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PDF LT3761 Data sheet ( Hoja de datos )
| Número de pieza | LT3761 | |
| Descripción | LED Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3761 (archivo pdf) en la parte inferior de esta página. Total 30 Páginas | ||
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Features
n 3000:1 True Color PWM™ Dimming for LEDs
n Wide VIN Range: 4.5V to 60V
n Rail-to-Rail Current Sense Range: 0V to 80V
n Programmable PWM Dimming Signal Generator
n Constant Current (±3%) and Constant-Voltage
(±2%) Regulation
n Analog Dimming
n Drives LEDs in Boost, SEPIC, Inverting, Buck Mode,
Buck-Boost Mode, or Flyback Configuration
n Output Short-Circuit Protected Boost
n Open LED Protection
n Adjustable Switching Frequency: 100kHz to 1MHz
n Programmable VIN UVLO with Hysteresis
n C/10 Indication for Battery Chargers
n Low Shutdown Current: <1µA
n Thermally Enhanced 16-Lead MSOP Package
n Open LED Status Pin (LT3761)
n Frequency Synchronization (LT3761-1)
Applications
n High Voltage LED Strings >100V with Ground
Referred Current Sense
n Grounded Anode LEDs
n Accurate Current Limited Voltage Regulators
LT3761/LT3761-1
60VIN LED Controller with
Internal PWM Generator
Description
The LT®3761/LT3761-1 is a DC/DC controller designed to
operate as a constant-current source and constant-voltage
regulator. It features a programmable internal PWM dim-
ming signal. The LT3761/LT3761-1 is ideally suited for
driving high current LEDs, but also has features to make
it suitable for charging batteries and supercapacitors.
The fixed frequency, current mode architecture results in
stable operation over a wide range of supply and output
voltages. A voltage feedback 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 LT3761-1
can be synchronized to an external clock.
The LT3761/LT3761-1 senses output current at the high
side or at the low side of the load. The PWM input can be
configured to self-oscillate at fixed frequency with duty
ratio programmable from 4% to 96%. When driven by
an external signal, the PWM input provides LED dimming
ratios of up to 3000:1. The CTRL input provides additional
analog dimming capability.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
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
94% Efficient Boost LED Driver for Automotive
Headlamp with 25:1 Internal PWM Dimming
VIN
8V TO
60V
10µH
2.2µF
×2
499k
90.9k
INTVCC
1M
140k
VIN
EN/UVLO
VREF
GATE
SENSE
LT3761
CTRL
GND
2.2µF 1M
×4
10mΩ
16.9k
0.25Ω
100k
DIM
124k
0.01µF
47nF
300Hz
FB
OPENLED
DIM/SS
ISP
ISN
PWM
PWMOUT
VC RT INTVCC
5.1k
4.7nF
28.7k
350kHz
INTVCC
1µF
1A
60W
LED
STRING
PWM Dimming Waveforms at
Various DIM Voltage Settings
ILED
1A/DIV
VDIM = 7.7V
DCPWM = 96%
VDIM = 4V
DCPWM = 50%
VDIM = 1.5V
DCPWM = 10%
VDIM = 0.4V
DCPWM = 4.3%
0.5ms/DIV
37611 TA01b
37611 TA01a
For more information www.linear.com/LT3761
37611fa
1
1 page  
LT3761/LT3761-1
Typical Performance Characteristics TA = 25°C, unless otherwise noted.
VvsISCP-TISRNLTVhorletashgoeld
300
250
FB < 1.2V
200
150
100
50
0
–50
0
0.5 1 1.5
CTRL VOLTAGE (V)
2
37611 G01
vVsISIPS-IPSNVoTlhtraegsehold
260
255
FB < 1.2V
250
245
240
0
20 40 60
ISP VOLTAGE (V)
80
37611 G02
vFsulTl-eSmcpaeleraVtuISrPe-ISN Threshold
260 CTRL = 2V
258
256
ISN = 0
254
252
250
ISP = 48V
248
246
244
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37611 G03
FB Regulation Voltage (VFB)
vs Temperature
1.270
CTRL > 0.1V
1.265
1.260
1.255
1.250
1.245
1.240
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37611 G04
VISP-ISN Threshold vs FB Voltage
300
CTRL = 2V
250
200
150
CTRL = 0.5V
100
50
0
1.20 1.21 1.22 1.23 1.24 1.25 1.26
FB VOLTAGE (V)
37611 G05
VREF Source Current
vs Temperature
200
190
180
170
160
150
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37611 G06
VREF Voltage vs Temperature
2.06
2.05
2.04
2.03
2.02
2.01
2.00
1.99
1.98
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37611 G07
Switching Frequency vs RT
1000
900
800
700
600
500
400
300
200
100
10
RT (kΩ)
100
37611 G08
Switching Frequency
vs Temperature
420 RT = 25.5k
415
410
405
400
395
390
385
380
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
37611 G09
For more information www.linear.com/LT3761
37611fa
5
5 Page 
LT3761/LT3761-1
Operation
The LT3761/LT3761-1 is a constant-frequency, current
mode controller with a low side NMOS gate driver. The
GATE pin and PWMOUT pin drivers and other chip loads
are powered from INTVCC, which is an internally regulated
supply. In the discussion that follows it will be helpful to
refer to the Block Diagram of the IC. In normal operation
with the PWM pin low, the GATE and PWMOUT pins are
driven to GND, the VC pin is high impedance to store the
previous switching state on the external compensation
capacitor, and the ISP and ISN pin bias currents are reduced
to leakage levels. When the PWM pin transitions high, the
PWMOUT pin transitions high after a short delay. At the
same time, the internal oscillator wakes up and gener-
ates a pulse to set the PWM latch, turning on the external
power MOSFET switch (GATE goes high). A voltage input
proportional to the switch current, sensed by an external
current sense resistor between the SENSE and GND input
pins, is added to a stabilizing slope compensation ramp
and the resulting switch current sense signal is fed into
the negative terminal of the PWM comparator. The current
in the external inductor increases steadily during the time
the switch is on. When the switch current sense voltage
exceeds the output of the error amplifier, labeled VC, the
latch is reset and the switch is turned off. During the
switch-off phase, the inductor current decreases. At the
completion of each oscillator cycle, internal signals such
as slope compensation return to their starting points and
a new cycle begins with the set pulse from the oscillator.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current or
voltage in the load. The VC signal is integrated over many
switching cycles and is an amplified version of the differ-
ence between the LED current sense voltage, measured
between ISP and ISN, and the target difference voltage set
by the CTRL pin. In this manner, the error amplifier sets
the correct peak switch current level to keep the LED cur-
rent in regulation. If the error amplifier output increases,
more current is demanded in the switch; if it decreases,
less current is demanded. The switch current is monitored
during the on-phase and the voltage across the SENSE
pin is not allowed to exceed the current limit threshold of
105mV (typical). If the SENSE pin exceeds the current limit
threshold, the SR latch is reset regardless of the output
state of the PWM comparator. The difference between
ISP and ISN is monitored to determine if the output is in
a short-circuit condition. If the difference between ISP
and ISN is greater than 600mV (typical), the SR latch will
be reset regardless of the PWM comparator. The DIM/SS
pin will be pulled down and the PWMOUT and GATE pins
forced low for at least 4µs. These functions are intended
to protect the power switch as well as various external
components in the power path of the DC/DC converter.
In voltage feedback mode, the operation is similar to that
described above, except the voltage at the VC pin is set by
the amplified difference of the internal reference of 1.25V
and the FB pin. If FB is lower than the reference voltage,
the switch current will increase; if FB is higher than the
reference voltage, the switch demand current will decrease.
The LED current sense feedback interacts with the FB
voltage feedback so that FB will not exceed the internal
reference and the voltage between ISP and ISN will not
exceed the threshold set by the CTRL pin. For accurate
current or voltage regulation, it is necessary to be sure that
under normal operating conditions the appropriate loop is
dominant. To deactivate the voltage loop entirely, FB can
be connected to GND. To deactivate the LED current loop
entirely, the ISP and ISN should be tied together and the
CTRL input tied to VREF.
Two LED specific functions featured on the LT3761/LT3761-1
are controlled by the voltage feedback pin. First, when the
FB pin exceeds a voltage 50mV lower (–4%) than the FB
regulation voltage, and the difference voltage between ISP
and ISN is below 25mV (typical), the pull-down driver on
the OPENLED pin is activated (LT3761 only). This function
provides a status indicator that the load may be discon-
nected and the constant-voltage feedback loop is taking
control of the switching regulator. The OPENLED pin de-
asserts only when PWM is high and FB drops below the
voltage threshold. FB overvoltage is the second protec-
tive function. When the FB pin exceeds the FB regulation
voltage by 60mV (plus 5% typical), the PWMOUT pin is
driven low, ignoring the state of the PWM input. In the
case where the PWMOUT pin drives a disconnect NFET,
this action isolates the LED load from GND, preventing
excessive current from damaging the LEDs.
For more information www.linear.com/LT3761
37611fa
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LT3761.PDF ] | |
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