DN99 - Floating CCFL with Dual Polarity Contrast

Floating CCFL with Dual Polarity Contrast –
Design Note 99
Anthony Bonte
fit within the LCD enclosure with a height restriction of
5mm to 10mm.
Current generation portable computers and instruments
use backlit liquid-crystal displays (LCDs). Cold-cathode
fluorescent lamps (CCFLs) provide the highest available
efficiency for backlighting the display. The lamp requires
high voltage AC to operate, mandating an efficient, high
voltage DC/AC converter. The LCD also requires a bias
supply for contrast control. The supply’s output must
regulate and provide adjustment over a wide range.
Linear Technology addresses these requirements by
introducing the LT®1182/LT1183/LT1184F/LT1184. The
LT1182/LT1183 are dual fixed frequency, current mode
switching regulators that provide the control function
for cold-cathode fluorescent lighting and liquid-crystal
display contrast. The LT1184F/LT1184 provide only
the CCFL function.
Manufacturers offer a wide array of monochrome and
color displays. These displays vary in size, lamp drive
current, contrast voltage polarity, operating voltage
range and power consumption. The small size and
battery-powered operation associated with LCDequipped apparatus dictate low component count and
high efficiency. Size constraints place limitations on
circuit architecture and long battery life is a priority. All
components, including PC board and hardware, must
ALUMINUM ELECTROLYTIC IS RECOMMENDED FOR C3B WITH AN
ESR ≥ 0.5Ω TO PREVENT DAMAGE TO THE LT1182 HIGH-SIDE
SENSE RESISTOR DUE TO SURGE CURRENTS AT TURN-ON.
UP TO 6mA
C1 MUST BE A LOW LOSS CAPACITOR, C1 = WIMA MKP-20
10
The ICs include high current, high efficiency switches,
an oscillator, a reference, output drive logic, control
blocks and protection circuitry. All of the devices support grounded lamp or floating lamp configurations
using a unique lamp current control circuit. The LT1182/
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
LAMP
C2
27pF
3kV
6
Q1, Q2 = ZETEX ZTX849 OR ROHM 2SC5001
L1
L1 = COILTRONICS CTX210605
3
L2 = COILTRONICS CTX100-4
+
L3 = COILTRONICS CTX02-12403
C5
1000pF
*DO NOT SUBSTITUTE COMPONENTS
R2
220k
COILTRONICS (407) 241-7876
0μA TO 45μA ICCFL
CURRENT GIVES
0mA TO 6mA
BULB CURRENT.
THIS IS EQUAL TO
0% TO 90% DUTY
CYCLE FOR THE
PWM SIGNAL.
V (PWM)
0V TO 5V
1kHz PWM
R4
46.4k
1%
R3
100k
4
5
C3B
2.2μF
35V
R1
750Ω
Q2*
L2
100μH
1
R5, 43.2k, 1%
2
3
C7, 1μF
5
6
C8, 0.15μF
7
R7, 10k 8
CCFL
PGND
CCFL VSW
ICCFL
BULB
DIO
BAT
LT1182
CCFL VC
ROYER
AGND
VIN
SHDN
FBP
LCD VC
FBN
LCD
PGND
LCD VSW
+
C3A
2.2μF
35V
+
C12
2.2μF
35V
Q1*
D1
BAT85
4
SHUTDOWN
1
C1*
0.068μF
C6 +
2.2μF
2
D3
1N5934A
4
24V
D1
1N5818
BAT
8V TO 28V
EITHER NEGCON OR POSCON
MUST BE GROUNDED.
GROUNDING NEGCON GIVES
VARIABLE POSITIVE CONTRAST
FROM 10V TO 30V.
GROUNDING POSCON GIVES
VARIABLE NEGATIVE CONTRAST
FROM –10V TO – 30V.
POSCON
L3
D2
2
1N914
16
+
7 C11
22μF
35V
9
NEGCON
15
N = 1:2
14
13
+
12
C4
2.2μF
D4
1N914
R12
20k
VIN
≥ 3V
R13
8.45k
1%
11
C9, 0.01μF
10
C10
0.01μF
9
R9, 4.99k, 1%
R10, 10k, 1%
R14
1.21k
1%
R11, 20k, 1%
5V
Figure 1. LT1182 Floating CCFL Configuration with Variable Positive/Variable Negative LCD Contrast
03/95/99_conv
1182/3 TA01
HV
LEAD WIRE
V/2
OUTPUT
CAPACITOR
TYPICALLY
15pF TO 47pF
FROM DRIVE
CIRCUITRY
–V/2
DISPLAY HOUSING
AND/OR
REFLECTIVE FOIL
ON LAMP
CCFL LAMP
STRAY
CAPACITANCE
HV
LEAD WIRE
DN99 • F02
Figure 2. Loss Path Due to Stray Capacitance in a Floating LCD Installation.
Differential, Balanced Lamp Drive Reduces This Loss Term and Improves Efficiency
Figure 1 is a complete floating CCFL circuit with variable
negative/variable positive contrast voltage capability
based on the LT1182. Lamp current is programmable
from 0mA to 6mA using a 0V to 5V 1kHz PWM signal
at 0% to 90% duty cycle. LCD contrast output voltage
polarity is determined by which side of the transformer
secondary (either POSCON or NEGCON) the output
connector grounds. In either case, LCD contrast output
voltage is variable from an absolute value of 10V to 30V.
The input supply voltage range is 8V to 28V. The CCFL
converter is optimized for photometric output per watt
of input power. CCFL electrical efficiency up to 90%
is possible and requires strict attention to detail. LCD
contrast efficiency is 82% at full power.
Achieving high efficiency for a backlight design requires
careful attention to the physical layout of the lamp,
its leads and the construction of the display housing.
Parasitic capacitance from any high voltage point to DC
or AC ground creates paths for unwanted current flow.
This parasitic current degrades electrical efficiency. The
loss term is related to 1/2CV2f where C is the parasitic
capacitance, V is the voltage at any point on the lamp
and f is the royer operating frequency. Losses up to
25% have been observed in practice. Figure 2 indicates
the loss paths present in a typical LCD enclosure for
a floating lamp configuration. Layout techniques that
increase parasitic capacitance include long high voltage
lamp leads, reflective metal foil around the lamp and
displays supplied in metal enclosures.
Lossy displays are the primary reason to use a floating
lamp configuration. Providing symmetric, differential
drive to the lamp reduces the total parasitic loss term by
one-half in comparison to a grounded lamp configuration. As an added benefit, floating lamp configurations
eliminate field imbalance along the length of the lamp.
Figure 3 illustrates this effect. Eliminating field imbalance
improves the illumination range from about 6:1 for a
grounded lamp configuration to 30:1 for a floating lamp
configuration. Figure 4 is a graph of normalized Nits/
Watt versus lamp current for a typical manufacturer’s
display with a 6mA lamp. Performance for the display
is compared in a floating lamp configuration versus a
grounded lamp configuration. The benefit of reduced
parasitic loss is readily apparent.
FIELD STRENGTH INCREASES
WITH INCREASING DISTANCE
FROM CENTER OF LAMP
HIGH
VOLTAGE
LAMP
TRANSFORMER
SECONDARY
HIGH
VOLTAGE
DN99 • F03
Figure 3. Field Strength vs Distance for a Floating Lamp.
Improving Field Imbalance Permits Extended Illumination
Range at Low Levels
1.0
FLOATING
CCFL
0.9
NORMALIZED NITs/WATT
LT1183 support negative voltage or positive voltage
LCD contrast operation with a new dual polarity error
amplifier. In short, this new family reduces system
power dissipation, requires fewer external components,
reduces overall system cost and permits a high level of
system integration for a backlight/LCD contrast solution.
0.8
0.7
0.6
GROUNDED
CCFL
0.5
0.4
0.3
0.2
0.1
0
0
1
3
4
2
LAMP CURRENT (mA)
5
6
DN99 • F04
Figure 4. Normalized Nits/Watts vs Lamp Current
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn99f_conv LT/GP 0395 160K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1995
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