CONSONANCE CN3705

CN3705 is a PWM step-down single-cell or multi-cell lithium-ion or LiFePO₄ battery charge management IC featuring constant current/constant voltage (CC/CV) charging modes. The constant current charge current is set by a current sense resistor R_CS connected between the CSP pin and BAT pin. In CV mode, the charge voltage is set by an external resistive voltage divider network.

When the VCC pin voltage exceeds the UVLO threshold and is greater than the battery voltage, the charger operates normally and charges the battery. If the battery voltage falls below 66.7% of the configured CV charge voltage, the charger automatically enters trickle charge mode, delivering a charge current equal to 15% of the configured CC charge current. When the battery voltage rises above 66.7% of the configured CV charge voltage, the charger enters CC mode, where the charge current is set by an internal 200mV reference and external resistor R_CS — i.e., charge current = 200mV / R_CS. As the battery voltage continues to rise and approaches the CV charge voltage, the charger transitions into CV mode and the charge current gradually decreases. When the charge current drops to the value set by the EOC pin resistor, charging terminates and the DRV pin outputs a high level. One open-drain output transistor turns off, presenting a high-impedance state; the other open-drain output transistor turns on, outputting a low level to indicate the end-of-charge condition.

In the end-of-charge state, disconnecting and reconnecting the input power supply initiates a new charge cycle. Alternatively, if the battery voltage falls to the recharge threshold (91.1% of the CV charge voltage), a new charge cycle begins automatically.

When the input voltage is removed, CN3705 automatically enters sleep mode and shuts down internal circuitry, minimizing battery current consumption and extending standby time.

To monitor battery temperature, a 10kΩ NTC thermistor must be connected between the TEMP pin and GND pin. If the battery temperature exceeds the normal operating range, charging is suspended until the battery temperature returns within the normal range.

CN3705 also incorporates an internal overvoltage comparator. If the BAT pin voltage rises — due to load transients, sudden battery removal, or other causes — to 1.08 times the configured CV charge voltage, the overvoltage comparator activates and turns off the external P-channel MOSFET, temporarily halting the charger until the BAT pin voltage drops back below the CV charge voltage.

• Constant current and constant voltage charging modes, suitable for Li-ion or LiFePO₄ battery charging
• Constant voltage set by external resistor divider network
• Constant current set by external resistor
• Trickle charging at 15% of the set constant current when battery voltage drops below 66.7% of the set constant voltage (for deeply discharged batteries)
• Charge termination current configurable via external resistor
• Automatic new charge cycle initiated when battery voltage drops to 91.1% of the set constant voltage after charge termination
• Automatic entry into low-power sleep mode when input power fails or input voltage falls below battery voltage
• Input undervoltage lockout
• Battery temperature monitoring
• Battery terminal overvoltage protection
• Charge status indication
• 16-pin TSSOP package

• LiFePO₄ battery charging
• Laptop computers
• Backup battery applications
• Portable industrial and medical instruments
• Power tools
• Standalone battery chargers