The switching topology uses its natural advantage of lower power consumption to achieve higher charging currents. Charger based on switching buck regulator is shown in figure 4
Figure 4: Switching buck regulator topology
The charging current is set by the duty cycle of the PWM driving the MOSFET.
Battery parameter measurement circuit: Feedback signals need to be measured using an ADC, a peripheral available in most modern day microcontrollers. In Figures 3 and 4, we saw how the battery voltage and current feedback is taken. However, these are differential signals that require a differential ADC for the measurement while typically single-ended ADCs are available in microcontrollers. The circuit shown in Figures 4 and 5 can be easily modified to produce a single-ended signal for all three parameters – voltage, current and temperature – by making the microcontroller ground and supply ground different.
Figure 5: Measurement using single ended ADC
The battery negative terminal is made as the microcontroller ground. This makes voltage, temperature, and current feedback referenced to the microcontroller ground and allows for single-ended ADC measurement. For current feedback, a positive offset voltage needs to be introduced, as the feedback voltage will be negative when the battery is being charged. As shown in Figure 5, resistor R3 and R4 provide the required offset voltage.
Charging algorithm: This action closes the loop. The CPU reads the ADC readings for battery voltage, charging current, and temperature and controls the PWM duty cycle according to the charging profile. The rate at which the CPU monitors the ADC results and controls the PWM depends upon the trade-off between loop response time and CPU bandwidth consumption.
ADC parameters and PWM resolution: ADC resolution and accuracy and PWM resolution are important parameters which should be considered while designing battery chargers. ADC resolution defines how precisely the input voltage can be measured (in this case, feedback voltages). PWM resolution defines how precisely you can change the duty cycle of the output signal, which in turn determines the output voltage of current control circuit. While charging Li-Ion batteries, the battery voltage needs to be accurately and precisely controlled. This is much more important when the battery voltage is near full charge. This controllability depends upon ADC resolution, accuracy in measurement, and granularity in duty cycle variation.
As an example, the charger architecture shown in Figure 5 has been implemented in a CY8C24x23 PSoC device from Cypress Semiconductors. The microcontroller is equipped with generic digital and analog blocks that can be configured to specific circuit functions. For example, continuous time analog blocks can be used to implement the programmable gain amplifier and comparator. Switched capacitor analog blocks have a wide variety of uses, including filters, digital to analog converters (DAC), and analog to digital converters (DAC). Digital basic blocks can be used to implement the PWM, counters, timers, and buffers while digital communication blocks are used to implement communication interfaces such as SPI, UART, IrDA RX and TX. The device also provides I2C block which can function as master or slave.