TB3097 Digital SMPS – Buck Converter Using the PIC12F1501 NCO Peripheral Author: Ionut Veche Microchip Technology Inc. INTRODUCTION The digital Switch Mode Power Supply (SMPS) is practically an asynchronous buck converter controlled by the PIC12F1501, and has a very high efficiency at light loads. It also offers hardware output overvoltage protection to avoid damaging the charged device. The charger is characterized by a very small and simple board and a very short Bill of Materials (BOM), which make it very easy to understand and use. BLOCK DIAGRAM The control board provides closed-loop proportional control in firmware to maintain the desired output voltage and limit the output current. The PIC® device (PIC12F1501) provides the necessary peripherals for A/D conversion, PWM generation and the amount of memory needed by the algorithm. The control-loop update rate is 4 kHz; the update rate is limited by the ADC acquisition speed. Figure 1 illustrates the block diagram of SMPS. FIGURE 1: BLOCK DIAGRAM 9%DW 92XW The features of the PIC12F1501 for this application are: /RDG 9 • • • • • Two 10-bit ADC One 5-bit DAC One Comparator One Numerically Controlled Oscillator (NCO) One Complementary Waveform Generator (CWG) 9LQ 3,&) 3:0 29SURWHFWLRQ $XWR6KXWGRZQ &:* *QG 92XWBGLY &203 1&2 '$& 92XWBGLY All these features are connected into the proper functions by software configuration. PERFORMANCE SPECIFICATIONS Electrical specifications over operating range: 8V ≤ VDD ≤ 16V. TABLE 1: ELECTRICAL SPECIFICATIONS Input Voltage Range Output Voltage 8-16 Volts DC 5 Volts DC Output Current 2 Amperes Output Power 10 Watts Code Size 452 Words Ram Size 31 Efficiency 88.5% %XWWRQ $'& ,6HQVH The overvoltage protection is assured by the CWG module, which is configured to shut down when the output of the comparator is high. The input in the CWG module is received from the NCO module, and the output is CWG1A. Thus, the main purpose of the CWG in this implementation is for auto-shutdown. The NCO module is running in Pulse Frequency mode. FIGURE 2: EFFICIENCY Bytes Measured at 2A Available Code Size 572 Words Available RAM Size 33 Bytes 2013 Microchip Technology Inc. 6: /223 &21752/ DS90003097A-page 1 TB3097 FUNCTIONAL DESCRIPTION APPLICATIONS The power supply uses a PIC12F1501 for implementing the proportional loop control. The algorithm for adjusting the output voltage and current is done in software, where the information about the output voltage and the load current (10-bit ADC channels) is used for controlling the closed loop. The circuit uses the NCO peripheral to generate a fixed ontime, variable frequency control signal for a buck converter. The “on time” is fixed to 2 µs and the duty cycle is limited in firmware to about 90% at 450 kHz. The NCO frequency step is about 16 Hz using a 16 MHz clock source. The power supply uses digital proportional control for regulating the output but the update rate is only 4 kHz. This makes it slow to respond to sudden load variations and input voltage changes. For this reason, output voltage is clamped by a very fast comparator-based, overvoltage protection mechanism. The control loop modifies the NCO frequency (essentially modifying duty cycle – fixed on-time, different period) to regulate the converter output. The decisions are made based on the ADC values read from the output voltage resistor divider and the output current shunt. Since only one control loop is running for both voltage and current regulation, a special function decides on each update which of the two needs to be regulated. In normal operating conditions, the control loop tries to match the output voltage to the reference. If the output current goes over the limit, the loop tries to match the output current to the maximum-allowed value by reducing the output voltage. A special counter prevents erratic behavior when transitioning from one mode to the other. On the other hand, a digital power supply has huge advantages. Output current and voltage can be modified during run-time by the application and complex algorithms like multi-step battery charging algorithms can be easily implemented. The peripheral configuration allows 15 bits of duty-cycle resolution for the transistor control signal, which enables very fine control of output voltage and current. The main application areas of this power supply are battery chargers, LED drivers, thermoelectric cell drivers, programmable bias generators, and so on. With an accurate voltage reference, the circuit is more than adequate for charging sensitive Li-Ion batteries. FIGURE 3: DUTY CYCLE vs. FREQUENCY USING FIXED ON-TIME (PF MODE) )UHT N+= 'XW\&\FOH )L[HGRQWLPH XV XV 3XOVH)UHTXHQF\0RGH 1&22XWSXW DS90003097A-page 2 2013 Microchip Technology Inc. TB3097 MCU PERIPHERAL CONFIGURATION DRAWING The application only needs two analog channels (one for output voltage and the other one for current sense) for the loop control algorithm. The comparator input for the overvoltage protection is configured on the same pin as the output voltage ADC input. At the run-time, the two programming pins (PGC and PGD) can be used for other purposes, if required. The configuration of the PIC12F1501 pins for implementing the proportional control loop application is listed in the following table (see Table 2): TABLE 2: Pin No. PIC12F1501 PERIPHERAL CONFIGURATION Name Function 1 VDD Supply voltage 2 RA5 NCO channel used for providing PWM signal for the proportional control loop 3 AN3 Analog input used for reading output voltage (VOUT) used in the feedback loop 4 RA3 Digital input – this input will provide start/stop functionality for the control loop algorithm 5 AN2 Analog input used for reading the current sense 6 PGC Programming clock 7 PGD Program data 8 VSS Ground reference 2013 Microchip Technology Inc. DS90003097A-page 3 TB3097 PERIPHERAL CONFIGURATION DS90003097A-page 4 FIGURE 4: 2013 Microchip Technology Inc. TB3097 SCOPE PLOTS OF KEY PARAMETERS POWERING THE DEMONSTRATION HW The following table (see Table 3) contains some characteristics of the charger obtained with an input of 14.3V and an output of 5V. TABLE 3: Sw. Frequency (kHz)(3) The output voltage (VOUT = 5V) is obtained at J2 connector and J4 connector (USB connector). WHERE TO FIND THE CODE AND SCHEMATICS POWER SUPPLY CHARACTERISTICS Output Current (mA)(2) The board is powered from the input J1 connector with an input voltage between 8V and 16V. Efficiency (%)(1) 0 0.1-0.5 50 11 78.0 100 22 81.3 250 56 84.6 500 108 86.4 1000 184 87.6 1200 184 88.1 1400 184 88.5 1600 184 88.8 1800 184 88.8 2000 180 88.5 The code can be downloaded from www.microchip.com. For a better understanding of the implemented algorithm, please see the following flowchart diagram (Figure 6): FIGURE 6: ALGORITHM FLOWCHART Read ADC channels Control Loop VOut > 5V Current regulation Note 1: Efficiency is calculated including power loss on current shunt. Voltage regulation ISense > 2A 2: At 2A, the converter is running in current limiting mode. 3: PWM is fixed on-time (2 µs) with variable frequency. FIGURE 5: OPERATING FREQUENCY vs. LOAD CURRENT 350 kHz GLOSSARY TABLE 4: ACRONYMS 300 kHz PWM 250 kHz ADC Analog-to-Digital Converter 200 kHz DAC Digital-to-Analog Converter NCO Numerically Controlled Oscillator PID Proportional Integral Derivative 14.3V Input 12V Input 150 kHz 9V Input 100 kHz CWG 50 kHz Pulse-Width Modulation Complementary Waveform Generator 0 kHz 50 mA 100 mA 250 mA 500 mA 750 mA 1000 1200 1400 1600 1800 2000 mA mA mA mA mA mA Figure 5 shows the converter operating frequency for 14.3V, 12V and 9V input at different output currents. Once the inductor current becomes continuous, the frequency/duty cycle changes very little, only to compensate for component power losses. 2013 Microchip Technology Inc. DS90003097A-page 5 TB3097 APPENDIX A: FIGURE 7: SCHEMATIC Vbat P2 C16 2 1 C19 10uF: 16V C21 10uF: 16V 1uF: 16V J1-Vbat Vout Vout P1 J2-Out 1 2 GND NTMS4177PR2G 8 7 6 5 L1 2 2 VDD 3 IN 4 GND OUT NC 15K 2 3 Murata 39S103C Q3 PWM 15K J4 1 1 PMEG3030EP 4 1 2 3 R1 C4 C5 C6 C9 C10 10uF 10uF 10uF 470uF: 10V 470uF: 10V 4 R8 R6 20K 10K VBUS DD+ GND 0 Vbat R7 5 GND C7 10nF 1 GND MCP1416 ISENSE GND GND R2 1K R5 0R050: 1W Vout J5 6 GND PGC 5 4 3 2 1 +5V PGD 1 MCLR +5V S1 C17 100nF PWM Vout_Div MCLR SW-DPST GND U15 GND 2 3 4 Vdd RA5 RA0/AN0/PGD RA1/AN1/PGC RA4/AN3 MCLR/VPP/RA3 RA2/AN2 Vss 7 6 5 PGC ISENSE 8 Vout_Div C8 1nF R4 10K PIC12F1501 GND Vbat R3 10K PGD GND +5V MCP1703 VOUT 2 GND C2 1uF VIN C1 1uF 1 3 GND DS90003097A-page 6 2013 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. 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All other trademarks mentioned herein are property of their respective companies. © 2013, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. ISBN: 9781620773741 QUALITY MANAGEMENT SYSTEM CERTIFIED BY DNV == ISO/TS 16949 == 2013 Microchip Technology Inc. Microchip received ISO/TS-16949:2009 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified. 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