TB1098

TB1098
Low-Power Techniques for LCD Applications
Author:
Brian Claveria
Microchip Technology Inc.
INTRODUCTION
Low power is often a requirement in LCD applications.
The low-power features of PIC® microcontrollers and
the ability to drive an LCD directly can help in meeting
this requirement. While the LCD Driver module makes
driving LCDs very easy, there are important factors to
take into account in configuring the module so that
application can be optimized for low power.
Two specific factors that affect power consumption with
respect to the LCD Driver module are the resistor ladder, which generates the bias voltages for the LCD
waveforms, and the clock source configuration. By
intelligently selecting a resistor ladder size and clock
source best suited for the application, an LCD application using PIC microcontrollers can be optimized for
low power.
Resistor Ladder Sizing
Voltage is provided to the LCD Driver module in one of
three configurations (Static, 1/2, and 1/3 biasing). In
1/2 and 1/3 biasing, a resistor ladder is used to provide
the bias levels for the LCD Driver. In Static mode, VDD
must be provided and no resistor ladder is needed. The
goal is to maximize the resistor ladder values, as the
resistor ladder will draw current at all times.
Resistor value is constrained by two factors: the refresh
rate of the LCD and the size of the LCD. The LCD module is essentially an analog multiplexer that connects
the LCD bias voltages to various segment and common
pins that connect to LCD pixels.
Each pixel/segment of the LCD display can be
modeled as a capacitor. Including both the internal
common resistance and switch circuit multiplexing
resistance, the circuit can be simplified to a Thévenin
equivalent circuit shown below. VTH is equal to either
2/3 VDD or 1/3 VDD for cases where the Thévenin
resistance in non-zero.
FIGURE 2:
SIMPLIFIED LCD CIRCUIT
RTH = (2R*R)/(2R+R) RTOTAL = RTH + RSW + RCOM
RTH = 2R2/3R
RTH = 2R/3
RSW = 4.7K
VTH
RCOM = 0.4K
Note:
RSW and RCOM are estimates.
Using the simplified circuit, the step response of the
voltage across each pixel can be described by the
following equation:
EQUATION 1:
V PIXEL = V TH ( 1 – e
VPIXEL/VTH = 1 - e-t/RC
ln (.02) = -t/RC
®
t = ~ 4 RC
LCD PIC Microcontroller
VLCD3
VLCD2
VLCD1
VSS
VPIXEL
VTH
0.98 VTH
SEGn
R
R
)
EQUATION 2:
LCD CIRCUIT
R
– t ⁄ ( R T OT A L C P I X EL )
Manipulation of this equation (see Equation 2) dictates
that it will take approximately 4 time constants (RC) for
the pixel voltage to reach 98% of VTH. Ideally, the resistor ladder should be sized such that pixel voltage
reaches at least 98% of VTH.
2% = e-t/RC
VDD
CPIXEL
98% = 1 -e-t/RC
Shown below is an LCD circuit.
FIGURE 1:
+
-
CPIXEL (n x m)
0
t = 4 RC
t
COMm
VLCD0
© 2007 Microchip Technology Inc.
DS91098A-page 1
TB1098
This analysis says that the resistor ladder can be made
larger if:
1.
2.
More time is allowed for the pixel to charge
(slower refresh rate), or
The capacitance of each pixel is smaller
(smaller size display).
Refresh rate should be as slow as possible in order to
maximize resistor ladder size. These settings are configured in <LP3:LP0> bits of the LCDPS register. Keep
in mind however, that human visual perception can
detect a frequency of 30 Hz or less.
The capacitance of each LCD pixel depends on its size.
A larger display will typically have more capacitance
than a smaller display. Therefore, a larger display will
require smaller resistors.
While it is possible to calculate the maximum resistor
value analytically (see LCD Tips n’ Tricks (DS41261) at
www.microchip.com/lcd), these calculations should be
used as a guide rather than a solution. Ultimately, it is
a reiterative bench testing exercise to determine
whether or not resistor size is optimal.
If the resistor value is too large, some pixels will be
darker than others and the contrast will not be constant.
Shown below are two screenshots of LCD segment
waveforms, with a 10K resistor ladder (Figure 3) and
220K resistor ladder (Figure 4).
FIGURE 3:
10K RESISTOR LADDER
WAVEFORM
In the next figure, Figure 4, the resistor value selected
for the resistor ladder was 220 kΩ. The output waveforms show a much more obvious RC charge and discharge that does not meet the 4 time constant
requirement. Nonetheless, the LCD display looked
exactly the same as the LCD display with a 10 kΩ
resistor ladder.
The lessons from this are that there is not an analytic
solution to calculating the ideal resistor values for the
voltage divider. Bench testing by stepping through various resistor values and visual inspecting the LCD
output is a better method.
In general, the larger the LCD display, the smaller the
resistors must be. In addition, the higher the refresh
rate the smaller the resistors. Consequently, a lower
refresh rate will allow for larger resistor values. These
are factors to be taken to account when selecting an
optimum resistor size.
LCD CLOCK SOURCE SELECTION
LCD clock source selection depends upon application.
Because the LCD glass can be modeled as a capacitive load, a higher frequency drive waveform will
require more current. Therefore, a lower frequency
clock source will consume less current.
Selecting the proper clock source can affect power
consumption in an application. This is because specific
low-power LCD modes can only be utilized by certain
clock sources. Therefore, each clock source has a
number of advantages and disadvantages.
Below, are some pros and cons of each available clock
source:
1.
FOSC
Pros:
- No External Circuitry
- Dual Purpose (also used for instruction
execution)
FIGURE 4:
Cons:
220K RESISTOR LADDER
WAVEFORM
2.
- Cannot drive LCD in Sleep Mode
- Typically draws more current than the
LFINTOSC
LFINTOSC
Pros:
-
The segment output waveforms from the PIC
microcontroller in Figure 3 are sized based on the PIC
microcontroller 10 kΩ data sheet design guidance. The
LCD had good contrast.
DS91098A-page 2
No External Circuitry
Low frequency (31 kHz)
Can drive the LCD is Sleep Mode
Dual Purpose (also used for instruction
execution)
Cons:
- May consume more current than an
external crystal
© 2007 Microchip Technology Inc.
TB1098
3.
External Crystal
Pros:
- Can drive the LCD in Sleep Mode
- Dual Purpose (can be used for accurate
time keeping)
Cons:
- Requires external circuitry
CONCLUSION
Two techniques that can significantly affect power consumption are resistor ladder sizing and clock source
selection. A deterministic guide for selecting resistor
ladder size has been demonstrated as well as a practical approach to finding an optimal resistor for an LCD
application. Finally, the pros and cons of specific clock
source selections have been shown. By following these
techniques and taking these factors into account, low
power can be achieved for specific LCD applications.
REFERENCES
LCD PIC® MCU Tips ‘n Tricks, DS41261
AN658, “LCD Fundamentals
Microcontroller”, DS00658
using
PIC16C92X
TB084, “Contrast Control Circuits for the PIC16F91X”,
DS91084
DS41250, “PIC16F946/917/916/914/913 Data Sheet”,
AN1070, “Driving Liquid Crystal Displays with the
PIC16F913/914/916/917/946”, DS01070
© 2007 Microchip Technology Inc.
DS91098A-page 3
TB1098
NOTES:
DS91098A-page 4
© 2007 Microchip Technology Inc.
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DS91098A-page 5
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