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MCP73871battery charger circuit design

Using the Microchip’s MCP73871 Li-Ion battery charger integrated circuit we can design a very simple , high efficiency battery charger solution for Li-Ion battery powered applications. The MCP73871 Li-Ion battery charger is a monolithic IC that offers compact size and rich features.
The MCP73871 device is a fully integrated linear solution for system load sharing and Li-Ion / Li-Polymer battery charge management with ac-dc wall adapter and USB port power sources selection. It is also capable of autonomous power source selection between input or battery.

The MCP73871 device automatically obtains power for the system load from a single-cell Li-Ion battery or an input power source (ac-dc wall adapter or USB port).
The MCP73871 device employs a constant current , constant voltage (CC/CV) charge algorithm with selectable charge termination point. The constant voltage regulation is fixed with four available options:

4.10V, 4.20V, 4.35V, or 4.40V to accommodate the new, emerging battery charging requirements. The MCP73871 device also limits the charge current based on die temperature during high power or high ambient conditions.
The MCP73871 device includes a low battery indicator, a power-good indicator and two charge status indicators that allows for outputs with LEDs or communication with host microcontrollers.
The Li-Ion , Li-Polymer charger can be used for a system that requires an average of 100 mA load current and consumes a maximum of 500 mA peak current for a short duration of time. A 950 mAh rated Li-Ion battery is used to operate the system.
The system continuously operates while charging the Li-Ion battery. The input power supply supplies the system load and charges battery when a battery is present in the system and when input power source is removed, the system is supported by the battery.
A fast charge current that equals 760 mA is calculated to meet the design specification for a 950 mAh at 0.8C .
ICHAREG=1000V/RPROG1, where RPROG1 in kilo-ohms and I in milli-ampere .
950mA × 0.8= 760mA, RPROG1=1000V/750mA= 1.3kΩ

The charge cycle is terminated when, during the constant voltage mode, the average charge current diminishes below a threshold established with the value of a resistor connected from PROG3 to VSS or the internal timer has expired: ITERMINATION =1000V/RPROG3
Voltage Proportional Charge Control is a key feature of MCP73871 that allows output to maintain proper voltage level even when the input varies .

VVPCC = [R2/(R1+R2)]× VIN = 1.23V (R2=110k, Vin=5)=> R1=337k (330k)
The MCP73871 compares the voltage at the THERM pin to the factory set thresholds of 1.23V and 0.25V, typically. Once a voltage outside the thresholds is detected during a charge cycle, the MCP73871 immediately suspends the charge cycle. The charge cycle resumes when the voltage at the THERM pin returns to the normal range.

An internal 50 μA current source provides the bias for a typical 10 kΩ negative-temperature coefficient thermistors (NTC ).
24kΩ=RT1 +(RT2 × RCOLD)/RT2 + RCOLD ; 5kΩ=RT1+(RT2 × RHOT)/(RT2 + RHOT) , where : RT1 is the fixed series resistance ,RT2 is the fixed parallel resistance ,RCOLD is the thermistor resistance at the lower temperature and RHOT is the thermistor resistance at the upper temperature .
MCP73871 Li-Ion Li-Poly battery charger schematic circuit diagram

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