DN462 - Reduce the Cost and Complexity of Medium LCD LED Backlights with a Single Inductor LED Driver for 60 LEDs

Reduce the Cost and Complexity of Medium LCD LED
Backlights with a Single Inductor LED Driver for 60 LEDs
Design Note 462
Daniel Chen
Introduction
One inductor, one IC, one string of LEDs. This is the
conventional way to build a boost LED driver for LCD
display backlights. Although this is a perfectly acceptable solution for small LCD displays that only require a
few strings, in larger displays the number of control ICs
and inductors multiplies quickly, as do the expenses and
PCB real estate requirements. This is a major hurdle in
the race to replace CCFLs with robust, spectrally superior
LEDs in medium sized, bright displays.
A better driver is needed to bring the cost and complexity of LED backlights in line with CCFLs. The LT3598
answers the call by driving six strings of ten LEDs at up
to 30mA per string. It also has a built-in power switch
to save space and design time. Efficiency is optimized
via an adaptive feedback loop that monitors all LED pin
voltages to provide an output voltage just high enough
to light all LED strings. The LED current is regulated even
when VIN is greater than VOUT . The LED current can be
derated based on programmed LED temperature through
an NTC resistor divider or by programming die junction
temperature.
Typical Application
Figure 1 shows the LT3598’s six channels driving 60 LEDs,
with each string programmed at 20mA. The CTRL pin and
PWM pin provide analog and digital dimming, respectively.
True Color PWM™ dimming delivers constant LED color
with a 3000:1 dimming ratio. Figure 2 shows the typical
±0.5% current matching between strings, which yields
the uniform light distribution that is so important in large
backlight applications.
Need More Current?
For applications that demand more than 30mA per string,
multiple channels of the LT3598 can be easily combined
for higher LED current. Figure 3 shows a configuration that
drives two strings at up to 90mA per string. The 1000:1
PWM dimming waveform at 125°C junction temperature
(worst case) is shown in Figure 4.
TSET Pin for Thermal Protection
The TSET pin voltage can be programmed to limit the
internal junction temperature of the LT3598. Once this
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
10μH
PVIN, 8V TO 40V
2.2μF
SW
1.5
1.0
OPENLED
SHDN
SHDN
PWM
PWM
SYNC
FB
30.9k
LT3598
RT
51.1k
VREF
10k
CTRL
100k
60.4k
TSET ISET SS
100k
47pF
14.7k
10nF
0.5
0
–0.5
–1.0
LED1
LED2
LED3
LED4
LED5
LED6
GND VC
20mA
dn4DC F01
–1.5
–50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
dn4DC F02
2.61k
15nF
Figure 1. LED Driver for 60 × 20mA LEDs
04/09/462
ALL SIX LED STRINGS
1.00M
100k
SYNC
4.7μF
VOUT
VO_SW
VIN
MATCHING (%)
VIN
5V
2.2μF
Figure 2. Current Matching for Figure 1
temperature is reached, the LED current will linerarly
decrease if the junction temperature keeps increasing, as
shown in Figure 5. This thermal regulation feature provides
important protection at high ambient temperatures, and
allows a given application to be optimized for typical,
instead of worst-case, ambient temperatures.
Conclusion
LT3598 is a versatile LED driver with a built-in power
switch for multiple LED strings. High PWM dimming is
possible even with its robust fault detection. Furthermore,
a voltage loop regulates the output voltage when all LED
strings are open.
Channel Disable Capability
Unused LED pins can be tied to VOUT to disable them, so
no current flows into the disabled channels. Fault detection ignores any channels tied to VOUT . Figure 6 shows
an application with two disabled channels that yields
efficiency as high as 90%.
L1, 4.7μH
PVIN, 6V TO 40V
VIN, 5V
VIN
C3
2.2μF
SW
VO_SW
SHDN
CTRL
PWM
SYNC
LT3598
RT
VREF
Figure 4. 1000:1 PWM Dimming for Figure 3 at 125°C
R4
1.00M
FB
R5
30.9k
25
20
R10
20k
R8
60.4k
C5
56pF
CTRL
TSET
ISET
LED1
LED2
LED3
LED4
LED5
LED6
R3
9.76k
SS
C4
0.1μF
dn4DC F04
5μs/DIV
C2
4.7μF
VOUT
OPENLED
R1
51.1k
R7
100k
D1
100k
SHDN
CTRL
PWM
ILED1
100mA/DIV
GND
VC
LED CURRENT (mA)
C1
2.2μF
PWM
5V/DIV
90mA
RC , 5.11k
CC , 6.8nF
15
10
5
dn4DC F03
C1: NIPPON CHEMI-CON KTS500B225M32NOTOO
C2: MURATA GRM32ER71H475KA88L
C3: TAIYO YUDEN LMK212BJ225MG
D1: VISHAY SS3H9
L1: WÜRTH ELEKTRONIK 7447785004
0
Figure 3. LED Driver for Two Strings of 90mA LEDs
C1
2.2μF
VIN
5V
C3
2.2μF
R6
100k
SHDN
PWM
SYNC
R1
51.1k
R8
60.4k
C1: TAIYO YUDEN GMK325BJ225ML
C2: MURATA GRM32ER71H475K
C3: TAIYO YUDEN LMK212BJ225MG
D1: DIODES, INC. B240A
L1: WÜRTH ELEKTRONIK 744777410
RNTC : MURATA NCP18WF104J03RB
125
75
100
50
JUNCTION TEMPERATURE (°C)
dn4DC F05
Figure 5. TSET Function Reduces LED Current
at High Temperatures
D1
L1, 10μH
PVIN
6V TO 40V
25
VIN
SW
C2
4.7μF
VOUT
VO_SW
OPENLED
SHDN
PWM
SYNC
RT
VREF
RHOT
10k
R4
1.00M
FB
R5
30.9k
LT3598
CTRL
RNTC
100k
LED1
LED2
LED3
LED4
LED5
LED6
TSET
ISET
R7
100k
C5
47pF
R3
14.7k
C4
0.1μF
SS
GND
VC
20mA
RC , 2.61k
dn4DC F06
CC , 15nF
Figure 6. Four LED Strings with Two Channels Disabled
Data Sheet Download
www.linear.com
For applications help,
call (408) 432-1900, Ext. 3513
Linear Technology Corporation
dn462 LT/TP 0409 155K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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