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LTC1732-8.4 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1732-8.4 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1732-8.4 17328f APPLICATIONS INFORMATION Charger Conditions The charger is off when any of the following conditions exist: the VCC pin is less than 8.2V, the dropout voltage (VCC – VBAT) is less than 54mV, or the PROG pin is floating. The DRV pin will be pulled to VCC and the internal resistor di- vider 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 below 4.9V, the charger goes into trickle mode. The charge current drops to 10% of the full-scale current. If the low voltage persists for one quarter of the total charge time, the battery is considered defective, the charge cycle is terminated and the CHRG pin output is forced to a high impedance state. Shutdown The LTC1732-8.4 can be forced into shutdown by floating the PROG pin and allowing the internal 2.5 µA current source to pull the pin above the 2.457V shutdown thresh- old voltage. The DRV pin is 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 = (IPROG)(800Ω/RSENSE) = (2.457V/RPROG)(800Ω/RSENSE) or RPROG = (2.457V/IBAT)(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 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 terminates the charge cycle. Typically, when charging at a 1C rate, a discharged Li-Ion battery will become fully charged in 3 hours. For lower charge current rates, extend the timer accordingly.The length of the timer is programmed by an external capaci- tor at the TIMER pin. The total charge time is: Time (Hours) = (3 Hours) • (CTIMER/0.1µF) or CTIMER = 0.1µF • Time (Hours)/3 (Hours) The timer starts when an 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 go into a high impedance state to indicate that charging has stopped. 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 a charge cycle starts, the CHRG pin is pulled to ground by an internal N-channel MOSFET that can drive an LED. When the battery current drops to 10% of the full- scale current (C/10), the N-channel MOSFET is turned off and a weak 35 µA current source to ground is connected to the CHRG pin. After a time-out occurs, the pin will go into a high impedance state. By using two different value pull- up resistors, a microprocessor can detect three states from this pin (charging, C/10 and stop charging). See Figure 1 and Table 1. |
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