ST6255 Low-Cost Li-Ion Battery Charger
Created: Mar 22, 2017
No description available.
Battery-powered portable devices have been rapidly increasing in the market nowadays. Li-ion batteries are widely used to power these devices because of their superior capacity for a given size and weight. This design utilizes ST6255C 8-bit microcontroller, which embeds an A/D converter, a PWM signal generator and 4K bytes of program memory, which is enough for the algorithm. The board also supports the ST6265C MCU, which adds 128 bytes of internal EEPROM to the features of the ST6255C.
This Li-ion battery charger has two slots. The front one can be used to plug in a simple battery or a mobile phone with an internal battery. The rear slot is for plugging in a stand-alone battery. A pair of LEDs (green/red) are assigned to each slot to indicate the charge status. The presence of battery in both slots is permanently monitored to implement the priority of the front slot over the rear slot. Whenever a battery is plugged into the front slot while the rear slot battery is being charged, rear charging is stopped and front charging begins. When front charging is terminated (ie., battery saturated, expire time reached, battery failure or battery removed), rear charging restarts from the beginning. Li-ion batteries should be charged using two different methods: constant voltage and constant current. During Stage 1 (constant current charge), the charging current is kept at a constant value (Iconst) until the battery voltage reaches the final cell voltage (VF). Then, in Stage 2 (constant voltage charge), the voltage is kept constant within this limit by slowly decreasing the current. Charging is stopped when the current drops below the manufacturer fixed threshold value (Isat). This current indicates that the battery is saturated.
In this design, the PWM signal generated by the ST6255C switches the PNP transistor on and off. A Schottky rectifier is needed to receive the current from the coil when the PNP transistor is off. If the charger is not powered on or if the battery is already fully charged, the PNP transistor is kept permanently off. The diode prevents the battery from discharging into the capacitor.