LINEAR TECHNOLOGY LTC3756 Technical data
DESIGN IDEAS L
V
IN
LT3756
L1A, B
22µH 2×
OVP = 95V
GNDV
C
INTV
CC
INTV
CC
SHDN/UVLO FB
V
REF
ISP
1M
499k
2.2µF
100V
×2
2.2µF
100V
×5
4700pF
0.1µF
10k
23.2k
10k
42.2k
250kHz
1%
4.7µF
30.9k
100k
CTRL
0.018Ω
Si7322DN
0.068Ω
1.8M
24.3k
I
LED
1.5A
83V
LED
STRING
OPENLED
PWM
SS
R
T
ISN
GATE
SENSE
PWMOUT
PDS5100
Q1A, B
L1 = 2× SERIES SLF12575T-220M4R0
Q1 = 2× PARALLEL Si7322DN
VIN= PV
IN
40V TO 60V
(LED CURRENT
REDUCED WHEN
VIN< 40V)
100V Controller in 3mm × 3mm QFN
or MSE Drives High Power LED Strings
from Just About Any Input
Introduction
Strings of high power solid-state
LEDs are replacing traditional lighting
technologies in large area and high
lumens light sources because of their
high quality light output, unmatched
durability, relatively low lifetime cost,
constant-color dimming and energy efficiency. The list of applications grows
daily, including LCD television backlights and projection system bulbs,
industrial and architectural lighting
systems, automotive headlamps, taillights and indicator lights, computer
monitors, street lights, billboards and
even stadium lights.
As the number of applications expands, so does the complexity of input
requirements for the LED drivers.
LED drivers must be able to handle
wide ranging inputs, including the
harsh transient voltage environment
presented by automotive batteries,
the wide voltage range of the Li-ion
cells and wallwart voltages. For LED
lighting manufacturers and designers, applying a different LED driver
for each application means stocking,
testing and designing with a wide
variety of LED controllers. This can
be an expensive and time-consuming
proposition. It would be far better to
use a controller that can be applied
to many solutions.
The LT3756 high voltage LED driver
features a unique topological versatility that allows it to be used in boost,
buck-boost mode, buck mode, SEPIC,
flyback and other topologies. Its high
power capability provides potentially
hundreds of watts of steady-state LED
power over a very wide input voltage
range. Its 100V floating LED current
sense inputs allow the LED string to
float above ground, as shown in the
buck mode and buck-boost mode topologies in this article. Excellent PWM
dimming architecture produces high
dimming ratios, up to 3000:1.
Linear Technology Magazine • January 2009
Figure 1. A 125W, 83V at 1.5A, 97% efficient boost LED driver for stadium lighting
A number of features protect the
LEDs and surrounding components.
Shutdown and undervoltage lockout,
when combined with analog dimming
derived from the input, provide the
standard ON/OFF feature as well as
a reduced LED current should the
battery voltage drop to unacceptably
low levels. Analog dimming is accurate and can be combined with PWM
dimming for an extremely wide range
of brightness control. The soft-start
feature prevents spiking inrush currents during start-up. The OPENLED
pin informs of open or missing LEDs
and the SYNC (LT3756-1) pin can be
used to sync switching to an external
clock.
The 16-pin IC is available in a
tiny QFN (3mm × 3mm) and an MSE
package, both thermally enhanced.
For applications with lower input voltage requirements, the 40VIN, 75V
LT3755 LED controller is a similar
option to the LT3756.
Although it is typically used as an
OUT
LED driver, the LT3756’s voltage FB
pin provides a well-regulated output
by Keith Szolusha
voltage if the constant current sense
voltage is not used. This is a side benefit
of the LT3756’s overvoltage protection
feature, in which the current control
loop is superceded by the FB voltage
loop in the case of an open LED string,
thus preventing the controller from a
running up the voltage in an effort to
maintain current.
125W Boost LED Driver for
Stadium Lights or Billboards
Lighting systems for stadiums, spotlights and billboards require huge
strings of LEDs running at high power.
The LT3756 controller can drive up
to 100V LED strings with its floating
sense resistor inputs ISP and ISN.
The 125W LED driver in Figure 1
accepts a wide-range 40V–60V input
taken from the output of a high power
transformer.
The LT3756’s high power GATE
driver switches two 100V MOSFETs
at 250kHz. This switching frequency
minimizes the size of the discrete components while maintaining high 97%
efficiency, thus producing a less-than-
3737
V
IN
LT3756
GNDV
C
INTV
CC
SHDN/UVLO
FB
V
REF
ISP
0.01µF
0.1µF
V
IN
10V TO
80V
PWM
69.8k
150kHz
100k
4.7µF
CTRL
0.05Ω
0.1Ω
M1
10µF
16V
6.2V
L1
33µH
D1
I
LED
1A
OPENLED
PWM
SS
R
T
ISN
GATE
PWMOUT
SENSE
196k
30.9k
D1: DIODES INC B2100
L1: SUMIDA CDRH8D38-330
M1: VISHAY SILICONIX Si4484EY
M2: VISHAY SILICONIX Si2307BDS
M3: VISHAY SILICONIX Si2328DS
Q1: MMBT5401
4.7k
12.4k
1 OR 2 LEDs
3.5V–7V
0A–1A
147k
1k
OPTIONAL
0V–12V FOR
0A–1A I
LED
Q1
51k
120k
M3
1N4448HWT
M2
10k
2.2µF
100V
×2
9.1k
EFFICIENCY (%)
VIN (V)
V
LED
= 7V
V
LED
= 3.5V
800
100
0
10 20 30 40 50 60 70
10
20
30
40
50
60
70
80
90
L DESIGN IDEAS
Figure 2. An 80VIN buck mode LED driver with PWM dimming for single or double LEDs
50°C discrete component temperature
rise—far more manageable than the
potential heat produced by the 83V
string of 1.5A LEDs.
Even if PWM dimming is not
required, the PWMOUT dimming
MOSFET is useful for LED disconnect
during shutdown. This prevents current from running through the string
of ground-connected LEDs—possible
under certain input conditions.
If an LED fails open or if the LED
string is removed from the high power
driver, the FB constant voltage loop
takes over and regulates the output at
95V until a proper string is attached
between LED+ and LED–. Without
overvoltage protection, the LED sense
resistor would see zero LED current
and the control loop would work hard
to increase its output. Eventually, the
output capacitor voltage would go over
100V, exceeding the maximum rating
of several components. While in OVP
the OPENLED status flag goes low.
High Voltage Buck Mode
LED Driver with High
PWM Dimming Ratio
When the input voltage is higher than
the LED string voltage, the LT3756
can serve equally well as a constant
current buck mode converter. For example, an automotive battery’s voltage
can present a wildly moving target,
38
38
from drooping voltages to dizzyingly
high voltage spikes, The buck mode
LED driver in Figure 2 is perfect for
such harsh environments. It operates
with a wide 10V-to-80V input range
to drive one or two 3.5V LEDs (7V) at
1A. In this case, both the V
IN
ISP and ISN current sense inputs can
go as high as 80V.
PWM dimming requires a level-shift
from the PWMOUT pin to the high
side LED string as shown in Figure 2. The maximum PWM dimming
ratio increases with higher switching frequency, lower PWM dimming
frequency, higher input voltage and
lower LED power. In this case, a 100:1
dimming ratio is possible with a 100Hz
dimming frequency, a 48V input, a
3.5V or 7V LED at 1A, and a 150kHz
switching frequency. Although higher
switching frequency is possible with
the LT3756, the duty cycle eventually
has its limits. Generous minimum
on-time and minimum off-time restrictions require a frequency on the lower
end of its range (150kHz) to meet both
the harsh high-VIN-to-low-V
to one 3.5V LED) and low-VIN-dropout
requirements (10V
particular converter.
The overvoltage protection of the
to 7V
IN
LED
LED
buck mode LED driver has a level
shift as well. Q1, a pnp transistor,
helps regulate the maximum allowable
pin and
(80VIN
) of this
Figure 3. Efficiency for the
buck mode converter in Figure 2
output capacitor voltage to a level just
beyond that of the LED string. Without
the level-shifted OVP network tied to
FB, an open LED string would result
in the output capacitor charging up to
the input voltage. Although the buck
mode components will survive this
scenario, the LEDs may not survive
being plugged back into a potential
equal to the input voltage. That is,
a single 3.5V LED might not survive
being connected directly to 80V.
Single Inductor Buck-Boost
Mode LED Driver
One increasingly common LED driver
requirement is that the ranges of both
the LED string voltage and the input
voltage are wide and overlapping. In
fact, some designers prefer to use the
same LED driver circuit for several
different battery sources and several
different LED string types. Such a
versatile configuration trades some
efficiency, component cost, and board
space for design simplicity, but the
tradeoffs are usually mitigated by the
significantly reduced time-to-market
by producing an essentially off-theshelf multipurpose LED driver.
The buck-boost mode topology
shown in Figure 4 uses a single
inductor and can both step-up and
step-down the input voltage to the LED
string voltage. It accepts inputs from
6V to 36V to drive 10V–50V LED strings
at up to 400mA. The PWM dimming
and OVP are level-shifted in a manner
similar to the buck mode for optimal
performance of these features.
The inductor current is the sum of
the input current and the LED string
Linear Technology Magazine • January 2009
Loading...