Datasheet LTC4071 (Analog Devices) - 10

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APPLICATIONS INFORMATION General Charging Considerations
Care must be taken in selecting the input resistor. Power dissipated in RIN under full charge current is given by the The LTC4071 uses a different charging methodology from following equation: previous chargers. Most Li-Ion chargers terminate the charging after a period of time. The LTC4071 does not V ( WALL – VBAT_MIN)2 (12V –3.2V)2 have a discrete charge termination. Extensive measure- PDISS = = = 0.48W ments on Li-Ion cells show that the cell charge current RIN 162Ω drops to very low levels with the shunt charge control The charge current decreases as the battery voltage circuit effectively terminating the charge. For improved increases. If the battery voltage is 40mV less than the battery lifetime choose 4.0V or 4.1V float voltage. programmed float voltage the LTC4071 consumes only The battery disconnect function requires some care in 550nA of current, and all of the excess input current flows selecting the input supply compliance for charging a bat- into the battery. As the battery voltage reaches the float tery while powering a load at VCC. The internal battery voltage, the LTC4071 shunts current from the wall adapter disconnect switch remains off while charging the battery and regulates the battery voltage to VFLOAT = VCC. The through the body diode of the internal switch until VCC more shunt current the LTC4071 sinks, the less charge exceeds VLBC_VCC. If the source voltage compliance is current the battery gets. Eventually, the LTC4071 shunts not greater than VLBC_VCC, then the battery will never re- all the current flowing through RIN; up to the maximum connect to VCC and the system load will not be able to shunt current. The maximum shunt current in this case, run on battery power. Users may detect that the battery is with no NTC adjustment is determined by the input resis- connected by monitoring the NTCBIAS pin as it will peri- tor and is calculated as: odically pulse high once VCC has risen above VLBC_VCC, and stops pulsing once V (V ) (12V – 4.1V) CC falls below VLBD. I WALL – VFLOAT SHUNT _MAX = = = 49mA RIN 162Ω The simplest application of the LTC4071 is shown in At this point the power dissipated in the input resistor is Figure 2. This application requires only an external resis- 388mW. tor to program the charge/shunt current. Assume the wall The LTC4071 can also be used to regulate series-con- RIN = 162Ω, 0.5W nected battery stacks as illustrated in Figure 3. Here two WALL LTC4071 devices are used to charge two batteries in series. ADAPTER VCC BAT A single resistor sets the maximum charge/shunt current. LTC4071 RIN WALL 1µF + ADAPTER Li-Ion BATTERY VCC BAT GND LTC4071 4071 F02 1µF +Li-Ion BATTERY
Figure 2. Single-Cell Battery Charger
GND adapter voltage (VWALL) is 12V and the maximum charge V current is calculated as: CC BAT LTC4071 V ( ) 1µF I WALL – VBAT _MIN +Li-Ion MAX _ CHARGE = R BATTERY IN GND (12V – 3.2V) 4071 F03 = = 54mA 162Ω
Figure 3. 2-Cell Battery Charger
Rev. D 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts