APPNOTE36 [ETC]
Electroluminescent Display Drivers ; 电致发光显示器驱动程序\n
| 型号： | APPNOTE36 |
| 厂家： | 
ETC |
| 描述： | Electroluminescent Display Drivers
|
| 文件： | 总8页 (文件大小：98K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Application Note 36
MIC4826/7 Electroluminescent Display Drivers
by William Mai and Andrew Cowell
This application note covers the MIC4826/7 Electrolumines-
cent (EL) lamp drivers and designing with EL lamps.
With most phosphors, the spectrum of emitted light will tend
to shift towards blue with an increase in excitation frequency.
Color can be controlled by selecting the phosphor type, by
adding fluorescent dyes in the phosphor layer, by using a
color filter over the lamp, or a combination of these pro-
cesses.ELlampbrightnessincreasesapproximatelywiththe
square of applied voltage. Increasing frequency, in addition
to affecting hue, will also increase EL lamp brightness, but
with a more linear relationship. Many EL lamp manufacturers
provide performance characteristics informing designers on
the relationships of frequency, voltage, and EL lamp bright-
ness for their EL lamps.
Electroluminescent Displays - The Basics
The design of an EL lamp circuit begins with the selection of
a lamp. A typical lamp will exhibit a capacitance on the order
of 2nF to 3.5nF per square inch. When a high voltage AC
signal is applied across the electrodes of an EL lamp, an
electric field is generated across the plates of the lamp. This
electric field excites the phosphor atoms to a higher energy
state. When the electric field is removed, the atoms fall back
to a lower energy state, emitting photons as visible light. The
wavelength of the emitted light is determined by the type of
phosphor used and the frequency of the excitation voltage.
Figure 1 shows a typical bridge configuration that is applied
to the EL electrodes to generate the AC signal. Typical AC
Increased voltage and/or frequency, however, adversely
affect lamp life. Higher frequencies generally decrease lamp
life moreso than increased voltages. EL lamps, unlike other
types of light sources, do not abruptly fail. Instead, their
brightness gradually decreases through use. Due to the
nature of the devices that EL lamps are used in, this is
normally not a concern.
voltages applied to the EL lamp are 50V to 250 V
, with
PK-PK
a frequency of 50Hz to 1KHz.
L1
220
VIN
D1
H
VDD
1
The MIC4826 and MIC4827 allow the user to select the EL
frequency and voltage driving the lamp to give the user
maximum flexibility during the design process.
5
CIN
RSW
COUT
SW
2
Switch
Oscillator
6
RSW
CS
Transparent Front Protective Cover
Transparent Front Electrode
Phosphor
Q1
Q2
8
REL
VA
V
Dielectric
EL
Oscillator
EL LAMP
VREF
Rear Electrode
Rear Protective Cover
Q3
Q4
7
VB
3
Figure 3. Typical EL Lamp Construction
How the MIC4826/7 Drives the EL Display
REL
4
GND
To generate the high voltages needed for driving EL lamps,
MICREL drivers employ switch-mode converters using a
boost converter to generate the high voltages needed. Fol-
lowing the boost converter is an H-bridge driver, this applies
the peak-to-peak voltage across the EL lamp at a user
Figure 1. MIC4826/7 Block Diagram
The basic AC signal applied to the EL lamp across VA and
VB, the two electrode pins, can be seen in Figure 2.
selectable frequency. The MIC4826 provides 160 V while
PP
the MIC4827 provides 180V for bigger EL lamps. Figure 1
PP
shows the internal block diagram of the MIC4826 and
MIC4827. The CS pin is the high voltage output of the boost
converter, which is half the peak-to-peak voltage across the
EL lamp. The second stage is the H-bridge circuit that
switches the boost voltage across the EL lamp. Both the
switching frequency of the boost converter and the switching
frequency of the EL lamp can be adjusted independently.
VIN = 3.0V
L = 220µH
C
OUT = 0.01µF
Lamp = 2in2
RSW = 332k
REL = 3.32M
TIME (2ms/div)
Figure 2. Typical AC Signal Applied to EL lamp
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
September 2001
1
MIC4826/4827
Application Note 36
Micrel
EL Frequency
MIC4826/7 Basic Operation
The EL lamp frequency is controlled via an external resistor
connectedbetweenR pinandV pinofthedevice. Asthe
the resistor value decreases the lamp frequency increases.
The EL frequency range is 60Hz to 1000Hz, with an accuracy
of ±20%. By using the below equation and a known value
resistor, the EL frequency can be determined.
The MIC4826 is a high voltage EL driver with an AC output
voltage of 160V peak-to-peak. The MIC4827 is a higher
voltage EL driver with an AC output voltage of 180V peak-to-
EL
DD
2
peak. Both parts are capable of driving EL lamps up to 6 in
(typically). Input supply current for the MIC4826/7 are typi-
cally 21µA. The MIC4826 and MIC4827 have a shutdown
current of 100nA. Both high voltage EL drivers have two
internal oscillators to control the switching MOSFET and the
H-bridge driver. The internal oscillators’ frequency can be
individually programmed through the external resistors to
maximize the efficiency and the brightness of the lamps.
360
fEL(Hz) =
REL(MΩ)
A typical EL frequency for a portable device is 100 to 400Hz,
depending on display size and type.
Inductor Selection
Referring to Figure 1, initially power is applied to V . The
DD
internal feedback voltage is less than the reference voltage
causing the internal comparator to go low which enables the
switchingMOSFET’soscillator.WhentheswitchingMOSFET
turns on, current flows through the inductor and into the
switch. The switching MOSFET will typically turn on for 90%
of the switching frequency. During the on time, energy is
storedintheinductor. WhentheswitchingMOSFETturnsoff,
current flowing into the inductor forces the voltage across the
inductor to reverse polarity. The voltage across the inductor
risesuntiltheexternaldiodeconductsandclampsthevoltage
In general, smaller value inductors, which can handle more
current, are more suitable to drive larger size lamps. As the
inductor value decreases, the switching frequency (con-
trolled by R ) should be increased to avoid inductor satura-
SW
tion, or the input voltage should be increased. Typically,
inductor values ranging from 220µH to 560µH can be used.
Murata offers the LQH3C series up to 560µH and LQH4C
series up to 470µH, with low DC resistance. A 220µH Murata
(LQH4C221K04)inductorisrecommendedfordrivingalamp
size of 3 square inches. It has a maximum DC resistance of
4.0Ω.
at V
+V . The energy in the inductor is then discharged
OUT
D1
intotheC
capacitor.Theinternalcomparatorcontinuesto
OUT
Diode
turn the switching MOSFET on and off until the internal
feedback voltage is above the reference voltage. Once the
internal feedback voltage is above the reference voltage, the
internal comparator turns off the switching MOSFET’s oscil-
lator.
The application circuits specify the 1N4148 or equivalent. It
has a forward current of 100mA and a typical forward voltage
of 930mV. For applications that are not cost driven, a fast
switching diode with lower forward voltage and higher re-
verse voltage can be used to enhance the efficiency.
When the EL oscillator is enabled, VA and VB switch in
opposite states to achieve a 160V peak-to-peak AC output
signal for the MIC4826 and 180V peak-to-peak for the
Output Capacitor
Low ESR capacitors should be used at the regulated boost
output (CS pin) of the MIC4826/7 to minimize the switching
output ripple voltage. Selection of the capacitor value will
dependuponthepeakinductorcurrent,inductorsize,andthe
load. MuRata offers the GRM40 series with up to 0.015.µF at
100V, with a X7R temperature coefficient in 0805 surface-
mount package. Typically, values ranging from 0.01µF to
0.1µF at 100V can be used for the regulated boost output
capacitor.
MIC4827. The external resistor that connects to the R pin
EL
determines the EL frequency.
Switching Frequency
The switching frequency of the converter is controlled via an
external resistor between R pin and V pin of the device.
SW
DD
The switching frequency increases as the resistor value
decreases.Theswitchingfrequencyrangeis8kHzto200kHz,
with an accuracy of ±20%. By using the below equation and
a known value resistor, the switching frequency can be
determined.
Remote Enable
Remote enable is implemented by connecting R
and R
EL
SW
to a signal that swings between ground and V . When the
remote enable is at ground, the power conversion and lamp
drive oscillators are halted and the driver becomes disabled.
DD
36
fSW(kHz) =
RSW(MΩ)
When the remote enable signal is at V , the oscillators
DD
There is a trade off in inductor size versus system efficiency.
Normally EL displays are in portable equipment and size is of
the utmost importance. A typical switching frequency recom-
mended is 108kHz, giving a recommended typical inductor
value of 220µH. See the “Pre-designed Circuits” section for
complete information.
function normally and the driver is enabled. Since R
and
SW
R
are typically high resistances, loading of the remote
EL
enable signal is minimal. However, to avoid interactions
betweenthepowerconversionandlampdriveoscillators, the
remote enable signal should be from a CMOS output of less
than 20KΩ.
MIC4826/4827
2
September 2001
Application Note 36
Micrel
Split Supplies
challenge. Manyapplicationsthatoperatefroma1.5Vsupply
employ a voltage booster to provide a nominal 3V. Although
this 3V, low current supply usually cannot deliver enough
power to drive an EL lamp, a split-supply driver circuit
circumvents this obstacle. See Circuit 3 in the “Pre-designed
Circuits” section for full information
Some applications require a high lamp drive capability but
operate from a 1.5V source. The MIC4826/7 family provides
high lamp drive, but does not operate directly from a 1.5V
source. A technique using split supplies overcomes this
Pre-designed Circuits
L1
Li-Ion Battery
VIN
3.0V to 4.2V
D1
220µH
Murata
Vishay Telefunken
MCL4148
LQH4C221K04
COUT
0.01µF/100V
GRM40X7R103K
C2
10µF/6.3V
MIC4826
Murata
1
2
3
4
5
6
7
8
GRM42-6X5R106K6.3
VDD
RSW
REL
SW
CS
VB
VA
C1
0.22µF/10V
R1
R2
3.32M
322k
Murata
GRM39X7R224K10
GND
3in2 LAMP
VIN
IIN
VA–VB
FEL
Lamp Size
3.3V
20mA
160VPP
100Hz
3in2
TIME (2ms/div)
2
Circuit 1. EL Driver for PDA Application (3in Lamp)
September 2001
3
MIC4826/4827
Application Note 36
Micrel
L1
220 H
D1
Diodes
Murata
VIN
BAV20WS
LQH4C221K04
2.5V to 5.5V
COUT
C2
0.1 F/100V
MIC4826
10 F/6.3V
Murata
GRM42-2X7R104K100
1
2
3
4
5
6
7
8
GRM42-6X5R106K6.3
VDD
RSW
REL
SW
CS
VB
VA
R1
R2
3.32M
332k
GND
EL LAMP
LSI
X533-13
VIN
IIN
14mA
VA–VB
FEL
100Hz
Lamp Size
2in2
3.3V
160VPP
TIME (2ms/div)
2
Circuit 2. EL Driver for 2in Lamp Using 1 Cell Li-Ion Battery
or 5V Fixed Input Voltage
MIC4826/4827
4
September 2001
Application Note 36
Micrel
L1
220µH
D1
VIN
1.5V
Murata
Diodes
LQH4C221K04
BAV20WS
C2
10µF/6.3V
COUT
MIC4826
Murata
VDD
0.01µF/100V
GRM40X7R103K100
GRM42-6X5R106K6.3
1.8V to 5.5V
1
2
3
4
5
6
7
8
VDD
RSW
REL
SW
CS
VB
VA
R1
C1
442k
0.01µF/50V
Murata
R2
3.32M
GRM40-X7R103K50
GND
EL LAMP
VIN
IIN
VDD
IDD
36µA
VA–VB
FEL
100Hz
Lamp Size
1.5V
22mA
3.0V
160VPP
1.6in2
TIME (2ms/div)
Circuit 3. Split Supply Applications
September 2001
5
MIC4826/4827
Application Note 36
Micrel
L1
220µH
D1
Diodes
Murata
VIN
BAV20WS
LQH4C221K04
1.8V to 3.3V
COUT
0.1µF/100V
GRM42-2X7R104K100
C2
MIC4827
10µF/6.3V
Murata
GRM42-6X5R106K6.3
1
2
3
4
5
6
7
8
VDD
RSW
REL
SW
R1
1M
R2
3.32M
CS
VB
VA
GND
EL LAMP
METROMARK
12607-N
VIN
IIN
VA–VB
FEL
104Hz
Lamp Size
5.3in2
3.0V
31mA
180VPP
TIME (2ms/div)
Circuit 4. EL Driver for Remote Control Lamp
Using 2 Cell Alkaline Batteries
MIC4826/4827
6
September 2001
Application Note 36
Micrel
L1
220µH
D1
Diodes
Murata
VIN
BAV20WS
LQH4C221K04
2.4V to 5.5V
COUT
C2
0.033µF/100V
GRM42-6X7R333K100
MIC4827
10µF/6.3V
Murata
GRM42-6X5R106K6.3
1
2
3
4
5
6
7
8
VDD
RSW
REL
SW
R1
R2
3.32M
CS
VB
VA
332k
GND
EL LAMP
LSI
X533-13
VIN
IIN
VA–VB
FEL
104Hz
Lamp Size
3.3V
18mA
180VPP
2in2
TIME (2ms/div)
2
Circuit 5. EL Driver for 2in Lamp with 180V Voltage
PP
September 2001
7
MIC4826/4827
Application Note 36
Micrel
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 2001 Micrel Incorporated
MIC4826/4827
8
September 2001
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