DN461 - 100V Controller Drives High Power LED Strings from Just about Any Input

100V Controller Drives High Power LED Strings from Just about
Any Input – Design Note 461
by Keith Szolusha
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 backlights and
projection, industrial and architectural lighting, automotive lights, streetlights, billboards and stadium lights.
As the list expands, so does the range of VIN for the LED
drivers. LED drivers must be able to handle wide ranging
inputs, including transient voltages of automotive batteries, a wide range of other batteries and wall wart voltages.
For LED lighting manufacturers, applying a different LED
driver for each application means stocking, testing and
designing with a number of controllers. It would be better
to use just one 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 LED power over a wide input voltage
range. Its 100V floating LED current sense inputs provide
accurate LED current sensing. Excellent PWM dimming
architecture produces high dimming ratios.
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 a wide
range of brightness control. Soft-start prevents spiking
inrush currents. 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 FB voltage
loop limits the max VOUT to protect the converter in the
case of open LEDs.
The 16-pin IC is available in a tiny QFN (3mm × 3mm)
and an MSE package, both thermally enhanced. For lower
input voltage requirements, the 40VIN , 75VOUT LT3755
LED controller is a similar option.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
VIN 10V TO 80V
L1A, B
22μH 2×
VIN 40V TO 60V
2.2μF
100V
×2
499k
SHDN/UVLO
FB
VREF
23.2k
OVP=
95V
VIN
24.3k
LT3756
0.068Ω
CTRL
100k
PWM
SS
RT
VC
83V
LED
STRING
1N4448HWT
Si7322DN
10k
4.7μF
L1 = 2× SERIES SLF12575T-220M4R0
Q1 = 2× PARALLEL Si7322DN
69.8k
150kHz
147k
ILED
1A
Q1
10μF
16V
4.7k
12.4k
M2
CTRL
1 OR 2
LEDs
3.5V
EACH
0A–1A
51k
1k
GND INTVCC
Figure 1. A 125W, 83V at 1.5A, 97% Efficient Boost LED
Driver for Stadium Lighting
03/09/461
0V–12V 9.1k
FOR
0A–1A ILED
0.1μF
10k
0.1Ω
6.2V
ISN
FB
LT3756
M3
PWMOUT
PWMOUT
4700pF
120k
VREF
PWM
0.018Ω
ISP
30.9k
SENSE
OPENLED
10k
VIN
SHDN/UVLO
Q1A, B
GATE
42.2k
250kHz
ILED
1.5A
196k
ISN
30.9k
0.1μF
2.2μF
100V
×5
1.8M
ISP
1M
INTVCC
2.2μF
100V
×2
PDS5100
PWM
OPTIONAL
L1
33μH D1
OPENLED
M1
SS
GATE
RT
VC
SENSE
GND INTVCC
0.01μF
0.05Ω
4.7μF
100k
D1: DIODES INC B2100
L1: SUMIDA CDRH8D38-330
M1: VISHAY SILICONIX Si4484EY
M2: VISHAY SILICONIX Si2307BDS
M3: VISHAY SILICONIX Si2328DS
Q1: MMBT5401
Figure 2. An 80VIN Buck Mode LED Driver With
PWM Dimming for Single or Double LEDs
Boost
Buck-Boost Mode
Lighting systems for stadiums, spotlights and billboards
require huge strings of LEDs running at high power. The
LT3756 controller drives up to 100V LED strings. The
125W LED driver in Figure 1 has a 40V–60V input.
A 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 strings.
Such a versatile configuration trades some efficiency,
component cost, and board space for design simplicity,
and time-to-market.
The 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, producing a less-than-50ºC discrete
component temperature rise—more manageable than
the heat produced by the 125W LEDs.
Even if PWM dimming is not required, the PWMOUT
MOSFET is useful for LED disconnect during shutdown. It
prevents current from running through the string of LEDs.
If the LED string is removed, the FB constant-voltage
loop takes over and regulates the output at 95V. Without
overvoltage protection, the LED sense resistor would
see zero current and the output cap voltage would go
over 100V, exceeding several max ratings. While in OVP
OPENLED goes low.
Buck Mode
When VIN is higher than VLED , the LT3756 can serve
equally well as a buck mode LED driver. The buck mode
LED driver in Figure 2 operates with a wide 10V-to-80V
input range to drive one or two LEDs at 1A.
PWM dimming requires a level-shift from the PWMOUT
pin to the high-side LED string. The max PWM dimming
ratio increases with higher switching frequency, lower
PWM dimming frequency, higher VIN and lower LED
power. In this case, a 100:1 dimming ratio is possible
with a 100Hz dimming frequency and a 48V input.
Although higher switching frequency is possible, the
duty cycle 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-VLED (80VIN to one 3.5V LED)
and low-VIN-dropout requirements (10VIN to 7VLED).
OVP of the buck mode LED driver has a level shift
as well. Without the level-shifted OVP network tied
to FB, an open LED string would result in the output
capacitor charging up to VIN . Although the buck mode
components will survive this scenario, the LEDs may not
survive being plugged into a potential equal to VIN .
Data Sheet Download
www.linear.com
The buck-boost mode driver in Figure 3 uses a single
inductor. It accepts inputs from 9V to 36V to drive
10V–50V LED strings at 400mA.
The inductor current is the sum of the input current
and the LED string current; the peak inductor current
is equal to the peak switching current. Below 9V input,
CTRL analog dimming scales back the LED current to
keep the inductor current under control. UVLO turns
off the LEDs below 6VIN . COUT, DI and MI can see
voltages as high as 95V here.
Conclusion
The LT3756 controller is a versatile high power LED
driver. It has all the features required for large (and small)
strings of high power LEDs. Its high voltage rating,
optimized LED driver architecture, high performance
PWM dimming, host of protection features and accurate
high side current sensing make the LT3756 a single-IC
choice for a variety of lighting systems.
10V–50V
LED–
L1
22μH
VIN 9V TO 36V
M1
110k
VIN
499k
GATE SENSE
130k
FB
4.7μF
COUT
2.2μF
100V
s2
VIN
4.7k
2.49k
LT3756
CTRL
0.25Ω
0.025Ω
3906
SHDN/UVLO
PWMOUT
PWM
INTVCC
VREF
ISP
ISN
100k
OPENLED
140k
ILED
400mA
D1
2.2μF
50V
s2
1M
LED+
SS
VC
0.1μF
10k
GND RT
5.1k
4700pF
M1: VISHAY SILICONIX Si7454DP
D1: DIODES INC. PDS3100
L1: SUMIDA CDRH127-220
28.7k
400kHz
Figure 3. A Buck-Boost Mode LED Driver With
Wide-Ranging VIN and VLED
For applications help,
call (408) 432-1900, Ext. 3801
Linear Technology Corporation
dn461 LT/TP 0309 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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