ZRD0013CHRGZRD

Reference Design Buck Converter Battery Charger Using the Z8F042A MCU RD001302-0814 Overview This document describes a Z8 Encore! XP-based Buck Converter Battery Charger reference design that employs Zilog’s Z8F042A MCU to control a step-down DC-DC converter (also known as a buck converter) that acts as a regulated power source. This buck converter battery charger hardware is capable of regulating charger output in a number of modes, such as constant voltage, constant current, or constant voltage with a current limit. The charger can be viewed as a complete control system. The type and capacity of the battery determines the mode of operation of the battery controller – namely, a constant current source or a constant voltage source. The voltage (VSET) and current (ISET) setpoints are also determined by the type and capacity of the battery. All battery control loop operations can be controlled by the user via the Z8F042A MCU’s UART block, and feedback is provided in the HyperTerminal console. Additionally, LEDs provide a visual status of the charging process. Note: The source code file associated with this reference design, RD0013-SC01.zip, is available free for download from the Zilog website. This source code has been tested with version 5.0.0 of ZDS II for Z8 Encore! XP MCUs. Subsequent releases of ZDS II may require you to modify the code supplied with this reference design. Features The key features of this buck converter battery charger are: • Z8F042A MCU featuring a highly accurate Sigma-Delta ADC • Buck converter with proportional/integral controlled constant current and voltage • Differential ADC for current measurements • Complete programmability • Current/voltage control independent of user input • UART-controlled operation • In-circuit programming for upgrading or modifying firmware Potential Applications This Buck Converter Battery Charger reference design can be used to develop a number of applications; the brief list below offers a few ideas. RD001302-0814 Page 1 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design • Security systems • Information systems • Warning systems Discussion The Buck Converter Battery Charger reference design consists of a buck converter controlled by a Z8F042A MCU, plus circuitry to provide feedback signals to the MCU. To save memory resources, the provided UART does not implement the stdio.h libraries. Instead, a simple UART using only integer values is used. As a result of this implementation, the user is asked to enter values from 10 to 40, representing values from 1.0 to 4.0, respectively, for a set voltage and set current. An external voltage source 5 V or 15 V, or USB can be used to supply the operating voltage. The USB 2.0 port current limit is 500 mA. When the MCU is powered up and if a battery was detected, the MCU will start charging the battery by generating the PWM pulses to the buck converter to the initial current and voltage settings as determined in the Userinput.h header file, until the user enters the desired values. The charge process is executed in the background and controlled via a timer0 interrupt service routine to alternate between current and voltage ADC channel sampling, keeping the UART as a foreground task in the main function. The PI then uses the sampled ADC information from the voltage and current samples to adjust the PWM for the battery charging process. During the charging process, the MCU measures battery charge current as a voltage drop across R4, a 1 Ohm resistor, in series with the battery and adjusts duty cycle of PWM to the buck converter to keep this voltage drop in respect with a battery-charging curve which is determined by the battery chemistry. When the battery charge current reaches the voltage charge threshold, the MCU decreases charging current and starts monitoring battery voltage. While the battery is charging, the red LED is on and the green LED is off. When the charging process is completed, the red LED is turned off and the green LED is turned on and a message to the hyper-terminal (GUI) indicates battery charge is completed. The buck converter off, but the PI control loop keeps monitoring the battery voltage and refreshes the charge, if necessary. The charging process can be stopped by the user at any time. Components and MCU Peripherals Switching MOSFET Q1 with 3 A current limit and low power MOSFET Q2 used as level shifter for Q1. RD001302-0814 Page 2 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Switching MOSFET Q1 with Inductor L1, capacitors C11, C12, C13, and diode D1 forms buck converter circuitry. Resistive dividers R6/R9 and R7/R10 provide battery voltage/current measurement points for MCU ADC circuitry. Capacitors C14, C15 may be used as HF filters, if switching noise affects ADC conversion. Capacitors C1 - C10 are input filter. Divider R2/R5 provides measurement point for input voltage. LEDs D2 (red) and D3 (green) are used as indicators to reflect battery charge state and fault conditions. Evaluation board assumes follow MCU ports assignment: • Analog inputs (ADC): – PB0/ANA0 - Thermistor Voltage (Single-ended mode) – PB3/ANA3 - Input Voltage (Single-ended mode) – PC0/ANA4 - Buck Converter Output Voltage (Single-ended mode) – PC1/ANA5 - Battery Voltage (Single-ended mode) – PC0/ANA4 - PC1/ANA5 - Battery Current (Differential Mode) • Digital outputs: – PA7/T1out: PWM pulses – PA1: Red LED D2 – PA2: Green LED D3 The Board features two power source options. It can be powered by connecting USB (B) connector J1 to the USB port of the development PC using the USB A to USB (B) cable included in the Kit. The other option is to connect a female plug to an external 6 VDC source with at least 300–400mA of current to external power supply connector J3. A drawing of an optional external power supply plug is shown in Figure 1. Figure 1. Female Plug RD001302-0814 Page 3 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Legend A = 5.5 mm B = 2.1 mm C = 8.8 mm or longer Figure 1. Female Plug When the Board is powered via a USB connection, communication with the development PC can be established through the FT232RL chip (U1) to provide a USB-to-serial interface. The UART0 block of the Z8F042A MCU is connected to this chip. An on-board USB power distribution switch (U6) provides overcurrent protection in the event of a short or if a device is connected to the Board that requires more than 500 mA @3.3 V. If either condition occurs, LED D4 will illuminate. The battery must be connected to terminals Vbat+ and Vbat–. The Battery Charger Reference Board, shown in Figure 2, features two IXYS MOSFETs. MOSFET Q1 exhibits a 3A current limit; the low-power MOSFET Q2 is used as a level shifter for Q1. The buck converter circuit consists of MOSFET Q1, inductor L1, capacitors C11, C12 and C13, and a diode. Resistor R4 is used as a battery current sense resistor, and resistive dividers R6/R9 and R7/R10 provide the battery voltage/current measurement points for the Z8F042A MCU’s ADC inputs. RD001302-0814 Page 4 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Figure 2. The Buck Converter Battery Charger Reference Design Module RD001302-0814 Page 5 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Battery Charger Hardware The battery charger hardware is built and tested with commonly available components. The main hardware component of this implementation is a Z8 Encore! XP-based F042A 20-pin microcontroller. Figure 3 displays a block diagram of the battery charger; see Appendix C. Schematic Diagrams on page 26 for a schematic illustration of the Battery Charger Reference Board. HyperTerminal UART PWM External Power Source Step-Down Converter Z8F042A MCU ADC V/I Converter Battery Feedback Status Indicator LED Battery Figure 3. Buck Converter Battery Charger Block Diagram Z8F082A Series Flash MCUs The Z8F042A MCU used in this reference design is a member of the Z8F082A Series of Z8 Encore! XP Flash MCUs; it is a 4 KB version of the Z8F082A MCU. Z8F082A Series MCUs offer the following features: • 20 MHz eZ8 CPU • 1 KB, 2 KB, 4 KB, or 8 KB Flash memory with in-circuit programming capability • 256 B, 512 B, or 1 KB register RAM • Up to 128 B nonvolatile data storage (NVDS) • Internal precision oscillator trimmed to ±1% accuracy • External crystal oscillator, operating up to 20 MHz • Optional 8-channel, 10-bit analog-to-digital converter (ADC) • Optional on-chip temperature sensor RD001302-0814 Page 6 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design • On-chip analog comparator • Optional on-chip low-power operational amplifier (LPO) • Full-duplex UART • The UART baud rate generator (BRG) can be configured and used as a basic 16-bit timer • Infrared Data Association (IrDA)-compliant infrared encoder/decoders, integrated with the UART • Two enhanced 16-bit timers with capture, compare and PWM capability • Watchdog Timer (WDT) with dedicated internal RC oscillator • Up to 20 vectored interrupts • 6 to 25 I/O pins depending upon package • Up to thirteen 5 V-tolerant input pins • Up to 8 ports capable of direct LED drive with no current limit resistor required • On-Chip Debugger (OCD) • Voltage Brown-Out (VBO) protection • Programmable low battery detection (LVD) (8-pin devices only) • Bandgap generated precision voltage references available for the ADC, comparator, • VBO and LVD • Power-On Reset (POR) • 2.7 V to 3.6 V operating voltage • 8-, 20- and 28-pin packages • 0°C to +70°C and –40°C to +105°C for operating temperature ranges FT232RL USB-to-Serial UART Interface The FT232RL (U1) device is a USB-to-serial UART interface between a host PC and the Z8F042A MCU with an optional clock generator output. In addition, asynchronous and synchronous bit-bang interface modes are available. USB-to-serial designs using the FT232RL device have been further simplified by fully integrating external EEPROM, clock circuit and USB resistors onto the device. Its inputs are connected to the USB connector J1, and its outputs are connected to the Z8F042A MCU’s UART serial communication ports PA4 and PA5. U3 U3 is a 3.3 V voltage regulator for the Z8F042A MCU and the debug connector. RD001302-0814 Page 7 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Buck Converter In general, a buck converter consists of an inductor, two switches, a transistor and a diode to control the inductor. The PWM alternates between connecting the inductor to a source of voltage to store energy in the inductor, and discharging the inductor into the load. In this reference design, the PWM is an input to the buck converter which is controlled by the MCU at an 80 kHz variable-PWM duty cycle. Depending on the charging speed, the output of the converter usually ranges from 1 V to 4 V; this voltage is supplied to the battery. Both the feedback charge current and charge voltage are controlled by the PI loop. R2 and R5 form a voltage divider on the input of the buck converter. The ratio of input voltage and battery voltage (Vbat) is used to determine the start-up duty cycle. Differential Voltage Divider Circuit The Differential Voltage Divider Circuit/Feedback Module provides battery voltage, battery current, and converter input using three ADC inputs. These feedback readings are used for constant current charging/voltage charging with a PI control loop, and to terminate the charging process. The constant current charging is applied using a differential resistor network. During the constant current charge, the differential voltage drop across a sense resistor is held constant, therefore the battery charge current is constant. The battery charging process can be monitored via UART and the GUI (HyperTerminal). Temperature Circuit A temperature (thermistor) circuit consists of resistors R11 and R12. If the battery features a temperature sensor, then this part of the circuit monitors the voltage drop across voltage divider R11, R12. The thermistor is parallel t o R12; its voltage drop determines the voltage drop across R12. As a result, the software acts upon this voltage drop threshold. LED Indicators Three different LEDs display the various functionalities of the charger: • Overcurrent indication LED D4 • Battery charging indication LED D2 • Charging complete indication LED D3 The red LED (D2) indicates when a battery charge is in progress; it illuminates the green LED (D3) when the charging process is complete. The Reference Board uses a 4.5 V to 15 V DC regulated external input power supply to drive the 5 V and 3.3 V internal supply. Battery Charger Software The software provided with this reference design is developed and tested on the hardware per the schematic diagram shown in Figure 12 on page 26. This section describes the functionality of this reference design’s different software blocks used for battery charging. RD001302-0814 Page 8 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design For additional details about this software, refer to the demo firmware contained in the RD0013-SC01.zip file, which includes the following files: • Main.c – Variable declarations – Initialization of peripherals – UART control • Control.c – Timer0 interrupt service routine (control loop for constant current/voltage charge) – PWM duty for battery charging – Constant current PI feedback loop – Constant voltage PI feedback loop – UART transmit function – UART receive function • Initialization.c – WDT initialization – Input/Output pin initialization – UART initialization – ADC initialization – Timer0 initialization – PWM initialization • UserInput.h – User input parameters for the PI control loop • Main.h – Function body declarations – External variable declarations – Constant definitions – Structure declarations Battery Charger Algorithm The battery charging process is a PWM PI-controlled, closed-loop algorithm based on battery current and voltage feedback. The input PWM frequency of the buck converter is 80 kHz. If the battery is not completely charged, then the duty cycle required for maintaining the setpoints at the converter outputs is calculated by the control algorithm. The control algorithm implements a Proportional, Integral (PI) loop to derive the necessary PWM duty cycle based on the following equation: u(t) = k1 * e(t) + k2 * e(t)dt RD001302-0814 Page 9 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design In this equation, K1 is the proportional constant coefficient, K2 is the integral constant coefficient, and e(t) is the error between the set value and the actual value as a function of time. The coefficients are determined empirically based on system response because they are contingent upon reactive/complex load conditions, which may vary. These coefficients should be chosen such that smooth convergence to a current setpoint and a voltage setpoint is achieved. If UART is enabled, then the current setpoint and voltage setpoint can be entered in HyperTerminal. If UART is not selected, then these voltage and current values must be entered in the UserInput.c file. The interrupt service routine (ISR) timer is invoked every 100 µs (at 10 kHz). The PWM value computed by the control algorithm is loaded into the PWM generators to be transmitted via the output pin. The 16-bit timer’s PWM Mode offers a programmable switching frequency based on the reload value; this flexibility allows a trade-off between accuracy and the frequency of the PWM switching signal. The higher the frequency, the lower the PWM Register reload value and resolution of the PWM; the converse is also true. The firmware heuristic is designed such that only UART code is executed in the main infinite loop. Timer0 is the main control engine, configured for interrupts at 10 kHz intervals to change the ADC channels, if the ADC data is ready, and to determine whether the current control or the voltage control PI loop is required. Battery Safety and Charge Termination For safe operation, termination threshold calculations must be based on battery parameters. The setpoints for the DC-DC step down (buck) converter voltage, the current, and the current limit are calculated. When the one-time calculations are complete, the charger software enters into an infinite loop which is terminated by a successful charge completion or safety error. Inside the Timer0 loop, the ADC reads the actual values of the converter output voltage, the battery voltage, and the current. The ADC measures the output voltage/current output of the buck converter as feedback to the controller. The ADC performs a basic measurement of output voltage, output current, and battery voltage at the battery’s terminals as an input to determine charge termination. To make this determination, the current across the battery terminals must be the same as the measured converter output current. This safety routine is responsible for the overall safety features associated with a battery charger. The charger ensures safety by comparing the actual converter voltage and the battery voltage with calculated thresholds. Crossing these thresholds switches the PWM output off, which turns off the converter output and terminates the charging functions. Such termination protects the batteries in case of a device failure. If all of the actual values are within limits, the battery is tested for full charge. RD001302-0814 Page 10 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Initializing the Z8 Encore! XP Peripherals All Z8Encore! peripherals are initialized from their power-on states to their required modes of operation via the following ports: • Port A, bits 1 and 2 are used for the status LEDs • Port A, bits 4 and 5 are the UART transmit and receive pins to communicate with HyperTerminal • Port B, bit 3 is used for single-ended input voltage mode • Port C, bit 0 and 1 are the battery charge voltage pins • PA 7 is configured to generate the 80 kHz PWM frequency for the buck converter Kit Contents The tools used to build this reference design are: • Buck Converter Battery Charger Design Module • USB cable, Type A to B • Buck Converter Battery Charger Reference Design Flyer (FL0144) Required Items Not Supplied • USB Smart Cable (Zilog PN: ZUSBSC0001ZACG) • 3.7 V lithium-ion battery Software Installation This project requires the ZDS II – Z8Encore! Integrated Development Environment. Observe the following procedure to download and install the ZDS II software. 1. Download the latest version of ZDS II for Z8 Encore! from the Downloadable Software category in the Zilog Store. 2. Run the software installation file and follow the on-screen instructions to install ZDS II. Viewing and Rebuilding the Battery Charger Software The source code/project is provided in this kit so that users can be familiar with the Buck Converter Battery Charger reference design. However, users are not expected to modify or change any parameters. 1. If you have not downloaded the software for this reference design, refer to the paper insert that was included in your kit (Document Control Number FL0144), and follow the 3-step software download instructions contained on it. RD001302-0814 Page 11 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design 2. Launch the ZDS II – Z8 Encore! application by navigating via the following default path: Start → Programs → Zilog ZDS II_Z8Encore!_ → ZDSII_Z8Encore! 3. From the File menu, select Open Project. The Open dialog box appears. 4. Browse to the src folder which, by default, is located in the following path: \ZRD0013CHRGZRD_ 5. Select the Applications.zdsproj file from the src folder, and click Open to display the initial ZDS II program screen. To view the source files, double-click the Project Files folder on the left side of the IDE interface. Double-click an individual file to open the file in the ZDS II file editor. 6. Click the Rebuild All toolbar icon, which is highlighted in red in Figure 4. RD001302-0814 Page 12 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Figure 4. Build Mode Configuration 7. When the rebuild is complete, a Build succeeded. message will appear, as highlighted in Figure 5. RD001302-0814 Page 13 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Figure 5. A Successful Build Note: The output checksum shown in Figure 5 is for reference only and may not display the same results at the time of release. RD001302-0814 Page 14 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Running the Demo Observe the following procedure to set up your reference design and host PC to run the demo/application software. 1. Connect a 3.7 V Lithium Ion battery to the Battery Charger Reference Module across Vbat– and Vbat+ terminals. 2. Connect a USB Smart Cable to J2 of the Reference Module and host PC USB port. See Appendix A. Installing the USB Smart Cable Driver on page 21 for assistance, if required. 3. Ensure that the S1 shunt is ON, and slide switch S5 to the USB position. Figure 6 indicates the locations of S1 and S5 on the Module. Figure 6. Shunt and Switch Locations on the Battery Charger Reference Module 4. Connect the included USB Cable Type A to B to J1 and to the host PC’s USB port. If necessary, see Appendix B. Installing the FTDI USB-to-UART Driver on page 24. 5. In your terminal emulation application, select the COM port assigned to the USB-toserial cable, and modify the settings to reflect an 57600, 8, N, 1, N configuration, as shown in the Tera Term example in Figure 7. RD001302-0814 Page 15 of 27 Buck Converter Battery Charger Using the Z8F042A MCU Reference Design Figure 7. Configuring the Serial Communication Settings 6. Launch ZDS II for Z8 Encore!. If you have not yet installed ZDS II, return to the Software Installation section on page 11. 7. From the File menu in ZDS II, select Open Project, and navigate to the following filepath: