This Lithium Ion Polymer charger circuit diagram is based on the LTC4078 standalone linear charger Manufactured by Linear Technology and capable of charging a single-cell Li-Ion Li-Polymer battery .
This Lithium Ion Polymer charger circuit works from both wall adapter and USB inputs. This charger can detect power at the inputs and automatically select the appropriate power source for charging.
The float voltage is fixed at 4.2V and the charge current is programmed with an external resistor.
The LTC4078 not require external sense resistor or blocking diode for charging due to the internal MOSFET architecture. The LTC4078 support a maximum input voltage up to 22V rating for both wall adapter and USB inputs ( charging stops if the selected power source exceeds the overvoltage limit ) .
The charge current can be programmed up to 950mA from wall adapter input .
IUSB pin is used for program the charge current for USB power that can be programmed by connecting a resistor to the ground.
The voltage on this pin can be used to measure the battery current delivered from the USB input using the following formula: IBAT = (VIUSB/RIUSB)*1000 .
ITERM pin is the termination current threshold program that is set by connecting a resistor to ground. ITERMINATE is set by the following formula:
ITERMINATE =100V/RITERM ; RITERM =100V/ITERMINATE
IDC pin is used for program the charge current for wall adapter power that is set by connecting a resistor to ground.
The voltage on this pin can be used to measure the battery current delivered from the DC input using the following formula: IBAT = (VIDC/RIDC)*1000
The charge current delivered to the battery from the wall adapter or USB supply is programmed using a single resistor from the IDC or IUSB pin to ground and can be calculated using the following equations:
RIDC =1000V/ICHRG-DC , ICHRG-DC = 1000V/RIDC - Wall adapter
RIUSB =1000V/ICHRG-USB , ICHRG-USB =1000V/RIUSB – USB port
The power dissipation of the circuit is calculated with the formula bellow :
PD = (VIN – VBAT) • IBAT , but when designing the battery charger circuit, it is not necessary to design for worst-case power dissipation scenarios because the LTC4078 automatically reduce the charge current during high power conditions .
The approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 120°C – (VIN – VBAT) • IBAT • θJA