Datasheet LTC4070 (Analog Devices) - 9

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APPLICATIONS INFORMATION
RIN Figure 2 shows a charge circuit that can boost the charge 110Ω 4W current as well as the shunt current with an external 24V WALL ADAPTER P-channel MOSFET, Q1. In this case, if the wall adapter NTC VCC voltage (VWALL) is 24V and the minimum battery voltage LTC4070 FLOAT NTCBIAS DRV Q1 (V IF NOT NEEDED BAT) is 3V, then the initial charge current is set to 191mA by selecting R ADJ IN = 110Ω. Note that this resistor dissipates + Li-Ion over 4W of power, so select the resistor taking power rating GND BATTERY into account. When the battery voltage reaches the float voltage, the LTC4070 and the external P-channel MOSFET Q1: FDN352AP 4070 F02 begin to shunt current from the wall adapter. Eventually,
Figure 2. Single-Cell Charger with Boosted Drive
the LTC4070 and the external P-channel MOSFET shunts all available current (182mA) and no current flows to the RIN WALL VCC1 battery. Take the full shunt current and power into account ADAPTER when selecting the external MOSFET. NTC VCC LTC4070 The LTC4070 can also be used to regulate series-connected FLOAT battery stacks as illustrated in Figure 3 and Figure 4. Here IF NOT NEEDED NTCBIAS + two LTC4070 devices are used to charge two batteries in FLOAT ADJ Li-Ion GND BATTERY series; with or without boosted drive. A single resistor sets VCC2 the maximum charge/shunt current. The GND pin of the top device is simply connected to the V NTC V CC pin of the bottom CC device. Care must be taken in observing the status output LTC4070 FLOAT pins of the top device as these signals are not ground ref- IF NOT NEEDED NTCBIAS erenced. Also, the wall adapter must have a high enough + FLOAT ADJ Li-Ion GND BATTERY voltage rating to charge both cells.
NTC Protection
4070 F03
Figure 3. 2-Cell Battery Charger
The LTC4070 measures battery temperature with a nega- tive temperature coefficient thermistor thermally coupled RIN WALL V to the battery. NTC thermistors have temperature char- CC1 ADAPTER acteristics which are specified in resistance-temperature NTC VCC conversion tables. Internal NTC circuitry protects the bat- LTC4070 FLOAT tery from excessive heat by reducing the float voltage for IF NOT NEEDED NTCBIAS DRV Q1 each 10°C rise in temperature above 40°C (assuming a FLOAT ADJ + Li-Ion Vishay thermistor with a B GND BATTERY 25/85 value of 3490). VCC2 The LTC4070 uses a ratio of resistor values to measure battery temperature. The LTC4070 contains an internal NTC VCC fixed resistor voltage divider from NTCBIAS to GND with LTC4070 FLOAT four tap points; NTC IF NOT NEEDED NTCBIAS DRV Q2 TH1-NTCTH4. The voltages at these tap points are periodically compared against the voltage FLOAT ADJ + Li-Ion at the NTC pin to measure battery temperature. To con- GND BATTERY serve power, the battery temperature is measured peri- Q1, Q2: Si3469DV odically by biasing the NTCBIAS pin to VCC about once 4070 F04 every 1.5 seconds.
Figure 4. 2-Cell Battery Charger with Boosted Drive
Rev. D For more information www.analog.com 9 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