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PDF LT3755-2 Data sheet ( Hoja de datos )
| Número de pieza | LT3755-2 | |
| Descripción | LED Controllers | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT3755/LT3755-1/LT3755-2
40VIN, 75VOUT LED Controllers
Features
n 3000:1 True Color PWM™ Dimming
n Wide Input Voltage Range: 4.5V to 40V
n Output Voltage Up to 75V
n Constant-Current and Constant-Voltage Regulation
n 100mV High Side Current Sense
n Drives LEDs in Boost, Buck Mode, Buck-Boost Mode,
SEPIC or Flyback Topology
n Adjustable Frequency: 100kHz to 1MHz
n Open LED Protection
n Programmable Undervoltage Lockout with Hysteresis
n Improved Open LED Status Pin (LT3755-2)
n Frequency Synchronization (LT3755-1)
n PWM Disconnect Switch Driver
n CTRL Pin Provides Analog Dimming
n Low Shutdown Current: <1µA
n Programmable Soft-Start
n Thermally Enhanced 16-Lead QFN (3mm × 3mm)
and MSOP Packages
Applications
n High Power LED
n Battery Chargers
n Accurate Current Limited Voltage Regulators
Description
The LT®3755, LT3755-1 and LT3755-2 are DC/DC control-
lers designed to operate as a constant-current source for
driving high current LEDs. They drive a low side external
N-channel power MOSFET from an internal regulated 7.15V
supply. The fixed frequency, current mode architecture
results in stable operation over a wide range of supply
and output voltages. A ground referenced voltage FB pin
serves as the input for several LED protection features,
and also makes it possible for the converter to operate
as a constant-voltage source. A frequency adjust pin
allows the user to program the frequency from 100kHz
to 1MHz to optimize efficiency, performance or external
component size.
The LT3755/LT3755-1/LT3755-2 sense output current at
the high side of the LED string. High side current sensing
is the most flexible scheme for driving LEDs, allowing
boost, buck mode or buck-boost mode configuration.
The PWM input provides LED dimming ratios of up to
3000:1, and the CTRL input provides additional analog
dimming capability.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 7199560, 7321203.
Typical Application
VIN
8V TO
40V
50W White Automotive LED Headlamp Driver
22µH
4.7µF 1M
185k
INTVCC
100k
0.01µF
332k
40.2k
VIN
SHDN/UVLO
VREF
FB
ISP
LT3755-2
CTRL
ISN
GATE
OPENLED
SENSE
PWM
SS
RT PWMOUT
VC
28.7k
GND INTVCC
375kHz 10k
4.7µF
0.001µF
0.1Ω
0.015Ω
1A
1M 4.7µF
23.7k
50W
LED
STRING
37551 TA01a
100
96
92
88
84
80
0
Efficiency vs VIN
10 20 30
VIN (V)
40
37551 TA01b
37551fd
1 page  
LT3755/LT3755-1/LT3755-2
Typical Performance Characteristics TA = 25°C, unless otherwise noted.
V(ISP – ISN) Threshold vs VCTRL
120
100
80
60
40
20
0
–20
0
0.5 1
VCTRL (V)
1.5 2
37551 G01
FB Regulation Voltage
vs Temperature
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37551 G04
Switching Frequency vs RT
10000
V(ISP – ISN) Threshold vs VISP
103 VCTRL = 2V
102
101
100
99
98
97
0
20 40 60
ISP VOLTAGE (V)
80
37551 G02
VREF Voltage vs Temperature
2.04
2.03
2.02
2.01
2.00
1.99
1.98
1.97
1.96
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37551 G05
Switching Frequency
vs Temperature
500
RT = 26.7k
vVs(ISTPem– IpSeNr)aTthurreeshold
103 VCTRL = 2V
102
101
100
99
98
97
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37551 G03
VREF Voltage vs VIN
2.04
2.03
2.02
2.01
2.00
1.99
1.98
1.97
1.96
0
10 20
VIN (V)
30 40
37551 G06
SHDN/UVLO Hysteresis Current
vs Temperature
2.4
1000
100
450
400
350
2.2
2.0
1.8
10
10
RT (k)
100
37551 G07
300
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37551 G08
1.6
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37551 G09
37551fd
5 Page 
LT3755/LT3755-1/LT3755-2
Applications Information
INTVCC Regulator Bypassing and Operation
The INTVCC pin requires a capacitor for stable operation
and to store the charge for the large GATE switching cur-
rents. Choose a 10V rated low ESR, X7R or X5R ceramic
capacitor for best performance. A 4.7µF capacitor will be
adequate for many applications. Place the capacitor close
to the IC to minimize the trace length to the INTVCC pin
and also to the IC ground.
An internal current limit on the INTVCC output protects
the LT3755 from excessive on-chip power dissipation.
The minimum value of this current should be considered
when choosing the switching NMOS and the operating
frequency.
IINTVCC can be calculated from the following equation:
IINTVCC = QG • fOSC
Careful choice of a lower QG FET will allow higher switch-
ing frequencies, leading to smaller magnetics. The INTVCC
pin has its own undervoltage disable (UVLO) set to 4.1V
(typical) to protect the external FETs from excessive power
dissipation caused by not being fully enhanced. If the
INTVCC pin drops below the UVLO threshold, the GATE
and PWMOUT pins will be forced to 0V and the soft-start
pin will be reset.
If the input voltage, VIN, will not exceed 8V, then the IN-
TVCC pin could be connected to the input supply. Be aware
that a small current (less than 12μA) will load the INTVCC
in shutdown. This action allows the LT3755 to operate
from VIN as low as 4.5V. If VIN is normally above, but
occasionally drops below the INTVCC regulation voltage,
then the minimum operating VIN will be close to 6V . This
value is determined by the dropout voltage of the linear
regulator and the INTVCC undervoltage lockout threshold
mentioned above.
Programming the Turn-On and Turn-Off Thresholds
with the SHDN/UVLO Pin
The falling UVLO value can be accurately set by the resistor
divider. A small 2.1µA pull-down current is active when
SHDN/UVLO is below the threshold. The purpose of this
current is to allow the user to program the rising hysteresis.
The following equations should be used to determine the
values of the resistors:
VIN,FALLING
=
1.22
•
R1+ R2
R2
VIN,RISING = 2.1µA •R1 + VIN,FALLING
VIN
LT3755
SHDN/UVLO
R1
R2
37551 F01
Figure 1. Resistor Connection to Set
VIN Undervoltage Shutdown Threshold
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor, RLED, in series with the LED
string. The voltage drop across RLED is (Kelvin) sensed
by the ISP and ISN pins. Typically, sensing of the current
should be done at the top of the LED string. If this option
is not available, then the current may be sensed at the
bottom of the string, but take caution that the minimum
ISN value does not fall below 3V, which is the lower limit of
the LED current regulation function. The CTRL pin should
be tied to a voltage higher than 1.2V to get the full-scale
100mV (typical) threshold across the sense resistor. The
CTRL pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
voltage sense threshold. When the CTRL pin voltage is
less than 1V, the LED current is:
ILED
=
VCTRL − 100mV
RLED • 10
When the CTRL pin voltage is between 1V and 1.2V the LED
current varies with CTRL, but departs from the equation
above by an increasing amount as CTRL voltage increases.
Ultimately, above CTRL = 1.2V the LED current no longer
varies with CTRL. At CTRL = 1.1V, the actual value of ILED
is ~98% of the equation’s estimate.
37551fd
11
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet LT3755-2.PDF ] | |
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