances on the battery terminal voltage, the voltage drop formed by the resistance in series in the charging circuit still affects the detection of the battery termination voltage, and the top cut-off charging plays an important role in maximizing the battery energy replenishment. In general, full charge and top cut-off charging can extend the battery life by 5% to 10%.

4 Charger parameter settings

4.1 Battery termination voltage setting

The battery charge termination voltage can be set by connecting an external voltage divider resistor with an accuracy of 1% and a resistance value of less than 100kΩ. The battery charge termination voltage is related to the chemical characteristics and battery structure of the battery, and the specific parameters are provided by the battery manufacturer. The relationship between the voltage VVADJ of the VADJ pin and the battery charge termination voltage (VBATTR), the number of battery cells (N), and the reference voltage (VREF) is determined by the following formula:

VVADJ = (9.5VBATTR/N) - (9.0VREF)

4.2 Charging Current Setting

The charging current of the fast charging process is determined by the voltage value of the ISETOUT pin (VIESTOUT), which is adjusted by the voltage divider resistor connected between REF and GND. When the ISETOUT pin is connected to REF, the current is the maximum value (1.5A). The relationship between ICHG and VIESTOUT is as follows:

ICHG=1.5(VIESTOUT/VREF)

4.3 Input current limit setting

The input current limit threshold IIN is determined by the voltage of the ISETTIN pin. The value of IIN can be determined according to the following formula.

ICHG=0.1(VISETIN/VREF)/R1

4.4 Selecting an Inductor

The inductance value is related to the size of the current ripple. When a larger inductance is selected, the current ripple is smaller. However, if the physical size of the inductor is the same, the larger the inductance value, the smaller the inductance value. Usually, the equivalent series resistance and rated current of the inductor are small. Considering the overall indicators, the current ripple is generally set to 30% to 50% of the average charging current. Assuming that the ratio of ripple current to DC charging current is LIR, the inductance value is determined by the following formula:

L=[VBATT(VDCIN(MAX)-VBATT)]/[VDCIN(MAX)foscICHGLIR]

In the formula: fosc is the switching frequency of the DC-DC converter inside the charger, which is 300kHz. The rated current of the inductor should be greater than ICHG[1+LIR/2].

4.5 Charging time setting

MAX1757 contains four timing setting functions, namely pre-charge, fast charge, full charge, and top cut-off charging time. The external capacitor of timer 1 can set the time limit of pre-charge, full charge, and top cut-off charging process, and the external capacitor of timer 2 can set the fast charge time limit. When the charging rate is 1C, the typical charging time is set (the external capacitors of timer 1 and timer 2 are both 1nF): pre-charge time is 7.5 minutes, fast charge time is 90 minutes, full charge time is 90 minutes, and top cut-off charging time is 45 minutes. The relationship between the external capacitors of timer 1 and timer 2 and the charging time is shown in Figures 3 and 4.

Figure 3 Relationship between external capacitance and charging time of timer 1

Figure 4 Relationship between external capacitance and charging time of timer 2

5. Considerations for selecting peripheral devices

When selecting MAX1757 peripheral components, pay attention to the following points:

(1) Since the size of the inductor is related to factors such as input voltage and charging current, when selecting an inductor, the saturation current of the inductor magnetic material should be greater than 2A, and the value can be adjusted appropriately.

(2) The capacitors near the rechargeable battery should be multilayer ceramic capacitors and electrolytic capacitors with low equivalent series resistance (ESR).

(3) The diode should be a Schottky diode with an operating current greater than 2A.

(4) The current limiting resistors at the pre-charge, fast charge, full charge and fault indicator lights have resistance values ​​that depend on VIN. Usually, they can be calculated using the following empirical formula:

R = (VIN -2) / 20 (kΩ)

6 Pulse Width Modulation Controller

A pulse width modulation (PWM) controller drives an internal high-side field effect transistor to control the charging current or voltage. It is a very effective technique for controlling analog circuits using the digital output of a microprocessor and is widely used in many fields from measurement and communication to power control and conversion [3]. PWM is a method of digitally encoding the level of an analog signal. By using a high-resolution counter, the duty cycle of a square wave is modulated to encode the level of a specific analog signal. The PWM signal is still digital because at any given moment, the full-scale DC power supply is either fully present (ON) or completely absent (OFF). The voltage or current source is applied to the analog load in a repetitive sequence of on (ON) or off (OFF) pulses. The on time is when the DC power supply is applied to the load, and the off time is when the power supply is disconnected. As long as the bandwidth is sufficient, any analog value can be encoded using PWM. Most loads (whether inductive or capacitive) require modulation frequencies above 10 Hz. [4] The specific working process of the charger is that when the switch cycles between the nominal voltages, the internal clamp voltage limits the non-control signal to prevent delay within the range of 200mV. The pulse width modulation controller in current mode measures the inductive current to adjust the output voltage or current, thereby simplifying the stability of the adjustment cycle. In addition, the respective compensation makes its adjustment circuit more stable, ensuring stable operation of the duty cycle in a wide range. The controller drives the internal N-channel FET switch to make the input voltage drop to the battery voltage. The gate of the high-side FET is driven by the input source voltage higher than the input source voltage close to the capacitor. When LX is low, this capacitor (between VL and LX) is charged through the diode from VL. When the high-side switch is closed and LX is connected to PGND to ensure that the capacitor is charged, the internal N-channel FET is immediately turned on. When the source voltage is close to the input voltage, the gate of the high-side FET is driven by BST, which provides sufficient voltage. The waveform of its working state is shown in Figure 5.

Figure 5 Pulse width modulation working state waveform

7 Conclusion

This design effectively utilizes the lithium-ion charging chip MAX1757. Its design process (peripheral circuit) is simple, and it has the advantages of small size, light weight, programmable number of charging cells, etc. It also has reliable and stable performance and strong applicability.

Reference address:Design and implementation of a lithium-ion intelligent charger

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