TB084 Contrast Control Circuits for the PIC16F91X Author: SYSTEM REQUIREMENTS Joseph Julicher Microchip Technology Inc. LCD glass requires a specific RMS voltage applied between a segment and a common to create the required polarization shift. Different pieces of glass are designed for different voltages with the most common type of glass being 5.0V glass. Lower voltage glass is available, though as the voltage gets below 3V, it is more difficult to create large panels with acceptable contrast. INTRODUCTION The PIC16F91X is a low-cost flash-based PICmicro® microcontroller that has the ability to drive LCD glass. This device also has a wide operational voltage range and low power, making it an ideal candidate for many battery powered applications. To get the most use from the battery, the device must operate over a wide voltage range. The wide voltage range can cause contrast problems with the LCD glass, so a solution is needed that will maintain constant LCD voltage and be very cost effective. In this technical brief we will discuss three different contrast control systems; external voltage regulator, comparator buck regulator and comparator buck/boost regulator. FIGURE 1: The PIC16F91X has a series of analog switches to connect the segment and common pins to 4 voltages. Three of the voltages are applied to the VLCD<3:0> pins. The fourth voltage is internally connected to VSS. To operate the LCD module, correct voltages must be applied to these pins. The voltages applied depend upon the type of LCD drive and the type of glass. VOLTAGE APPLICATION Static Bias VLCD 3 To VLCD 2 LCD VLCD 1 Driver VLCD 0(1) LCD Bias 3 LCD Bias 2 LCD Bias 1 1/2 Bias 1/3 Bias VLCD 0 VSS VSS VSS VLCD 1 — VDD VDD VLCD 2 — VDD VDD VLCD 3 VDD VDD VDD Connections for External R-ladder VDD* Status Bias VDD* 10 k Ω * VDD* 10 k Ω * * Note 1: 10 k Ω * 10 k Ω * 10 k Ω * 1/2 Bias VSS 1/3 Bias VSS These values are provided for design guidance only and should be optimized for the application by the designer. Internal connection. © 2004 Microchip Technology Inc. DS91084A-page 1 TB084 EXTERNAL VOLTAGE REGULATOR Circuit Description A simple solution for contrast control is to use an external voltage regulator to generate the required glass voltage. If low voltage glass is used, the voltage regulator simply steps the power supply down to the required glass voltage. The basic format for the external regulator voltage generator is as shown in Figure 2. The regulator must be chosen to satisfy the requirements of glass voltage and power usage. FIGURE 2: EXTERNAL VOLTAGE REGULATOR GENERATOR PIC16F91X VDD VLCD3 Regulator R1 LCD Glass VLCD2 R2 VLCD1 R3 COMPARATOR BUCK REGULATOR Circuit Description The external comparator solution is a simple one, but not without its issues. The first issue is that most battery applications are cost sensitive, so the addition of a regulator IC may not be practical. Another one is the regulator’s dropout voltage. Linear regulators require a higher input voltage than their output. This difference may be very small, but it will limit the useful battery life of a product. The last issue is power consumption. The linear regulator will burn a small amount of current to provide its function. These problems can be solved with a PFM switched capacitor regulator, if it can use features already present in the microcontroller. Fortunately, the comparator in the PIC16F91X devices is designed to accommodate this function. The circuit in Figure 4 uses the internal comparator to create a PFM buck power supply. The voltage between R4 and R5 is compared to the internal 0.6V reference. The comparator output is filtered by the capacitors and R6. R1-R3 divide the output voltage to an appropriate value for the VLCD inputs. When Comparator mode 5 (CMCON<2:0> = 101) is used, it is possible to use an internal 0.6V band gap as the voltage reference. With this mode, creating a PFM buck regulator is simple. DS91084A-page 2 Software Description No software is required for running this system, but the comparator must be initialized into the correct mode. The following code can be used to initialize this circuit. EXAMPLE 1: banksel MOVLW MOVWF INITIALIZATION CODE CMCON0 0x0D CMCON0 ;enter the contrast ;control mode © 2004 Microchip Technology Inc. TB084 FIGURE 3: COMPARATOR MODE 5 DETAIL One Independent Comparator with Reference Option CM<2:0> = 101 RA0/AN0/C1-/SEG12 RA3/AN3/C1+/VREF+/SEG15 D VIN- D VIN+ RA2/AN2/C2+/VREF-/COM2 Off (Read as ‘0’) VIN- A RA1/AN1/C2-/SEG7 C1 A CIS = 0 A VIN+ C2 C2OUT CIS = 1 RA5 Internal 0.6V reference Legend: A = Analog Input, port reads zeros always CIS = Comparator Input Switch (CMCON0<3>) D = Digital Input FIGURE 4: COMPARATOR BUCK REGULATOR VDD RA1 RA5 R6 VLCD3 0.6V R1 LCD Glass VLCD2 R4 R2 VLCD1 R5 © 2004 Microchip Technology Inc. R3 PIC16F91X DS91084A-page 3 TB084 COMPARATOR BUCK/BOOST REGULATOR Circuit Description There are two basic methods to boost a voltage. Fortunately, it is easy to use the on-board comparators to regulate both methods of boosting voltage. The basic architecture of the circuit is shown in Figure 5. Comparator C1 is used to build an oscillator that drives the boost circuit. Comparator C2 is used to regulate the activity of C1 by shutting off the oscillator when the voltage exceeds the limit programmed by R6 and R7. R5 and D3 create a simple voltage reference for the entire system. C2 is a bulk capacitor that is maintained at the desired voltage level by the entire regulator. D1 allows the rise and fall time of the oscillator to be tuned independently by adjusting R2 and R4. A comparator buck circuit is beneficial for many applications, but if the VDD is too low for the glass, a boost converter may be a better option. Fortunately, it is possible to use the internal comparators to build a simple buck/boost. There are many trade-offs with a boost circuit, but the primary issue is the voltage limits of the I/O pins. The data sheet specifies that the input range of the VLCD pins is VSS to VDD. Therefore, if the voltage to the LCD is to be boosted, then that boosted voltage must also be applied to VDD. With an increase in VDD comes an increase in current consumption so software must be used to control when the boosted voltage is to be used. Also, the device must be powered before the boost is active so the minimum battery voltage is still 2.0V. FIGURE 5: . COMPARATOR BUCK/BOOST REGULATOR D1 R1 R2 Boost Circuit R3 D2 (Figure 6 or Figure 7) R4 C1 Q1 C1 R5 VDD R6 D3 C2 C2 R7 PIC16F91X DS91084A-page 4 © 2004 Microchip Technology Inc. TB084 There are two boost circuits that can be easily inserted into the boost block as shown in Figure 5. The first circuit is a simple switched capacitor doubler. The capacitor and diode work with the circuit as drawn in Figure 6 to produce a doubler. This circuit works best with a 50% duty cycle. Therefore, a few components can be removed from the first schematic. To meet the start-up requirement, the minimum VBAT is 2 diode drops over 2.0V. Assuming schottky diodes, this circuit will convert 2.6V to any voltage from 2.6-5.2V. If a slightly lower minimum voltage is required, then an inductor-based switcher is required (Figure 7). To adjust the amount of energy transferred to the output capacitor, the duty cycle of Q2 must be tuned. This is done by adjusting the values of R2 and R4 in the first schematic. FIGURE 6: Software Description There is no special software required for this application. Simply initialize the comparators by writing 0x06 to the CMCON0 register. CONCLUSION Managing the contrast of an LCD in an environment with a wide range of voltages can be challenging. The designs presented in this technical brief provide a good basis to start designing low-cost, low-voltage systems. The PIC16F91X device provides all the features necessary to ensure excellent low-power performance. SIMPLE SWITCHED CAPACITOR DOUBLER VBAT FIGURE 7: INDUCTOR BASED SWITCHER VBAT L1 Q2 R8 © 2004 Microchip Technology Inc. DS91084A-page 5 TB084 NOTES: DS91084A-page 6 © 2004 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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Analog-for-the-Digital Age, Application Maestro, dsPICDEM, dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, MPASM, MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net, PICLAB, PICtail, PowerCal, PowerInfo, PowerMate, PowerTool, rfLAB, rfPICDEM, Select Mode, Smart Serial, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2004, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received ISO/TS-16949:2002 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona and Mountain View, California in October 2003. 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