DN449 - Triple LED Driver in 4mm × 5mm QFN Supports LCD Backlights in Buck, Boost or Buck-Boost Modes and Delivers 3000:1 PWM Dimming Ratio

Triple LED Driver in 4mm × 5mm QFN Supports LCD
Backlights in Buck, Boost or Buck-Boost Modes and
Delivers 3000:1 PWM Dimming Ratio
Design Note 449
Hua (Walker) Bai
Introduction
By integrating three independent LED drivers, the LT®3496
offers a highly efficient, compact and cost-effective
solution to drive multiple LED strings. All three drivers
have independent on/off and PWM dimming control, and
can drive different numbers or types of LEDs. High side
current sensing and built-in gate drivers for PMOS LED
disconnect allow the LT3496 to operate in buck, boost,
SEPIC or buck-boost modes with up to 3000:1 True Color
PWM™ dimming ratio.
The LT3496 is offered in a single 4mm × 5mm QFN or
FE28 package. The efficiency of each driver can exceed
95%.
Integrated PMOS Drivers Improve PWM Dimming
Ratio to 3000:1
A high PWM dimming ratio is critical in many display
applications, especially in high end LCD panels. Beware,
though, the definition of dimming ratio varies among
suppliers. When comparing dimming ratios, pay close
attention to the PWM dimming frequency and linearity
of the LED average current at different PWM duty cycles.
For instance, the LT3496’s high 3000:1 PWM dimming
ratio can be achieved at a 100Hz PWM frequency—high
enough to keep the display flicker-free over the entire
dimming range.
L, LT, LTC and LTM are registered trademarks and True Color PWM is a trademark of
Linear Technology Corporation. All other trademarks are the property of their respective
owners.
PVIN
12V
L1
15μH
PVIN
42V MAX
TG1
CAP1
CAP2
CAP3
200mΩ
200mΩ
200mΩ
LED1
LED2
LED3
TG2
M1
M2
C1 TO C3
1μF
s3
L2
15μH
D1
C2
1μF
50V
TG3
M3
C1
1μF
50V
D2
CAP1
0.5Ω
C3
1μF
50V
0.5A
C4
0.47μF
0.5A
0.5A
0.5Ω
VIN
3.3V OR 5V
PWM1-3
SHDN
C7, 1μF
D1
SW1
CAP1-3
LED1-3
VIN
PWM1-3
SHDN
L2
L3
10μH 10μH
D2
SW2
LT3496
GND
SW3
D3
LED3
TG3
M2
TG1-3
OVP1-3
VC1-3
VREF
FADJ
CTRL1-3
C1-C3, C7: MURATA GRM31MR71H105KA88
C4-C6: MURATA GRM188R71C474KA88
D1-D3: DIODES DFLS160
7.15M
200mA
OVP1
7 LEDs
7.15M
200mA
OVP2
191k
22k
470pF
DN449 F01
L1-L3: TAIYO YUDEN NP04SZB 100M
M1-M3: ZETEX ZXMP6A13F
Figure 1. Triple Buck Mode Can Drive 3x 500mA LED Strings
09/08/449
0.5Ω
M3
C6
0.47μF
C5
0.47μF
7 LEDs
L1
10μH
CAP3
C4
1μF
50V
LED2
TG2
M1
D3
CAP2
LED1
TG1
L3
15μH
VIN
3.3V OR 5V
SW1
CAP1-3
LED1-3
VIN
PWM1-3
SHDN
7 LEDs
7.15M
200mA
OVP3
191k
SW2
191k
SW3
LT3496
TG1-3
OVP1-3
VC1-3
VREF
FADJ
CTRL1-3
PWM1-3
SHDN
GND
C5
DN449 F02
1μF
6.3V C1: TAIYO YUDEN UMK325BJ105MH L1-L3: SUMIDA CDRH3D14/HP-150
C2-C4: MURATA GRM21BR71H105K M1-M3: ZETEX ZXMP6A13F
D1-D3: ROHM RB160M-60
Figure 2. Triple Boost Mode Can Drive 200mA LEDs
18.2k
1nF
Buck Mode Circuit Drives Three 500mA LED Strings
Figure 1 shows a triple buck mode LED driver. Each channel drives 500mA of current to its LEDs. Each string can
have from eight to twelve LEDs, depending on type. The
2.1MHz switching frequency minimizes the solution size
by allowing the use of low profile inductors and capacitors. The overall size of the circuit is less than 16mm ×
16mm, with a maximum height of 1.5mm.
Efficiency can be above 95% for a LT3496 buck mode
driver. A further reduction in the parts count is possible
by removing M1, M2 and M3. However, the dimming ratio
drops without those MOSFETs. To improve the efficiency,
the VIN pin should be biased from a 3.3V or 5V supply.
Energy to the LEDs is supplied by PVIN. OVP protection
is omitted in Figure 1.
Boost Mode Circuit Drives Three 200mA LED Strings
Figure 2 shows a triple boost mode driver that delivers
200mA to each LED string from a regulated 12V. Figure 3
shows the superior PWM dimming performance of the
circuit. The LED current reaches a programmed 200mA
in less than 500ns. The efficiency of this circuit is 90%
at a 2.1MHz switching frequency. Unlike the buck mode
driver, the boost mode and buck-boost mode drivers
always require an OVP circuit at the output for open LED
protection.
Buck-Boost Mode Circuit Survives Load Dump Events
In automotive applications, load dump is a condition under which an IC is expected to experience 40V transient.
In such applications, the LED string voltage often falls
in the middle of the 8V to 40V input supply range, thus
requiring buck-boost mode.
In a buck-boost circuit, the switch voltage is the sum of
the input voltage and the LED voltage. Therefore, it is
necessary to turn off the internal power switch before
the input voltage gets too high. The LT3496 circuit in
Figure 4 drives four LEDs, at 200mA per channel. The
circuit monitors the Schottky diodes’ cathode voltage
(VSC). The OVP logic turns off the main switch when VSC
is above 38V, preventing the switch voltage from rising
further. Since no IC pin experiences absolute maximum
voltage, the circuit survives the load dump event.
Conclusion
Multiple output LED drivers, such as the LT3496, offer
excellent current matching, efficiency and space savings. The flexibility to operate in buck, boost or buckboost mode makes the LT3496 feasible in many rugged
applications.
PVIN, 8V TO 16V
40V TRANSIENT
C1
1μF
50V
TG1
L1
15μH
LED1
PWM
5V/DIV
(100Hz)
M1
7.15M
OVP1
TG2
L2
15μH
LED2
191k
0.5Ω
D1
0.5μs/DIV
4 LEDs
D2
VIN
3.3V OR 5V
C8
1μF
6.3V
PWM
SHDN
SW1
CAP1-3
LED1-3
VIN
PWM1-3
SHDN
7.15M
OVP3
191k
CAP3
4 LEDs
4 LEDs
200mA
D3
VSC
C3
2.2μF
PVIN 25V
M3
0.5Ω
191k
200mA
VSC
DN449 F03
Figure 3. Achieving Greater Than 3000:1 PWM
Dimming Ratio with a PMOS Disconnect
OVP2
TG3
L3
15μH
LED3
CAP2
200mA
ILED
200mA/DIV
7.15M
0.5Ω
CAP1
IL
500mA/DIV
M2
VSC
C5
2.2μF
PVIN 25V
SW2
SW3
LT3496
GND
C7
2.2μF
PVIN 25V
TG1-3
OVP1-3
VC1-3
VREF
FADJ
CTRL1-3
18.2Ω
1nF
L1-L3: SUMIDA CDRH3D14/HP-150
C1: TAIYO YUDEN UMK325BJ105MH
C3, C5, C7: TAIYO YUDEN TMK212BJ225MG M1-M3: ZETEX ZXMP6A13F
DN449 F04
D1-D3: ROHM RB160M-60TR
Figure 4. Triple Buck-Boost Mode Can Drive
200mA LEDs While Surviving Load Dump
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2759
www.linear.com
Linear Technology Corporation
dn449 LT/TP 0908 246K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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