CN3717

CN3717 is a PWM step-down lead-acid battery charge management IC featuring trickle, constant current, overcharge, and float charge modes. The constant current charge current is set by a current sense resistor R_CS connected between the CSP pin and BAT pin. In overcharge and float charge modes, the charge voltage is set by an external resistor 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. At the start of charging, if the battery voltage is below 75.6% of the configured overcharge voltage, the charger automatically enters trickle charge mode, where the charge current is 19% of the configured constant current charge current. When the battery voltage exceeds 75.6% of the configured overcharge voltage, the charger enters constant current charge mode, where the charge current is set by an internal 120mV reference voltage and external resistor R_CS, i.e., charge current = 120mV/R_CS. During the constant current charge phase, even if the battery voltage drops back below 75.6% of the overcharge voltage, CN3717 does not re-enter trickle charge mode but remains in constant current charge mode. As the battery voltage continues to rise toward the overcharge voltage, the charger enters overcharge mode and the charge current gradually decreases. When the charge current decreases to the overcharge termination current set by the EOC pin resistor, CN3717 enters float charge mode, at which point the BAT pin voltage is regulated to the float charge voltage. The internal transistor of one open-drain output pin turns off, presenting a high-impedance state; the internal transistor of the other open-drain output pin turns on, pulling the output low to indicate float charge status. In float charge state, if the input power supply is disconnected and then reconnected, a new charge cycle begins. If the battery voltage drops to the recharge threshold (82.2% of the overcharge voltage), a new charge cycle also starts automatically. When the input voltage is lost, CN3717 automatically enters sleep mode and the internal circuitry is shut down, reducing 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 range, charging is suspended during trickle charge, constant current charge, and overcharge phases until the battery temperature returns to the normal range. CN3717 also incorporates an overvoltage comparator. When the BAT pin voltage rises due to load changes or sudden battery removal, if the BAT pin voltage rises to 1.08 times the overcharge 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 constant voltage charge voltage.

Applications

• Lead-acid battery charging
• Uninterruptible power supplies
• Backup battery applications
• Portable industrial and medical instruments
• Standalone battery chargers

Key features

• When charging begins, if the battery voltage is below 75.6% of the set overcharge voltage, the CN3717 trickle-charges the battery at 19% of the set constant-current charge value. During the overcharge phase, the charge current gradually decreases; when it drops to the value set by the external resistor, the CN3717 enters float charge mode. In float charge mode, if the battery voltage drops to 82.2% of the set overcharge voltage, a new charge cycle begins automatically. When the input power is lost or the input voltage falls below the battery voltage, the CN3717 automatically enters a low-power sleep mode. Additional features include input undervoltage lockout, battery temperature monitoring, battery terminal overvoltage protection, and charge status indication. The CN3717 is available in a 16-pin TSSOP package.