LT3755IUD-PBF [Linear]
40VIN, 75VOUT LED Controller; 40VIN ， 75VOUT LED控制器
| 型号： | LT3755IUD-PBF |
| 厂家： | 
Linear |
| 描述： | 40VIN, 75VOUT LED Controller |
| 文件： | 总24页 (文件大小：266K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3755/LT3755-1
40V , 75V LED Controller
IN
OUT
FEATURES
n
DESCRIPTION
3000:1 True Color PWMTM Dimming
TheLT®3755andLT3755-1areDC/DCcontrollersdesigned
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 7V supply. The
fixedfrequency,current-modearchitectureresultsinstable
operationoverawiderangeofsupplyandoutputvoltages.
A ground referenced voltage FB pin serves as the input for
severalLEDprotectionfeatures,andalsomakesitpossible
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.
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
Open LED Status Pin (LT3755)
n
Frequency Synchronization (LT3755-1)
n
PWM Disconnect Switch Driver
The LT3755/LT3755-1 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.
Both parts are available in the 16-lead (3mm × 3mm) QFN
and MSOP packages.
n
CTRL Pin Provides Analog Dimming
n
Low Shutdown Current: <1μA
Programmable Soft-Start
n
n
Thermally Enhanced 16-Lead QFN (3mm × 3mm)
and MSOP Packages
APPLICATIONS
n
High Power LED Applications
L, LT, LTC and LTM 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.
n
Industrial
n
Automotive
TYPICAL APPLICATION
50W White Automotive LED Headlamp Driver
Efficiency vs VIN
22μH
V
IN
100
8V TO
40V
4.7μF
1M
4.7μF
1M
V
IN
SHDN/UVLO
FB
96
V
REF
ISP
23.7k
185k
332k
LT3755
0.1Ω
1A
92
88
84
80
CTRL
ISN
INTV
CC
40.2k
GATE
100k
SENSE
OPENLED
PWM
SS
0.015Ω
50W
LED
STRING
0.01μF
PWMOUT
RT
D2
V
C
GND INTV
CC
28.7k
400kHz
0
10
30
40
20
(V)
10k
10k
0.001μF
4.7μF
V
IN
37551 TA01b
37551 TA01a
37551fa
1
LT3755/LT3755-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V ............................................................................40V
SYNC ..........................................................................8V
RT ............................................................................1.5V
SENSE......................................................................0.5V
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Maximum Junction Temperature........................... 125°C
Storage Temperature Range................... –65°C to 125°C
IN
SHDN/UVLO............................................ 40V, V + 0.3V
IN
ISP, ISN .....................................................................75V
INTV ...................................................... 8V, V + 0.3V
CC
IN
GATE, PWMOUT........................................INTV + 0.3V
CC
CTRL, PWM, OPENLED.............................................12V
V , V , SS, FB..........................................................3V
C
REF
PIN CONFIGURATION
TOP VIEW
16 15 14 13
TOP VIEW
V
1
2
3
4
12 FB
REF
1
2
3
4
5
6
7
8
PWMOUT
FB
16 GATE
15 SENSE
PWM
SYNC OR OPENLED
SS
11 PWMOUT
17
ISN
14 V
IN
GATE
10
9
ISP
13 INTV
CC
17
VC
12 SHDN/UVLO
11 RT
SENSE
CTRL
5
6
7
8
V
10 SS
REF
PWM
9
SYNC OR OPENLED
MSE PACKAGE
16-LEAD PLASTIC MSOP
T
= 125°C, θ = 43°C/W, θ = 4°C/W
JA JC
JMAX
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
T
= 125°C, θ = 68°C/W, θ = 4.2°C/W
JA JC
JMAX
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3755EUD#PBF
LT3755IUD#PBF
TAPE AND REEL
PART MARKING*
3755
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3755EUD#TRPBF
LT3755IUD#TRPBF
LT3755EUD-1#TRPBF
LT3755IUD-1#TRPBF
LT3755EMSE#TRPBF
LT3755IMSE#TRPBF
LT3755EMSE-1#TRPBF
LT3755IMSE-1#TRPBF
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
16-Lead (3mm × 3mm) Plastic QFN
16-Lead (3mm × 3mm) Plastic QFN
16-Lead (3mm × 3mm) Plastic QFN
16-Lead (3mm × 3mm) Plastic QFN
16-Lead Plastic MSOP
3755
LT3755EUD-1#PBF
LT3755IUD-1#PBF
LT3755EMSE#PBF
LT3755IMSE#PBF
LT3755EMSE-1#PBF
LT3755IMSE-1#PBF
37551
37551
3755
3755
16-Lead Plastic MSOP
37551
16-Lead Plastic MSOP
37551
16-Lead Plastic MSOP
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
37551fa
2
LT3755/LT3755-1
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
PARAMETER
CONDITIONS
Tied to INTV
CC
MIN
TYP
MAX
UNITS
V
Minimum Operating Voltage
V
IN
4.5
V
IN
IN
V
Shutdown I
SHDN/UVLO = 0V, PWM = 0V
SHDN/UVLO = 1.15V, PWM = 0V
0.1
1
5
μA
μA
Q
V
V
V
Operating I (Not Switching)
VC = 0V, RT = 100k to GND
1.4
2.00
0.006
108
1.7
mA
V
IN
Q
l
Voltage
100μA ≤ I
≤ 0μA
VREF
1.965
98
2.045
REF
REF
Line Regulation
4.5V ≤ V ≤ 40V
%/V
mV
μA
IN
SENSE Current Limit Threshold
SENSE Input Bias Current
SS Pull-Up Current
118
13
Current Out of Pin
Current Out of Pin
40
8
10.5
μA
Error Amplifier
l
LED Current Sense Threshold (V – V
)
FB = 0V, V = 48V
96
–13
0
100
–10
103
–8
mV
mV
V
ISP
ISN
ISP
LED Current Sense Threshold at CTRL = 0V (V – V
)
ISN
CTRL = 0V, FB = 0V, V = 48V
ISP
ISP
CTRL Threshold Linear Programming Range
CTRL Input Bias Current
1.1
100
75
Current Out of Pin
50
nA
V
LED Current Sense Amplifier Input Common Mode
2.9
Range (V )
ISP
ISP/ISN Short-Circuit Threshold (V – V
)
V = 0V
ISN
115
0
150
200
3
mV
V
ISP
ISN
ISP/ISN Short-Circuit Fault Sensing Common Mode
Range (V
)
ISN
ISP/ISN Input Bias Current
PWM = 5V (Active), V = 48V
50
0
μA
μA
ISP
PWM = 0V (Standby), V = 48V
0.1
ISP
LED Current Sense Amplifier g
VC Output Impedance
V
– V = 100mV
120
μS
kΩ
nA
m
ISP
ISN
1V < V < 2V
15000
VC
VC Standby Input Bias Current
PWM = 0V
–20
20
FB Regulation Voltage (V
)
FB
1.232
1.220
1.250
1.250
1.265
1.270
V
V
l
V
ISP
= V
ISN
FB Amplifier g
FB = V , V = V
ISN
480
40
μS
nA
V
m
FB ISP
FB Pin Input Bias Current
FB Open LED Threshold
Current Out of Pin
100
OPENLED Falling (LT3755 Only)
V
V
V
FB
FB
FB
– 60mV – 50mV – 40mV
FB Overvoltage Threshold
PWMOUT Falling
V
V
V
FB
V
FB
FB
+ 50mV + 60mV + 70mV
4
V/V
VC Current Mode Gain – ΔV /ΔV
VC
SENSE
Oscillator
l
Switching Frequency
R = 100k
T
90
925
105
1000
125
1050
kHz
kHz
T
R = 10k
Minimum Off-Time
170
ns
37551fa
3
LT3755/LT3755-1
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Linear Regulator
INTV Regulation Voltage
7
7.15
350
4.1
34
7.3
V
mV
V
CC
Dropout (V – INTV
)
I
= –10mA, V = 7V
INTVCC IN
IN
CC
INTV Undervoltage Lockout
3.9
29
4.3
40
12
CC
INTV Current Limit
mA
μA
CC
INTV Current in Shutdown
SHDN/UVLO = 0V, INTV = 7V
8
CC
CC
Logic Inputs/Outputs
PWM Input High Voltage
PWM Input Low Voltage
PWM Pin Resistance to GND
1.5
45
V
V
0.4
50
60
0
kΩ
mV
V
PWMOUT Output Low (V
)
OL
PWMOUT Output High (V
)
OH
INTV
CC
– 50mV
l
SHDN/UVLO Threshold Voltage Falling
SHDN/UVLO Rising Hysteresis
1.185
1.220
20
1.245
V
mV
V
SHDN/UVLO Input Low Voltage
SHDN/UVLO Pin Bias Current Low
SHDN/UVLO Pin Bias Current High
I
Drops Below 1μA
0.4
2.5
VIN
SHDN/UVLO = 1.15V
SHDN/UVLO = 1.30V
1.7
1.5
2.05
10
μA
nA
mV
kΩ
V
100
200
OPENLED Output Low (V
)
OL
I
= 0.5mA (LT3755 Only)
OPENLED
SYNC Pin Resistance to GND
SYNC Input High
SYNC Input Low
LT3755-1 Only
LT3755-1 Only
LT3755-1 Only
30
0.4
V
Gate Driver
t GATE Driver Output Rise Time
C = 3300pF
35
35
ns
ns
V
r
L
t GATE Driver Output Fall Time
f
C = 3300pF
L
GATE Output Low (V
)
0.05
OL
GATE Output High (V
)
OH
INTV
V
CC
– 50mV
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT3755E and LT3755E-1 are guaranteed to meet performance
specifications from 0°C to 125°C junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization and correlation with statistical process controls. The
LT3755I and LT3755I-1 are guaranteed to meet performance specifications
over the –40°C to 125°C operating junction temperature range.
37551fa
4
LT3755/LT3755-1
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
VISP – VISN Threshold
vs Temperature
VISP – VISN Threshold vs VCTRL
VISP – VISN Threshold vs VISP
103
102
101
100
99
120
100
80
103
102
101
100
99
V
CTRL
= 2V
V
= 2V
CTRL
60
40
20
98
98
0
97
–20
97
–50
0
20
40
60
80
0
0.5
1
1.5
2
0
25
50
75 100 125
–25
ISP VOLTAGE (V)
V
(V)
TEMPERATURE (°C)
CTRL
37551 G02
37551 G01
37551 G03
FB Voltage vs Temperature
VREF Voltage vs Temperature
VREF Voltage vs VIN
2.04
2.03
2.02
2.01
2.00
1.99
1.98
1.97
1.96
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
2.04
2.03
2.02
2.01
2.00
1.99
1.98
1.97
1.96
0
10
20
(V)
30
40
–50
0
25
50
75 100 125
–50
0
25
50
75 100 125
–25
–25
V
TEMPERATURE (°C)
TEMPERATURE (°C)
IN
37551 G06
37551 G05
37551 G04
Switching Frequency
vs Temperature
Switching Frequency vs RT
SHDN/UVLO Current vs Voltage
10000
1000
100
1400
1300
1200
1100
1000
900
2.4
2.2
2.0
1.8
1.6
R
= 10k
T
800
700
600
10
10
100
–50
0
25
50
75 100 125
–60
–20
0
20
40
–25
–40
R
(k)
TEMPERATURE (°C)
TEMPERATURE (°C)
T
37551 G07
37551 G08
37551 G09
37551fa
5
LT3755/LT3755-1
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
SENSE Current Limit Threshold
vs Temperature
SHDN/UVLO Threshold
Quiescent Current vs VIN
vs Temperature
1.28
1.26
1.24
1.22
1.20
1.18
2.0
1.5
1.0
0.5
0
110
105
100
95
PWM = 0V
SHDN/UVLO RISING
SHDN/UVLO FALLING
90
0
10
20
(V)
30
40
–50
0
25
50
75 100 125
–25
–50
0
25
50
75 100 125
–25
V
TEMPERATURE (°C)
TEMPERATURE (°C)
IN
37551 G10
37551 G12
37551 G11
INTVCC Current Limit
vs Temperature
INTVCC Voltage vs VIN
INTVCC Voltage vs Temperature
40
38
36
34
32
30
7.4
7.3
7.2
7.1
7.0
8
6
4
2
0
0
10
20
(V)
30
40
50
TEMPERATURE (°C)
125
50
TEMPERATURE (°C)
125
–50
0
25
75 100
–50
0
25
75 100
–25
–25
V
IN
37551 G13
37551 G14
37551 G15
SENSE Current Limit Threshold
vs Duty Cycle
Gate Rise/Fall Time
vs Capacitance
V
(ISP-ISN) Threshold vs FB Voltage
125
100
75
50
25
0
100
80
60
40
20
0
110
105
100
95
V
= 2V
10% TO 90%
CTRL
GATE RISE
TIME
GATE
FALL TIME
90
1.2
1.22
1.24
1.26
1.28
0
2
4
6
8
10
0
25
50
75
100
FB VOLTAGE (V)
CAPACITANCE (nF)
DUTY CYCLE (%)
37551 G17
37551 G18
37551 G16
37551fa
6
LT3755/LT3755-1
PIN FUNCTIONS (MSOP/QFN)
PWMOUT(Pin1/Pin11):BufferedVersionofPWMSignal
forDrivingLEDLoadDisconnectNMOSorLevelShift.This
pinalsoservesaprotectionfunctionfortheFBovervoltage
condition—will toggle if the FB input is greater than the FB
VC (Pin 5/Pin 15): Transconductance Error Amplifier
Output Pin Used to Stabilize the Voltage Loop with an RC
Network. This pin is high impedance when PWM is low, a
feature that stores the demand current state variable for
thenextPWMhightransition.Connectacapacitorbetween
this pin and GND; a resistor in series with the capacitor is
recommended for fast transient response.
regulationvoltage(V )plus60mV(typical).ThePWMOUT
FB
pin is driven from INTV . Use of a FET with gate cut-off
CC
voltage higher than 1V is recommended.
CTRL(Pin6/Pin16):CurrentSenseThresholdAdjustment
FB (Pin 2/Pin 12): Voltage Loop Feedback Pin. FB is
intendedforconstant-voltageregulationorforLEDprotec-
tion/open LED detection. The internal transconductance
amplifierwithoutputVCwillregulateFBto1.25V(nominal)
through the DC/DC converter. If the FB input is regulating
the loop, the OPENLED pull-down is asserted. This ac-
tion may signal an open LED fault. If FB is driven above
the FB threshold (by an external power supply spike, for
example),theOPENLEDpull-downwillbede-assertedand
the PWMOUT pin will be driven low to protect the LEDs
from an overcurrent event. Do not leave the FB pin open.
If not used, connect to GND.
Pin. Regulating threshold V – V is 1/10th V plus
ISP
ISN
CTRL
an offset. CTRL linear range is from GND to 1.1V. Connect
CTRL to V for the 100mV default threshold. Do not
REF
leave this pin open.
V
(Pin7/Pin1):VoltageReferenceOutputPin,Typically
REF
2V.ThispindrivesaresistordividerfortheCTRLpin,either
foranalogdimmingorfortemperaturelimit/compensation
of LED load. Can supply up to 100ꢀA.
PWM (Pin 8/Pin 2): A signal low turns off switcher, idles
oscillator and disconnects VC pin from all internal loads.
PWMOUT pin follows PWM pin. PWM has an internal
ISN (Pin 3/Pin 13): Connection Point for the Negative
Terminal of the Current Feedback Resistor. The LED cur-
pull-down resistor. If not used, connect to INTV .
CC
OPENLED (Pin 9/Pin 3, LT3755 Only): An open-drain
pull-down on OPENLED asserts if the FB input is greater
than the FB regulation threshold minus 50mV (typical).
To function, the pin requires an external pull-up resistor.
When the PWM input is low and the DC/DC converter is
idle, the OPENLED condition is latched to the last valid
state when the PWM input was high. When PWM input
goes high again, the OPENLED pin will be updated. This
pin may be used to report an open LED fault.
rent can be programmed by I
= 100mV/R
when
LED
LED
V
> 1.2V or I
= V
–100mV/(10 • R ). If ISN
CTRL
LED
CTRL LED
is greater than 2.9V, input bias current is typically 20μA.
Below 3V, ISN is an input to the short-circuit protection
feature that forces GATE to 0V if ISN is more than 150mV
(typ) below ISP.
ISP(Pin4/Pin14):ConnectionPointforthePositiveTermi-
naloftheCurrentFeedbackResistor. Inputbiascurrentfor
thispinistypically30μA.ISPisaninputtotheshort-circuit
protection feature when ISP is less than 3.1V.
37551fa
7
LT3755/LT3755-1
PIN FUNCTIONS (MSOP/QFN)
SYNC (Pin 9/Pin 3, LT3755-1 Only):The SYNC pin is used
Below the falling threshold, a 2μA pull-down current is
enabled so the user can define the hysteresis with the
external resistor selection. An undervoltage condition
resets soft-start. Tie to 0.4V, or less, to disable the device
to synchronize the internal oscillator to an external logic
level signal. The R resistor should be chosen to program
T
aninternalswitchingfrequency20%slowerthantheSYNC
pulse frequency. Gate turn-on occurs a fixed delay after
the rising edge of SYNC. For best PWM performance, the
PWM rising edge should occur at least 200ns before the
SYNC rising edge. Use a 50% duty cycle waveform to
drive this pin. This pin replaces OPENLED on LT3755-1
option parts. If not used, tie this pin to GND.
and reduce V quiescent current below 1μA. Do not tie
IN
SHDN/UVLO to a voltage higher than V .
IN
INTV (Pin13/Pin7):RegulatedSupplyforInternalLoads,
CC
GATE Driver and PWMOUT Driver. Supplied from V and
IN
regulates to 7V (typical). INTV must be bypassed with
CC
a 4.7μF capacitor placed close to the pin. Connect INTV
CC
SS (Pin 10/Pin 4): Soft-Start Pin. This pin modulates
oscillator frequency and compensation pin voltage (VC)
clamp.Thesoft-startintervalissetwithanexternalcapaci-
tor. The pin has a 10μA (typical) pull-up current source
to an internal 2.5V rail. The soft-start pin is reset to GND
by an undervoltage condition (detected by SHDN/UVLO
pin) or thermal limit.
directly to V if V is always less than or equal to 7V.
IN
IN
V
(Pin 14/Pin 8): Input Supply Pin. Must be locally
IN
bypassed with a 0.22μF (or larger) capacitor placed close
to the IC.
SENSE (Pin 15/Pin 9): The current sense input for the
control loop. Kelvin connect this pin to the positive ter-
minal of the switch current sense resistor, R
, in the
SENSE
RT (Pin 11/Pin 5): Switching Frequency Adjustment Pin.
Set the frequency using a resistor to GND (for resistor
values, see the Typical Performance curve or Table 1). Do
not leave the RT pin open.
source of the NFET. The negative terminal of the current
sense resistor should be connected to the GND plane
close to the IC.
GATE (Pin 16/Pin 10): N-Channel FET Gate Driver Output.
SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage
Detect Pin. An accurate 1.22V falling threshold with ex-
ternally programmable hysteresis detects when power is
OK to enable switching. Rising hysteresis is generated by
the external resistor divider and an accurate internal 2μA
pull-down current. Above the 1.24V (nominal) threshold
(but below 6V), SHDN/UVLO input bias current is sub-μA.
Switches between INTV and GND. Driven to GND during
CC
shutdown, fault or idle states.
ExposedPad(Pin17/Pin17):Ground.Thispinalsoserves
as current sense input for control loop, sensing negative
terminal of current sense resistor. Solder the Exposed Pad
directly to ground plane.
37551fa
8
LT3755/LT3755-1
BLOCK DIAGRAM
SHDN/UVLO
–
+
A6
FB
VC
PWMOUT PWM
1.25V
V
IN
–
+
SHDN
1.22V
2μA
1.3V
LDO
–
+
OVFB
COMPARATOR
A8
INTV
CC
7V
A5
+
–
10μA AT
FB = 1.25V
g
m
1.25V
SHORT-CIRCUIT
DETECT
SCILMB
10μA
+
SCILMB
A10
GATE
SENSE
GND
+
+
–
150mV
–
R
Q
–
g
m
A2
DRIVER
S
EAMP
ISN
ISP
PWM
COMPARATOR
+
–
10μA AT
5k
A1
I
+
–
SENSE
A1 = A1
+
–
CTRL
A4
BUFFER
CTRL
+
–
1.1V
A3
Q2
+
RAMP
GENERATOR
VC
SSCLAMP
10μA
50k
50KHz TO 1MHz
OSCILLATOR
OPENLED
FAULT
–
+
140μA
LOGIC
1.25V
+
+
–
1.2V
FB
OPTION
FOR
LT3755
V
REF
–
+
FREQ
PROG
TSD
A7
2V
OPTION FOR
LT3755-1
SS
RT
SYNC
37551 BD
37551fa
9
LT3755/LT3755-1
OPERATION
TheLT3755isaconstant-frequency,currentmodecontrol-
ler with a low side NMOS gate driver. The GATE pin and
PWMOUT pin drivers and other chip loads are powered
of the output state of the PWM comparator. Likewise, at
an ISP/ISN common mode voltage less than 3V, the dif-
ference 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 150mV (typical), the
SR latch will be reset regardless of the PWM comparator.
These functions are intended to protect the power switch
as well as various external components in the power path
of the DC/DC converter.
from INTV , which is an internally regulated supply. In
CC
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 positive 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
asslopecompensationreturntotheirstartingpointsanda
new cycle begins with the set pulse from the oscillator.
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 (nominal) 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 V
.
REF
Two LED specific functions featured on the LT3755 are
controlled by the voltage feedback pin. First, when the
FB pin exceeds a voltage 50mV lower (–4%) than the FB
regulation voltage, the pull-down driver on the OPENLED
pin is activated. This function provides a status indicator
thattheloadmaybedisconnectedandtheconstant-voltage
feedback loop is taking control of the switching regulator.
WhentheFBpinexceedstheFBregulationvoltageby60mV
(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 damag-
ing the LEDs. If the FB input exceeds both the open LED
and the overvoltage (OV) thresholds, then an externally
driven overvoltage event has caused the FB pin to be too
high and the OPENLED pull-down will be deactivated and
locked out until the FB pin drops below both thresholds.
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 current 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 108mV (typical). If the SENSE pin exceeds
the current limit threshold, the SR latch is reset regardless
37551fa
10
LT3755/LT3755-1
APPLICATIONS INFORMATION
INTV Regulator Bypassing and Operation
Programming the Turn-On and Turn-Off Thresholds
with the SHDN/UVLO Pin
CC
The INTV pin requires a capacitor for stable operation
CC
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. The value of the capacitor
is determined primarily by the stability of the regulator
The falling UVLO value can be accurately set by the resis-
tor divider. A small 2μA pull-down current is active when
SHDN/UVLO is below the 1.24V 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:
ratherthanthegatecharge,Q ,oftheswitchingNMOS—a
G
4.7μF capacitor will be adequate for many applications.
Place the capacitor close to the IC to minimize the trace
R1+R2
VIN,FALLING =1.24•
R2
VIN,RISING HYST = 2μA •R1
length to the INTV pin and also to the IC ground.
CC
An internal current limit on the INTV output protects
CC
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.
V
IN
R1
LT3755
SHDN/UVLO
I
can be calculated from the following equation:
R2
INTVCC
37551 F01
I
= Q • f
G OSC
INTVCC
Careful choice of a lower Q FET will allow higher switch-
Figure 1
G
ingfrequencies, leadingtosmallermagnetics. TheINTV
CC
pin has its own undervoltage disable (UVLO) set to 4.3V
(typical)toprotecttheexternalFETsfromexcessivepower
dissipation caused by not being fully enhanced. If the
LED Current Programming
The LED current is programmed by placing an appropriate
valuecurrentsenseresistorbetweentheISPandISNpins.
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 1.1V 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 1.1V, the LED
current is:
INTV pin drops below the UVLO threshold, the GATE
CC
and PWMOUT pins will be forced to 0V and the soft-start
pin will be reset.
If the input voltage, V , will not exceed 7V, then the
IN
INTV pin should be connected to the input supply. Be
CC
aware that a small current (typically less than 10ꢀA) will
load the INTV in shutdown. If V is normally above, but
CC
IN
occasionally drops below the INTV regulation voltage,
CC
then the minimum operating V will be close to 6V. This
IN
value is determined by the dropout voltage of the linear
regulator and the 4.5V (4.3V typical) INTV undervoltage
CC
lockout threshold mentioned above.
VCTRL − 100mV
ILED
=
RLED • 10
37551fa
11
LT3755/LT3755-1
APPLICATIONS INFORMATION
CTRL
regulated to:
When V
is higher than 1.1V, the LED current is
ForaboosttypeLEDdriver,settheresistorfromtheoutput
to the FB pin such that the expected V during normal
FB
operationwillnotexceed1.1V. ForanLEDdriverofbuckor
a buck-boost configuration, the output voltage is typically
level-shifted to a signal with respect to GND as illustrated
in Figure 3. The output can be expressed as:
100mV
RLED
ILED
=
The LED current programming feature can increase total
dimming range by a factor of 10. The CTRL pin should
not be left open (tie to V
R1
VOUT = VBE + 1.25 •
R2
if not used). The CTRL pin
REF
can also be used in conjunction with a thermistor to
provide overtemperature protection for the LED load, or
ISP/ISN Short-Circuit Protection Feature
with a resistor divider to V to reduce output power and
IN
The ISP and ISN pins have a protection feature indepen-
dent of the LED current sense feature that operates at
ISN below 3V. The purpose of this feature is to provide
continuous current sensing when ISN is below the LED
current sense common mode range (during start-up or
an output short-circuit fault) to prevent the development
of excessive switching currents that could damage the
power components. The action threshold (150mV, typ) is
above the default LED current sense threshold, so that no
interference will occur over the ISN voltage range where
these two functions overlap. This feature acts in the same
manner as SENSE current limit—it prevents GATE from
going high (switch turn-on) until the ISP/ISN difference
falls below the threshold.
switching current when V is low. The presence of a time
IN
varying differential voltage signal (ripple) across ISP and
ISN at the switching frequency is expected. The amplitude
of this signal is increased by high LED load current, low
switching frequency and/or a smaller value output filter
capacitor. Some level of ripple signal is acceptable: the
compensation capacitor on the VC pin filters the signal so
the average difference between ISP and ISN is regulated
to the user-programmed value. Ripple voltage amplitude
(peak-to-peak) in excess of 20mV should not cause mis-
operation, but may lead to noticeable offset between the
average value and the user-programmed value.
Programming Output Voltage (Constant Voltage
Regulation) or Open LED/Overvoltage Threshold
Dimming Control
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 2) according
to the following equation:
There are two methods to control the current source for
dimming using the LT3755. One method uses the CTRL
pin to adjust the current regulated in the LEDs. A second
method uses the PWM pin to modulate the current source
R1+R2
VOUT = 1.25 •
R2
+
R1
R
SEN(EXT)
LED
V
V
OUT
OUT
ARRAY
100k
–
R1
LT3755
LT3755
FB
FB
R2
R2
37551 F03
37551 F02
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
37551fa
12
LT3755/LT3755-1
APPLICATIONS INFORMATION
between zero and full current to achieve a precisely pro-
grammedaveragecurrent. Tomakethismethodofcurrent
controlmoreaccurate,theswitchdemandcurrentisstored
on the VC node during the quiescent phase when PWM is
low. This feature minimizes recovery time when the PWM
signal goes high. To further improve the recovery time, a
disconnect switch may be used in the LED current path to
prevent the ISP node from discharging during the PWM
signal low phase. The minimum PWM on or off time will
depend on the choice of operating frequency through the
RT input. For best current accuracy, the minimum PWM
low or high time should be at least six switching cycles
maynotallowsufficientlyhighorlowdutycycleoperation.
Lowerfrequencyoperationgivesbetterperformanceatthe
cost of larger external component size. For an appropri-
ate R resistor value see Table 1 or Figure 4. An external
T
resistor from the RT pin to GND is required—do not leave
this pin open.
Table 1. Switching Frequency vs RT Value (1% Resistors)
f
(kHz)
R (kΩ)
T
OSC
1000
10
400
200
100
28.7
53.6
100
(6ꢀsforf =1MHz).MaximumPWMperiodisdetermined
SW
by the system and is unlikely to be longer than 12ms.
Duty Cycle Considerations
The maximum PWM dimming ratio (PWM
) can be
) and the
(RATIO)
Switching duty cycle is a key variable defining converter
operation, therefore, its limits must be considered when
programming the switching frequency for a particular
application. The fixed minimum on-time and minimum
off-time (see Figure 5) and the switching frequency define
the minimum and maximum duty cycle of the switch,
respectively. The following equations express the mini-
mum/maximum duty cycle:
calculated from the maximum PWM period (t
MAX
minimum PWM pulse width (t ) as follows:
MIN
tMAX
tMIN
PWMRATIO
=
t
= 9ms, t
MIN
= 6ꢀs (f = 1MHz)
MAX
SW
PWM
= 9ms/6ꢀs = 1500:1
RATIO
Min Duty Cycle = (minimum on-time) • switching
frequency
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz to optimize
efficiency/performanceorexternalcomponentsize.Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
Max Duty Cycle = 1 – (minimum off-time) • switching
frequency
When calculating the operating limits, the typical values
for on/off-time in the datasheet should be increased by
300
10000
1000
100
C
= 3300pF
GATE
250
200
150
100
50
MINIMUM ON-TIME
MINIMUM OFF-TIME
0
–50
10
0
25
50
75 100 125
–25
10
100
TEMPERATURE (°C)
R
(k)
37551 F04
T
37551 F05
Figure 5. Typical Minimum On and Off
Pulse Width vs Temperature
Figure 4. Switching Frequency vs RT
37551fa
13
LT3755/LT3755-1
APPLICATIONS INFORMATION
at least 100ns to allow margin for PWM control latitude,
GATE rise/fall times and SW node rise/fall times.
20% lower than the external clock frequency. The SYNC
pin is disabled during the soft-start period.
Observation of the following guidelines about the SYNC
waveform will ensure proper operation of this feature.
Driving SYNC with a 50% duty cycle waveform is always
agoodchoice, otherwise, maintainthedutycyclebetween
20%and60%. WhenusingbothPWMandSYNCfeatures,
the PWM signal rising edge should occur at least 200ns
Thermal Considerations
The LT3755 and LT3755-1 are rated to a maximum input
voltageof40V.Carefulattentionmustbepaidtotheinternal
power dissipation of the IC at higher input voltages to en-
sure that a junction temperature of 125°C is not exceeded.
This junction limit is especially important when operating
at high ambient temperatures. The majority of the power
dissipationintheICcomesfromthesupplycurrentneeded
todrivethegatecapacitanceoftheexternalpowerMOSFET.
This gate drive current can be calculated as:
before the SYNC rising edge (V ) for optimal PWM
IH
performance. If the SYNC pin is not used, it should be
connected to GND.
Open LED Detection (LT3755)
The LT3755 provides an open-drain status pin, OPENLED,
that pulls low when the FB pin is within ~50mV of its
1.25V regulated voltage. If the open LED clamp voltage
is programmed correctly using the FB pin, then the FB
pin should never exceed 1.1V when LEDs are connected,
therefore, the only way for the FB pin to be within 50mV
of the 1.25V regulation voltage is for an open LED event to
have occurred when an open LED fault occurs, the output
may initially overshoot the FB regulation point by several
percent,duetoslewratelimitationsonVCandtheabsence
of any load on the output. In order to ensure the voltage
on switching components remains below programmed
limits, and to guarantee accurate reporting of the open
LED fault, adding a silicon diode between OPENLED and
SS is recommended, as well as a 10k resistor in series
with the soft-start capacitor, if one is used.
I
= f • Q
SW G
GATE
A low Q power MOSFET should always be used when op-
G
eratingathighinputvoltages,andtheswitchingfrequency
should also be chosen carefully to ensure that the IC does
not exceed a safe junction temperature. The internal junc-
tion temperature of the IC can be estimated by:
T = T + [V (I + f • Q ) • θ ]
J
A
IN
Q
SW
G
JA
where T is the ambient temperature, I is the quiescent
A
Q
current of the part (maximum 1.7mA) and θ is the
JA
package thermal impedance (68°C/W for the 3mm × 3mm
QFN package). For example, an application with T
A(MAX)
= 85°C, V
= 40V, f = 400kHz, and having a FET
IN(MAX)
SW
with Q = 20nC, the maximum IC junction temperature
G
will be approximately:
T = 85°C + [40V (1.7mA + 400kHz • 20nC) • 68°C/W]
J
Input Capacitor Selection
= 111°C
Theinputcapacitorsuppliesthetransientinputcurrentfor
the power inductor of the converter and must be placed
andsizedaccordingtothetransientcurrentrequirements.
Theswitchingfrequency,outputcurrentandtolerableinput
voltage ripple are key inputs to estimating the capacitor
value. An X7R type ceramic capacitor is usually the best
choicesinceithastheleastvariationwithtemperatureand
DC bias. Typically, boost and SEPIC converters require a
lower value capacitor than a buck mode converter. As-
suming that a 100mV input voltage ripple is acceptable,
The Exposed Pad on the bottom of the package must be
soldered to a ground plane. This ground should then be
connectedtoaninternalcoppergroundplanewiththermal
vias placed directly under the package to spread out the
heat dissipated by the IC.
Frequency Synchronization (LT3755-1 Only)
TheLT3755-1switchingfrequencycanbesynchronizedto
anexternalclockusingtheSYNCpin.Forproperoperation,
theR resistorshouldbechosenforaswitchingfrequency
T
37551fa
14
LT3755/LT3755-1
APPLICATIONS INFORMATION
the required capacitor value for a boost converter can be
estimated as follows:
current ripple. Use of X7R type ceramic capacitors is
recommended.
To achieve the same LED ripple current, the required filter
capacitor is larger in the boost and buck-boost mode ap-
plications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values.
1μF
A • μs
VOUT
CIN(μF) = ILED(A)
•
• TSW(μs) •
V
IN
Therefore, a 4.7μF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 1A load.
Soft-Start Capacitor Selection
WiththesameV voltagerippleof100mV,theinputcapaci-
tor for a buck converter can be estimated as follows:
IN
For many applications, it is important to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltageovershoot. Thesoft-startintervalissetbythesoft-
start capacitor selection according to the equation:
4.7μF
CIN(μF) = ILED(A) • TSW(μs)
•
A • μs
A 10μF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 1A load.
2V
10μA
TSS = CSS
•
In the buck mode configuration, the input capacitor has
large pulsed currents due to the current returned through
the Schottky diode when the switch is off. In this buck
convertercaseitisimportanttoplacethecapacitorasclose
as possible to the Schottky diode and to the GND return
of the switch (i.e., the sense resistor). It is also important
to consider the ripple current rating of the capacitor. For
best reliability, this capacitor should have low ESR and
ESL and have an adequate ripple current rating. The RMS
input current for a buck mode LED driver is:
A typical value for the soft-start capacitor is 0.01μF. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch. The soft-start capacitor
is discharged when SHDN/UVLO falls below its threshold,
during an overtemperature event or during an INTV
CC
undervoltage event. During start-up with SHDN/UVLO,
charging of the soft-start capacitor is enabled after the
first PWM high period.
Power MOSFET Selection
IIN(RMS) = ILED
•
1–D •D
(
)
Forapplicationsoperatingathighinputoroutputvoltages,
the power NMOS FET switch is typically chosen for drain
where D is the switch duty cycle.
voltage V rating and low gate charge Q . Consideration
DS
G
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER PHONE
of switch on-resistance, R
, is usually secondary be-
DS(ON)
WEB
cause switching losses dominate power loss. The INTV
CC
TDK
516-535-2600
www.tdk.com
www.kemet.com
www.murata.com
www.t-yuden.com
regulator on the LT3755 has a fixed current limit to protect
the IC from excessive power dissipation at high V , so the
Kemet
408-986-0424
814-237-1431
408-573-4150
IN
Murata
Taiyo Yuden
FET should be chosen so that the product of Q at 7V and
G
switching frequency does not exceed the INTV current
CC
limit. For driving LEDs be careful to choose a switch with
Output Capacitor Selection
a V rating that exceeds the threshold set by the FB pin
DS
in case of an open-load fault. Several MOSFET vendors
are listed in Table 3. The MOSFETs used in the application
circuits in this datasheet have been found to work well
with the LT3755. Consult factory applications for other
recommended MOSFETs.
The selection of the output capacitor depends on the load
and converter configuration, i.e., step-up or step-down
and the operating frequency. For LED applications, the
equivalent resistance of the LED is typically low and the
output filter capacitor should be sized to attenuate the
37551fa
15
LT3755/LT3755-1
APPLICATIONS INFORMATION
Table 3. MOSFET Manufacturers
The placement of R
should be close to the source of
SENSE
the NMOS FET and GND of the LT3755. The SENSE input
to LT3755 should be a Kelvin connection to the positive
VENDOR
PHONE
WEB
Vishay Siliconix
Fairchild
402-563-6866
972-910-8000
310-252-7105
www.vishay.com
www.fairchildsemi.com
www.irf.com
terminal of R
.
SENSE
International Rectifier
Inductor Selection
Schottky Rectifier Selection
TheinductorusedwiththeLT3755shouldhaveasaturation
current rating appropriate to the maximum switch current
The power Schottky diode conducts current during the
interval when the switch is turned off. Select a diode rated
forthemaximumSWvoltage. IfusingthePWMfeaturefor
dimming, it is important to consider diode leakage, which
increaseswiththetemperature,fromtheoutputduringthe
PWM low interval. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 4 has some
recommended component vendors.
selectedwiththeR
resistor.Chooseaninductorvalue
SENSE
based on operating frequency, input and output voltage
to provide a current mode ramp on SENSE during the
switch on-time of approximately 20mV magnitude. The
following equations are useful to estimate the inductor
value (T = 1/f ):
SW
OSC
TSW •RSENSE • VLED V – V
(
)
IN
LED
LBUCK
=
V • 0.02V
Table 4. Schottky Rectifier Manufacturers
IN
VENDOR
PHONE
WEB
TSW •RSENSE • VLED • V
IN
On Semiconductor
Diodes, Inc.
888-743-7826
805-446-4800
www.onsemi.com
www.diodes.com
www.centralsemi.com
LBUCK-BOOST
=
V
LED + V • 0.02V
(
)
IN
Central Semiconductor 631-435-1110
TSW •RSENSE • V VLED – VIN
(
)
IN
LBOOST
=
Sense Resistor Selection
VLED • 0.02V
The resistor, R
, between the source of the exter-
SENSE
Table 5 provides some recommended inductor vendors.
nal NMOS FET and GND should be selected to provide
adequate switch current to drive the application without
exceeding the 108mV (typical) current limit threshold on
the SENSE pin of LT3755. For buck mode applications,
select a resistor that gives a switch current at least 30%
greater than the required LED current. For buck mode,
select a resistor according to:
Table 5. Inductor Manufacturers
VENDOR
PHONE
WEB
Sumida
408-321-9660
605-886-4385
561-998-4100
402-563-6866
847-639-6400
www.sumida.com
www.we-online.com
www.cooperet.com
www.vishay.com
www.coilcraft.com
Würth Elektronik
Coiltronics
Vishay
Coilcraft
0.07V
RSENSE,BUCK
≤
ILED
For buck-boost, select a resistor according to:
V • 0.07V
Loop Compensation
TheLT3755usesaninternaltransconductanceerrorampli-
fier whose VC output compensates the control loop. The
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
IN
RSENSE,BUCK-BOOST
≤
V + V
I
(
)
IN
LED LED
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are selected to optimize control loop
response and stability. For typical LED applications, a
2.2nF compensation capacitor at VC is adequate, and
37551fa
For boost, select a resistor according to:
V • 0.07V
IN
RSENSE,BOOST
≤
VLED •ILED
16
LT3755/LT3755-1
APPLICATIONS INFORMATION
a series resistor should always be used to increase the
slew rate on the VC pin to maintain tighter regulation of
LED current during fast transients on the input supply to
the converter.
filter capacitor to GND should be minimized. The ground
points of these two switching current traces should come
toacommonpointthenconnecttothegroundplaneunder
the LT3755. Likewise, the ground terminal of the bypass
capacitor for the INTV regulator should be placed near
CC
Board Layout
the GND of the switching path. Typically this requirement
will result in the external switch being closest to the IC,
The high speed operation of the LT3755 demands careful
attention to board layout and component placement. The
Exposed Pad of the package is the only GND terminal of
the IC and is also important for thermal management of
the IC. It is crucial to achieve a good electrical and thermal
contact between the Exposed Pad and the ground plane of
theboard.Toreduceelectromagneticinterference(EMI),it
isimportanttominimizetheareaofthehighdV/dtswitching
node between the inductor, switch drain and anode of the
Schottky rectifier. Use a ground plane under the switching
node to eliminate interplane coupling to sensitive signals.
The lengths of the high dI/dt traces: 1) from the switch
node through the switch and sense resistor to GND, and
2) from the switch node through the Schottky rectifier and
along with the INTV bypass capacitor. The ground for
CC
the compensation network and other DC control signals
should be star connected to the underside of the IC. Do
not extensively route high impedance signals such as FB
and VC, as they may pick up switching noise. In particular,
avoid routing FB and PWMOUT in parallel for more than
a few millimeters on the board. Since there is a small
variable DC input bias current to the ISN and ISP inputs,
resistance in series with these pins should be minimized
to avoid creating an offset in the current sense threshold.
Likewise, minimize resistance in series with the SENSE
inputtoavoidchanges(mostlikelyreduction)totheswitch
current limit threshold.
Efficiency vs VIN
20W SEPIC LED Driver
100
C4
V
I
= 18V
OUT
LED
D1
5A
100V
1μF
= 1A
L1A
100V
22μH
V
IN
95
90
85
80
8V TO
40V
C3
4.7μF
50V
C1
4.7μF
50V
1M
V
IN
511k
SHDN/UVLO
FB
L1B
V
25k
187k
REF
CTRL
ISP
INTV
CC
LT3755
0.1Ω
1A
100k
ISN
0
20
(V)
30
40
10
M1
OPENLED
PWM
SS
GATE
V
IN
D2
20W
37551 TA04b
SENSE
PWMOUT
LED
R
T
0.015Ω
0.01μF
STRING
V
GND INTV
C
CC
28.7k
C2
4.7μF
10V
400kHz
30k
10k
0.001μF
M2
37551 TA04a
L1: WÜRTH ELEKTRONIK 744870220
M1: VISHAY SILICONIX SI754DP
D1: DIODES INC. - PDS5100
M2: VISHAY SILICONIX SI2318DS
37551fa
17
LT3755/LT3755-1
TYPICAL APPLICATIONS
50W White LED Headlamp Driver
L1
22μH
D1
V
IN
8V TO
40V
C2
4.7μF
C1
4.7μF
1M
1M
V
IN
SHDN/UVLO
FB
V
ISP
23.7k
187k
REF
16.9k
LT3755
0.1Ω
1A
CTRL
ISN
INTV
100k
NTC
RT1
CC
M1
GATE
100k
SENSE
OPENLED
PWM
SS
0.015Ω
50W
LED
D2
STRING
RT
PWMOUT
0.01μF
V
C
GND INTV
CC
28.7k
400kHz
10k
0.001μF
4.7μF
10k
M2
L1: COILTRONICS DR127-220
37551 TA02a
M1: VISHAY SILICONIX SI7850DP
D1: DIODES INC. PDS5100
M2: VISHAY SILICONIX SI2308DS
RT1: MURATA NCP18WM1045
D2: IN4448HWT
Waveforms for 50W LED Driver with
PWM Disconnect NFET
VISP-VISN vs Temperature
for NTC Resistor Divider
Efficiency vs Load
100
96
92
88
84
80
120
V
= 12V
PWM
V
= 12V
IN
IN
0V TO 5V
100
80
60
40
20
0
V
= 50V
10V/DIV
OUT
I
L1
2A/DIV
I
LED
500mA/DIV
37551 TA02b
50μs/DIV
0.0
0.4
0.6
0.8
1.0
0.2
25
45
65
85
105
125
LOAD (A)
TEMPERATURE (°C)
37551 TA02d
37551 TA02c
37551fa
18
LT3755/LT3755-1
TYPICAL APPLICATIONS
Buck Mode 1.4A LED Driver
V
IN
15V TO
40V
+
C1
1M
ISP
V
C3
4.7μF
IN
249k 14V
1μF
–
SHDN/UVLO
0.068Ω
1.4A
100k
Q1
187k
V
ISN
FB
REF
CTRL
1.5k
22.1k
M3
INTV
CC
LT3755
M2
1k
PWMOUT
3 LUXEON K2
- WHITE
100k
PWM
OPENLED
L1
33μH
SS
RT
D1
V
IN
C4
M1
GATE
4.7μF
0.1μF
V
C
GND INTV
SENSE
CC
0.033Ω
28.7k
47k
10k
C2
4.7μF
400kHz
0.001μF
37551 TA03a
L1: COILTRONICS DR125-330
M1: VISHAY SILICONIX SI7850DP
D1: ON SEMICONDUCTOR MBRS360
M2: ZETEX ZXMN4A06G
M3: ZETEX ZXM62P03E6
Q1: ZETEX FMMT558
1000:1 PWM Dimming at 120Hz
with Buck Mode
Efficiency vs VIN
100
96
92
88
84
80
V
V
= 24V
LED
IN
= 10V
PWM
0V TO 5V
I
LED
1A/DIV
I
L1
1A/DIV
37551 TA03b
2μs/DIV
15
25
30
35
40
20
V
(V)
IN
37551 TA03c
37551fa
19
LT3755/LT3755-1
TYPICAL APPLICATIONS
Buck Mode 1.5A LED Driver with PWM Dimming
V
IN
10V TO
36V
C1
1μF
0.068Ω
510k
1M
ISP
ISN
V
IN
SHDN/UVLO
1.5A
8.5V MAX
267k
249k
Q2
V
REF
100k
CTRL
PWMOUT
Q1
INTV
CC
LT3755
16V
PWM
22μF
100k
L1
10μH
1k
22.1k
D1
OPENLED
SS
RT
M1
GATE
0.1μF
10k
SENSE
INTV
V
C
GND
FB
CC
28.7k
400kHz
0.033Ω
15k
2200pF
4.7μF
37551 TA06a
M1: VISHAY SILICONIX SI7850DP
M2: VISHAY SILICONIX SI1471DH
Q1: MMBT2222A NPN
Q2: FMMT558 PNP
D1: VISHAY 30BQ060
L1: SUMIDA CDRH8D43-100
1500:1 PWM Dimming at 100Hz
Buck Mode Efficiency vs Input Voltage
100
95
90
85
80
75
70
I
LED
500mA/DIV
PWM
37551 TA06b
10μs/DIV
0
5
10 15 20
25 30 35 40
INPUT VOLTAGE (V)
37551 TA06c
37551fa
20
LT3755/LT3755-1
TYPICAL APPLICATIONS
21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection
L1
15μH
0.1k
D1
V
IN
M2
8V TO
36V
50V
2.2μF
2s
100V
2.2μF
2s
21.5V
1A
392k
0.020k
1.5k
V
IN
Q1
100k
499k
GATE SENSE
LT3755
V
IN
FB
SHDN/UVLO
20.0k
549k
93.1k
V
REF
PWMOUT
Q2
CTRL
4.7μF
PWM
PWM
75.0k
1k
INTV
CC
ISP
ISN
100k
OPENLED
37551 TA07a
SS
V
GND
RT
C
M1: VISHAY SILICONIX SI7850DP
M2: VISHAY SILICONIX SI2319DS
Q1: ZETEX FMMT558
28.7K
400KHz
4.7k
4700pF
0.01μF
10k
Q2: MMBTA42
D1: DIODES INC. PDS560
L1: SUMIDA CDRH127/LD-150
Buck-Boost Mode Efficiency vs Input Voltage
Buck-Boost Mode LED Current vs Input Voltage
100
1.10
1.05
1
95
90
0.95
0.9
85
80
0.85
0.8
0
5
10 15 20
25 30 35 40
8
9
10 11 12
13 14 15 16
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
37551 TA07b
37551 TA07c
37551fa
21
LT3755/LT3755-1
PACKAGE DESCRIPTION
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev Ø)
BOTTOM VIEW OF
EXPOSED PAD OPTION
3.556 p 0.102
(.140 p .004)
2.845 p 0.102
(.112 p .004)
3.835 p 0.102
(.151 p .004)
0.889 p 0.127
(.035 p .005)
1
8
5.23
(.206)
MIN
1.651 p 0.102 1.905 p 0.102
(.065 p .004) (.075 p .004)
2.159 p 0.102 3.20 – 3.45
(.085 p .004) (.126 – .136)
16
4.039 p 0.102
(.159 p .004)
(NOTE 3)
9
0.50
(.0197)
BSC
0.305 p 0.038
(.0120 p .0015)
TYP
0.280 p 0.076
(.011 p .003)
REF
RECOMMENDED SOLDER PAD LAYOUT
16151413121110
9
DETAIL “A”
0o – 6o TYP
0.254
(.010)
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
1 2 3 4 5 6 7 8
DETAIL “A”
0.86
(.034)
REF
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 p 0.0508
(.004 p .002)
0.50
(.0197)
BSC
MSOP (MSE16) 0907 REV Ø
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
37551fa
22
LT3755/LT3755-1
PACKAGE DESCRIPTION
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 0.05
3.50 0.05
2.10 0.05
1.45 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER
R = 0.115
TYP
0.75 0.05
3.00 0.10
(4 SIDES)
15 16
PIN 1
TOP MARK
(NOTE 6)
0.40 0.10
1
2
1.45 0.10
(4-SIDES)
(UD16) QFN 0904
0.25 0.05
0.50 BSC
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
37551fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3755/LT3755-1
TYPICAL APPLICATION
Buck-Boost LED Driver for Automotive
Efficiency vs VIN
V
IN
100
96
92
88
84
80
6V TO
36V
4.7μF
x2
LTC4440-5
0.22μF
D2
INTV
CC
V
BOOST
TG
CC
GND
M1
TS
INP
22μF
D1
V
IN
1M
SHDN/UVLO
V
M2
GATE
1M
REF
330k
4.7μF
383k
LT3755
SENSE
CTRL
0.025Ω
40k
INTV
CC
47k
0
20
(V)
30
40
10
FB
V
IN
100k
37551 TA05b
ISP
OPENLED
PWM
SS
0.1Ω
1A
D2
ISN
RT
PWMOUT
0.01μF
V
C
GND INTV
CC
28.7k
400kHz
1%
INTV
CC
10k
0.01μF
10k
4.7μF
M1, M2: VISHAY SILICONIX SI7850DP
D1, D2: DIODES, INC SBM540
37551 TA05a
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 4V to 36V, V
LT3474
36V, 1A (I ), 2MHz, Step-Down LED Driver
= 13.5V, True Color PWM Dimming = 400:1,
LED
IN
OUT(MAX)
OUT(MAX)
I
< 1μA, TSSOP16E Package
SD
LT3475
Dual 1.5A (I ), 36V, 2MHz Step-Down LED Driver
V : 4V to 36V, V
= 13.5V, True Color PWM Dimming = 3000:1,
LED
IN
SD
I
< 1μA, TSSOP20E Package
LT3476
Quad Output 1.5A, 36V, 2MHz High Current
LED Driver with 1000:1 Dimming
V : 2.8V to 16V, V
SD
= 36V, True Color PWM Dimming = 1000:1,
IN
OUT(MAX)
OUT(MAX)
I
< 10μA, 5mm × 7mm QFN Package
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
V : 2.5V to 25V, V
= 40V, Dimming = Analog/PWM,
IN
SD
I
< 1μA, QFN and TSSOP20E Packages
LT3478/LT3478-1
LT3486
4.5A, 42V, 2.5MHz High Current LED Driver with
3000:1 Dimming
V : 2.8V to 36V, V
SD
= 42V, True Color PWM Dimming = 3000:1,
IN
OUT(MAX)
I
< 3μA, TSSOP16E Package
Dual 1.3A, 2MHz High Current LED Driver
Triple 0.75A, 2.1MHz, 45V LED Driver
1.3A, 2.5MHz, 45V LED Driver
V : 2.5V to 24V, V
SD
= 36V, True Color PWM Dimming = 1000:1,
IN
OUT(MAX)
I
< 1μA, 5mm × 3mm DFN and TSSOP16E Packages
LT3496
V : 3V to 30V, V
SD
= 45V, Dimming = 3000:1,
IN
OUT(MAX)
I
< 1μA, 4mm × 5mm QFN and TSSOP16E Packages
LT3517
V : 3V to 30V, V
SD
= 45V, Dimming = 3000:1,
IN
OUT(MAX)
I
< 1μA, 4mm × 4mm QFN and TSSOP16E Packages
LT3518
2.3A, 2.5MHz, 45V LED Driver
V : 3V to 30V, V
SD
= 45V, Dimming = 3000:1,
OUT(MAX)
IN
I
< 1μA, 4mm × 4mm QFN and TSSOP16E Packages
LT3756
100V , 100V
LED Controller
V : 6V to 100V, V
SD
= 100V, True Color PWM Dimming = 3000:1,
IN
OUT
IN
OUT(MAX)
I
< 1μA, 3mm × 3mm QFN-16 and MS16E Packages
LTC®3783
High Current LED Controller
V : 3V to 36V, V
SD
= Ext FET, True Color PWM Dimming = 3000:1,
IN
OUT(MAX)
I
< 20μA, 5mm × 4mm QFN10 and TSSOP16E Packages
37551fa
LT 1008 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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