LT3650-8.4 based Li-ion 2S battery charger and UPS with 9 to 32Volt input and open collector status signals for control electronics.
Maybe you have a Raspberry based project, or an Arduino, or a dragon that needs to be always on, thus the need for a UPS circuit.
The solution here presented is based on a LT3650-8.4 chip from Linear that offers multiple pins to suit every need regarding charging, low power consumption and battery/system overload protection.
The schematic can be partially understood by looking at Linear’s datasheet. Charging current is set to 1.5Amp circa via R5 and will stop at 150mA; soft start charging is controlled via feeding 50uA into C4, thus creating a linear ramp from 0 to 1V that controls the percentage of current to feed into the battery based on the nominal 1.5A.
Something to notice in Figure 9 of the regulator’s datasheet (CLICK HERE) is the path of noisy and sensitive currents inside the circuit. If you’re going to modify my design, be careful to keep SW separated from SENSE and BAT as much as possible, and try to always think as an electron flowing into the circuit: “where would I like to go? > where it’s easy to go, meaning low resistance (DC) or Inductance (HF components)”.
It is possible to connect an NTC to the NTC1 header, it must be a 10kΩ B=3380 NTC Thermistor. Charging will be then be halted at temperatures below 0°C or above 40°C to avoid damage to the battery pack.
No system overload protection is here implemented ( CLP is shorted to VIN ). D5 and D6 are used to clamp to 3V the outgoing open collector signals for CHARGING and FAULT status.
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Nadim Conti 