Contrast Control Circuits for the PIC16F91X

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. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED,
WRITTEN OR ORAL, STATUTORY OR OTHERWISE,
RELATED TO THE INFORMATION, INCLUDING BUT NOT
LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE,
MERCHANTABILITY OR FITNESS FOR PURPOSE.
Microchip disclaims all liability arising from this information and
its use. Use of Microchip’s products as critical components in
life support systems is not authorized except with express
written approval by Microchip. No licenses are conveyed,
implicitly or otherwise, under any Microchip intellectual property
rights.
Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
dsPIC, KEELOQ, microID, MPLAB, PIC, PICmicro, PICSTART,
PRO MATE, PowerSmart, rfPIC, and SmartShunt are
registered trademarks of Microchip Technology Incorporated
in the U.S.A. and other countries.
AmpLab, FilterLab, Migratable Memory, MXDEV, MXLAB,
PICMASTER, SEEVAL, SmartSensor and The Embedded
Control Solutions Company are registered trademarks of
Microchip Technology Incorporated in the U.S.A.
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. The Company’s quality system processes and
procedures are for its PICmicro® 8-bit MCUs, 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.
© 2004 Microchip Technology Inc.
DS91084A-page 7
WORLDWIDE SALES AND SERVICE
AMERICAS
ASIA/PACIFIC
ASIA/PACIFIC
EUROPE
Corporate Office
2355 West Chandler Blvd.
Chandler, AZ 85224-6199
Tel: 480-792-7200
Fax: 480-792-7277
Technical Support:
http://support.microchip.com
Web Address:
www.microchip.com
Australia - Sydney
Tel: 61-2-9868-6733
Fax: 61-2-9868-6755
India - Bangalore
Tel: 91-80-2229-0061
Fax: 91-80-2229-0062
China - Beijing
Tel: 86-10-8528-2100
Fax: 86-10-8528-2104
India - New Delhi
Tel: 91-11-5160-8631
Fax: 91-11-5160-8632
Austria - Weis
Tel: 43-7242-2244-399
Fax: 43-7242-2244-393
Denmark - Ballerup
Tel: 45-4450-2828
Fax: 45-4485-2829
China - Chengdu
Tel: 86-28-8676-6200
Fax: 86-28-8676-6599
Japan - Kanagawa
Tel: 81-45-471- 6166
Fax: 81-45-471-6122
France - Massy
Tel: 33-1-69-53-63-20
Fax: 33-1-69-30-90-79
China - Fuzhou
Tel: 86-591-8750-3506
Fax: 86-591-8750-3521
Korea - Seoul
Tel: 82-2-554-7200
Fax: 82-2-558-5932 or
82-2-558-5934
Germany - Ismaning
Tel: 49-89-627-144-0
Fax: 49-89-627-144-44
Atlanta
Alpharetta, GA
Tel: 770-640-0034
Fax: 770-640-0307
Boston
Westford, MA
Tel: 978-692-3848
Fax: 978-692-3821
Chicago
Itasca, IL
Tel: 630-285-0071
Fax: 630-285-0075
Dallas
Addison, TX
Tel: 972-818-7423
Fax: 972-818-2924
Detroit
Farmington Hills, MI
Tel: 248-538-2250
Fax: 248-538-2260
Kokomo
Kokomo, IN
Tel: 765-864-8360
Fax: 765-864-8387
China - Hong Kong SAR
Tel: 852-2401-1200
Fax: 852-2401-3431
China - Shanghai
Tel: 86-21-5407-5533
Fax: 86-21-5407-5066
China - Shenyang
Tel: 86-24-2334-2829
Fax: 86-24-2334-2393
China - Shenzhen
Tel: 86-755-8203-2660
Fax: 86-755-8203-1760
China - Shunde
Tel: 86-757-2839-5507
Fax: 86-757-2839-5571
Singapore
Tel: 65-6334-8870
Fax: 65-6334-8850
Taiwan - Kaohsiung
Tel: 886-7-536-4818
Fax: 886-7-536-4803
Taiwan - Taipei
Tel: 886-2-2500-6610
Fax: 886-2-2508-0102
Italy - Milan
Tel: 39-0331-742611
Fax: 39-0331-466781
Netherlands - Drunen
Tel: 31-416-690399
Fax: 31-416-690340
England - Berkshire
Tel: 44-118-921-5869
Fax: 44-118-921-5820
Taiwan - Hsinchu
Tel: 886-3-572-9526
Fax: 886-3-572-6459
China - Qingdao
Tel: 86-532-502-7355
Fax: 86-532-502-7205
Los Angeles
Mission Viejo, CA
Tel: 949-462-9523
Fax: 949-462-9608
San Jose
Mountain View, CA
Tel: 650-215-1444
Fax: 650-961-0286
Toronto
Mississauga, Ontario,
Canada
Tel: 905-673-0699
Fax: 905-673-6509
10/20/04
DS91084A-page 8
© 2004 Microchip Technology Inc.
Similar pages