LINEAR TECHNOLOGY LT3492 Technical data
L DESIGN IDEAS
0.3A
C4
0.47µFC50.47µF
C6
0.47µF
D1 D2 D3
5.6k
0Ω
5.6k
68.1k
68.1k
2k 2k
ISN1
M1 M2 M3
330mΩ 330mΩ
TG1
PV
IN
43V to 58V
L1
33µH
L2
33µH
L3
33µH
M4
OVP1
OVP2
M5
M6
ISP1
0.3A
ISN2
TG2
ISP2
0.3A
ISN3
C1
1µF
s3
330mΩ
TG3
ISP3
5.6k
68.1k
2k
OVP3
C1: MURATA GRM31CR72A105KA01L
C4–C6: MURATA GRM21BR72A474KA73
D1–D3: DIODES B1100
L1–L3: WE 7447789133
M1–M3: ZETEX ZXMP6A13F
M4–M6: PHILIPS BC858B
1nF
SW1 SW2
LT3492
GND
SW3
FADJ
TG1-3
OVP1-3
V
C1-3
V
REF
CTRL1-3
ISP1-3
ISN1-3
V
IN
PWM1-3
SHDN
PWM1-3
SHDN
V
IN
5V
1µF
6.3V
430k
100k
8 LEDs 8 LEDs 8 LEDs
(1MHz)
14 LEDs
C2
1µF
100V
C3
1µF
100V
C4
1µF
100V
PV
IN
9V TO 40V
60mA
1020k
20k
OVP1
ISN1
TG1
1.65Ω
2.2nF
L1
22µH
L2
22µH
L3
22µH
ISP1
D1 D2 D3
M1 M2 M3
SW1 SW2
LT3492
GND
SW3
TG1-3
OVP1-3
V
C1-3
V
REF
CTRL1-3
ISP1-3
ISN1-3
V
IN
PWM1-3
SHDN
PWM1-3
SHDN
V
IN
5V
C1–C4: MURATA GRM31CR72A105KA01L
D1–D3: DIODES B1100
L1–L3: COILCRAFT MSS6132
M1–M3: ZETEX ZXMP6A13F
1µF
6.3V
14 LEDs
60mA
1020k
20k
OVP2
ISN2
TG2
1.65Ω
ISP2
14 LEDs
60mA
1020k
20k
OVP3
215k
18.2k
49.9k
ISN3
TG3
1.65Ω
ISP3
C1
1µF
s3
FADJ
(1MHz)
Triple Output LED Driver Works
with Inputs to 60V and Delivers
3000:1 PWM Dimming
Introduction
The LT3492 is a 60V triple output
LED driver for high input and/or high
output voltage backlighting or direct
lighting applications. A single 4mm ×
5mm IC can drive a large number of
LEDs, reducing overall solution cost
when compared to less capable drivers.
A built-in gate driver for a disconnect
PMOS in series with the LED string,
along with other techniques, enables
a 3000:1 PWM dimming ratio. When
coupled with the part’s analog dimming functions, the overall dimming
ratio can be as high as 30,000:1. The
LT3492 can be configured into buckmode, boost-mode or buck-boost
mode, depending on the available input voltage source and the number and
configuration of LEDs to be driven.
High Input Voltage Triple
Buck Mode LED Driver
Many “regulated” supplies actually
have fairly loose tolerances. For ex-
Figure 1. Triple buck mode LED driver with open LED protection
by Hua (Walker) Bai
DESIGN IDEAS
Triple Output LED Driver Works with
Inputs to 60V and Delivers 3000:1
PWM Dimming ...................................26
Hua (Walker) Bai
Bidirectional Power Manager Provides
Efficient Charging and Automatic
USB On-The-Go with a Single Inductor
.........................................................28
Sauparna Das
Supercapacitor Charger with
Adjustable Output Voltage and
Adjustable Charging Current Limit
.........................................................30
Jim Drew
Monolithic Triple Output Converter for
Li-Ion Powered Handheld Devices ......32
Chuen Ming Tan
Ultralow Power Boost Converters
Require Only 8.5µA of Standby
Quiescent Current .............................34
Xiaohua Su
15VIN, 4MHz Monolithic Synchronous
Buck Regulator Delivers 5A in
4mm × 4mm QFN ...............................35
Tom Gross
26
26
Linear Technology Magazine • June 2009
Figure 2. Triple boost mode 60mA × 14 LED driver
DESIGN IDEAS L
PWM
5V/DIV
0V
0V
I
LED
50mA/DIV
1µs/DIV
PV
IN
9V TO 40V
2.2nF
SW1 SW2
PV
IN
LT3492
GND
SW3
ISP1-3
ISN1-3
V
IN
PWM1-3
SHDN
PWM
SHDN
V
IN
5V
1µF
6.3V
4 LEDs
150mA
1150k
20k
OVP3
18.2k
ISN3
TG3
0.68Ω
L1
27µH
L2
27µH
L3
27µH
ISP3
ISN2
0.68Ω
ISP2
C5
2.2µF
25V
PV
IN
C3
2.2µF
25V
PV
IN
C7
2.2µF
25V
ISN1
D1 D2 D3
0.68Ω
ISP1
1150k
20k
OVP2
1150k
20k
OVP1
C1
2.2µF
50V
4 LEDs
150mA
TG2TG1
4 LEDs
150mA
M1 M2 M3
C1: MURATA GRM31CR71H225KA88L
C3, C5, C7: MURATA GRM21BR71E225K
D1–D3: DIODES B1100
L1–L3: COILCRAFT MSS6132
M1–M3: ZETEX ZXMP6A13F
FADJ
TG1-3
OVP1-3
V
C1-3
V
REF
CTRL1-3
215k
49.9k
(1MHz)
350
300
250
200
150
100
50
0
10 20 30 40 50 60
OUTPUT VOLTAGE (V)
OUTPUT CURRENT(mA)
PVIN= 9V, 85% EFFICIENCY
ample, a 48V supply can range between
43V and 58V, well above most LED
drivers’ safe operating voltage ratings.
The LT3492’s 60V input voltage rating
makes it an easy fit in such volatile
voltage environments.
Figure 1 shows a triple buck-mode
LED driver for high voltage inputs.
Each channel can drive up to eight
300mA white LEDs in series, a limit
set by assuming 4V maximum forward
voltage and a 43V minimum input voltage. Red LEDs or infrared LEDs have
much lower forward voltage, therefore
each output can drive as many as 20
infrared LEDs. The VIN pin in Figure
1 is tied to a 5V supply, as opposed to
PVIN, to improve circuit efficiency.
Triple Boost Mode Driver
Supports 14 LEDs per Output
from a 9V–40V Input
Figure 2 shows a triple boost mode
LED driver that delivers 60mA to each
LED string. Due to the LT3492’s 60V
switch rating, each output can support up to 14 LEDs. The 9V-to-40V
input range covers a diverse range
of applications, including regulated
12V, 24V, 32V to 36V, etc. Unlike in a
buck mode regulator, where the output
current capability is determined by
the switch current limit, the current
driving capability of a boost regulator
Figure 3. Maximum output current
capability of an LT3492 boost circuit
is a function of the ratio of output voltage to minimum input voltage. Figure
3 shows the maximum output current
vs output voltage for a 9V minimum
input (assuming 85% efficiency at
1MHz). For applications that require
less than 40V output, the LT3496
should be considered instead.
Triple Buck-Boost Mode LED
Driver Regulates During Load
Dump Events
Buck-boost mode is used when the
LED string voltage falls within the
input voltage range. Figure 4 shows a
buck-boost application that uses one
inductor per driver. The LED string
is returned to the input—returning
all LED strings to the same potential
allows easy heat sinking. To prevent
body diode conduction, the drain of
the disconnect PMOS is tied to the
anode of the LED string. The high input
voltage of the circuits in Figure 2 and
Figure 4 is a real benefit in automotive
applications, where the ability to ride
through 40V load dump events while
maintaining LED current regulation is
required. Figure 5 shows the greater
than 3000:1 PWM dimming ratio
achievable with the LT3492. This high
PWM dimming ratio helps improve the
picture quality of an LCD display under
various dynamic conditions.
Conclusion
The LT3492 is a high voltage triple
output LED driver with 60V rated
switches, allowing high input voltage
and/or high output voltage operations
with accurate LED current. It can run
in buck mode, boost mode or buckboost mode with 3000:1 PWM dimming
capability.
Figure 5. High dimming ratio (>3000:1)
improves quality of an LCD display
L
Linear Technology Magazine • June 2009
Figure 4. Triple buck-boost mode 150mA × 4 LED driver
2727
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