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LTC1731-4.1 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1731-4.1 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1731-4.1/LTC1731-4.2 pull-up resistors, a microprocessor can detect three states from this pin (charging, C/10 and stop charging). See Figure 1. When the LTC1731 is in charge mode, the CHRG pin is pulled down by an internal N-channel MOSFET. To detect this mode, force the digital output pin, OUT, high and measure the voltage at the CHRG pin. The N-channel MOSFET will pull the pin low even with a 2k pull-up resis- tor. Once the charge current drops to 10% of the full-scale current (C/10), the N-channel MOSFET is turned off and a 100 µA current source is connected to the CHRG pin. The IN pin will then be pulled high by the 2k pull-up. By forcing the OUT pin into a high impedance state, the current source will pull the pin low through the 100k resistor. When the internal timer has expired, the CHRG pin will change to high impedance state and the 100k resistor will then pull the pin high to indicate the charging has stopped. Refer to Table 1 for the summary. Table 1. CHRG Pin Interface with Microprocessor IN OUT STATUS Low High Charging Low Hi-Z Charging High High C/10 Low Hi-Z C/10 High Hi-Z Stop Charging End of Charge (C/10) The LTC1731 includes a comparator to monitor the charge current to detect an end-of-charge condition. When the battery current falls below 10% of full scale, the compara- tor trips and turns off the N-channel MOSFET at the CHRG pin and switches in a 100 µA current source to ground. APPLICATIONS INFORMATION 2k 100k CHRG 1731 F01 VCC LTC1731 V + VDD OUT µPROCESSOR IN 2 7 Figure 1. Microprocessor Interface voltage. The DRV pin will then be pulled up to VCC and turn 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) 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 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 = (3 Hours)(CTIMER/0.1µF) The timer starts when the input voltage greater than 4.1V is applied and the program resistor is connected to ground. After a time-out occurs, the CHRG output will turn into a high impedance state to indicate that the 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 the charge cycle starts, the CHRG pin is pulled down 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 100 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 |
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