DN264 - High Power Desktop LCD Backlight Controller Supports Wide Dimming Ratios While Maximizing Lamp Lifetime

High Power Desktop LCD Backlight Controller Supports Wide
Dimming Ratios While Maximizing Lamp Lifetime – Design Note 264
Rich Philpott
Introduction
Liquid crystal displays (LCDs), long standard in laptop
computers and handheld instruments, are gaining
in popularity as desktop computer displays. Larger
displays require multiple high power cold cathode
fluorescent lamps (CCFLs). The lamps must have a
dimming range and life expectancy comparable to
those of previous generations of desktop displays. To
achieve maximum lamp lifetime and dimming range
while maintaining efficiency, CCFL drive should be
sinusoidal, contain zero DC component, and not exceed
the CCFL manufacturer’s current ratings. Providing a
low crest-factor sinusoidal CCFL drive also maximizes
current-to-light conversion efficiency, reduces display
flicker and minimizes EMI and RFI emissions. The
LT®1768 high power CCFL controller, with its unique
Multimode Dimming, provides the necessary drive to
enable a wide dimming range, while maximizing lamp
lifetime.
LT1768 Dual CCFL Backlight Inverter
The circuit in Figure 1 is a dual, grounded-lamp backlight inverter that operates from an input of 9V to 24V,
delivers current from 0mA to 9mA per CCFL and has
a dimming ratio greater than 100:1. The LT1768 in
the circuit is a 350kHz fixed frequency, current mode,
pulse width modulator that provides the lamp-current
control function.
The CCFL current is controlled by a DC voltage on
the PROG pin of the LT1768. This voltage feeds the
LT1768’s Multimode Dimming block, which converts
it to a current and feeds it to the VC pin. As the VC pin
voltage rises, the LT1768’s GATE pin is pulse-width
modulated at 350kHz. The pulse width modulation
produces an average current in inductor L1 proportional
to the voltage on the VC pin. The CCFL’s are driven by
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
6
33pF
10
Lamp Output and Dimming
Ratio vs Lamp Current
LAMP
VIN
8V TO 24V
33pF
T1
10000
4
5
3
C1
33μF
1
2
C4
0.33μF
1000
DIMMING RATIO (NITS/NITS)
PGND
GATE
100
C2
0.033μF
DI02
VIN
DI01
VREF
SENSE FAULT
LT1768
VC
SHDN
AGND
RMIN
CT
RMAX
PROG
PWM
PROG
0V TO 5V OR
1kHz PWM
R1
49.9k
LAMP OUTPUT (NITS)
Q1
0.1μF
Q1
10
1
R6 125k
R4 16.2k
C3
0.1μF
R7
250Ω
1/4W
5V
L1
68μH
MBRS130T3
LAMP MANUFACTURER’S
SPECIFIED CURRENT RANGE
0.1
0
R2
40.2k
6
8
4
LAMP CURRENT (mA)
10
dn264f01b
C4
10μF
Si3456DY
R3
60.4k
100Ω
2200pF
R5*
0.025Ω
dn264F01a
C4: PANASONIC ECHU
L1: COILTRONICS UP4-680
T1: COILTRONICS CTX110607 (2 IN PARALLEL)
Q1: ZETEX ZDT1048
*R5 CAN BE METAL PCB TRACE
Figure 1. 14W CCFL Supply Produces a 100:1 Dimming Ratio While
Maintaining Minimum and Maximum Lamp-Current Specifications
08/01/264_conv
2
(914) 374-2474
(561) 752-5000
(631) 543-7100
the Royer-class converter comprised of T1, C4 and Q1.
The Royer converter produces a 90% efficient, zero DC
component, 60kHz sinusoidal waveform based on the
average current in L1. Sinusoidal currents from both
CCFLs are returned to the LT1768 through the DIO1/
DIO2 pins. A fraction of the CCFL current pulls against
the VC pin closing the loop. A single capacitor on the
VC pin provides loop compensation and CCFL current
averaging, which results in constant CCFL current regardless of line and load conditions. Varying the value of
the VC current source via the Multimode Dimming block
varies the CCFL current and resultant light intensity.
Multimode Dimming
Previous solutions used intensity control schemes that
were limited to either linear or PWM control. Linear
control schemes provide the highest efficiency circuits
but either limit dimming range or violate lamp specifications to achieve wide dimming ratios. PWM control
schemes offer wide dimming range but produce high
crest-factor waveforms detrimental to CCFL life and
waste power at higher currents. The LT1768’s patented
Multimode Dimming combines the best of both control
schemes to extend CCFL life while providing the widest
possible dimming range.
The circuit in Figure 1 accepts either a 0V to 5V DC voltage, or a 0V to 5V, 1kHz PWM waveform and converts
to a DC voltage. The filtered input voltage is sent to the
LT1768 PROG pin, which controls lamp intensity by
placing the LT1768 into one of five distinct modes of
operation. Referring to Figure 2, which mode is in use
is determined by the voltages on the PROG and PWM
pins and by the currents that flow out of the RMAX and
RMIN pins.
Off mode (VPROG < 0.5V) sets the CCFL current to zero.
Minimum current mode (0.5V < VPROG < 1.0V) sets
the CCFL current to a precise minimum level set by
the RMIN resistor. This mode determines the minimum
lamp current and intensity.
Maximum current mode (VPROG > 4V) sets the CCFL
current to a precise maximum level set by the RMAX
resistor. Setting the CCFL current in this mode to the
manufacturers maximum rating achieves maximum
intensity and ensures no degradation in the lamp lifetime.
In linear mode (VPWM < VPROG < 4V), CCFL current is
controlled linearly with the voltage on the PROG pin.
Data Sheet Download
www.linear.com
Linear Technology Corporation
MIN
CURRENT
9mA
MAX
LINEAR CURRENT
PWM
(FREQ = 220Hz)
0%
6mA
100%
ICCFL (mA)
OFF
0mA
0.5 1.0
3V (VPWM) 4.0
VPROG(V)
5.0
dn264f02
Figure 2. Lamp Current vs PROG Voltage
Linear mode provides the best current-to-light conversion and highest efficiency.
In PWM mode (1V < VPROG < VPWM), the CCFL current
is modulated between the minimum CCFL current and
the value for CCFL current in linear mode with VPROG =
VPWM. The PWM frequency is set by a single capacitor
on the CT pin. The PWM duty cycle is set by the voltage on the PROG pin with 1V equal to 0%, and 100%
(linear mode) equal to VPWM. The LT1768’s PWM mode
enables wide dimming ratios while reducing the high
crest factor found in PWM-only dimming solutions.
When combined, these five modes of operation allow
the creation of a DC-controlled CCFL current profile that
can be tailored to enable the widest possible dimming
ratio while maximizing CCFL lifetime.
LT1768 Fault Modes
The LT1768 also has fault detection to ensure that lamp
current and Royer transformer ratings are not exceeded
under fault conditions. If one CCFL lamp is open, the
LT1768 activates a fault flag and adjusts the current in
the remaining so that it never exceeds the maximum
current set by the RMAX resistor. If both lamps are open
circuit, the LT1768 shuts down the Royer section to
avoid any hazardous high voltage conditions.
Additional Features
The LT1768 also provides a temperature-compensated
5V reference, an undervoltage lockout feature, thermal
shutdown and a logic-compatible shutdown pin that
reduces supply current when activated. The LT1768 is
available in a 16-pin SSOP package.
For applications help,
call (408) 432-1900
dn264f_conv LT/TP 0801 345K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
Similar pages