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LTC1731ES8-8.2 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1731ES8-8.2 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1731-8.2/LTC1731-8.4 sn1731 17318fs For best stability over temperature and time, 1% resistors are recommended. The closest 1% resistor value is 19.6k. Programming the Timer The programmable timer is used to terminate the charge. The length of the timer is programmed by an external capacitor at the TIMER pin. The total charge time is: Time (Hours) = (3 Hours)(CTIMER/0.1µF) The timer starts when the input voltage greater than 8.2V is applied and the program resistor is connected to ground. After a time-out occurs, the CHRG output will become high impedance indicating that the charge cycle has ended. Connecting the TIMER pin to VCC disables the timer and also puts the charger into a constant-current mode. To only disable the timer function, short the TIMER pin to GND. CHRG Status Output Pin When the charge cycle starts, the CHRG pin is pulled to ground by an internal N-channel MOSFET that can drive an LED. When the charge current drops to 10% of the full- scale current (C/10), the N-channel MOSFET turns off and a weak 100 µAcurrentsourcetogroundisconnectedtothe CHRG pin. After a time-out occurs, the CHRG pin goes high impedance indicating that the charge cycle has ended. By using two different value pull-up resistors, a micropro- cessor can detect three states from this pin (charging, C/10 and stop charging). See Figure 1. APPLICATIONS INFORMATION Charger Conditions The charger is off when any of the following conditions exist: the VCC pin is less than 8.2V, the droput voltage (VCC - VBAT) is less than 54mV, the PROG pin is floating, or the timer has ended. The DRV pin will be pulled to VCC and the internal resistor divider is disconnected to reduce the current drain on the battery. Undervoltage Lockout (UVLO) An internal undervoltage lockout circuit monitors the input voltage and keeps the charger in shutdown mode until VCC rises above 8.2V. To prevent oscillation around VCC = 8.2V, the UVLO circuit has built-in hysteresis. Trickle Charge and Defective Battery Detection At the beginning of the charging sequence, if the battery voltage is less than 4.95V the charger goes into trickle charge mode. The charge current is reduced to 10% of the full-scale current. If the low battery voltage persists for one quarter of the total charging time, the battery is considered defective, charging will be terminated and the CHRG pin output is forced to a high impedance state. Shutdown The LTC1731-8.2/LTC1731-8.4 can be forced into shut- down by floating the PROG pin and allowing the internal 2.5 µA current source to pull the pin above the 2.457V shutdown threshold voltage. The DRV pin is then pulled up to VCC turning off the external P-channel MOSFET. The internal timer is reset in the shutdown mode. Programming Charge Current The formula for the battery charge current (see Block Diagram) is: IBAT = (2.457V/RPROG)(800Ω/RSENSE) where RPROG is the total resistance from the PROG pin to ground. For example, if 0.5A charge current is needed, select a value for RSENSE that will drop 100mV at the maximum charge current. RSENSE = 0.1V/0.5A = 0.2Ω, then calculate: RPROG = (2.457V/500mA)(800Ω/0.2Ω) = 19.656k 2k 100k CHRG 1731 F01 VCC LTC1731-8.4 V + VDD OUT µPROCESSOR IN 2 7 Figure 1. Microprocessor Interface |
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