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LTC1729CS8-8.4 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1729CS8-8.4 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LTC1729 Series APPLICATIONS INFORMATION Block Diagram Figure 2 shows the block diagram of the LTC1729. The IC consists of six comparators, a resistive divider, a UVLO block and a logic and timer block. The comparator COMP1 is used to indicate C/10 charge current threshold. (“C” is a term used to indicate the Ampere-hour capacity of a cell.) Figure 3 details the C/10 detection circuit. The resistor and capacitor on the SENSE pin filter the signal from the 0.2 Ω current sense resistor R1. The 100 µA current sink on the SENSE pin develops 20mV across the 200 Ω resistor R2. This sets a 20mV offset on COMP1 that sense resistor R1 must equal before C/10 current is indicated. The C/10 is not recognized by the logic until it passes through a 3-second digital filter. For cases where there are multiple batteries to be charged, the LTC1729 detects when a battery is removed and resets the timers for the next charge cycle. The battery present detection is the primary function of comparator COMP2. When the battery is disconnected from the LTC1729, the voltage at the BAT pin will begin ramping upward. This is due to a current source inside the charger or an external pull-up resistor. Once the voltage on the BAT pin exceeds the 4.5V/cell limit, COMP2 signals a VMAX = true to the logic and turns on a transistor that clamps the voltage. There is the possibility of an overvoltage transient at the charging terminals. This occurs if the battery protection circuit, which is often included in the battery pack, opens the connection to the battery during high current charging (constant-current mode). This is a result of energy storage in the inductor. COMP2 senses the overvoltage and clamps 100 µA 50 µA 1729 F03 COMP1 STATUS VCC CHARGE LED LTC1729 SENSE BAT + + – – R2 200 Ω 20mV IBIAS 20mV R1 0.2 Ω CHARGING CURRENT C1 1 µF 3-SEC FILTER LOGIC AND TIMERS Figure 3. C/10 Detection Circuit the VC pin, reducing the duty cycle of the charger. If the battery protection circuit quickly returns battery connec- tion to the charger (in less than one second), then the LTC1729 continues charging without resetting the timers. For a battery that has been over-discharged, the LTC1729 provides a 16mA 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. |
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