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LTC4069EDC-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC4069EDC-PBF Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTC4069 9 4069fb not exceed the recharge threshold voltage when the timer ends, the timer resets and a 2.25 hour recharge cycle begins. The CHRG output assumes a strong pull-down state during recharge cycles until C/10 is reached when it transitions to a high impendance state. Trickle Charge and Defective Battery Detection At the beginning of a charge cycle, if the battery voltage is low (below 2.9V), the charger goes into trickle charge, reducing the charge current to 10% of the full-scale current. If the low-battery voltage persists for one quarter of the total time (1.125 hour), the battery is assumed to be defective, the charge cycle is terminated and the CHRG pin output pulses at a frequency of 2Hz with a 75% duty cycle. If for any reason the battery voltage rises above 2.9V, the charge cycle will be restarted. To restart the charge cycle (i.e., when the defective battery is replaced with a discharged battery), simply remove the input voltage and reapply it or momentarily float the PROG pin and reconnect it. CHRG Status Output Pin The charge status indicator pin has three states: pull-down, pulse at 2Hz (see Trickle Charge and Defective Battery Detection and Battery Temperature Monitoring) and high impedance. The pull-down state indicates that the LTC4069 is in a charge cycle. A high impedance state indicates that the charge current has dropped below 10% of the full-scale current or the LTC4069 is disabled. Figure 2 shows the CHRG status under various conditions. Charge Current Soft-Start and Soft-Stop The LTC4069 includes a soft-start circuit to minimize the inrush current at the start of a charge cycle. When a charge cycle is initiated, the charge current ramps from zero to the full-scale current over a period of approximately 170μs. Likewise, internal circuitry slowly ramps the charge current from full-scale to zero when the charger is shut off or self terminates. This has the effect of minimizing the transient current load on the power supply during start-up and charge termination. OPERATION Constant-Current/Constant-Voltage/ Constant-Temperature The LTC4069 uses a unique architecture to charge a battery in a constant-current, constant-voltage and constant- temperature fashion. Figure 1 shows a Simplified Block Diagram of the LTC4069. Three of the amplifier feedback loops shown control the constant-current (CA), constant- voltage (VA), and constant-temperature (TA) modes. A fourth amplifier feedback loop (MA) is used to increase the output impedance of the current source pair, M1 and M2 (note that M1 is the internal P-channel power MOSFET). It ensures that the drain current of M1 is exactly 1000 times greater than the drain current of M2. Amplifiers CA and VA are used in separate feedback loops to force the charger into constant-current or constant- voltage mode, respectively. Diodes D1 and D2 provide priority to either the constant-current or constant-voltage loop, whichever is trying to reduce the charge current the most. The output of the other amplifier saturates low which effectively removes its loop from the system. When in constant-current mode, CA servos the voltage at the PROG pin to be precisely 1V. VA servos its inverting input to an internal reference voltage when in constant-voltage mode and the internal resistor divider, made up of R1 and R2, ensures that the battery voltage is maintained at 4.2V. The PROG pin voltage gives an indication of the charge current during constant-voltage mode as discussed in “Programming Charge Current”. Transconductance amplifier, TA, limits the die temperature to approximately 115°C when in constant-temperature mode. Diode D3 ensures that TA does not affect the charge current when the die temperature is below approximately 115°C. The PROG pin voltage continues to give an indication of the charge current. In typical operation, the charge cycle begins in constant- current mode with the current delivered to the battery equal to 1000V/RPROG. If the power dissipation of the LTC4069 results in the junction temperature approaching 115°C, the amplifier (TA) will begin decreasing the charge current to limit the die temperature to approximately 115°C. As the battery voltage rises, the LTC4069 either returns to constant-current mode or enters constant-voltage mode straight from constant-temperature mode. |
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