3.7 volt battery 18650 charge protection integrated circuit

　　1. Characteristics of lithium-ion batteries

　　Compared with other batteries, lithium-ion batteries have the following main advantages.

　　1. high voltage

　　The marked open circuit voltage is usually 3.6V, while the open circuit voltage of nickel metal hydride and nickel cadmium batteries is 1.2V.

　　2. large capacity

　　High energy and high storage energy density are the core values of lithium batteries. For the same output power, lithium-ion batteries are not only half lighter than nickel-metal hydride batteries, but also 20% smaller in volume.

　　3. discharge rate

　　The charging speed of lithium-ion batteries is very fast, and it only takes 1 to 2 hours (h) to charge and reach the best state; at the same time, the leakage of lithium-ion batteries is very small, even if it is left casually for 1 to 2 weeks before being picked up again When used out, it can still exert power and work as usual; the self-discharge rate of lithium-ion batteries is as low as <8%/month, which is much lower than the 30% of nickel-cadmium batteries and 40% of nickel-metal hydride batteries.

　　4. Lithium-ion batteries do not have a "memory effect", so they can be charged while not fully discharged without reducing their capacity. However, if the lithium-ion battery is fully charged and continues to be charged (overcharged), the battery will be damaged. Lithium-ion batteries are currently widely used rechargeable batteries.

　　2. Charging characteristics of lithium-ion batteries

　　During the charging process of lithium-ion batteries, the voltage and charging current of the battery will change with the charging time. The change pattern is shown in Figure 1.

　　Charging a lithium-ion battery requires controlling its charging voltage, limiting the charging current and accurately detecting the battery voltage. The charging characteristics of lithium-ion batteries are completely different from those of cadmium-nickel and nickel-metal hydride. Lithium-ion batteries can be charged at any point during its discharge cycle, and can maintain its charge very effectively. The retention time is more than twice as long as that of nickel-metal hydride batteries. It is lightweight, and its weight is only 1/2 of a cadmium-nickel battery of the same capacity. , the specific mass density is 4 times that of cadmium-nickel batteries. When the lithium-ion battery starts to charge, the voltage rises slowly and the charging current gradually decreases. When the battery voltage reaches about 4.2V, the battery voltage remains basically unchanged and the charging current continues to decrease. The way to determine whether the lithium-ion battery charging is completed is to detect it. charging current, and ends charging when its charging current drops to a certain value. For example, charging ends when the charging current of a lithium-ion battery drops to 40mA (typically about 5% of the initial charging current). You can also start a timer when it detects that the lithium-ion battery reaches 4.2V and end it after a certain delay. Charge. At this time, the charging circuit should have a battery voltage detection circuit with higher accuracy to prevent overcharging of the lithium-ion battery. It should be noted that lithium-ion batteries do not require trickle charging.

　　3. Main features of UCC3957

　　UCC3957 is a 3/4-cell lithium-ion battery pack charger protection control integrated circuit using BiCMOS technology. It works with an external p-channel MOSFET transistor to charge the battery pack to achieve two-level overcurrent protection. If the first-level overcurrent threshold potential is reached, the protection circuit will discharge the external capacitor according to the protection time set by the user. If The first-level protection time is up, and the battery overcharge and discharge faults have not been eliminated. The external protection timing capacitor discharge MOSFET turns off at 17 times the first-level protection time to implement the second-level protection, which is very useful for capacitive loads. . The power consumption of UCC3957 in sleep mode is only 3.5μA, the typical operating current is 30μA, and the DC operating voltage range is 6.5∽20V. The charging overcurrent protection delay time can be achieved by adjusting the parameters of external components. The advantage of using an external p-channel MOSFET transistor is that it protects any cell from overdischarge and overcharge, as well as protecting the battery pack and the UCC3957 integrated circuit itself.

　　3.1 UCC3957 working principle block diagram and pin functions

　　The working principle block diagram of UCC3957 is shown in Figure 2.

　　Figure 2 UCC3957 working principle block diagram

　　As can be seen from Figure 2, the working status of UCC3957 can be selected using the internal working status selector of UCC3957. When working in the continuous working status, each lithium-ion battery can be protected from overcharge and overdischarge. The over-current controller is used to protect the battery pack from generating excessive discharge current and damaging the battery pack.

　　In order to match the lithium batteries produced by different manufacturers, the UCC3957 series integrated circuits have 4 different overvoltage protection thresholds as shown in Table 1.

　　Table 1 UCC3957-X overvoltage protection threshold value (V)

　　The pin diagram of UCC3957 is shown in Figure 3.

　　Figure 3 Pin diagram of UCC3957

　　The functions of each pin of UCC3957 are as follows:

　　Pin 1VDD: This pin is the power supply input pin of UCC3957. The input voltage range is 6.5~20V and is connected to the high potential end of the battery pack.

　　Pin 2CLCNT: This pin is the pin that sets the UCC3957 to work in the charging state of three or four cells.

　　Pin 3WU: This pin is when the UCC3957 is in the dormant working state. Adding a signal to this pin can wake up the UCC3957 and enter the normal working state. This pin should be connected to the drain of the N-channel level shift MOSFET transistor.

　　Pin 4AN1: This pin is connected to the negative electrode of the highest potential battery cell 1 and the positive electrode of the second battery cell.

　　Pin 5AN2: This pin is the pin connecting the negative electrode of the second battery with the highest potential and the positive electrode of the third battery.

　　Pin 6AN3: This pin is the pin connecting the negative electrode of the 3rd battery with the lowest potential and the positive electrode of the 4th battery. When there are only 3 batteries, it is connected to the low potential end of the battery pack and the AN4 pin.

　　Pins 7 and 11 AN4: This pin is connected to the low potential end of the battery pack and to the high potential end of the current sensing resistor.

　　Pin 8BATLO: This pin is connected to the negative potential end of the battery pack and at the same time connected to the low potential end of the current detection resistor.

　　Pin 9CHGEN: This pin is the charging enable pin. When this pin is high level, the battery pack starts charging.

　　Pin 10CDLY1: This pin is the delay time control pin for short circuit protection. The value of the capacitor connected between this pin and the AN4 pin determines the overcurrent time. When the overcurrent occurs, it controls the turn-off time of the discharge MOSFET transistor. The capacitor The value also determines the hiccup over-current protection time.

　　Pin 12CHG: This pin is connected to an external controllable N-channel MOSFET transistor, and the external N-channel MOSFET transistor can be used to drive an external p-channel MOSFET. If any battery voltage is higher than If the overvoltage protection threshold potential is reached, this pin will be set to a low potential relative to the AN4 pin; only when the voltage of all single-cell batteries being charged is lower than the threshold potential, this pin will be set to a high potential.

　　Pin 13DCHG: This pin is used to prevent battery over-discharge. If the working status detector inside UCC3957 determines that any battery is in an under-voltage state, the pin DCHG is set to a high potential to turn off the external discharge p-channel MOSFET transistor. However, when the voltage of all batteries is higher than the minimum threshold potential, the pin DCHG is set to low potential.

　　Pin 14CDLY2: Connect a capacitor between this pin and the AN4 pin to extend the protection setting time of the second-level overcurrent protection.

　　Pin 15AVDD: This pin is connected to the AN4 pin through a 0.1μF capacitor. The normal operating voltage is 7.3V.

　　Pin 16DVDD: This pin is connected to the AN4 pin through a 0.1μF capacitor, which is the power supply pin for the internal digital circuit. The normal operating voltage is 7.3V.

　　4. Working principle and typical application circuit of UCC3957

　　4.1 Working principle of UCC3957

　　UCC3957 can provide comprehensive protection functions for 3-cell or 4-cell lithium battery packs to prevent battery overcharge, over-discharge, over-current charge and discharge, etc. It samples the voltage of each cell in the battery pack and compares it with the internal precision reference The voltage is compared, and when any battery is in an overvoltage or undervoltage state, the UCC3957 will take appropriate measures to prevent the battery from further charging or discharging. UCC3957 is externally connected with two independent p-channel MOSFET transistors to control the charging and discharging current of the battery respectively.

　　The following takes Figure 4 as an example to introduce the characteristics of the 4-cell lithium battery charging protection circuit using UCC3957.

　　1. Battery pack connection

　　Pay attention to the order when connecting the battery pack to the UCC3957. The low potential end of the battery pack is connected to pin 7AN4, the high potential end is connected to pin VDD, and the connection points of every two batteries are connected to pins 4AN1, 5AN2, and 6AN3 in the corresponding order.

　　2. Choose 3 or 4 battery charging working status

　　When the battery pack is 3 cells, pin 2CLCNT should be connected to pin 16DVDD, and pin 6AN3 and pin 7AN4 should be connected together. When the battery pack is 4 cells, pin 2CLCNT should be connected to ground (that is, connected to pin Pin 7AN4), AN3 pin is connected to the positive terminal of the bottom battery of the battery pack.

　　3. Under voltage protection

　　When it is detected that any battery is in an over-discharge state (lower than the under-voltage threshold potential), the status detector turns off two MOSFET transistors at the same time, causing the UCC3957 to enter the sleep mode. At this time, the power consumption of the UCC3957 is only 3.5μA. , only when the voltage of pin 3WU rises to 1VDD, UCC3957 exits the sleep mode.

　　4. Charging batteries

　　When the charger is connected to the charging power supply, as long as the voltage of pin 9CHGEN is pulled to 16DVDD, the charging FET transistor VT1 is turned on and the battery pack is charged. But if pin 9CHGEN is left open or connected to pin 7AN4, the charging FET transistor VT1 is turned off. During charging, if the UCC3957 is in the sleep mode, the discharge FET transistor VT2 is still turned off, and the charging current flows through the body diode of the discharge FET transistor VT2; until the voltage of each battery is higher than the undervoltage threshold voltage, the discharge FET transistor VT2 conduction. During sleep operation, the charging FET transistor VT1 is in a periodic turn-on and turn-off mode, with a turn-on time of 7ms and a turn-off time of 10ms.

　　5. Protection against abnormal battery connection

　　UCC3957 has a protection function in case of abnormal battery connection in the charged battery box. If the pins 4AN1, 5AN2 or 6AN3 connected to the battery are not connected properly or disconnected, UCC3957 can detect and prevent the battery pack from overcharging.

　　6. Overvoltage protection and intelligent discharge characteristics

　　If the charging voltage of a certain battery exceeds the normal overcharge threshold potential, the charging FET transistor VT1 is turned off to prevent the battery from overcharging. Shutdown remains until the battery voltage drops to the overcharge threshold level. In most protection circuit designs, the charging FET transistor VT1 is always fully protected within this overvoltage protection band (between normal value ∽ overcharge threshold, or conversely, between overcharge threshold ∽ normal value). In the off working state, the discharge current must pass through the body diode of the charging FET transistor VT1. The voltage drop of the diode is as high as 1V, which causes great power consumption in the charging FET transistor VT1 and consumes precious battery power.

　　UCC3957 has a unique intelligent discharge characteristic, which can make the charging FET transistor VT1 conductive to the discharge current (only for discharge) while still being within the over-voltage hysteresis range. This greatly reduces the power dissipated in charging FET transistor VT1. This measure is accomplished by sampling the voltage drop flowing through the current sensing resistor RSENES. If this voltage drop exceeds 15mV (0.025Ω current sensing resistor corresponds to a discharge current of 0.6A), the charging FET transistor VT1 is turned on again. In this example, if the body diode voltage drop of a 20mW FET transistor is 1V, corresponding to a 1A load, the power consumption of VT1 is reduced from 1W to 0.02W.

　　7. Over current protection

　　UCC3957 uses a secondary overcurrent protection mode to protect the battery pack from overcharging current and battery pack short circuit. When the voltage drop on the current detection resistor RSENSE (connected between pin AN4 and pin BATLO) exceeds a certain threshold potential, The over-current protection enters the hiccup protection working mode. In this operating mode, the discharge FET transistor VT2 is turned off and on periodically until the fault is eliminated. Once the fault is eliminated, the UCC3957 automatically resumes normal operation.

　　In order to adapt to large capacitive loads, UCC3957 has two over-current threshold voltages, and different delay times can be set corresponding to each threshold voltage. This secondary overcurrent protection provides fast response to short circuits while allowing the battery pack to withstand certain surge currents. This can prevent unnecessary overcurrent protection actions caused by large filter capacitance.

　　The first-level overcurrent protection threshold potential is 150mV, corresponding to a 0.025Ω current detection resistor, and the overcurrent threshold is 6A. If the peak discharge current lasts longer than the time set by this value (set by the capacitor connected between CDLY1 and ground), UCC3957 enters the hiccup protection operating mode. The duty cycle of the hiccup protection working mode is about 6%, that is, the off time is about 17 times the on time.

　　The second-level overcurrent threshold potential is 375mV, corresponding to a 0.025Ω current detection resistor, and the overcurrent threshold is 15A. If the peak discharge current exceeds the time set by this potential value (set by the capacitor connected between CCDLY2 and ground), UCC3957 enters the hiccup protection working mode, and the duty cycle is generally less than 1%. The off time tOFF is still determined by the capacitor connected between CCDLY1 and ground. This technology greatly reduces the power consumption on the FET transistor VT2 during a short circuit, thereby reducing the requirements for the use of the FET transistor VT2.

　　When CDLY1=0.022μF, the on-time tON corresponding to the first-level overcurrent (when the current is greater than 6A and less than 15A) is approximately 10ms, the off-time tOFF is approximately 160ms, and the duty cycle is 5.9%; when If CDLY2 is not used when the current exceeds 15A, the duty cycle of the second-level overcurrent protection is 0.1%; if CDLY2 is 22pF, the corresponding conduction time is 800μs and the duty cycle is 0.5%.

　　4.2 Typical application circuit of UCC3957

　　A typical protection circuit for a 4-cell battery pack using UCC3957 is shown in Figure 4.

　　Figure 44 Typical protection circuit for lithium battery charging circuit

　　In Figure 4, VR1 and R2 are used to protect the charging FET transistor VT1 when the charger's open circuit charging voltage is too high. In this application circuit, when short-circuitedWhen the discharge FET transistor VT2 is turned off, due to the distributed inductance of the battery pack output, a negative sudden change in voltage will occur; this negative sudden change will exceed the withstand voltage value of the discharge FET transistor VT2, and this negative sudden change will also damage the UCC3597. VD1 in Figure 4 clamps this negative mutation to protect the discharge FET transistor VT2, C5 should be placed directly on the top and bottom of the battery pack.

　　Because when the discharge overcurrent protection occurs, the negative voltage overcharge generated by the discharge FET transistor VT2 is related to the size and the rise and fall time of the turn-on and turn-off drive pulses of the discharge FET transistor VT2. Therefore, R3, C5, and R4 are used to control the size in Figure 4.

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