Driven by the need forunt ethered mobility and ease of use, many system srely on rechargable batteries as their primary power source. The battery charging circuitry for these systems is typically implemented using a fixed-function IC to control the charging current/voltage profile.The C8051F30x family provides a flexible alternative to fixed-function battery chargers. This application note discusses how to use the C8051F30x family in Li-Ion battery charger applications. TheLi-Ion charging algorithms can be easily adapted toother battery chemistries, but an understanding of other battery chemistries is required to ensure proper charging for those chemistries.The code accompanying this application note was originally written for C8051F30x devices. Thecode can also be ported toother devices in the Sili-con Labs microcontroller range.
• On-chip high-speed, 8-bit ADC provides superior accuracy in monitoring charge voltage(critical to prevent overcharging in Li-Ion applications), maximizing charge effectivenes sand battery life.
• On-chip PWM provides means to implement buck converter with a very small external inductor.
• On-chip Temp sensor provides an accurate and stable drive voltage for determining battery temperature. An external RTD (resistive temperature device) can also be used via the flexi-ble analog input AMUX.
• A single C8051F30x platform provides full product range for multi-chemistry chargers,expediting time to market and reducing inventory Charging Basics Batteries are exhaustively characterized to deter-mine safe yet time-efficient charging profiles. The optimum charging method for a battery is dependent on the battery’s chemistry (Li-Ion, NiMH,NiCd, SLA, etc.). However, most charging strategies implement a 3-phase scheme:
1. Low-current conditioning phase
2. Constant-current phase
3. Constant-voltage phase/charge termination All batteries are charged by transferring electrical energy into them (refer to the references at the end of this note for a battery primer). The maximum charge current for a battery is dependent on the battery’s rated capacity
(C).For example, a battery with a cell capacity of 1000 mAh is referred to as being charged at 1C (1 times the battery capacity) if the charge current is 1000 mA. A battery can be charged at 1/50C (20 mA) or lower if desired.However, this is a common trickle-charge rate andis not practical in fast charge schemes where short charge time is desired.Most modern chargers utilize both trickle-charge and rated charge (also referred to as bulk charge)while charging a battery.
The trickle-charge currentis usually used in the initial phases of charging to minimize early self heating which can lead to pre-mature charge termination. The bulk charge is usu-ally used in the middle phase where the most of the battery’s energy is restored.During the final phase of battery charge, which generally takes the majority of the charge time,either the current or voltage or a combination of both are monitored to determine when charging is complete. Again, the termination scheme depend son the battery’s chemistry. For instance, most Lithium Ion battery chargers hold the battery voltage constant, and monitor for minimum current. NiCd batteries use a rate of change in voltage or temperature to determine when to terminate.Note that while charging a battery,most of the electrical energy is stored ina chemical process, but not all as no system is 100 percent efficient. Some of the electrical energy is converter to thermal energy,heating up the battery. This is fine until the battery reaches full charge at which time allt he electrical energy is converted to thermal energy. In this case,if charging isn’t terminated, the battery can be damaged or destroyed. Fast chargers (chargers that charge batteries fully in less than a couple hours)compound this issue, as these chargers use a high charge current to minimize charge time. As one can see, monitoring a battery’s temperature is critical(especially for Li-Ion as they explode if over-charged). Therefore, the temperature is monitored during all phases. Charge is terminated immediately if the temperature rises out of range.Li-Ion Battery Charger –Hardware Currently, Li-Ion batteries are the battery chemistry of choice for most applications due to their high energy/space and energy/weight characteristics when compared to other chemistries. Most modern Li-Ion chargers use the tapered charge termination,minimum current (see Figure 2), method to ensure the battery is fully charged as does the example code provided at the end of this note.The most economical way to create a tapered termination charger is to use a buck converter. A buck converter is a switch ingregulator that uses an inductor and/or a transformer (if isolation is desired), as an energy storage element to transfer energy from the input to the output in discrete packets (for our example we use an inductor the capacitor in Figure 3 is used for ripple reduction).Feedback circuitry regulates the energy transfer via the transistor, also referred to as the pass switch, to maintain a constant voltage or constant current within the load limits of the circuit. See Figure 3for details.Tapered Charger Using the F3 0x Figure 3 illustrates an example buck converter using the ‘F30x. The pass switch is controlled via the on-chip 8-bit PWM (Pulse Width Modulator)output of the PCA. When the switch is on, current will flow like in Figure 3A. The capacitor is charged from the input through the inductor. The inductor is also charged. When the switch is opened (Figure 3B), the inductor will try to main-tain its current flow by inducing a voltage as the current through an inductor can’t change instantaneously. The current then flows through the diode and the inductor charges the capacitor. Then the cycle repeats itself. On alarger scale, if the duty cycle is decreased (shorter“on” time), the average voltage decreases and vice versa. Therefore, con-trolling the duty cycles allows one to regulate the voltage or the current to within desired limits.Selecting the Buck Converter Inductor To size the inductor in the buck converter, one first assumes a 50 percent duty cycle, as this is where the converter operates most efficiently.Duty cycle is given by Equation 1, where T is the period of the PWM (in our example T = 10.5 S).Charge Current Charge Voltage Time Conditioning Phase Current regulation Voltage regulation Figure 2. Lithium Ion Charge Profile. Inductor Capacitor Power Source Battery Inductor Pass Switch Off Capacitor Power Source Battery(A)(B)Pass Switch On Shottky Diode Shottky Diode Figure 3. Buck Converter. Duty Cycle ton t Equation 1. Duty Cycle.