New lithium-ion battery charge and discharge protector MAX1665S/V/X

    Abstract: MAX1665S/V/Xj is a special lithium-ion battery pack charge and discharge control protector produced by MAXIM Company. It can be used to provide effective over-voltage, under-voltage and over-voltage protection for lithium-ion battery packs. The article introduces the performance parameters of MAX1665S/V/X, the working principle of a typical circuit, and also provides the application circuit of a 2 to 4-cell lithium-ion battery pack protector.

    Keywords: Li-ion battery overcharge and over-discharge protector MAX1665

1 Overview

Lithium-ion battery is a new type of available rechargeable battery. It has the characteristics of high single working voltage, small size, light weight, high energy density, long cycle life (up to 1200 times), rapid full charging within two hours and allowable discharge temperature. Wide range (-20～+60℃) and other advantages. In addition, lithium-ion batteries have the advantages of small self-discharge current, no memory effect and no environmental pollution. Its comprehensive performance is better than lead-acid, nickel-cadmium, nickel-metal hydride and metal lithium batteries, so it is currently the best performing battery. It can be widely used in portable electronic devices, mobile communications, mobile phones and wireless stations.

The main disadvantage of lithium-ion batteries is that they do not tolerate overcharging and over-discharging. In order to prevent overcharging and over-discharging of lithium-ion batteries, single cells or battery packs must be equipped with charge, discharge and over-current protection circuits, and a dedicated charger is required to charge the lithium-ion battery in the safest and best manner. to extend its service life.

MAX1665S, MAX1665V, and MAX1665X are charge and discharge process controllers that can be used for 2, 3, and 4-cell lithium-ion battery packs respectively. These devices can detect the voltage and charge and discharge current of each cell, and can be used for lithium-ion battery packs. Provide over-voltage, under-voltage and over-current maintenance, they realize the charging and charging protection of the battery by controlling the on and off of two external N-channel MOS field effect transistors. The protector composed of the MAX1665 requires few components and operates at very low current, ensuring that the battery will not over-discharge during long-term storage.

2 Main characteristic parameters and pin functions

The main functions of the MAX1665 lithium-ion battery charge protector are as follows:

●With protection function, it can protect batteries and battery packs from overcharge and overdischarge;

●Typical power supply current value is 16μA;

●Sleep current is less than 1μA;

●The matching input bias current of the battery pack is less than 500pA, which can maintain the balance of the battery pack;

●The maximum voltages between B4P, B3P, B2P, B1P and PKN are +24V, +18V, +12V and +6V respectively;

●The working temperature range is: -40～+80℃;

●Storage temperature range is -60～+150℃.

MAX1665 is packaged in an 8-pin SO package, and its pin arrangement is shown in Figure 1. The pin functions of each device are listed in Table 1.

Table 1 MAX1665 pin functions

3 Protection working principle

3.1 Overvoltage protection

When the voltage of any battery rises above Vov (overvoltage limit value), the CHG terminal in the chip outputs a low level to control the external MOSFET to turn off, thus disconnecting the battery charging circuit. MAX1665 can measure the voltage of each cell in the battery pack one by one to prevent overcharging. When the battery voltage drops below Vov-100mV, the charging process starts again.

3.2 Undervoltage or overdischarge protection

In the battery pack, when the voltage of each cell is higher than the undervoltage threshold VUV (typical value 2.5V), the device will control the external MOSFET to connect the discharge circuit to allow the battery pack to discharge. During the discharge process, as long as the voltage of one of the batteries is lower than VUV, the device will lock CHG to PKN and DSG to BN, so that the MOSFET's turn-off discharge circuit will cause the device to enter sleep mode. At this time, the quiescent current is less than 1μA.

During long-term storage, the MAX1665 enters sleep mode and the battery is safely discharged (less than 1µA) until the battery reaches the undervoltage threshold. When the external battery is connected and the BN terminal voltage rises 18mV above the PKN terminal voltage, the circuit re-enters the normal working state.

3.3 Overcurrent protection

When the MAX1665 detects an overcurrent, it turns off the external MOSFET by connecting CHG and DSG, thereby stopping the charging or discharging process. In charging mode, when the voltage from BN terminal to PKN terminal exceeds +250mV, overcurrent charging will be entered. In discharge mode, when the voltage from BN terminal to PKN terminal is lower than -250mV, it enters overcurrent discharge. Under any overcurrent condition, the CHG and DSG terminals give a 50ms/12Hz strobe pulse (sampling the battery current regularly) until the overcurrent fault is eliminated. If overvoltage and overcurrent conditions exist at the same time, the overvoltage condition takes precedence; if overcharge (undervoltage) and overcharge conditions exist at the same time, the discharge condition takes precedence.

3.4 Battery current balancing

When the cells are matched (the parameters of each cell are consistent), the MAX1665 only draws current from the top point, and the current drawn from the middle point is zero, which keeps the battery pack current balanced. Figure 2 shows a voltage sampled simplified battery diagram.

In actual work, the MAX1665 can input a current of 70nA to 150nA from the external contact, and when the battery is matched, this current does not exist. During the battery sampling period (taking up 1/32 of a machine cycle), a current of 2~5nA can be obtained.

3.5 External field effect transistor selection

External N-channel MOSFETs are used as switching gates to control the charging and discharging processes. The CHG and DSG of MAX1665 are used to control the gate of the external MOS field effect transistor to prevent damage to the lithium-ion battery due to overcurrent conditions. It should be noted that the MAX1665X limits the maximum gate-source voltage VGS to 20V.

4 Series battery pack charging control circuit

Figure 3(a), (b), and (c) respectively show the application circuits of charging and discharging holders for 2, 3, and 4 series battery packs. In order to prevent the body diode in the MOS tube from turning on, the dual N-channel MOSFET of IRF7101 is sampled in a directional series connection (source connected to source), and the polarity symbol indicates the polarity of the externally connected charging power supply. The IRF7101 in the picture is available in a small 8-pin narrow package. How to choose MOSFET depends on the maximum charge and discharge current value of the battery. Using different MOSFETs can achieve different application effects. Table 2 gives recommended MOSFETs.

Table 2 Recommended MOSFETs

When the voltage of each battery is lower than 4.3V (overvoltage charging limit), the MAX1665 charges the battery. During the charging process, if the voltage of any battery rises to 4.3V, the protector will turn off the charging MOSFET to prevent overcharging of the batteries in the battery pack; when the voltage of each battery is 2.5V (undervoltage limit) Above, the protector allows the battery to discharge the load. When the voltage of any battery is lower than 2.5V, the protector will turn off the discharge MOSFET to prevent the battery in the battery pack from over-discharging; if the voltage difference between the PKN terminal and the BN terminal is lower than 250mV, the battery will charge or discharge normally, otherwise it will be charged or discharged normally. Disabled, this feature prevents the battery pack from overcurrent.

The control pins CHG and DSG of the MAX1665 are used to drive the gates of two series-connected N-channel MOSFETs respectively. Whether the battery pack can be charged or discharged depends on the state of the battery. In the battery-holding circuit, the MAX1665 uses measurements of B1P, B2P, B3P, and B4P to determine whether the battery pack can be charged or discharged.

In the working state, the power consumption of the MAX1665 series devices is less than the quiescent current (25μA), and in the sleep state, it is less than 1μA. This circuit allows the battery to be stored for a long time without affecting the battery life.