Datasheet LTC4070 (Analog Devices) - 8

link to page 8 LTC4070
OPERATION
For example, if the NTC thermistor requires the float volt-
Low Battery Status Output: LBO
age to be dropped by 100mV (ADJ = VCC and 0.29•VNTCBIAS When the battery voltage drops below 3.2V, the LBO pin < VNTC < 0.36•VNTCBIAS) then the HBO rising threshold pulls high. Otherwise, the LBO pin pulls low when the is detected when VCC rises past VFLOAT – �VFLOAT(NTC) – battery voltage exceeds about 3.5V. VHBTH = 4.2V – 100mV – 40mV = 4.06V. The HBO falling threshold in this case is detected when VCC falls below While the low battery condition persists, NTC and ADJ VFLOAT – �VFLOAT(NTC) – VHBTH – VHBHY = 4.2V – 100mV pins are no longer sampled—the functions are disabled— – 40mV – 100mV = 3.96V. and total supply consumption for the LTC4070 drops to less than 300nA (typ).
APPLICATIONS INFORMATION General Charging Considerations
The charge current decreases as the battery voltage The LTC4070 uses a different charging methodology from increases. If the rising battery voltage is 40mV less than previous chargers. Most Li-Ion chargers terminate the the programmed float voltage, the LTC4070 consumes charging after a period of time. The LTC4070 does not only 450nA of current, and all of the input current flows have a discrete charge termination. Extensive measure- into the battery. As the battery voltage reaches the float ments on Li-Ion cells show that the cell charge current voltage, the LTC4070 shunts current from the wall adapter drops to nanoamps with the shunt charge control circuit and regulates the battery voltage to VFLOAT. The more effectively terminating the charge. For long cell life, oper- shunt current the LTC4070 sinks, the less charge cur- ate the charger at 100mV lower charge voltage normally rent the battery gets. Eventually, the LTC4070 shunts all used. the current from the battery; up to the maximum shunt current. The maximum shunt current in this case, with no The simplest application of the LTC4070 is shown in NTC adjustment, is determined by the input resistor and Figure 1. This application requires only an external resis- is calculated as: tor to program the charge/shunt current. Assume the wall adapter voltage (V (V ) (12V – 4.1V) WALL) is 12V and the minimum battery I WALL – VFLOAT = = 49mA voltage (V SHUNT _MAX = BAT_MIN) is 3V, then the maximum charge cur- RIN 162Ω rent is calculated as: At this point the power dissipated in the input resistor is V ( ) 388mW. I WALL – VBAT _MIN MAX _ CHARGE = RIN RIN 162Ω (12V – 3V) 0.5W 12V WALL = = 55.5mA ADAPTER 162Ω NTC VCC Care must be taken in selecting the input resistor. Power LTC4070 dissipated in RIN under ful charge current is given by the FLOAT NTCBIAS IF NOT NEEDED following equation: + FLOAT ADJ Li-Ion GND BATTERY V ( WALL – VBAT_MIN)2 (12V – 3V)2 PDISS = = = 0.5W 4070 F01 RIN 162Ω
Figure 1. Single-Cell Battery Charger
Rev. D 8 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 Typical Applications Package Description Revision History Typical Application Related Parts