September 2009 - 4W LED Driver Includes Power Switch, Compensation Components and Schottky in 16-Pin MSOP

L DESIGN IDEAS
4W LED Driver Includes Power Switch,
Compensation Components and
by Keith Szolusha
Schottky in 16-Pin MSOP
Introduction
As the number of applications for
medium power (1W–4W) LED strings
grows, so does the need for compact, efficient, high performance LED drivers.
The LT3519 LED driver satisfies the
needs of a wide variety of applications,
including LCD displays, automotive
and avionic applications, architectural and industrial lighting, portable
projection and scanners. It’s 16-pin
MSOP package includes accurate LED
current regulation, small size, high
efficiency, PWM and analog dimming
for brightness control and open circuit
protection with fault detect.
Easy Layout: Integrated
Power Switch, Compensation
Components and Schottky
The 400kHz LT3519 LED driver features an integrated 750mA 45V peak
power switch, integrated compensation components and an integrated
low leakage Schottky diode, making
designs simple and small. Despite
this high level of integration, it can be
used in a wide variety of topologies,
including boost, SEPIC, buck mode
or buck-boost mode. For maximum
versatility, the Schottky diode anode
(ANODE) and internal power switch
emitter (SW) pins are separately pinned
out, so a SEPIC coupling capacitor can
be inserted between these two.
The internal compensation components are chosen to match the 2.2µF
VIN
6V TO 30V
68µH
VOUT
1µF
4.7µF
SW ANODE CATHODE
VIN
ISP
2.49Ω
1M
100k
ISN
SHDN/UVLO
243k
1M
1M
LT3519
VREF
FB
CTRL
OPENLED
GND
PWM
M1
PWM
Figure 1. A 4W boost LED driver with 89% efficiency at 12VIN
to 4.7µF output capacitors in all of
the topologies mentioned above. The
integrated compensation network
combined with current mode control yields fast and stable transient
response.
OPENLED detection and fault reporting are included. A simple resistor
divider sets the overvoltage protection
output voltage in case of an open LED
string and a small pull-up resistor is
all that is needed to assert the open
collector OPENLED output pin during
a fault.
4W Boost LED Driver
The simple boost LED driver in Figure
1 drives up to 38V of LEDs at 100mA
from an automotive input voltage
range. The 400kHz switching frequency is common for automotive, avionic
and industrial solutions; it combines
continued on page 39
VIN
4V TO 24V
C3
2.2µF
L1A
68µH
L1B
68µH
t
C1
1µF
t
RSENSE
1.68Ω
1M
100k
SHDN/UVLO
1M
432k
VREF
1M
FB
CTRL
C1: TDK C3216X7R1H105K
C2: TDK C3216X7R1E475K
C3: TDK C3216X7R1E225K
L1: COILTRONICS DRQ74-680-R (COUPLED INDUCTOR)
M1: VISHAY SILICONIX Si2318DS
IL
0.3A/DIV
VIN = 12V
2µs/DIV
Figure 2. Integrated PWM dimming yields
1000:1 dimming at 120Hz
36
C2
4.7µF
ISN
LT3519
16V LED
150mA
69.8k
OPENLED
GND
ILED
0.1A/DIV
VOUT
SW ANODE CATHODE
VIN
ISP
158k
PWM
5V/DIV
38V LED
100mA
29.4k
137k
PWM
M1
PWM
5V
NOTE:
VIN = 6V RISING TURN ON
VIN = 4V FALLING UVLO
VIN > 9V FULL LED CURRENT AND FOLDBACK BELOW
VOUT 18.5V OVERVOLTAGE PROTECTION
Figure 3. A SEPIC LED driver with short-circuit protection
Linear Technology Magazine • September 2009
DESIGN IDEAS L
VIN
3.3V to 5V
VIN1
C2
150µF
C1
22µF
x2
VOUT1
CLKOUT1 CLKIN2
SVIN1
FB1
RUN1
ITH1
LTM4616
(15mm s 15mm s 2.8mm)
PLLLPF1
MODE1
C3
22µF
x2
ITHM1
VOUT
1.8V
16A MAX
C4
100µF
x4
PHMODE1
TRACK1
MGN1
VIN2
VOUT2
SVIN2
FB2
RUN2
ITH2
RSET
2.49k
ITHM2
MODE2
PHMODE2
GND
TRACK2
MGN2
Figure 3. LTM4616 with 16A parallel operation
100
Whether you require a single 16A high
current output or dual 8A outputs with
sequencing, the LTM4616 provides a
simple and efficient solution. L
IOUT1
5VIN
80
EFFICIENCY (%)
Conclusion
3.3VIN
90
IOUT2
70
60
50
0A
40
0A
OUTPUT CURRENT (2A/DIV)
two regulators drive equal output current even during soft-start, as shown
in Figure 5.
30
TIME (2ms/DIV)
20
10
0
0
2
4
6
8
10
12
14
16
18
Figure 5. Balanced current sharing for even
heat dissipation [5VIN to 1.8VOUT at 16A]
LOAD CURRENT (A)
Figure 4. Efficiency: single 1.8V output
LT3519, continued from page 36
high efficiency, small inductor and
capacitor size, and high PWM dimming
capability while avoiding frequencies
in the AM broadcast band. A small
inductor with about 750mA saturation current rating, a few ceramic
capacitors and several tiny resistors
are all that are needed to complete the
design. As shown in Figure 2, the tiny
PWM dimming MOSFET can be used
to provide over 1000:1 pwm dimming
at 120Hz using the integrated LT3519
PWM dimming architecture and an
extremely low leakage integrated
Schottky diode.
A 1000:1 dimming ratio at 120Hz
is exceptionally high for a 400kHz
switching regulator. It can be tempting to bump up the dimming ratio by
choosing a higher frequency driver,
since in general, higher switching
frequency corresponds to higher
PWM dimming ratios. In this case,
avoiding the AM band means jumping
Linear Technology Magazine • September 2009
to 2MHz, which in the end reduces
the maximum duty cycle and the
efficiency. The 400kHz switching
frequency of the LT3519 does what
2MHz converters cannot do: it provides
high duty cycle for operation down to
6VIN with 38VLED and as high as 89%
efficiency at 12VIN. If PWM dimming
is not needed, the MOSFET M1 can
be removed and the analog dimming
(CTRL) pin can be used to adjust the
regulated LED current below 100mA
for simple brightness control.
2.4W SEPIC LED Driver
When the LED string voltage is within
the input rail voltage range, a SEPIC
topology is called for. The SEPIC produces a high PWM dimming ratio and
also gives short-circuit protection. The
SEPIC in Figure 3 drives 16V LEDs at
150mA from a 4V to 24V input range.
Since the anode of the integrated catch
diode (ANODE) is made available at
a pin independent of the npn power
switch emitter (SW), the coupling
capacitor is easily inserted between
the two. The maximum voltage that
the SW pin sees is a little above the
input voltage plus the output voltage,
so the 45V 750mA integrated power
switch is a perfect match for these
specifications.
Conclusion
The 400kHz LT3519 is a 4W LED
driver that integrates a number of
required components, including a 45V,
750mA power switch, a low leakage
Schottky diode and compensation
components. It also features PWM
dimming, overvoltage protection and
OPENLED fault detection, making it a
small, simple, and efficient choice for
automotive, avionic, industrial and
other LED driver applications. L
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