View

P
A T E N T
P
LIN D O C #: 1592
E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
T
H E
I
N F I N I T E
P
O W E R
O F
I
P
N N O VA T I O N
R E L I M I N A R Y
DESCRIPTION
The LXM1592/93 series of floating output drive CCFL (Cold Cathode Fluorescent Lamp) Inverter Modules are specifically designed to drive large LCD displays
(11.3" and larger), which are used in notebook computers. These new inverters
were specifically designed to reduce the
leakage currents from the lamp to the
reflector or the metal frame of the panels. The floating output architecture of
these inverters also permits a much wider
dimming range when compared to nonfloating designs, and an additional 10%
efficiency improvement is realized.
Both the LXM1592 and LXM1593 are
fully customizable (electronically and mechanically) to specific customer requirements.
The modules convert unregulated DC
voltage from the system battery or AC
adapter directly to high-frequency, highvoltage sine waves required to ignite and
operate CCFL lamps. The module design is based on a proprietary Linfinity IC
that provides important new performance
advances.
Remarkable improvements in efficiency
and RF emissions result from these single
stage resonant inverters, featuring a patent
pending Current Synchronous, Zero Voltage Switching (CS-ZVS) topology. CSZVS produces nearly pure sine wave cur-
rents in the lamp, enabling maximum light
delivery, while reducing both conducted
and radiated noise. This topology simultaneously performs two tasks including
line voltage regulation and lamp dimming
through lamp current regulation. These
two functions are performed in a single
power stage made up of a pair of lowloss MOSFETs. The MOSFETs drive a low
current resonant circuit that feeds the
primary of a high voltage transformer with
a sinusoidal voltage.
Required L and C values in the resonant circuit are such that very low-loss
components can be used to obtain higher
electrical efficiency than is possible with
previous topologies.
Two module versions are available.
The half-bridge LXM1592 provides peak
efficiency when operated at input voltages above 7 volts. The LXM1593
achieves higher efficiency at input voltages above 4.5V with its full-bridge drive
circuit.
The modules are equipped with a
dimming input that permits full range
brightness control from an external
potentiometer, and a sleep input that
r educes module power to a few
microwatts in shut-down mode.
Each module features output open and
short circuit protection.
S
H E E T
■ FULLY FLOATING OUTPUT
■ 35% MORE LIGHT OUTPUT AT 2.5 WATTS
■ GREATER EFFICIENCY THAN GROUNDED
OUTPUT DESIGNS
■ 4.5V TO 30V INPUT VOLTAGE RANGES
■ VERSATILE BRIGHTNESS CONTROL INPUT
■ 3 MICROAMP SLEEP CURRENT
■ OUTPUT SHORT CIRCUIT PROTECTION AND
AUTOMATIC OVER VOLTAGE LIMITING
■ 8mm MAX HEIGHT, NARROW FOOTPRINTS
■ MINIMIZE THERMOMETER EFFECTS
■ MINIMIZE LAMP TO PANEL LEAKAGE
CURRENT
A P P L I C AT I O N S
■ 11.3" LCD PANELS AND LARGER
■ NOTEBOOK AND SUB-NOTEBOOK
COMPUTERS
■ PERSONAL DIGITAL ASSISTANTS
■ PORTABLE INSTRUMENTATION
■ AUTOMOTIVE DISPLAYS
■ DESKTOP DISPLAYS
■ AIRLINE ENTERTAINMENT CENTERS
BENEFITS
■ ULTRA-HIGH EFFICIENCY, LINE VOLTAGE
REGULATION AND SLEEP MODE EXTEND
COMPUTER BATTERY LIFE
■ COOL OPERATION PERMITS CLOSE
PROXIMITY TO LCD PANEL WITHOUT
DISPLAY DISTORTION
FLOATING OUTPUT A RCHITECTURE
■ SMOOTH, FULL-RANGE BRIGHTNESS
CONTROL GIVES YOUR PRODUCT A HIGH
QUALITY IMAGE
High Voltage
Transformer
CCFL
CS-ZVS
Inverter
A T A
K E Y F E AT U R E S
PRODUCT HIGHLIGHT
DC VIN
D
■ LOW EMI / RFI DESIGN MINIMIZES
SHIELDING REQUIREMENTS
■ NARROW, LOW-PROFILE STANDARD
MODULES FIT INTO MOST LCD
ENCLOSURES
■ SINGLE-SIDED PCB SAVES EXPENSIVE
HIGH VOLTAGE INSULATING TAPES
M O D U L E O R D E R I N F O R M AT I O N
HALF-BRIDGE DRIVE
FULL-BRIDGE DRIVE
LXM1592-xxxxx-zz
LXM1593-xxxxx-zz
See instructions inside for completing module part number.
F O R F U R T H E R I N F O R M AT I O N C A L L ( 7 1 4 ) 8 9 8 - 8 1 2 1
Copyright © 1996
Rev. 0.2 6/96
11861 WESTERN A VENUE , G ARDEN G ROVE , CA. 92841
1
P AT E N T P E N D I N G
PRODUCT DATABOOK 1996/1997
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
A B S O L U T E M A X I M U M R AT I N G S
(Note 1)
Input Supply Voltage (VIN)........................................................................................ LXM1592 = -0.3V to 30V / LXM1593 = -0.3 to 7.0V
Output Voltage, no load .............................................................................................................................. Internally Limited to 1700VRMS
Output Current .............................................................................................................................................. 7.0mARMS (Internally Limited)
Output Power ........................................................................................................................................................................................ 4.5W
Input Signal Voltage, (SLEEP and BRITE Inputs) .................................................................................................................. -0.3V to 6.5V
Ambient Operating Temperature, zero airflow ........................................................................................................................ 0°C to 60°C
Storage Temperature Range ................................................................................................................................................... -40°C to 85°C
Note 1.
Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of
the specified terminal.
R E C O M M E N D E D O P E R A T I N G C O N D I T I O N S (R.C.)
This module has been designed to operate over a wide range of input and output conditions. However, best efficiency and performance
will be obtained if the module is operated under the condition listed in the 'R.C.' column. Min. and Max. columns indicate values beyond
which the inverter, although operational, will not function optimally.
Parameter
Symbol
Input Supply Voltage
LXM1592
LXM1593
Output Power
Brightness Control Input Voltage Range
Lamp Operating Voltage
Lamp Current - Full Brightness
Operating Ambient Temperature Range
VIN
PO
VBRITE
VLAMP
IOLAMP
TA
Recommended Operating Conditions
Min.
R.C.
Max.
7
4.5
12
30
6.5
4.2
2.5
650
6.5
60
2.5
0.8
240
Units
500
5
0
V
V
W
V
VRMS
mA RMS
°C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, these specifications apply over the recommended operating conditions and 25°C ambient temperature for the LXM1592/1593.
Parameter
Symbol
Test Conditions
LXM1592/1593
Min. Typ.
Max.
Units
Output Pin Characteristics
Full Bright Lamp Current
Minimum Lamp Current
Lamp Start Voltage
Operating Frequency
IL (MAX)
IL (MIN)
VLS
fO
VBRITE = 2.5 V DC, SLEEP = Logic High
VBRITE = 0.8 V DC, SLEEP = Logic High
0°C < TA < 60°C
VBRITE = 2.5VDC, SLEEP = Logic High, VIN = 12V
5.9
6.2
2.0
6.5
mA
mA RMS
VRMS
KHz
-1000
2.6
nADC
VDC
VDC
50
5.5
0.8
100
VDC
VDC
µADC
2.50
2.60
VDC
µADC
3
92
90
10
µADC
%
%
1200
70
Brightness Control
Input Current
Input Voltage for Max. Lamp Current
Input Voltage for 50% Lamp Current
IBRITE
VC
VC
VBRITE = 0V DC
IO (LAMP) = 100%
IO (LAMP) = 50%
2.4
-200
2.5
1.25
SLEEP Input
Input Logic 1
Input Logc 0
Input Current
VIH
VIL
IIN
2.2
0
VSLEEP = 0 - 5VDC
Voltage Reference
Output Voltage
Output Current
V REF
IREF
0 < IREF < 500µA
2.40
500
Power Characteristics
Sleep Current
Electrical Efficiency (calculated values)
2
IIN (MIN)
η
VIN = 5VDC , SLEEP = Logic 0
LXM1592, VIN = 12VDC, IO (LAMP) = 5mARMS
LXM1593, VIN = 5VDC, IO (LAMP) = 5mARMS
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
FUNCTIONAL PIN DESCRIPTION
Conn.
CN1
Pin
Description
CN1-1
CN1-2
VIN
Input voltage. (+4.5 to +30VDC )
CN1-3
CN1-4
GND
Power supply return.
CN1-5
SLEEP
Logical high on this pin enables inverter operation. Logical low removes power from the module and
the lamp. A floating input is sensed as a logical low and will disable inverter operation. If not used,
connect SLEEP through a 33kΩ resistor to VIN or directly to any voltage between 2.5 and 5.5V.
CN1-6
BRITE
Brightness control input. Apply 0.9 to 2.5 volts DC to control lamp brightness. Lamp current varies
linearly with input voltage. Open circuit or 2.5V gives maximum brightness.
CN1-7
AGND
Brightness control signal return. For best results do not run VIN power supply current return through this pin.
CN1-8
VREF
Reference Voltage Output. 2.5V @ 500µA max. For use with external dimming circuit.
CN2-1
LAMP HI
High-voltage connection to high side of lamp. Connect to lamp terminal with shortest lead length. Do not
connect to ground.
CN2-2
LAMP LO
High-voltage connection to low side of lamp. Connect to lamp terminal with longer lead length. Do not
connect to ground.
CN2
Copyright © 1996
Rev. 0.3 6/96
3
P AT E N T P E N D I N G
PRODUCT DATABOOK 1996/1997
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A LY S I S I N F O R M AT I O N
INTRODUCTION
LIGHTING CHARACTERISTICS OF CCFLs
The duration of time that it takes for the light output to stabilize
must first be determined before any meaningful measurements
can be made. This is important when trying to maintain consistency between measurements, and is also important in minimizing the required testing time.
Several factors affect the light output of the CCFL’s, such as
operating current waveshape and frequency, proximity of the
lamp to conducting surfaces, inverter output configuration, and
ambient temperature, among other things. In addition, the newer
lamps have very small diameters and operate at higher gas pressures. It appears that this makes these lamps electrically more
unstable.
In order to determine the time required to reach steady
state for a particular lighting system in this test, a completely
automated data acquisition system has been set up that is capable of taking light output data at uniform time intervals. The
power supply, the ammeter and voltmeter are all controlled by
the computer. The photometer’s RS-232 port is connected to
the RS-232 port of the computer. Figure 1 shows a block diagram of this setup.
With this setup, the calculation of the power input and efficiencies is greatly simplified, because automation and data gathering consistency are assured.
4
LCD Panel
Power
Supply
Photo Sensor Middle of Screen
V
CCFL
A
Inverter
J1803
Photometer
Instrument Controller
(Light out in Nits)
Tektronix J17
Desktop
Computer
Parallel
Port
RS-232
FIGURE 1 — MEASUREMENT SETUP
100 samples are taken from a system at 3 second intervals
consisting of the following:
1. Lamp type: 560Vrms Operating at 5-6mA.
2. Inverter type: Half-bridge floating output CS-ZVS
inverter at 10V input.
3. Lamp is housed in a 11.3” active matrix LCD panel.
4. The panel is laid flat on a desk with the photometer
placed at the center of the panel.
The result of these measurements is shown in Figure 2. This
figure shows the initial turn-on profile of the lamp under specific environmental conditions.
155
135
VIN = 10V
115
PIN = 3.72W
Nits
This section discusses some general topics in testing and evaluating Cold Cathode Fluorescent Lamps (CCFL) along with the
inverters that drive them as they are used in active and passive
matrix LCD displays. In particular, this discussion will concentrate on the testing of the Current Synchronous Zero Voltage
Switching Inverter.
The past two years have seen a rapid change in the types of
available LCD displays, as well as their lighting and inverter
systems.
Significant strides have been made in the light transmission
efficiencies of the optical systems in addition to efficiency gains
in their lighting and inverter systems. At the same time, some
of these improvements, especially in the reflector and lamp
housing systems, now pose difficulties when driving these lamps.
The discussion which follows will examine lighting characteristics of the CCFL’s and experimental data which can be used
to determine the duration of time that it takes for the light output from the CCFL to stabilize. In addition, light output efficiency calculation methods will be presented that can help
sort out various efficiency claims from different inverter manufacturers.
As part of the following discussion, the parasitics of the
CCFL/Panel system will be modeled and SPICE simulations will
reveal the current profile in the lamp. Finally, actual performance data will be presented comparing non-floating versus
floating secondaries, with an analysis of this data.
95
75
55
35
3
103
203
303
Time - (sec)
FIGURE 2 — INITIAL TURN-ON CHARACTERISTICS OF THE
CCFL IN A HIGH-EFFICIENCY 11.3" LCD PANEL
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
LIGHTING CHARACTERISTICS OF CCFLs (continued)
Eff =
3.8
3.78
3.76
3.74
Nits / Watt
The rapid increase of light output during the first few seconds
of this test is due to the fact that mercury vapor inside the CCFL
reaching steady state concentration. The continual increase of
light output from the lamp at a slower rate is a result of the
thermal time constants of the system. Essentially, as the lamp
gets warmer, it tends to become more efficient.
Figure 3 shows the light output efficiency of the system as
calculated by using the following formula.
Light Out (Nits)
Power In (Watts)
3.72
3.7
3.68
3.66
3.64
40
3.62
3.6
35
3
103
203
303
Time - (sec)
Nits / Watt
30
FIGURE 4 — INVERTER POWER INPUT PROFILE
DURING INITIAL TURN ON
25
20
15
10
3
103
203
303
Time - (sec)
FIGURE 3 — LIGHT OUTPUT EFFICIENCY PROFILE DURING
INITIAL TURN ON
Figure 3 clearly shows the increase in efficiency as the lamp
in the panel is self heating. This graph also shows that 303
seconds is not a sufficient amount of time for this system to
reach a steady state. Figure 5 shows what the required amount
of time is for this system to reach a steady state.
Figure 4 shows the inverter power input profile during initial turn on. It is interesting to note that, when the inverter is
first turned on, the input power is lower. This is a result of the
higher impedance of the lamp. It takes a finite amount of time
for the mercury to fully vaporize, thereby reducing the impedance of the lamp and permitting it to reach a steady state in
terms of power.
Figures 5 and 6 show the above-mentioned system at a
slightly different input power taken at a different time than the
previous graphs. The light output and efficiency data is probably different because of a different ambient temperature. The
sampling interval for these graphs was set at 10 seconds.
172
152
132
Nits
112
92
72
52
32
0
200
400
600
800
1000
Time - (sec)
FIGURE 5 — LIGHT OUTPUT VERSUS TIME AT INITIAL
TURN ON, 10sec SAMPLING PERIOD
Copyright © 1996
Rev. 0.3 6/96
5
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
LIGHTING CHARACTERISTICS OF CCFLs (continued)
3.95
43
38
3.9
3.85
28
Watts
Nits / Watt
33
23
3.8
18
3.75
13
8
0
200
400
600
800
3.7
10
1000
12
14
16
18
20
Time - (sec)
Input Voltage - (V)
FIGURE 6 — LIGHT OUTPUT EFFICIENCY VERSUS TIME AT
INITIAL TURN ON, 10sec SAMPLING PERIOD
FIGURE 8 — POWER INPUT VERSUS INPUT VOLTAGE
Based on the graphs of Figures 5 and 6, it can be determined
that this system reaches steady state in approximately 17 minutes.
INVERTER INPUT VOLTAGE CONSIDERATIONS
Almost all power conversion devices lose some efficiency when
operated at voltages beyond their nominal values. In order to
investigate the effect of input voltage variation on the light output efficiency, the input voltage to the inverter has been varied
from its minimum to its maximum operating condition. The
results of this effort are shown in Figures 7 and 8.
The total input power increases by approximately 200mW
when input voltage is increased from 10V to 20V. This corresponds to a light output efficiency change of 5.8%. This efficiency degradation is, of course, smaller when the input voltage is from 10V to 14V, as is the case in most systems, where
the efficiency change is only 2.8%. The higher input voltages
depicted in Figures 7 and 8 correspond to operating from an
AC power source.
Figure 9 shows the light output regulation versus input voltage. This graph shows the excellent light output (line) regulation characteristic of a CS-ZVS inverter with the floating output.
The total line regulation is only ±0.23% because of this the
purity of the lamp drive current as well as the true load current
sensing capability of this circuit.
42.5
156.2
42
156
PIN = 3.8W
155.8
41
Nits
Nits / Watt
41.5
40.5
40
155.4
39.5
155.2
39
10
12
14
16
18
20
Input Voltage - (V)
FIGURE 7 — LIGHT OUTPUT EFFICIENCY VERSUS
INPUT VOLTAGE
6
155.6
155
10
12
14
16
18
20
Input Voltage - (V)
FIGURE 9 — LIGHT OUTPUT VERSUS INPUT VOLTAGE
LIGHT OUTPUT REGULATION
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
INVERTER INPUT VOLTAGE CONSIDERATIONS (continued)
All the information discussed thus far can be very useful
when trying to design the power subsystem. The minimal decrease in efficiency of the CS-ZVS inverter enables the system
designer to have a relatively wide operating input voltage range
without a significant efficiency penalty.
The inverter is normally designed for the minimum battery
voltage. Efficiency is optimized when the minimum operating
voltage is as close as possible to the nominal operating voltage.
A FEW WORDS ON NEW LCD PANEL DISPLAYS
Significant efficiency improvements have been made to the
optical systems of newer, larger LCD panels, panels that are
typically 11.3" inches and larger. However, these improvements,
including improvements in the lightpipe, the reflector and the
CCFL itself, have caused increased leakage currents from the
lamp to the reflector and/or panel's metal frame. This condition results in degraded light output and reduced dimming
ranges, when used with backlight inverters equipped with nonfloating (or grounded) high voltage sides. Further compounding the leakage current problem is an increase of the operating
voltages of CCFL's, with some lamps requiring as high as 650VRMS
to operate.
In a non-floating or grounded inverter, the output of the
high-voltage transformer is referenced to ground, permitting
leakage currents to circulate between the panel, the system
ground and the inverter ground. In order to address these
leakage currents, a new inverter configuration has been designed by Linfinity, which uses a floating output drive, coupled
with Linfinity's patent pending CS-ZVS technology.
Generally speaking, in a floating output drive, the high-voltage side of the inverter transformer is not referenced to ground
and, therefore, interrupts the path of the leakage currents, preventing them from flowing into the system ground. Because
the Linfinity LXM1592 and LXM1593 are configured with a unique
COUT
Reflector
CS-ZVS
Inverter
DC VIN
CP1
CCFL
T1
CPN
RS
combination of Linfinity's floating output architecture and CSZVS technique, they significantly reduce the leakage currents
from the lamp to the reflector of the metal frame of the panel,
further improving the efficiency of these newer inverters over
non-floating, or grounded designs. The LXM1592 and LXM1593,
equipped with the Linfinity floating output drive architecture,
yield an additional 10% improvement in light output and also
permit a wider dimming range, resulting in a more uniformlylighted, as well as more efficient and brighter panel. Linfinity's
floating output drive scheme, which currently is the only design which senses the secondary side lamp current, achieves
very accurate lamp current regulation and, as such, is unique
and superior even to other floating output implementations.
SIMPLIFIED MODELING OF THE CCFL-PANEL SYSTEM
Non-Floating Configuration
Figure 10 shows the electrical configuration of a non-floating
drive. In this system, the CCFL current is being sensed with a
resistor referenced to the inverter ground. The panel, along
with the reflector, is also electrically “tied” to the inverter ground.
A “thermometer effect” (or brightness gradient) is created
when the parasitic capacitance and the reflector are diverting
useful current from the lamp to ground. This effect is very
intense in some of the newer panels because the reflector is
metal or metal-coated plastic or is situated very close to the
panel itself. An additional side effect of this leakage is a marked
reduction of efficiency.
The following experiment was performed in order to quantify this capacitance. A lamp was broken at both cathode ends
and an AWG#18 bus wire was inserted through the tube. This
assembly was then placed in the cavity of a metal reflector and
the capacitance was measured using a standard RLC bridge.
The measured parasitic capacitance was approximately 15pF.
Normally this capacitance is distributed along the length of the
tube. Also, the lamp wiring formed a parasitic capacitance with
the metal frame, which in this case was about 14pF.
With the above information, a simple discrete distributed
electrical model was constructed to help analyze the system.
This electrical model of the non-floating configuration is shown
in Figure 11.
The parasitic shunt capacitors shown as CP1-CPN produce a
current gradient across the length of the lamp that results in the
“thermometer effect” that exhibits itself as a brightness gradient. In extreme cases, this exhibits itself as partial lighting of
the lamp with the “hot” side of the lamp being the brightest.
FIGURE 10 — NON-FLOATING OUTPUT CONFIGURATION.
CP1 THROUGH CPN REPRESENT DISCRETIZED
DISTRIBUTED PARASITIC CAPACITANCE
Copyright © 1996
Rev. 0.3 6/96
7
P AT E N T P E N D I N G
PRODUCT DATABOOK 1996/1997
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
S
A T A
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
SIMPLIFIED MODELING OF THE CCFL-PANEL SYSTEM (con't.)
1
IL
Resistive Element
RP1
RPN
CP1
Resistive Element Current
Current through
Resistive Element
Normalized
to 6.132mA
0.995
0.99
0.985
0.98
0.975
1
11
6
Discrete Distributed
Electrical Model of the Lamp
In order to study the effects of the parasitic capacitance, the
lamp was divided into 20 identical segments, consisting of both
resistive and capacitive elements. Assuming a full brightness
operating impedance of 100KΩ, each individual resistive element would be 5KΩ and each capacitance would be 0.75pF.
The circuit then was solved by using a circuit simulator, such as
SPICE. The capacitance of the wiring in the non-floating drive
was inconsequential and was ignored. The accuracy of the
above model is thought to be limited because of the nonlinear
nature of the lamp impedance along the lamp length as a result
of the thermometer effect (resulting in impedance modulation).
Figure 12 shows the result of SPICE simulation on the 20
element model. Impedance was adjusted for 600V and 6mA
operation. This graph shows the variation of the current flow
in the resistive elements of the lamp that produces light output.
Furthermore, it shows that the current is higher at the “hot” end
of the lamp by 2.4%. The effect of this is minimal light nonuniformity from one end of the lamp to the other. This is also
apparent in the real circuit.
21
FIGURE 12 — CURRENT GRADIENT ASSUMING 20 ELEMENT
ANALYSIS. FULL BRIGHTNESS
Figure 13 shows the result of SPICE simulation on the above
20 element model when the lamp is dimmed to 1/3 brightness.
Impedance was adjusted in this case for 600V and 2mA operation. The current differential in this case was 20%. The consequence of this is that the brightness change from one end of the
lamp to the other will likely be more than 25%, a variance
which is clearly visible to the human eye.
1
Normalized
to 2.36mA
0.98
Resistive Element Current
FIGURE 11 — NON-FLOATING OUTPUT CONFIGURATION.
ELECTRICAL MODEL OF LAMP THAT SHOWS
THE CURRENT PROFILE AS A RESULT OF THE
PARASITIC SHUNT CAPACITANCE
16
Resistive Element
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0.82
1
6
11
16
21
Resistive Element
FIGURE 13 — CURRENT GRADIENT ASSUMING 20 ELEMENT
ANALYSIS. ONE-THIRD BRIGHTNESS LEVEL
8
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
SIMPLIFIED MODELING OF THE CCFL-PANEL SYSTEM (con't.)
CH
COUT
0.999
Normalized
to 6.036mA
0.998
0.997
0.996
0.995
0.994
Reflector
DC VIN
CP1
CCFL
CS-ZVS
Inverter
T1
1
Resistive Element Current
Figure 14 shows the electrical configuration of a floating
drive. In this system, the CCFL current is being sensed either in
the primary side of the high-voltage transformer or at the secondary side. The panel and the reflector are electrically connected to the primary side (of T1) inverter ground.
0.993
1
6
11
16
21
Resistive Element
CPN
FIGURE 16 — CURRENT GRADIENT WITH FLOATING
DRIVE MODELING
CL
CH & CL Are Wiring
To Panel Capacitances
FIGURE 14 — FLOATING OUTPUT CONFIGURATION
Figure 16 shows the result of SPICE simulation on the circuit
of Figure 15, again with a 20-element model.
Current through
Resistive Element
IL
The total current deviation in this case is 0.7%. Although
this a small deviation, it is expected that in a real physical circuit, the difference would be higher as a result of other unmodelled parasitics and lamp non-linearities.
Figure 17 shows the simulation results for the dimmed case
of the floating drive. The total current deviation in this case is
6.1%. Thus, the floating drive introduces a smaller brightness
gradient than the non-floating drive, resulting in a more uniformly
lighted panel.
1
Resistive Element
RP1
RPN
CH
CP1
CPN
Discrete Distributed
Electrical Model of the Lamp
CL
Resistive Element Current
0.99
Normalized
to 2.105mA
0.98
0.97
0.96
0.95
0.94
1
6
11
16
21
Resistive Element
FIGURE 15 — FLOATING OUTPUT CONFIGURATION
ELECTRICAL MODEL OF LAMP
Copyright © 1996
Rev. 0.3 6/96
FIGURE 17 — CURRENT GRADIENT WITH FLOATING
DRIVE MODELING
9
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
NON-FLOATING INVERTER MEASUREMENTS
152
132
3.8
3.75
Watts
A series of measurements were made to determine the performance of the previously used inverter with non-floating output.
The exact same components were used to make the comparison.
The light out versus time test was first run to determine at
which point the light output reaches a steady state. Figure 18
shows the results of this test in graphical form.
3.7
3.65
Nits
112
3.6
0
92
200
400
600
800
1000
Time - (sec)
72
FIGURE 20 — CURRENT GRADIENT WITH FLOATING
DRIVE MODELING
52
32
0
200
400
600
800
1000
FINAL ANALYSIS, FLOATING VERSUS
NON-FLOATING LAMP DRIVE
Time - (sec)
FIGURE 18 — LIGHT OUT VERSUS TIME AT INITIAL TURN ON
FOR NON-FLOATING INVERTER
The light out efficiency versus time curve then has been
calculated by using the light out and the power input data (Figure 20). The results of this effort are shown in Figure 19. As
expected, light out efficiency improves as the lamp warms up
reaching a maximum value of 38.86 Nits/Watt.
39
34
Nits / Watt
29
24
19
14
9
0
200
400
600
800
1000
Table 1 summarizes the performance differences between the
floating and non-floating drive configurations evaluated in the
testing discussed above. As has been seen, the performance
gains strongly depend on the physical configuration of the lamp
and the reflector assembly. One of the panels that was tested
exhibited higher leakage, along with a significant improvement
in the dimming range and the thermometer visual effect. The
improvement in efficiency between floating versus non-floating
in this case was 20%.
Of course, as is usually the case, improvements in performance often come with an increase in circuit complexity. While
inverters employing floating output design tend to be somewhat more complex, because of the advantage realized through
their use, they often are the most optimum or only viable way
to drive the latest generation LCD panels.
The Current Synchronous Zero Voltage topology employed
in the inverters manufactured by Linfinity Microelectronics is
used in floating designs that exhibit “True Current Sense”, i.e. a
reflection of the actual lamp current is being sensed to provide
superior lamp current regulation. This can be contrasted with
the average primary current sense of Buck-Royer oscillator- based
inverters. It is noteworthy that the efficiency gains of the singlepower stage CS-ZVS topology compared to the double power
stage Buck-ROYER combinations is more that 20%.
Time - (sec)
FIGURE 19 — LIGHT OUT EFFICIENCY VERSUS TIME AT INITIAL
TURN ON FOR NON-FLOATING INVERTER
10
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T E C H N I C A L / A N A L Y S I S I N F O R M AT I O N
(continued)
TABLE 1
Parameter
Floating
Non-Floating
Input Power
Light Output
Light Out Efficiency
Percent Max-Min Current Difference
Because of Parasitics (note 1)
3.175 Watts
154.3 Nits
41.5 Nits/Watt
0.7%
6.1%
at
at
Full Bright
1/3 Dimmed
To 50% of full
Brightness Current
3.726 Watts
144.8 Nits
38.86 Nits/Watt
2.4%
20%
at
at
Full Bright
1/3 Dimmed
To 65% of full
Brightness Current
Dimming Range (note 2)
Note 1.
Note 2.
Improvement over
Non-Floating
-0.3%
6.56%
6.8%
242%
at
Full Bright
227%
at
1/3 Dimmed
This refers to the max and min currents in resistive elements of the 20 element analysis. The parasitics used did not pertain exactly to the 11.3"
LCD panel used to make the measurements. The results are provided for comparison purposes. It is expected that the parasitics of the panel
used to make the light measurements are lower than those depicted.
Dimming range here is defined as the point where a visible "thermometer" effect just takes place.
SUMMARY
Several new ways for testing CCFL’s and inverters have been
presented. The emphasis throughout these tests has been on
how to make fair comparisons. To that end, a method has been
presented that makes certain that the light output has reached
steady state with all inverters tested, thus guaranteeing fair comparisons.
The lamp/reflector parasitics were modeled and the lamp
current profile was calculated based on these models. This
gave insights on the effect of the parasitics either when the
lamp is at full brightness or dimmed. Both the non-floating and
floating inverter designs were considered and analyzed.
The use of a floating lamp architecture resulted in approximately a 6.8% improvement in light output efficiency when it
was compared to a non-floating design. The dimming range
with the floating drive was also better by more than 15%.
The comparison between the floating versus the non-floating drive designs were presented in tabular form for easy evaluation.
Copyright © 1996
Rev. 0.3 6/96
11
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
T Y P I C A L A P P L I C AT I O N S
AC/DC
Adapter
VIN
VREF
LAMP HI
R1
100k
Sytem Battery
(NiCd, NiMH, etc.)
BRITE
LXM1592/93
3.3 or 5V
From Power
Management
Logic
SLEEP
CMOS
or TTL
R2
56k
LAMP LO
AGND
GND
CFL
TUBE
Inside
LCD
Panel
FIGURE 21 — NOTEBOOK SYSTEM APPLICATION
CMOS or TTL gate
From Power
Management
Logic
VIN
SLEEP
VREF
R1
BRITE
Lamp Current (%) =
LAMP HI
CFL
TUBE
LXM1592/93
R2
LAMP LO
AGND
VBRITE
x 100
VREF
R1 = 100k typical, 5k minimum
R2 = Value optional to determine lowest
brightness setting
R2 = 0.5 R1 minimum
GND
Longest Lead
FIGURE 22 — POTENTIOMETER BRIGHTNESS CONTROL & SLEEP MODE
+5V
S1
"Increase"
+5V
U1
47k
N.O.
UC
From Logic
(optional)
D
N.O.
S2
"Decrease" 47k
DC
RH
VREF
RW
RL
BRITE
R1
AGND
• S1 & S2 are momentary push buttons,
normally open contacts.
• U1 = 100k digital pot.
• R1 value optional for choosing dimming
range:
for R1 = 100k, Range = 100% to 50%
for R1 = 50k, Range = 100% to 33%
• R1 minimum value is 0.5 RPOT.
LXM1592/93
+5V
FIGURE 23 — NONVOLATILE DIGITAL BRIGHTNESS CONTROL
12
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
S
A T A
T Y P I C A L A P P L I C AT I O N S
R1
100k
BRITE
A
Logic
Inputs
R3
402k
(continued)
• Drivers are open collector.
• Run separate ground from driver chip to LXM1592/93
AGND to prevent noise pickup.
VREF
R2
150k
H E E T
• This scheme may be expanded to 3 or 4 bits for more
resolution.
Lamp Current (% Full On) =
AGND
LXM1592/93
B
A
B
Lamp Current
0
0
1
1
0
1
0
1
52%
60%
80%
100%
VBRITE
x 100
VREF
FIGURE 24 — LOW COST DIGITAL BRIGHTNESS CONTROL
1kHz
Standard
CMOS Gate
100k
20k
1N914
Brightness (% Full On) =
VREF
BRITE
1µF
VBRITE
x 100
VREF
AGND
LXM1592/93
Input Duty
Cycle
Lamp Current
* 100%
50%
0%
100%
70%
33%
* 100% DC is a logic HI at CMOS
gate input.
FIGURE 25 — PWM BRIGHTNESS CONTROL
Copyright © 1996
Rev. 0.3 6/96
13
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT DRIVE , C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
S
A T A
H E E T
COMPLETING THE MODULE PART NUMBER
LXM159
Module Type
=
=
-
Half-Bridge Drive (7.0V to 30V Battery Voltage)
Full-Bridge Drive (4.5V to 6.5V Battery Voltage)
Nominal Input Voltage (4 thru 11 NiMH cells)
=
=
=
=
4.8 VDC
6.0 VDC
7.2 VDC
8.4 VDC
5
6
7
8
=
=
=
=
9.6 VDC
10.8 VDC
12.0 VDC
13.2 VDC
9
0
=
=
5
6
7
8
=
=
=
=
8.1 VDC
9.0 VDC
9.9 VDC
10.8 VDC
9
0
=
=
RSVD *
RSVD *
450 VRMS
500 VRMS
550 VRMS
600 VRMS
9
0
=
=
RSVD *
RSVD *
900 VRMS
1000 VRMS
1100 VRMS
7
8
9
=
=
=
1200 VRMS
1300 VRMS
1400 VRMS
=
=
RSVD *
RSVD *
=
=
=
=
4.5 VDC
5.4 VDC
6.3 VDC
7.2 VDC
M
Minimum Input Voltage
1
2
3
4
X
X
X
X
R X
-
Z
Z
RSVD *
RSVD *
P
1
2
3
4
X
L
2
3
X
E
1. Choose either the half or full-bridge version by determining your operating conditions. See Recommended Operating Conditions Table.
2. Choose the nominal input voltage you will be using, that is, the voltage where you want efficiency to be highest. Selections are in 1.2V increments to
match the 1.2V/cell potential of NiCd and NiMH batteries. If a different type of power source is being used, select the closest nominal voltage.
3. Choose the minimum input voltage where full lamp brightness is needed. For convenience, selections are in 0.9V increments, corresponding to end
of discharge potential for NiCd and NiMH cells. Your selection need not correspond to the number of cells selected for nominal voltage input.
4. Specify lamp running voltage.
5. Specify maximum lamp start voltage.
6. Specify lamp running current.
Nominal Lamp Operating Voltage
1
2
3
4
=
=
=
=
250 VRMS
300 VRMS
350 VRMS
400 VRMS
5
6
7
8
=
=
=
=
1
2
3
=
=
=
600 VRMS
700 VRMS
800 VRMS
A
Maximum Lamp Start Voltage
4
5
6
=
=
=
Nominal Lamp Operating Current at Full Brightness
=
=
=
=
2 mARMS
3 mARMS
4 mARMS
5 mARMS
S
1
2
3
4
5
6
7
8
=
=
=
=
6 mARMS
7 mARMS
10 mARMS
12 mARMS
9
0
Reserved
Mechanical Configuration
Factory Assigned
RSVD = Reserved for Special Requirements
14
Copyright © 1996
Rev. 0.3 6/96
PRODUCT DATABOOK 1996/1997
P AT E N T P E N D I N G
LXM1592/LXM1593
F LOATING O UTPUT D RIVE, C USTOMIZABLE CCFL INVERTER MODULES
P
R E L I M I N A R Y
D
A T A
S
H E E T
PHYSICAL DIMENSIONS
LXM1592
4.61 (117)
2.60 (66.0)
1.75
1.75
Ø 0.069 (1.75) Location Hole
2-1
0.41 (10.5)
L1
1-1
1
T1
1
Connector CN-1
Ø 0.118 (3.0) Mtg. Hole
Grounded Both Sides
0.512
(13.0)
Connector CN-2
0.287 (7.30) Max.
Warning!!
High Voltage
Present
0.039 (1.0)
All dimensions in inches (mm)
CN-1 = JST P/N: 05FMS-1.0SP
CN-2 = JST P/N: SM02-(8.0)B-BHS-1-TB
Copyright © 1996
Rev. 0.3 6/96
15