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LTC4079 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4079 Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page LTC4079 9 4079f For more information www.linear.com/LTC4079 operaTion The LTC4079 is a full featured constant-current, constant- voltage charger designed to charge multiple chemistry types of batteries from voltage sources up to 60V. The low quiescent current of the device minimizes power drain on the source and the battery, making it suitable for a variety of applications including backup and energy harvesting from an intermittent power source. The battery charge voltage is set using an external resistor divider. Charge duration can be set using a capacitor on the TIMER pin. For safety and improved battery life, the LTC4079 includes a thermistor input for temperature qualified charging. Charge current starts when the EN pin is brought above 1.190V. Figure 2 shows a flow chart of the primary states and state transitions of the LTC4079. A typical charge cycle includes: 1. Constant-Current (CC) Charging: The programmed charge current is used to charge the battery until the battery voltage reaches the charge voltage set using the feedback divider. For a low ESR battery, this mode provides the bulk of the charge. The charge timer should generally be set long enough to charge the battery above the recharge threshold, otherwise another charge cycle would immediately follow. 2. Constant-Voltage (CV) Charging: Once the battery reaches the set charge voltage, constant voltage is maintained across the battery by controlling the charge current. The charge current reduces with time in this mode as the battery nears its full charge capacity. 3.ChargingTermination:TheLTC4079canbeconfigured to terminate charging automatically based on time or current. The CHRG status pin goes to high impedance when the charge current reduces below 1/10th of the programmed current, indicating that the battery is almost fully charged. The charge current continues to top-off the batteryuntilthetimerterminatesthechargecurrent.Timer termination can be disabled by connecting the TIMER pin to ground. In this case, charging terminates when the charge current falls below 1/10th of the programmed charge current. A 6ms filter (tTERMINATE) is used on the C/10 detector to prevent premature termination due to transient loads on the battery during charging. 4. Automatic Recharge: When VBAT drops below the recharge threshold (97.6% of the charge voltage), whether by battery drainage or replacement of the battery, the charger automatically re-engages and starts charging. Setting The Battery Charge Voltage The battery charge voltage is set by connecting a resistor divider from the battery to the FB and FBG pins as shown in Figure 3. The charge voltage is determined as follows: VCHG =1.170V • 1+ RFB1 RFB2 +RFBG where RFB1 is the resistor from BAT to FB, RFB2 is the resistor from FB to FBG and RFBG is the resistance of the internal switch of the FBG pin (160Ω typical). + BAT FB LTC4079 RFB1 RFB2 4079 F03 BATTERY FBG ENABLE Figure 3. Setting the Battery Charge Voltage Setting and Monitoring the Charge Current The charge current delivered to the battery in constant- current mode, ICHG is set using a resistor from the PROG pintoground.Thevalueofthisresistoriscalculatedusing: RPROG = 297.5V ICHG |
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