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LTC4069 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4069 Datasheet(HTML) 9 Page - Linear Technology |
9 / 18 page LTC4069 9 4069fc For more information www.linear.com/LTC4069 operaTion below 4.1V (which corresponds to 80% to 90% battery capacity), a new charge cycle is initiated and a 2.25 hour timer begins. This ensures that the battery is kept at, or near, a fully charged condition and eliminates the need for periodicchargecycleinitiations.Also,ifthebatteryvoltage does 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, reducingthechargecurrentto10%ofthefull-scalecurrent. Ifthelow-batteryvoltagepersistsforonequarterofthetotal 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 anyreasonthebatteryvoltagerisesabove2.9V,thecharge 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 Thechargestatusindicatorpinhasthreestates:pull-down, pulse at 2Hz (see Trickle Charge and Defective Battery Detection and Battery Temperature Monitoring) and high impedance.Thepull-downstateindicatesthattheLTC4069 is in a charge cycle. A high impedance state indicates that thechargecurrenthasdroppedbelow10%ofthefull-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 zerotothefull-scalecurrentoveraperiodofapproximately 170µs.Likewise,internalcircuitryslowlyrampsthecharge 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. Constant-Current/Constant-Voltage/ Constant-Temperature The LTC4069 uses a unique architecture to charge a battery in a constant-current, constant-voltage and con- stant-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 toforcethechargerintoconstant-currentorconstant-volt- age 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-currentmode,CAservosthevoltageatthePROG 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”. Transconductanceamplifier,TA,limitsthedietemperature 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.ThePROGpinvoltagecontinuestogiveanindication of the charge current. |
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