LT3755IMSE#PBF [Linear]
LT3755/LT3755-1/LT3755-2 - 40VIN, 75VOUT LED Controllers; Package: MSOP; Pins: 16; Temperature Range: -40°C to 85°C;
| 型号： | LT3755IMSE#PBF |
| 厂家： | 
Linear |
| 描述： | LT3755/LT3755-1/LT3755-2 - 40VIN, 75VOUT LED Controllers; Package: MSOP; Pins: 16; Temperature Range: -40°C to 85°C 驱动 光电二极管 接口集成电路 |
| 文件： | 总24页 (文件大小：262K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3755/LT3755-1/LT3755-2
40V , 75V LED Controllers
IN
OUT
FEATURES
DESCRIPTION
n
3000:1 True Color PWM™ Dimming
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 7V
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.
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
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
dimmingcapability.Thesepartsareavailableinthe16-lead
(3mm × 3mm) QFN and MSOP packages.
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
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.
n
Battery Chargers
n
Accurate Current Limited Voltage Regulators
Protected by U.S. Patents including 7199560, 7321203.
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
LT3755-2
0.1Ω
1A
92
88
84
80
CTRL
ISN
INTV
CC
40.2k
GATE
100k
SENSE
OPENLED
PWM
SS
0.015Ω
50W
LED
STRING
PWMOUT
RT
V
C
GND INTV
CC
28.7k
400kHz
0.01μF
0
10
30
40
20
(V)
10k
0.001μF
4.7μF
V
IN
37551 TA01b
37551 TA01a
37551fc
1
LT3755/LT3755-1/LT3755-2
(Note 1)
ABSOLUTE MAXIMUM RATINGS
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 (Note 3)................................................40V
ISP, ISN .....................................................................75V
INTV ......................................................V + 0.3V, 8V
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
GND
GATE
10
9
ISP
13 INTV
CC
17
GND
VC
12 SHDN/UVLO
11 RT
SENSE
CTRL
V
10 SS
5
6
7
8
REF
PWM
9
SYNC OR OPENLED
MSE PACKAGE
16-LEAD PLASTIC MSOP
T
= 125°C, θ = 43°C/W, θ = 4°C/W
JMAX
JA JC
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
TAPE AND REEL
PART MARKING*
LDGC
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3755EUD#PBF
LT3755EUD#TRPBF
LT3755IUD#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
–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 (3mm × 3mm) Plastic QFN
16-Lead (3mm × 3mm) Plastic QFN
16-Lead Plastic MSOP
LT3755IUD#PBF
LDGC
LT3755EUD-1#PBF
LT3755IUD-1#PBF
LT3755EUD-2#PBF
LT3755IUD-2#PBF
LT3755EMSE#PBF
LT3755IMSE#PBF
LT3755EMSE-1#PBF
LT3755IMSE-1#PBF
LT3755EMSE-2#PBF
LT3755IMSE-2#PBF
LT3755EUD-1#TRPBF
LT3755IUD-1#TRPBF
LT3755EUD-2#TRPBF
LT3755IUD-2#TRPBF
LT3755EMSE#TRPBF
LT3755IMSE#TRPBF
LT3755EMSE-1#TRPBF
LT3755IMSE-1#TRPBF
LT3755EMSE-2#TRPBF
LT3755IMSE-2#TRPBF
LDMS
LDMS
LFJZ
LFJZ
3755
3755
16-Lead Plastic MSOP
37551
37551
37552
37552
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
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/
37551fc
2
LT3755/LT3755-1/LT3755-2
ELECTRICAL CHARACTERISTICS The ldenotes the specifications which apply over the full operating
temperature 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
l
V
V
Minimum Operating Voltage
V
IN
4.5
V
IN
IN
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, R = 100k to GND
1.4
2.00
0.006
108
1.7
mA
V
IN
Q
T
l
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
118
13
SENSE Input Bias Current
Current Out of Pin
Current Out of Pin
40
SS Pull-Up Current
8
10.5
μA
Error Amplifier
l
l
l
l
ISP/ISN Full-Scale Current Sense Threshold
ISP/ISN Full-Scale Current Sense Threshold at CTRL = 0V
CTRL Threshold Linear Programming Range
CTRL Input Bias Current
FB = 0V, ISP = 48V
96
–12
0
100
103
–7
mV
mV
V
CTRL = 0V, FB = 0V, ISP = 48V
–9.5
1.1
100
75
Current Out of Pin
ISN = 0V
50
nA
V
LED Current Sense Amplifier Input Common Mode Range
ISP/ISN Short-Circuit Threshold
2.9
115
0
150
200
3
mV
V
ISP/ISN Short-Circuit Fault Sensing Common Mode Range
ISP/ISN Input Bias Current
PWM = 5V (Active), ISP = 48V
PWM = 0V (Standby), ISP = 48V
50
0
μA
μA
0.1
LED Current Sense Amplifier g
VC Output Impedance
V
= 100mV
120
μS
kΩ
nA
m
(ISP – ISN)
1V < VC < 2V
PWM = 0V
15000
VC Standby Input Bias Current
–20
20
FB Regulation Voltage (V
)
FB
1.232
1.220
1.250
1.250
1.265
1.270
V
V
l
ISP = ISN
FB Amplifier g
FB = V , ISP = ISN
480
40
μS
nA
V
m
FB
FB Pin Input Bias Current
FB Open LED Threshold
Current Out of Pin
100
OPENLED Falling (LT3755 and LT3755-2)
V
V
FB
V
FB
–40mV
FB
–65mV
–50mV
FB Overvoltage Threshold
PWMOUT Falling
V
V
V
FB
+75mV
V
FB
FB
+50mV
+60mV
4
V/V
VC Current Mode Gain – (ΔV /ΔV
)
SENSE
VC
Oscillator
l
Switching Frequency
R = 100k
T
90
925
105
1000
125
1050
kHz
kHz
T
R = 10k
Minimum Off-Time
170
ns
37551fc
3
LT3755/LT3755-1/LT3755-2
ELECTRICAL CHARACTERISTICS The ldenotes the specifications which apply over the full operating
temperature 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
)
CC
I
= –10mA, V = 7V
INTVCC IN
IN
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
l
l
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 and LT3755-2)
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
)
OL
0.05
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, LT3755E-1 and LT3755-2 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, LT3755I-1 and LT3755I-2 are
guaranteed to meet performance specifications over the –40°C to 125°C
operating junction temperature range.
Note 3: For V below 6V, the SHDN/UVLO pin must not exceed V for
IN
IN
proper operation.
37551fc
4
LT3755/LT3755-1/LT3755-2
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
V(ISP – ISN) Threshold
vs Temperature
V(ISP – ISN) Threshold vs VCTRL
V(ISP – ISN) 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 Regulation 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
–50
0
25
50
75 100 125
–50
0
25
50
75 100 125
–25
0
10
20
(V)
30
40
–25
TEMPERATURE (°C)
TEMPERATURE (°C)
V
IN
37551 G05
37551 G04
37551 G06
SHDN/UVLO Hysteresis Current
vs Temperature
Switching Frequency
vs Temperature
Switching Frequency vs RT
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
–50
0
25
50
75 100 125
–25
–25
R
(k)
TEMPERATURE (°C)
TEMPERATURE (°C)
T
37551 G07
37551 G08
37551 G09
37551fc
5
LT3755/LT3755-1/LT3755-2
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
SENSE Current Limit Threshold
vs Temperature
SHDN/UVLO Threshold
Quiescent Current vs VIN
vs Temperature
110
105
100
95
1.28
1.26
1.24
1.22
1.20
1.18
2.0
1.5
1.0
0.5
0
PWM = 0V
SHDN/UVLO RISING
SHDN/UVLO FALLING
90
50
75 100 125
–50
–25
0
25
50
75 100 125
–50
0
25
0
10
20
(V)
30
40
–25
TEMPERATURE (°C)
TEMPERATURE (°C)
V
IN
37551 G11
37551 G12
37551 G10
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
115
110
105
100
95
V
= 2V
10% TO 90%
CTRL
GATE RISE
TIME
GATE
FALL TIME
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
37551fc
6
LT3755/LT3755-1/LT3755-2
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
ISP/ISN Input Bias Current
vs CTRL Voltage
INTVCC Dropout Voltage
vs Current, Temperature
0
–0.5
–1.0
–1.5
–2.0
–2.5
40
30
20
10
0
T
= –45°C
A
ISP
ISN
T
= 125°C
T
A
= 25°C
A
V
= 7V
IN
0
5
10
15
20
25
30
0
0.5
1
1.5
2
LDO CURRENT (mA)
CTRL (V)
37551 G20
37551 G19
PIN FUNCTIONS
(MSOP/QFN)
PWMOUT(Pin1/Pin11):BufferedVersionofPWMSignal
forDrivingLEDLoadDisconnectNMOSorLevelShift.This
pinalsoservesaprotectionfunctionfortheFBovervoltage
condition—will toggle if the FB input is greater than the FB
ISN (Pin 3/Pin 13): Connection Point for the Negative
TerminaloftheCurrentFeedbackResistor. IfISNisgreater
than 2.9V, the LED current can be programmed by I
=
LED
–100mV)/
100mV/R whenV
>1.2VorI =(V
LED
CTRL
LED
CTRL
(10 • R ). Input bias current is typically 20μA. Below
regulationvoltage(V )plus60mV(typical).ThePWMOUT
LED
FB
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.
pin is driven from INTV . Use of a FET with gate cut-off
CC
voltage higher than 1V is recommended.
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.
ISP(Pin4/Pin14):ConnectionPointforthePositiveTermi-
naloftheCurrentFeedbackResistor. Inputbiascurrentfor
thispinistypically30μA.ISPisaninputtotheshort-circuit
protection feature when ISP is less than 3.1V.
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.
37551fc
7
LT3755/LT3755-1/LT3755-2
PIN FUNCTIONS (MSOP/QFN)
CTRL(Pin6/Pin16):CurrentSenseThresholdAdjustment
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.
Pin. Regulating threshold 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.
SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage
Detect Pin. An accurate 1.22V falling threshold with exter-
nally 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
below6V),SHDN/UVLOinputbiascurrentissub-μA.Below
thefallingthreshold,a2μApull-downcurrentisenabledso
the user can define the hysteresis with the external resis-
tor selection. An undervoltage condition resets soft-start.
V
REF
(Pin7/Pin1):VoltageReferenceOutputPin,Typically
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
pull-down resistor. If not used, connect to INTV .
CC
OPENLED (Pin 9/Pin 3, LT3755 and LT3755-2): An open-
collector 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
current less than 1mA. 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.
Tie to 0.4V, or less, to disable the device and reduce V
IN
quiescent current below 1μA. For proper operation, do not
tie 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
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
SYNC (Pin 9/Pin 3, LT3755-1 Only):The SYNC pin is used
to the IC.
to synchronize the internal oscillator to an external logic
level signal. The R resistor should be chosen to program
T
SENSE (Pin 15/Pin 9): The current sense input for the
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.
control loop. Kelvin connect this pin to the positive ter-
minal of the switch current sense resistor, R
, in the
SENSE
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.
Switches between INTV and GND. Driven to GND during
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.
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.
37551fc
8
LT3755/LT3755-1/LT3755-2
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
AND
LT3755-2
V
REF
–
+
FREQ
PROG
TSD
170k
A7
2V
OPTION FOR
LT3755-1
SS
RT
SYNC
37551 BD
37551fc
9
LT3755/LT3755-1/LT3755-2
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
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 (LT3755 and LT3755-2 only). This func-
tion provides a status indicator that the load may be
disconnected and the constant-voltage feedback loop is
taking control of the switching regulator. When the FB pin
exceeds the FB regulation voltage by 60mV (5% typical),
the PWMOUT pin is driven low, ignoring the state of the
PWM input. In the case where the PWMOUT pin drives a
disconnectNFET,thisactionisolatestheLEDloadfromGND
preventing excessive current from damaging the LEDs. If
theFBinputexceedsboththeopenLEDandtheovervoltage
thresholds, then an externally driven overvoltage event
has caused the FB pin to be too high and the OPENLED
pull-downwillbede-asserted. TheLT3755-2willre-assert
the OPENLED signal when FB falls below the overvoltage
thresholdandremainsabovetheOPENLEDthreshold.The
LT3755 is prevented from re-asserting OPENLED until FB
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
of the output state of the PWM comparator. Likewise, at
an ISP/ISN common mode voltage less than 3V, the dif-
37551fc
10
LT3755/LT3755-1/LT3755-2
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
length to the INTV pin and also to the IC ground.
CC
An internal current limit on the INTV output protects
CC
VIN,RISING = 2µA •R1 + V
IN,FALLING
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.
Iftheinputvoltage, V , willnotexceed8V, thentheINTV
IN
CC
pin could be connected to the input supply. Be aware that
a small current (typically less than 10ꢀA) will load the
INTV in shutdown. This action allows the LT3755 to
CC
operate from V as low as 4.5V. If V is normally above,
IN
IN
butoccasionallydropsbelowtheINTV regulationvoltage,
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
37551fc
11
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
When V
is higher than 1.1V, the LED current is
ForaboosttypeLEDdriver,settheresistorfromtheoutput
CTRL
to the FB pin such that the expected V during normal
regulated to:
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 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
37551fc
12
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
Table 1. Switching Frequency vs RT Value (1% Resistors)
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 is
affected by choice of operating frequency and external
component selection. The data sheet application titled
“Buck Mode 500mA LED Driver for 20kHz PWM Dim-
ming” demonstrates regulated current pulses as short as
1μs are achievable. The best overall combination of PWM
and analog dimming capability is available if the minimum
PWM pulse is at least six switching cycles.
f
(kHz)
R (kΩ)
OSC
T
1000
10
400
200
100
28.7
53.6
100
Duty Cycle Considerations
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:
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
maynotallowsufficientlyhighorlowdutycycleoperation.
Lowerfrequencyoperationgivesbetterperformanceatthe
cost of larger external component size. For an appropri-
Max Duty Cycle = 1 – (minimum off-time) • switching
frequency
When calculating the operating limits, the typical values
for on/off-time in the data sheet should be increased by
at least 60ns to allow margin for PWM control latitude,
GATE rise/fall times and SW node rise/fall times.
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.
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
37551fc
13
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
Thermal Considerations
agoodchoice, otherwise, maintainthedutycyclebetween
20%and60%. WhenusingbothPWMandSYNCfeatures,
the PWM signal rising edge should occur at least 200ns
The LT3755 is rated to a maximum input voltage of 40V.
Careful attention must be paid to the internal power dis-
sipation of the IC at higher input voltages to ensure that
a junction temperature of 125°C is not exceeded. This
junction limit is especially important when operating at
highambienttemperatures. Themajorityofthepowerdis-
sipationintheICcomesfromthesupplycurrentneededto
drive the gate capacitance of the external power MOSFET.
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 and LT3755-2)
The LT3755 and LT3755-2 provide 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. The key difference
between the LT3755 and LT3755-2 is the behavior of the
OPENLED pin when the FB pin crosses and re-crosses
the FB overvoltage threshold (1.3V typ). The LT3755-2
asserts/de-asserts OPENLED freely when crossing the
1.3V threshold. The LT3755, by comparison, de-asserts
OPENLED when FB exceeds 1.3V and is prevented from
re-asserting OPENLED until the FB pin falls below the
1.2V (typ) open LED threshold and clears the fault. The
LT3755-2 has the more general purpose behavior and is
recommended for applications using OPENLED.
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 required capacitor value for a boost converter can be
estimated as follows:
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
20% lower than the external clock frequency. The SYNC
pin is disabled during the soft-start period.
VOUT
V
IN
⎛ µF ⎞
CIN(µF) = ILED(A) •
• TSW(µs) •
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
⎜
⎟
⎝
⎠
A • µs
37551fc
14
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
Therefore, a 10μF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 1A load.
operating frequencies will require proportionately higher
capacitor values.
Soft-Start Capacitor Selection
WiththesameV voltagerippleof100mV,theinputcapaci-
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:
tor for a buck converter can be estimated as follows:
⎛ µF ⎞
CIN(µF) = ILED(A) • TSW(µs) • 4.7 •
⎜
⎟
⎝
⎠
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
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
current ripple. Use of X7R type ceramic capacitors is
recommended.
in case of an open-load fault. Several MOSFET vendors
are listed in Table 3. The MOSFETs used in the application
circuits in this data sheet have been found to work well
with the LT3755. Consult factory applications for other
recommended MOSFETs.
Table 3. MOSFET Manufacturers
VENDOR
PHONE
WEB
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
Vishay Siliconix
Fairchild
402-563-6866
972-910-8000
310-252-7105
www.vishay.com
www.fairchildsemi.com
www.irf.com
International Rectifier
37551fc
15
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
Schottky Rectifier Selection
cycle, and those with discontinuous conduction mode
(DCM) switching. It is always prudent to verify the peak
inductor current in the application to ensure the sense
resistor selection provides margin to the SENSE current
limit threshold.
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.
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
terminal of R
.
SENSE
Table 4. Schottky Rectifier Manufacturers
Inductor Selection
VENDOR
PHONE
WEB
TheinductorusedwiththeLT3755shouldhaveasaturation
current rating appropriate to the maximum switch current
On Semiconductor
Diodes, Inc.
888-743-7826
805-446-4800
www.onsemi.com
www.diodes.com
www.centralsemi.com
selectedwiththeR
resistor.Chooseaninductorvalue
SENSE
Central Semiconductor 631-435-1110
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 for
continuous conduction mode operation:
Sense Resistor Selection
The resistor, R
, between the source of the exter-
SENSE
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:
RSENSE • VLED V – V
(
)
IN
LED
LBUCK
=
V • 0.02V • f
IN
OSC
RSENSE • VLED • V
IN
LBUCK-BOOST
=
V
LED + V • 0.02V • f
IN
OSC
(
)
0.07V
ILED
RSENSE,BUCK
≤
RSENSE • V
V
LED – VIN
(
)
IN
LBOOST
=
VLED • 0.02V • fOSC
For buck-boost, select a resistor according to:
Table 5 provides some recommended inductor vendors.
V • 0.07V
IN
RSENSE,BUCK-BOOST
≤
Table 5. Inductor Manufacturers
V + V
I
(
)
IN
LED LED
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
For boost, select a resistor according to:
Würth Elektronik
Coiltronics
Vishay
V • 0.07V
IN
RSENSE,BOOST
≤
V
LED
•ILED
Coilcraft
These equations provide an estimate of the sense resistor
value based on reasonable assumptions about induc-
tor current ripple during steady state switching. Lower
values of sense resistor may be required in applications
where inductor ripple current is higher. Examples include
applications with current limited operation at high duty
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.
37551fc
16
LT3755/LT3755-1/LT3755-2
APPLICATIONS INFORMATION
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
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.
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
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
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. Minimize resistance in
series with the SENSE input to avoid changes (most likely
reduction) to the switch current limit threshold.
Efficiency vs VIN
20W SEPIC LED Driver
100
C4
D1
V
LED
= 18V
OUT
1μF
5A
L1A
I
= 1A
100V
100V
22μH
V
IN
8V TO
40V
95
90
85
80
C3
4.7μF
50V
C1
4.7μF
50V
1M
V
IN
511k
SHDN/UVLO
FB
L1B
V
25k
187k
REF
CTRL
ISP
INTV
CC
LT3755-2
0.1Ω
1A
100k
ISN
M1
OPENLED
GATE
0
20
(V)
30
40
10
PWM
SS
RT
V
IN
20W
SENSE
PWMOUT
37551 TA04b
LED
0.015Ω
0.01μF
STRING
V
GND INTV
C
CC
28.7k
C2
4.7μF
10V
400kHz
30k
0.001μF
M2
37551 TA04a
L1: WÜRTH ELEKTRONIK 744870220
M1: VISHAY SILICONIX SI7454DP
D1: DIODES INC. - PDS5100
M2: VISHAY SILICONIX SI2318DS
37551fc
17
LT3755/LT3755-1/LT3755-2
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
LT3755-2
0.1Ω
1A
CTRL
ISN
100k
NTC
RT1
INTV
CC
M1
GATE
100k
SENSE
OPENLED
PWM
SS
0.015Ω
50W
LED
STRING
RT
PWMOUT
0.01μF
V
C
GND INTV
CC
28.7k
400kHz
10k
0.001μF
4.7μF
M2
L1: COILTRONICS DR127-220
37551 TA02a
M1: VISHAY SILICONIX SI7850DP
D1: DIODES INC. PDS5100
M2: VISHAY SILICONIX SI2308DS
RT1: MURATA NCP18WM1045
Waveforms for 50W LED Driver with
PWM Disconnect NFET
V
= 12V
PWM
IN
0V TO 5V
V
= 50V
OUT
10V/DIV
I
L1
2A/DIV
I
LED
500mA/DIV
37551 TA02b
50μs/DIV
V(ISP-ISN) Threshold vs Temperature
for NTC Resistor Divider
Efficiency vs Load
100
96
92
88
84
80
120
100
80
60
40
20
0
V
= 12V
IN
0.0
0.4
0.6
0.8
1.0
0.2
25
45
65
85
105
125
LOAD (A)
TEMPERATURE (°C)
37551 TA02c
37551 TA02d
37551fc
18
LT3755/LT3755-1/LT3755-2
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
107k
V
ISN
FB
REF
CTRL
1.5k
22.1k
M3
INTV
CC
LT3755-2
M2
1k
PWMOUT
3 LUXEON K2
- WHITE
PWM
100k
OPENLED
L1
SS
RT
33μH
D1
V
IN
C4
M1
GATE
4.7μF
0.1μF
V
C
GND INTV
SENSE
CC
28.7k
0.033Ω
400kHz
47k
C2
4.7μF
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
37551fc
19
LT3755/LT3755-1/LT3755-2
TYPICAL APPLICATIONS
Buck Mode 500mA LED Driver for 20kHz PWM Dimming
V
IN
22V TO
36V
1M
ISP
ISN
V
IN
SHDN/UVLO
0.2Ω
500mA
16V
68.1k
V
REF
CTRL
0.22μF
M2
PWMOUT
4.7μF 2s
25V
6V
1M
LT3755-2
PWM
INTV
L1
CC
3.3μH
4.7μF
0.1μF
2200pF
D1
GATE
M1
OPENLED
SENSE
2.2μF 2s
SS
V
C
GND
22k
FB
RT
0.033Ω
50V
13k
800kHz
37551 TA03a
470pF
L1: TOKO 962BS_3R3M
M1: VISHAY SILICONIX SI7850DP
M2: VISHAY SILICONIX SI2306DS
D1: DIODES, INC SBM540
Minimum PWM Pulse Switching Waveform
Efficiency vs VIN
100
V
= 16V
= 0.5A
LED
LED
I
GATE
96
92
88
84
80
INDUCTOR
CURRENT
1A/DIV
PWM
I
LED
500mA/DIV
37551 TA06b
500ns/DIV
15
20
25
30
35
40
V
(V)
IN
37551 TA06c
37551fc
20
LT3755/LT3755-1/LT3755-2
TYPICAL APPLICATIONS
21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection
L1
15μH
0.1Ω
D1
V
IN
M2
8V TO
36V
50V
2.2μF
2s
100V
2.2μF
2s
21.5V
1A
M1
392k
1.5k
0.02Ω
V
Q1
IN
100k
499k
GATE SENSE
LT3755-2
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
C
GND
RT
M1: VISHAY SILICONIX SI7850DP
M2: VISHAY SILICONIX SI2319DS
Q1: ZETEX FMMT558
28.7k
400kHz
4.7k
4700pF
0.01μF
Q2: MMBTA42
D1: DIODES INC. PDS560
L1: SUMIDA CDRH127/LD-150
Buck-Boost Mode Efficiency vs Input Voltage
Buck-Boost Mode LED Current vs Low Input Voltage
100
1.10
1.05
1
95
90
0.95
0.9
0.85
0.8
85
80
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
37551fc
21
LT3755/LT3755-1/LT3755-2
PACKAGE DESCRIPTION
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev A)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
2.845 p 0.102
(.112 p .004)
0.889 p 0.127
(.035 p .005)
1
8
0.35
REF
5.23
(.206)
MIN
1.651 p 0.102
(.065 p .004)
1.651 p 0.102
(.065 p .004)
3.20 – 3.45
(.126 – .136)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
16
9
0.305 p 0.038
0.50
(.0197)
BSC
NO MEASUREMENT PURPOSE
4.039 p 0.102
(.159 p .004)
(NOTE 3)
(.0120 p .0015)
TYP
0.280 p 0.076
(.011 p .003)
RECOMMENDED SOLDER PAD LAYOUT
16151413121110
9
REF
DETAIL “A”
0.254
(.010)
3.00 p 0.102
(.118 p .004)
(NOTE 4)
0o – 6o TYP
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)
MSOP (MSE16) 0608 REV A
0.50
(.0197)
BSC
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
37551fc
22
LT3755/LT3755-1/LT3755-2
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
0.200 REF
0.25 0.05
0.50 BSC
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
37551fc
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/LT3755-2
TYPICAL APPLICATION
Buck-Boost LED Driver for Automotive
Efficiency vs VIN
V
IN
100
96
92
88
84
80
6V TO
36V
4.7μF
s2
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
SENSE
CTRL
0.025Ω
40k
INTV
LT3755-2
CC
0
20
(V)
30
40
47k
10
FB
V
IN
37551 TA05b
100k
ISP
OPENLED
PWM
SS
0.1Ω
1A
ISN
RT
PWMOUT
0.01μF
V
C
GND INTV
CC
28.7k
400kHz
1%
INTV
CC
10k
0.01μF
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/LT3756-1/
LT3756-2
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
37551fc
LT 0909 REV C • 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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