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LTC1729CMS8-4.2 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1729CMS8-4.2 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC1729 Series VMAX = 1V VMIN = 0V 4.5V/CELL BATTERY CLAMP PULL-DOWN ON VMAX = 0V VMIN = 0V 2.7V/CELL NORMAL CHARGE VMAX = 0V VMIN = 1V TRICKLE CHARGE PRECONDITIONING 1729 F04 HOLD MODE 2.79V HOLD MODE 0.2V RESET MODE NORMAL CHARGE 0.405V 1729 F05 When the input supply is removed in step-down charger designs, the VCC pin does not drop to 0V, but is held to a diode drop below the battery voltage. This is due to the internal parasitic diode from the SW pin to the VCC pin that is present in the LT1510, LT1511, LT1769, and the body diode of the top MOSFET in the LT1505 charger. Remov- ing the input supply will result in the VCC pin dropping below the UVLO threshold, thus shutting down the LTC1729 and minimizing battery drain. Once the input supply is connected, the LTC1729 resets the timers and begins another charge cycle. There is a disconnect transistor that will remove the OVP resistor divider from the battery when the VCC voltage drops below the UVLO threshold. If external circuitry is connected directly to the VCC pin and the input power is removed, battery current will flow through the charger’s internal parasitic diode (SW pin to VCC pin), discharging the battery and possibly damaging the charger if currents greater than 1A are present. If external circuitry is used, connect it directly to the input power source (which is the anode of the input Schottky diode). The logic and timer block consists of a state machine, oscillator and counter. The state machine will be dis- cussed in detail in the next section. The oscillator is internally trimmed to 131kHz. The counter is composed of a chain of 25 flip-flops that includes self-test circuitry. Figure 4. BAT Voltage Operating Range Figure 5. NTC Voltage Operating Range For a battery that has been deeply discharged, the LTC1729 provides a 12mA start-up current. This preconditioning trickle charge begins when the battery voltage drops below 2.7V/cell. Comparator COMP3 is used to determine when this condition exists. The battery operating range is summarized in Figure 4. With a 10k negative TC thermistor connected to the NTC pin, the LTC1729 will disable charging if the temperature is not in the 0 °C to 50°C range. A thermistor must be present at this pin, or if the temperature qualification feature is not used, a 10k resistor must be used in its place. This pin should not be left open. The NTC thermistor monitoring circuits consist of COMP4, COMP5 and COMP6. The NTC pin voltage range is shown in Figure 5. Comparators COMP4 and COMP5 signal the logic when the temperature is below 0 °C or above 50°C. The 100µA bias current through the 10k Ω thermistor will generate 2.79V at 0 °C and 0.405V at 50°C. The voltage hysteresis on COMP4 and COMP5 is designed to give 10 °C of temperature hysteresis. If the voltage on the NTC pin drops below 0.2V, the comparator COMP6 generates a reset to the logic and timer block. To provide accurate charge voltages, the internal resistor divider is trimmed to provide a precise ±0.2% voltage of 2.465V at the OVP pin for the four battery voltage options of 4.1V, 4.2V, 8.2V and 8.4V. APPLICATIO S I FOR ATIO |
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