LINER LT3755-2 40vin, 75vout led controller Datasheet

LT3755/LT3755-1/LT3755-2
40VIN, 75VOUT LED Controllers
Features
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
Description
3000:1 True Color PWM™ Dimming
Wide Input Voltage Range: 4.5V to 40V
Output Voltage Up to 75V
Constant-Current and Constant-Voltage Regulation
100mV High Side Current Sense
Drives LEDs in Boost, Buck Mode, Buck-Boost Mode,
SEPIC or Flyback Topology
Adjustable Frequency: 100kHz to 1MHz
Open LED Protection
Programmable Undervoltage Lockout with Hysteresis
Improved Open LED Status Pin (LT3755-2)
Frequency Synchronization (LT3755-1)
PWM Disconnect Switch Driver
CTRL Pin Provides Analog Dimming
Low Shutdown Current: <1µA
Programmable Soft-Start
Thermally Enhanced 16-Lead QFN (3mm × 3mm)
and MSOP Packages
The LT®3755, LT3755-1 and LT3755-2 are DC/DC controllers designed to operate as a constant-current source for
driving high current LEDs. They drive a low side external
N-channel power MOSFET from an internal regulated 7.15V
supply. The fixed frequency, current mode architecture
results in stable operation over a wide range of supply
and output voltages. A ground referenced voltage FB pin
serves as the input for several LED protection features,
and also makes it possible for the converter to operate
as a constant-voltage source. A frequency adjust pin
allows the user to program the frequency from 100kHz
to 1MHz to optimize efficiency, performance or external
component size.
The LT3755/LT3755-1/LT3755-2 sense output current at
the high side of the LED string. High side current sensing
is the most flexible scheme for driving LEDs, allowing
boost, buck mode or buck-boost mode configuration.
The PWM input provides LED dimming ratios of up to
3000:1, and the CTRL input provides additional analog
dimming capability.
Applications
n
n
n
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 7199560, 7321203.
High Power LED
Battery Chargers
Accurate Current Limited Voltage Regulators
Typical Application
50W White Automotive LED Headlamp Driver
Efficiency vs VIN
22µH
4.7µF
1M
185k
VIN
SHDN/UVLO
VREF
332k
40.2k
23.7k
0.1Ω
1A
ISN
GATE
100k
0.01µF
96
FB
ISP
LT3755-2
CTRL
INTVCC
100
4.7µF
1M
EFFICIENCY (%)
VIN
8V TO
40V
SENSE
28.7k
375kHz
OPENLED
PWM
SS
PWMOUT
RT
VC
GND INTVCC
10k
0.015Ω
50W
LED
STRING
92
88
84
80
4.7µF
0.001µF
0
10
20
VIN (V)
30
40
37551 TA01b
37551 TA01a
37551fd
LT3755/LT3755-1/LT3755-2
Absolute Maximum Ratings
(Note 1)
VIN.............................................................................40V
SHDN/UVLO (Note 3).................................................40V
ISP, ISN......................................................................75V
INTVCC....................................................... VIN + 0.3V, 8V
GATE, PWMOUT (Note 4).......................... INTVCC + 0.3V
CTRL, PWM, OPENLED..............................................12V
VC, VREF, SS, FB...........................................................3V
SYNC...........................................................................8V
RT.............................................................................1.5V
SENSE.......................................................................0.5V
Operating Junction Temperature Range (Notes 2, 5)
LT3755E/LT3755I................................–40°C to 125°C
LT3755H............................................–40°C TO 150°C
Storage Temperature Range.................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP................................................................ 300°C
Pin Configuration
ISN
ISP
VC
CTRL
TOP VIEW
16 15 14 13
VREF 1
TOP VIEW
12 FB
PWM 2
11 PWMOUT
17
GND
SYNC OR OPENLED 3
PWMOUT
FB
ISN
ISP
VC
CTRL
VREF
PWM
10 GATE
5
6
7
8
RT
INTVCC
VIN
9 SENSE
SHDN/UVLO
SS 4
1
2
3
4
5
6
7
8
17
GND
16
15
14
13
12
11
10
9
GATE
SENSE
VIN
INTVCC
SHDN/UVLO
RT
SS
SYNC OR OPENLED
MSE PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 125°C (E, I GRADES), TJMAX = 150°C (H GRADE), θJA = 43°C/W, θJC = 4°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
UD PACKAGE
16-LEAD (3mm s 3mm) PLASTIC QFN
TJMAX = 125°C, θJA = 68°C/W, θJC = 4.2°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
order information
LEAD FREE FINISH
LT3755EUD#PBF
TAPE AND REEL
LT3755EUD#TRPBF
PART MARKING*
LDGC
PACKAGE DESCRIPTION
LT3755IUD#PBF
LT3755IUD#TRPBF
LDGC
16-Lead (3mm × 3mm) Plastic QFN
–40°C to 125°C
LT3755EUD-1#PBF
LT3755EUD-1#TRPBF
LDMS
16-Lead (3mm × 3mm) Plastic QFN
–40°C to 125°C
LT3755IUD-1#PBF
LT3755IUD-1#TRPBF
LDMS
16-Lead (3mm × 3mm) Plastic QFN
LT3755EUD-2#PBF
LT3755EUD-2#TRPBF
LFJZ
LT3755IUD-2#PBF
LT3755IUD-2#TRPBF
LFJZ
16-Lead (3mm × 3mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
16-Lead (3mm × 3mm) Plastic QFN
–40°C to 125°C
16-Lead (3mm × 3mm) Plastic QFN
LT3755EMSE#PBF
LT3755EMSE#TRPBF
3755
16-Lead Plastic MSOP
–40°C to 125°C
LT3755IMSE#PBF
LT3755IMSE#TRPBF
3755
16-Lead Plastic MSOP
–40°C to 125°C
LT3755EMSE-1#PBF
LT3755EMSE-1#TRPBF
37551
16-Lead Plastic MSOP
–40°C to 125°C
LT3755IMSE-1#PBF
LT3755IMSE-1#TRPBF
37551
16-Lead Plastic MSOP
–40°C to 125°C
LT3755EMSE-2#PBF
LT3755EMSE-2#TRPBF
37552
16-Lead Plastic MSOP
–40°C to 125°C
LT3755IMSE-2#PBF
LT3755IMSE-2#TRPBF
37552
16-Lead Plastic MSOP
–40°C to 125°C
LT3755HMSE-2#PBF
LT3755HMSE-2#TRPBF
37552
16-Lead Plastic MSOP
–40°C to 150°C
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/
37551fd
LT3755/LT3755-1/LT3755-2
Electrical Characteristics
The l denotes 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
VIN Minimum Operating Voltage
VIN Tied to INTVCC
VIN Shutdown IQ
SHDN/UVLO = 0V, PWM = 0V
SHDN/UVLO = 1.15V, PWM = 0V
VIN Operating IQ (Not Switching)
PWM = 0V
VREF Voltage
100µA ≤ IVREF ≤ 0µA
VREF Line Regulation
4.5V ≤ VIN ≤ 40V
SENSE Current Limit Threshold
MIN
l
Current Out of Pin
SS Pull-Up Current
Current Out of Pin
1.965
MAX
98
UNITS
4.5
V
0.1
1
5
µA
µA
1.4
1.7
mA
2.00
2.045
0.006
l
SENSE Input Bias Current
TYP
l
108
V
%/V
118
40
mV
µA
8
10
13
µA
96
100
103
mV
–12
–9.5
–7
mV
1.1
V
50
100
nA
75
V
200
mV
Error Amplifier
ISP/ISN Full-Scale Current Sense Threshold
FB = 0V, ISP = 48V
ISP/ISN Full-Scale Current Sense Threshold at CTRL = 0V
CTRL = 0V, FB = 0V, ISP = 48V
CTRL Pin Range for Current Sense Threshold Adjustment
CTRL Input Bias Current
l
0
Current Out of Pin
LED Current Sense Amplifier Input Common Mode Range (VISN)
ISP/ISN Short-Circuit Threshold
l
l
ISN = 0V
ISP/ISN Short-Circuit Fault Sensing Common Mode Range (VISN)
2.9
115
l
150
0
ISP/ISN Input Bias Current (Combined)
PWM = 5V (Active), ISP = ISN = 48V
PWM = 0V (Standby), ISP = ISN = 48V
55
0
LED Current Sense Amplifier gm
V(ISP – ISN) = 100mV
120
VC Output Impedance
1V < VC < 2V
VC Standby Input Bias Current
PWM = 0V
FB Regulation Voltage (VFB)
ISP = ISN
3
V
0.1
µA
µA
µS
15000
–20
l
1.232
1.220
1.250
1.250
kΩ
20
nA
1.265
1.270
V
V
FB Amplifier gm
FB = VFB, ISP = ISN
480
µS
FB Pin Input Bias Current
Current Out of Pin
40
100
nA
FB Open LED Threshold
OPENLED Falling (LT3755 and LT3755-2)
VFB –
65mV
VFB –
50mV
VFB –
40mV
V
FB Overvoltage Threshold
PWMOUT Falling
VFB +
50mV
VFB +
60mV
VFB +
75mV
V
4
VC Current Mode Gain – (∆VVC /∆VSENSE)
V/V
Oscillator
Switching Frequency
Minimum Off-Time
RT = 100k
RT = 10k
l
90
925
100
1000
170
125
1050
kHz
kHz
ns
37551fd
LT3755/LT3755-1/LT3755-2
Electrical Characteristics
The l denotes 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
7
7.15
7.3
UNITS
Linear Regulator
INTVCC Regulation Voltage
Dropout (VIN – INTVCC)
IINTVCC = –10mA, VIN = 7V
350
INTVCC Undervoltage Lockout
INTVCC Current Limit
INTVCC Current in Shutdown
3.9
4.1
29
SHDN/UVLO = 0V, INTVCC = 7V
V
mV
4.3
V
34
40
mA
8
12
µA
Logic Inputs/Outputs
PWM Input High Voltage
l
PWM Input Low Voltage
l
PWM Pin Resistance to GND
1.5
45
PWMOUT Output Low (VOL)
0.4
V
50
mV
60
0
PWMOUT Output High (VOH)
SHDN/UVLO Threshold Voltage Falling
V
kΩ
INTVCC –
0.05
E-, I-Grades
H-Grade
l
l
1.185
1.175
SHDN/UVLO Rising Hysteresis
V
1.220
1.245
1.245
20
SHDN/UVLO Input Low Voltage
IVIN Drops Below 1µA
SHDN/UVLO Pin Bias Current Low
SHDN/UVLO = 1.15V
SHDN/UVLO Pin Bias Current High
SHDN/UVLO = 1.30V
OPENLED Output Low (VOL)
IOPENLED = 0.5mA (LT3755 and LT3755-2)
SYNC Pin Resistance to GND
LT3755-1 Only
SYNC Input High
LT3755-1 Only
SYNC Input Low
LT3755-1 Only
1.7
V
V
mV
0.4
V
2.05
2.5
µA
10
100
nA
200
mV
30
kΩ
1.5
V
0.4
V
Gate Driver
tr GATE Driver Output Rise Time
CL = 3300pF
35
ns
tf GATE Driver Output Fall Time
CL = 3300pF
35
ns
GATE Output Low (VOL)
GATE Output High (VOH)
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 LT3755E-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. The LT3755H-2 is guaranteed to
meet performance specifications over the full –40°C to 150°C operating
junction temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
0.05
V
INTVCC –
0.05
V
Note 3: For VIN below 6V, the SHDN/UVLO pin must not exceed VIN for
proper operation.
Note 4: GATE and PWMOUT pins are driven either to GND or INTVCC by
internal switches. Do not connect these pins externally to a power supply.
Note 5: The LT3755 includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operating above
the specified maximum operating junction temperature may impair device
reliability.
37551fd
LT3755/LT3755-1/LT3755-2
Typical Performance Characteristics
103
100
102
80
60
40
20
0
0
0.5
1
101
100
99
98
97
2
1.5
103
VCTRL = 2V
20
0
VCTRL (V)
40
60
ISP VOLTAGE (V)
100
99
98
97
–50 –25
80
VREF Voltage vs Temperature
1.27
2.03
2.03
1.26
2.02
2.02
1.25
2.01
2.01
VREF (V)
2.04
2.00
1.99
2.00
1.99
1.22
1.98
1.98
1.21
1.97
1.97
1.20
–50 –25
0
1.96
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
25 50 75 100 125 150
TEMPERATURE (°C)
SWITCHING FREQUENCY (kHz)
500
10
100
RT (k)
37551 G07
10
30
40
2.4
RT = 26.7k
450
2.2
400
350
300
–50 –25
20
VIN (V)
SHDN/UVLO Hysteresis Current
vs Temperature
ISHDN/UVLO (µA)
10000
100
0
37551 G06
Switching Frequency
vs Temperature
Switching Frequency vs RT
10
1.96
37551 G05
37551 G04
1000
25 50 75 100 125 150
TEMPERATURE (°C)
VREF Voltage vs VIN
2.04
1.23
0
37551 G03
1.28
VREF (V)
VFB (V)
FB Regulation Voltage
vs Temperature
SWITCHING FREQUENCY (kHz)
101
37551 G02
37551 G01
1.24
VCTRL = 2V
102
V(ISP – ISN) THRESHOLD (mV)
120
–20
V(ISP – ISN) Threshold
vs Temperature
V(ISP – ISN) Threshold vs VISP
V(ISP – ISN) THRESHOLD (mV)
V(ISP – ISN) THRESHOLD (mV)
V(ISP – ISN) Threshold vs VCTRL
TA = 25°C, unless otherwise noted.
2.0
1.8
0
25 50 75 100 125 150
TEMPERATURE (°C)
37551 G08
1.6
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
37551 G09
37551fd
LT3755/LT3755-1/LT3755-2
Typical Performance Characteristics
SENSE Current Limit Threshold
vs Temperature
Quiescent Current vs VIN
110
SENSE THRESHOLD (mV)
PWM = 0V
VIN CURRENT (mA)
1.5
1.0
0.5
0
0
10
20
VIN (V)
30
1.28
105
100
95
90
–50 –25
40
0
2
30
38
7.3
36
34
0
125
25
50
75
DUTY CYCLE (%)
100
37551 G16
25 50 75 100 125 150
TEMPERATURE (°C)
Gate Rise/Fall Time
vs Capacitance
100
VCTRL = 2V
100
75
50
25
0
1.2
10% TO 90%
80
TIME (ns)
V(ISP – ISN) THRESHOLD (mV)
SENSE THRESHOLD (mV)
100
0
37551 G15
V(ISP-ISN) Threshold vs FB Voltage
115
0
7.0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
37551 G14
SENSE Current Limit Threshold
vs Duty Cycle
95
7.2
7.1
32
37551 G13
105
25 50 75 100 125 150
TEMPERATURE (°C)
7.4
30
–50 –25
40
110
0
INTVCC Voltage vs Temperature
INTVCC (V)
INTVCC CURRENT LIMIT (mA)
VINTVCC (V)
4
20
VIN (V)
1.20
37551 G12
40
6
SHDN/UVLO FALLING
1.22
INTVCC Current Limit
vs Temperature
8
10
SHDN/UVLO RISING
1.24
37551 G11
INTVCC Voltage vs VIN
0
1.26
1.18
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
37551 G10
0
SHDN/UVLO Threshold
vs Temperature
SHDN/UVLO VOLTAGE (V)
2.0
TA = 25°C, unless otherwise noted.
GATE RISE
TIME
60
GATE
FALL TIME
40
20
1.22
1.24
1.26
FB VOLTAGE (V)
1.28
37551 G17
0
0
2
4
6
CAPACITANCE (nF)
8
10
37551 G18
37551fd
LT3755/LT3755-1/LT3755-2
Typical Performance Characteristics
ISP/ISN Input Bias Current
vs CTRL Voltage
INTVCC Dropout Voltage
vs Current, Temperature
0
40
TA = –45°C
–0.5
30
LDO DROPOUT (V)
INPUT BIAS CURRENT (µA)
ISP
ISN
20
10
0
TA = 25°C, unless otherwise noted.
TA = 25°C
TA = 125°C
–1.0
TA = 150°C
–1.5
–2.0
0
0.5
1
CTRL (V)
1.5
2
37551 G19
Pin Functions
–2.5
VIN = 7V
0
5
10
15
20
LDO CURRENT (mA)
25
30
37551 G20
(MSOP/QFN)
PWMOUT (Pin 1/Pin 11): Buffered Version of PWM Signal
for Driving LED Load Disconnect NMOS or Level Shift.
This pin also serves in a protection function for the FB
overvoltage condition—will toggle if the FB input is greater
than the FB regulation voltage (VFB) plus 60mV (typical).
The PWMOUT pin is driven from INTVCC. Use of a FET with
gate cut-off voltage higher than 1V is recommended.
FB (Pin 2/Pin 12): Voltage Loop Feedback Pin. FB is
intended for constant-voltage regulation or for LED protection/open LED detection. The internal transconductance
amplifier with output VC will regulate FB to 1.25V (nominal)
through the DC/DC converter. If the FB input is regulating
the loop, the OPENLED pull-down is asserted. This action may signal an open LED fault. If FB is driven above
the FB threshold (by an external power supply spike, for
example), the OPENLED pull-down will be de-asserted and
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.
ISN (Pin 3/Pin 13): Connection Point for the Negative
Terminal of the Current Feedback Resistor. If ISN is greater
than 2.9V, the LED current can be programmed by ILED =
100mV/RLED when VCTRL > 1.2V or ILED = (VCTRL –100mV)/
(10 • RLED) when VCTRL ≤ 1V. Input bias current is typically 25µA. Below 3V, ISN is an input to the short-circuit
protection feature that forces GATE to 0V if ISP exceeds
ISN by more than 150mV (typ).
ISP (Pin 4/Pin 14): Connection Point for the Positive
Terminal of the Current Feedback Resistor. Input bias
current is dependent upon CTRL pin voltage as shown
in the TPC. ISP is an input to the short‑circuit protection
feature when ISN is less than 3V.
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
the next PWM high transition. Connect a capacitor between
this pin and GND; a resistor in series with the capacitor is
recommended for fast transient response.
37551fd
LT3755/LT3755-1/LT3755-2
Pin Functions
(MSOP/QFN)
CTRL (Pin 6/Pin 16): Current Sense Threshold Adjustment
Pin. Regulating threshold V(ISP – ISN) is 1/10th VCTRL plus
an offset for 0V < VCTRL < 1V. For VCTRL > 1.2V the current
sense threshold is constant at the full-scale value of 100mV.
For 1V < VCTRL < 1.2V, the dependence of current sense
threshold upon VCTRL transitions from a linear function
to a constant value, reaching 98% of full-scale value by
VCTRL = 1.1V. Do not leave this pin open.
VREF (Pin 7/Pin 1): Voltage Reference Output Pin, Typically
2V. This pin drives a resistor divider for the CTRL pin, either
for analog dimming or for temperature limit/compensation
of LED load. Can supply up to 100μA.
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 INTVCC.
OPENLED (Pin 9/Pin 3, LT3755 and LT3755-2): An opencollector 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.
SYNC (Pin 9/Pin 3, LT3755-1 Only): The SYNC pin is used
to synchronize the internal oscillator to an external logic
level signal. The RT resistor should be chosen to program
an internal switching frequency 20% slower than the SYNC
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.
SS (Pin 10/Pin 4): Soft-Start Pin. This pin modulates
oscillator frequency and compensation pin voltage (VC)
clamp. The soft-start interval is set with an external capacitor. 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.
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.
SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage
Detect Pin. An accurate 1.22V falling threshold with externally programmable hysteresis detects when power is
OK to enable switching. Rising hysteresis is generated
by the external resistor divider and an accurate internal
2.1µA pull-down current. Above the threshold (but below
6V), SHDN/UVLO input bias current is sub‑µA. Below the
falling threshold, a 2.1µA 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 and reduce VIN
quiescent current below 1µA.
INTVCC (Pin 13/Pin 7): Regulated Supply for Internal
Loads, GATE Driver and PWMOUT Driver. Supplied from
VIN and regulates to 7.15V (typical). INTVCC must be
bypassed with a 4.7µF capacitor placed close to the pin.
Connect INTVCC directly to VIN if VIN is always less than
or equal to 8V.
VIN (Pin 14/Pin 8): Input Supply Pin. Must be locally
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 terminal of the switch current sense resistor, RSENSE, in the
source of the NFET. The negative terminal of the current
sense resistor should be Kelvin connected to the GND
plane of the IC.
GATE (Pin 16/Pin 10): N-channel FET Gate Driver Output.
Switches between INTVCC and GND. Driven to GND during
shutdown, fault or idle states.
Exposed Pad (Pin 17/Pin 17): Ground. This pin also serves
as current sense input for control loop, sensing negative
terminal of current sense resistor. Solder the Exposed Pad
directly to ground plane.
37551fd
LT3755/LT3755-1/LT3755-2
Block Diagram
SHDN/UVLO
1.22V
–
+
A6
VC
FB
1.3V
2.1µA
A5
+
gm
–
1.25V
SHORT-CIRCUIT
DETECT
+
–
+
A10
–
150mV
gm
EAMP
ISN
+
A1
–
5k
ISP
CTRL
BUFFER
CTRL
1.1V
+
+ A3
–
OVFB
COMPARATOR
2V
170k
R
7.15V
INTVCC
GATE
Q
DRIVER
S
PWM
COMPARATOR
10µA AT
A1+ = A1–
ISENSE
+
–
SENSE
A4
FAULT
LOGIC
GND
RAMP
GENERATOR
VC
SSCLAMP
–
+A7
– LDO
+A8
1.25V
+
A2
–
50k
VREF
VIN
SCILMB
Q2
140µA
PWM
10µA AT
FB = 1.25V
10µA
SCILMB
PWMOUT
–
+
SHDN
100KHz TO 1MHz
OSCILLATOR
10µA
1.25V
TLIM
165°C
SS
+
+
–
1.2V
FB
FREQ
PROG
RT
–
+
OPENLED
OPTION
FOR
LT3755
AND
LT3755-2
SYNC OPTION FOR
LT3755-1
37551 BD
37551fd
LT3755/LT3755-1/LT3755-2
Operation
The LT3755 is a constant-frequency, current mode controller with a low side NMOS gate driver. The GATE pin and
PWMOUT pin drivers and other chip loads are powered
from INTVCC, which is an internally regulated supply. In
the discussion that follows it will be helpful to refer to
the Block Diagram of the IC. In normal operation with the
PWM pin low, the GATE and PWMOUT pins are driven to
GND, the VC pin is high impedance to store the previous
switching state on the external compensation capacitor,
and the ISP and ISN pin bias currents are reduced to
leakage levels. When the PWM pin transitions high, the
PWMOUT pin transitions high after a short delay. At the
same time, the internal oscillator wakes up and generates 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
as slope compensation return to their starting points and a
new cycle begins with the set pulse from the oscillator.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current or
voltage in the load. The VC signal is integrated over many
switching cycles and is an amplified version of the difference 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-
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.
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 VREF .
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 function 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
disconnect NFET, this action isolates the LED load from GND
preventing excessive current from damaging the LEDs. If
the FB input exceeds both the open LED and the overvoltage
thresholds, then an externally driven overvoltage event
has caused the FB pin to be too high and the OPENLED
pull-down will be de-asserted. The LT3755-2 will re-assert
the OPENLED signal when FB falls below the overvoltage
threshold and remains above the OPENLED threshold. The
LT3755 is prevented from re-asserting OPENLED until FB
drops below both thresholds.
37551fd
10
LT3755/LT3755-1/LT3755-2
Applications Information
INTVCC Regulator Bypassing and Operation
The INTVCC pin requires a capacitor for stable operation
and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R or X5R ceramic
capacitor for best performance. A 4.7µF capacitor will be
adequate for many applications. Place the capacitor close
to the IC to minimize the trace length to the INTVCC pin
and also to the IC ground.
The following equations should be used to determine the
values of the resistors:
VIN,RISING
VIN
An internal current limit on the INTVCC output protects
the LT3755 from excessive on-chip power dissipation.
The minimum value of this current should be considered
when choosing the switching NMOS and the operating
frequency.
LT3755
Careful choice of a lower QG FET will allow higher switching frequencies, leading to smaller magnetics. The INTVCC
pin has its own undervoltage disable (UVLO) set to 4.1V
(typical) to protect the external FETs from excessive power
dissipation caused by not being fully enhanced. If the
INTVCC pin drops below the UVLO threshold, the GATE
and PWMOUT pins will be forced to 0V and the soft-start
pin will be reset.
If the input voltage, VIN, will not exceed 8V, then the INTVCC pin could be connected to the input supply. Be aware
that a small current (less than 12μA) will load the INTVCC
in shutdown. This action allows the LT3755 to operate
from VIN as low as 4.5V. If VIN is normally above, but
occasionally drops below the INTVCC regulation voltage,
then the minimum operating VIN will be close to 6V . This
value is determined by the dropout voltage of the linear
regulator and the INTVCC undervoltage lockout threshold
mentioned above.
Programming the Turn-On and Turn-Off Thresholds
with the SHDN/UVLO Pin
The falling UVLO value can be accurately set by the resistor
divider. A small 2.1µA pull-down current is active when
SHDN/UVLO is below the threshold. The purpose of this
current is to allow the user to program the rising hysteresis.
R1
SHDN/UVLO
R2
37551 F01
Figure 1. Resistor Connection to Set
VIN Undervoltage Shutdown Threshold
IINTVCC can be calculated from the following equation:
IINTVCC = QG • fOSC
R1+R2
R2
= 2.1µA •R1 + VIN,FALLING
VIN,FALLING = 1.22 •
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor, RLED, in series with the LED
string. The voltage drop across RLED is (Kelvin) sensed
by the ISP and ISN pins. Typically, sensing of the current
should be done at the top of the LED string. If this option
is not available, then the current may be sensed at the
bottom of the string, but take caution that the minimum
ISN value does not fall below 3V, which is the lower limit of
the LED current regulation function. The CTRL pin should
be tied to a voltage higher than 1.2V to get the full-scale
100mV (typical) threshold across the sense resistor. The
CTRL pin can also be used to dim the LED current to zero,
although relative accuracy decreases with the decreasing
voltage sense threshold. When the CTRL pin voltage is
less than 1V, the LED current is:
ILED =
VCTRL − 100mV
RLED • 10
When the CTRL pin voltage is between 1V and 1.2V the LED
current varies with CTRL, but departs from the equation
above by an increasing amount as CTRL voltage increases.
Ultimately, above CTRL = 1.2V the LED current no longer
varies with CTRL. At CTRL = 1.1V, the actual value of ILED
is ~98% of the equation’s estimate.
37551fd
11
LT3755/LT3755-1/LT3755-2
Applications Information
When VCTRL is higher than 1.2V, the LED current is
regulated to:
ILED =
100mV
RLED
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 VREF if not used). The CTRL pin
can also be used in conjunction with a thermistor to
provide overtemperature protection for the LED load, or
with a resistor divider to VIN to reduce output power and
switching current when VIN is low. The presence of a time
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 misoperation, 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
For a boost application, the output voltage can be set by
selecting the values of R3 and R4 (see Figure 2) according
to the following equation:
VOUT = 1.25 •
R3 + R4
R4
For a boost type LED driver, set the resistor from the output
to the FB pin such that the expected VFB during normal
VOUT
R3
LT3755
FB
R4
37551 F02
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
operation will not exceed 1.1V. For an LED driver of buck or
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:
VOUT = VBE + 1.25 •
R3
R4
R3
+
RSEN(EXT)
VOUT
–
LT3755
100k
LED
ARRAY
COUT
37551 F03
FB
R4
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
ISP/ISN Short-Circuit Protection Feature (for SEPIC)
The ISP and ISN pins have a protection feature independent 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 in a SEPIC converter. 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. If the
load has appreciable series inductance, use of a Schottky
clamp from GND to ISN is recommended for the SEPIC
to prevent excessive current flowing from the ISN pin in
a fault.
Dimming Control
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 between
37551fd
12
LT3755/LT3755-1/LT3755-2
Applications Information
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz to optimize
efficiency/performance or external component size. Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
may not allow sufficiently high or low duty cycle operation.
Lower frequency operation gives better performance at the
cost of larger external component size. For an appropriate
RT resistor value see Table 1. An external resistor from the
RT pin to GND is required—do not leave this pin open.
Table 1. Switching Frequency vs RT Value
fOSC (kHz)
RT (kΩ)
1000
10.0
900
11.8
800
13.0
700
15.4
600
17.8
500
21.0
400
26.7
300
35.7
200
53.6
100
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 4) and the switching frequency define
the minimum and maximum duty cycle of the switch,
respectively. The following equations express the minimum/maximum duty cycle:
Min Duty Cycle = (minimum on-time) • switching
frequency
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.
300
CGATE = 3300pF
250
MINIMUM ON-TIME
200
TIME (ns)
zero and full current to achieve a precisely programmed
average current. To make PWM dimming more accurate,
the switch demand current is stored 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 Dimming” 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.
150
MINIMUM OFF-TIME
100
50
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
37551 F04
Figure 4. Typical Minimum On and Off
Pulse Width vs Temperature
Thermal Considerations
The LT3755 is rated to a maximum input voltage of 40V.
Careful attention must be paid to the internal power dissipation of the IC at higher input voltages to ensure that
a junction temperature of 125°C (150°C for H-Grade) is
not exceeded. This junction limit is especially important
37551fd
13
LT3755/LT3755-1/LT3755-2
Applications Information
when operating at high ambient temperatures. The majority of the power dissipation in the IC comes from the
supply current needed to drive the gate capacitance of
the external power MOSFET. This gate drive current can
be calculated as:
IGATE = fSW • QG
A low QG power MOSFET should always be used when operating at high input voltages, and the switching frequency
should also be chosen carefully to ensure that the IC does
not exceed a safe junction temperature. The internal junction temperature of the IC can be estimated by:
TJ = TA + [VIN (IQ + fSW • QG) • θJA]
where TA is the ambient temperature, IQ is the quiescent
current of the part (maximum 1.7mA) and θJA is the
package thermal impedance (68°C/W for the 3mm × 3mm
QFN package). For example, an application with TA(MAX)
= 85°C, VIN(MAX) = 40V, fSW = 400kHz, and having a FET
with QG = 20nC, the maximum IC junction temperature
will be approximately:
TJ = 85°C + [40V (1.7mA + 400kHz • 20nC) • 68°C/W]
= 111°C
The Exposed Pad on the bottom of the package must be
soldered to a ground plane. This ground should then be
connected to an internal copper ground plane with thermal
vias placed directly under the package to spread out the
heat dissipated by the IC.
If LT3755 junction temperature reaches 165°C, the GATE
and PWMOUT pins will be driven to GND and the softstart (SS) pin will be discharged to GND. Switching will
be enabled after device temperature is reduced 10°C. This
function is intended to protect the device during momentary
thermal overload conditions.
Frequency Synchronization (LT3755-1 Only)
The LT3755-1 switching frequency can be synchronized to
an external clock using the SYNC pin. For proper operation,
the RT resistor should be chosen for a switching frequency
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
a good choice, otherwise, maintain the duty cycle between
20% and 60%. When using both PWM and SYNC features,
the PWM signal rising edge should occur at least 200ns
before the SYNC rising edge (VIH) for optimal PWM
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 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
(1.31V typ). The LT3755-2 asserts/de-asserts OPENLED
freely when crossing the 1.31V threshold. The LT3755,
by comparison, de-asserts OPENLED when FB exceeds
1.31V 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.
Input Capacitor Selection
The input capacitor supplies the transient input current for
the power inductor of the converter and must be placed
and sized according to the transient current requirements.
The switching frequency, output current and tolerable input
voltage ripple are key inputs to estimating the capacitor
value. An X7R type ceramic capacitor is usually the best
choice since it has the least variation with temperature and
DC bias. Typically, boost and SEPIC converters require a
lower value capacitor than a buck mode converter. Assuming that a 100mV input voltage ripple is acceptable,
the required capacitor value for a boost converter can be
estimated as follows:
CIN (µF) = ILED (A) •
 µF 
VOUT
• t SW (µs) • 
 A • µs 
VIN
37551fd
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.
With the same VIN voltage ripple of 100mV, the input capacitor for a buck converter can be estimated as follows:
 µF 
CIN (µF) = ILED (A) • t SW (µs) • 4.7 • 
 A • µs 
A 10µF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 1A load.
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
converter case it is important to place the capacitor as close
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:
IIN(RMS) = ILED •
(1– D) • D
where D is the switch duty cycle.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
WEB
TDK
www.tdk.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Output Capacitor Selection
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.
To achieve the same LED ripple current, the required filter
capacitor is larger in the boost and buck-boost mode applications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values.
Soft-Start Capacitor Selection
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
voltage overshoot. The soft-start interval is set by the softstart capacitor selection according to the equation:
TSS = CSS •
2V
10µA
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 INTVCC
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
For applications operating at high input or output voltages,
the power NMOS FET switch is typically chosen for drain
voltage VDS rating and low gate charge QG. Consideration
of switch on-resistance, RDS(ON), is usually secondary because switching losses dominate power loss. The INTVCC
regulator on the LT3755 has a fixed current limit to protect
the IC from excessive power dissipation at high VIN, so the
FET should be chosen so that the product of QG at 7V and
switching frequency does not exceed the INTVCC current
limit. For driving LEDs be careful to choose a switch with
a VDS rating that exceeds the threshold set by the FB pin
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
WEB
Vishay Siliconix
www.vishay.com
Fairchild
www.fairchildsemi.com
International Rectifier
www.irf.com
37551fd
15
LT3755/LT3755-1/LT3755-2
Applications Information
Schottky Rectifier Selection
The power Schottky diode conducts current during the
interval when the switch is turned off. Select a diode rated
for the maximum SW voltage. If using the PWM feature for
dimming, it is important to consider diode leakage, which
increases with the temperature, from the output during the
PWM low interval. Therefore, choose the Schottky diode
with sufficiently low leakage current. Table 4 has some
recommended component vendors.
Table 4. Schottky Rectifier Manufacturers
VENDOR
WEB
On Semiconductor
www.onsemi.com
Diodes, Inc.
www.diodes.com
Central Semiconductor
www.centralsemi.com
Sense Resistor Selection
The resistor, RSENSE, between the source of the external 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,BUCK ≤
0.07V
ILED
VIN • 0.07V
( VIN + VLED )ILED
For boost, select a resistor according to:
RSENSE,BOOST
The placement of RSENSE should be close to the source of
the NMOS FET and GND of the LT3755. The SENSE input
to LT3755 should be a Kelvin connection to the positive
terminal of RSENSE.
Inductor Selection
The inductor used with the LT3755 should have a saturation
current rating appropriate to the maximum switch current
selected with the RSENSE resistor. Choose an inductor value
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:
LBUCK =
RSENSE • VLED ( VIN – VLED )
VIN • 0.02V • fOSC
LBUCK-BOOST =
LBOOST =
For buck-boost, select a resistor according to:
RSENSE,BUCK-BOOST ≤
(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.
V • 0.07V
≤ IN
VLED • ILED
These equations provide an estimate of the sense resistor
value based on reasonable assumptions about inductor 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
cycle, and those with discontinuous conduction mode
RSENSE • VLED • VIN
( VLED + VIN ) • 0.02V • fOSC
RSENSE • VIN ( VLED – VIN )
VLED • 0.02V • fOSC
Table 5 provides some recommended inductor vendors.
Table 5. Inductor Manufacturers
VENDOR
WEB
Sumida
www.sumida.com
Würth Elektronik
www.we-online.com
Coiltronics
www.cooperet.com
Vishay
www.vishay.com
Coilcraft
www.coilcraft.com
Loop Compensation
The LT3755 uses an internal transconductance error amplifier whose VC output compensates the control loop. The
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
37551fd
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.
Board Layout
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
the board. To reduce electromagnetic interference (EMI), it
is important to minimize the area of the high dV/dt switching
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
filter capacitor to GND should be minimized. The ground
points of these two switching current traces should come
to a common point then connect to the ground plane under
the LT3755. Likewise, the ground terminal of the bypass
capacitor for the INTVCC regulator should be placed near
the GND of the switching path. Typically this requirement
will result in the external switch being closest to the IC,
along with the INTVCC bypass capacitor. The ground for
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.
20W SEPIC LED Driver
C1
4.7µF
50V
1M
VIN
SHDN/UVLO
FB
100k
95
ISP
0.1Ω
1A
ISN
OPENLED
GATE
PWM
SENSE
SS
RT
PWMOUT
VC
GND INTVCC
28.7k
375kHz
0.001µF
L1: WÜRTH ELEKTRONIK 744870220
M1: VISHAY SILICONIX SI7454DP
D1: DIODES INC. - PDS5100
M2: VISHAY SILICONIX SI2318DS
30k
VOUT = 18V
ILED = 1A
L1B
LT3755-2
0.01µF
100
25k
CTRL
INTVCC
Efficiency vs VIN
C3
4.7µF
50V
511k
VREF
187k
D1
5A
100V
EFFICIENCY (%)
VIN
8V TO
40V
C4
2.2µF
50V
L1A
22µH
C2
4.7µF
10V
M1
20W
LED
STRING
0.015Ω
90
85
80
M2
0
10
20
VIN (V)
30
40
37551 TA04b
37551 TA04a
37551fd
17
LT3755/LT3755-1/LT3755-2
CTRL
VREF
OPENLED
PWM
Applications Information
VIAS TO GROUND PLANE
CSS
RT
R2
4
3
2
1
5
16
CC
RC
R1
6
15
CVCC
7
14
8
13
x VOUT VIA
9 10 11 12
L1
R3
x
R4
5
4
1
6
3
M2
M1
7
2
8
1
3
LED–
2
RSENSE
COUT
COUT
D1
CIN
RLED
VIN
GND
COMPONENT DESIGNATIONS REFER TO “50W WHITE LED HEADLAMP DRIVER” SCHEMATIC
LED+
37551 F05
Figure 5. Boost Converter Suggested Layout
37551fd
18
LT3755/LT3755-1/LT3755-2
Typical Applications
50W White LED Headlamp Driver
VIN
8V TO
40V
L1
22µH
CIN
4.7µF
R1
1M
D1
SHDN/UVLO
R2
187k
16.9k
FB
VREF
100k
CSS
0.01µF
RT
28.7k
375kHz
RLED
0.1Ω
LT3755-2
1A
ISN
100k
NTC
RT1
INTVCC
R4
23.7k
ISP
CTRL
COUT
4.7µF
R3
1M
VIN
M1
GATE
SENSE
OPENLED
PWM
SS
RT
PWMOUT
VC
GND INTVCC
RC
10k
CC
0.001µF
L1: COILTRONICS DR127-220
M1: VISHAY SILICONIX SI7850DP
D1: DIODES INC. PDS5100
M2: VISHAY SILICONIX SI2308DS
RT1: MURATA NCP18WM1045
RSENSE
0.015Ω
50W
LED
STRING
CVCC
4.7µF
M2
SEE SUGGESTED LAYOUT, FIGURE 5
37551 TA02a
Waveforms for 50W LED Driver with
PWM Disconnect NFET
PWM VIN = 12V
0V TO 5V
VOUT = 50V
10V/DIV
IL1
2A/DIV
ILED
500mA/DIV
50µs/DIV
37551 TA02b
V(ISP-ISN) Threshold vs Temperature
for NTC Resistor Divider
Efficiency vs Load
100
120
VIN = 12V
V(ISP-ISN) THRESHOLD (mV)
EFFICIENCY (%)
96
92
88
84
80
0.0
0.2
0.4
0.6
LOAD (A)
0.8
1.0
37551 TA02c
100
80
60
40
20
0
25
45
65
85
TEMPERATURE (°C)
105
125
37551 TA02d
37551fd
19
LT3755/LT3755-1/LT3755-2
Typical Applications
Buck Mode 1.4A LED Driver
VIN
15V TO
40V
C1
1µF
1M
ISP
VIN
110k
SHDN/UVLO
107k
0.068Ω
CTRL
200k
Q1
1.5k
INTVCC
20k
M3
LT3755-2
M2
PWMOUT
PWM
100k
1.4A
ISN
FB
VREF
C3
4.7µF
200k
3 LUXEON K2
- WHITE
1k
OPENLED
L1
33µH
SS
D1
VIN
0.1µF
28.7k
375kHz
VC
GND INTVCC
47k
0.001µF
C4
4.7µF
M1
GATE
RT
SENSE
0.033Ω
C2
4.7µF
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
VIN = 24V
VLED = 10V
PWM
0V TO 5V
EFFICIENCY (%)
96
ILED
1A/DIV
IL1
1A/DIV
2µs/DIV
37551 TA03b
92
88
84
80
15
20
25
30
35
40
VIN (V)
37551 TA03c
37551fd
20
LT3755/LT3755-1/LT3755-2
Typical Applications
Buck Mode 500mA LED Driver for 20kHz PWM Dimming
VIN
22V TO
36V
1M
ISP
VIN
SHDN/UVLO
68.1k
0.2Ω
ISN
VREF
CTRL
16V
0.22µF
M2
PWMOUT
6V
4.7µF 2s
25V
1M
LT3755-2
PWM
INTVCC
4.7µF
L1
3.3µH
2200pF
SENSE
VC
GND
22k
0.1µF
470pF
FB
D1
M1
GATE
OPENLED
SS
500mA
RT
0.033Ω
13k
800kHz
2.2µF 2s
50V
37551 TA03a
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
GATE
VLED = 16V
ILED = 0.5A
96
EFFICIENCY (%)
INDUCTOR
CURRENT
1A/DIV
PWM
ILED
500mA/DIV
92
88
84
500ns/DIV
37551 TA06b
80
15
20
25
30
VIN (V)
35
40
37551 TA06c
37551fd
21
LT3755/LT3755-1/LT3755-2
Typical Applications
21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection
L1
15µH
VIN
8V TO
36V
0.1Ω
D1
50V
2.2µF
2s
M2
100V
2.2µF
2s
M1
392k
21.5V
1A
1.5k
0.02Ω
VIN
Q1
100k
499k
GATE SENSE
VIN
FB
SHDN/UVLO
549k
93.1k
VREF
LT3755-2
PWMOUT
CTRL
75.0k
20.0k
4.7µF
Q2
PWM
PWM
1k
INTVCC
ISP
100k
ISN
OPENLED
SS
VC
0.01µF
GND
4.7k
37551 TA07a
RT
M1: VISHAY SILICONIX SI7850DP
M2: VISHAY SILICONIX SI2319DS
Q1: ZETEX FMMT558
Q2: MMBTA42
D1: DIODES INC. PDS560
L1: SUMIDA CDRH127/LD-150
28.7k
375kHz
4700pF
Buck-Boost Mode Efficiency vs Input Voltage
Buck-Boost Mode LED Current vs Low Input Voltage
100
1.10
1.05
LED CURRENT (A)
EFFICIENCY (%)
95
90
1
0.95
0.9
85
0.85
80
0
5
10
15 20
25 30
INPUT VOLTAGE (V)
35
40
37551 TA07b
0.8
8
9
10
11 12
13 14
INPUT VOLTAGE (V)
15
16
37551 TA07c
37551fd
22
LT3755/LT3755-1/LT3755-2
Package Description
MSE Package
MSE Package
16-Lead Plastic MSOP,
Exposed Die Pad
16-Lead
, Exposed
(Reference
LTCPlastic
DWG #MSOP
05-08-1667
RevDie
A) Pad
(Reference LTC DWG # 05-08-1667 Rev A)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
5.23
(.206)
MIN
2.845 p 0.102
(.112 p .004)
0.889 p 0.127
(.035 p .005)
8
1
1.651 p 0.102
(.065 p .004)
1.651 p 0.102 3.20 – 3.45
(.065 p .004) (.126 – .136)
0.305 p 0.038
(.0120 p .0015)
TYP
16
0.50
(.0197)
BSC
4.039 p 0.102
(.159 p .004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 p 0.076
(.011 p .003)
REF
16151413121110 9
DETAIL “A”
0o – 6o TYP
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)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
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
0.86
(.034)
REF
0.1016 p 0.0508
(.004 p .002)
MSOP (MSE16) 0608 REV A
37551fd
23
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
1.45 ± 0.05
2.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER
R = 0.115
TYP
0.75 ± 0.05
15
PIN 1
TOP MARK
(NOTE 6)
16
0.40 ± 0.10
1
1.45 ± 0.10
(4-SIDES)
2
(UD16) QFN 0904
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
0.25 ± 0.05
0.50 BSC
37551fd
24
LT3755/LT3755-1/LT3755-2
Revision History
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
PAGE NUMBER
D
03/10
Revised Entire Data Sheet to Include H-Grade
1-26
37551fd
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
25
LT3755/LT3755-1/LT3755-2
Typical Application
Buck-Boost LED Driver for Automotive
VIN
6V TO
36V
Efficiency vs VIN
100
4.7µF
s2
LTC4440-5
VCC BOOST
TG
GND
TS
INP
330k
D1
SHDN/UVLO
VREF
383k
M2
GATE
0.025Ω
LT3755-2
47k
1M
100k
OPENLED
PWM
SS
ISN
RT
PWMOUT
VC
GND INTVCC
28.7k
375kHz
1%
88
84
4.7µF
40k
80
FB
ISP
0.01µF
92
SENSE
CTRL
INTVCC
D2
22µF
VIN
1M
96
0.22µF
M1
EFFICIENCY (%)
INTVCC
10k
0.01µF
0.1Ω
0
10
20
VIN (V)
30
40
37551 TA05b
1A
INTVCC
4.7µF
M1, M2: VISHAY SILICONIX SI7850DP
D1, D2: DIODES, INC SBM540
37551 TA05a
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LT3474
36V, 1A (ILED), 2MHz, Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 400:1,
ISD < 1µA, TSSOP16E Package
LT3475
Dual 1.5A (ILED), 36V, 2MHz Step-Down LED Driver
VIN: 4V to 36V, VOUT(MAX) = 13.5V, True Color PWM Dimming = 3000:1,
ISD < 1µA, TSSOP20E Package
LT3476
Quad Output 1.5A, 36V, 2MHz High Current
LED Driver with 1000:1 Dimming
VIN: 2.8V to 16V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1,
ISD < 10µA, 5mm × 7mm QFN Package
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
VIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM,
ISD < 1µA, QFN and TSSOP20E Packages
LT3478/LT3478-1
4.5A, 42V, 2.5MHz High Current LED Driver with
3000:1 Dimming
VIN: 2.8V to 36V, VOUT(MAX) = 42V, True Color PWM Dimming = 3000:1,
ISD < 3µA, TSSOP16E Package
LT3486
Dual 1.3A, 2MHz High Current LED Driver
VIN: 2.5V to 24V, VOUT(MAX) = 36V, True Color PWM Dimming = 1000:1,
ISD < 1µA, 5mm × 3mm DFN and TSSOP16E Packages
LT3496
Triple 0.75A, 2.1MHz, 45V LED Driver
VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1,
ISD < 1µA, 4mm × 5mm QFN and TSSOP16E Packages
LT3517
1.3A, 2.5MHz, 45V LED Driver
VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1,
ISD < 1µA, 4mm × 4mm QFN and TSSOP16E Packages
LT3518
2.3A, 2.5MHz, 45V LED Driver
VIN: 3V to 30V, VOUT(MAX) = 45V, Dimming = 3000:1,
ISD < 1µA, 4mm × 4mm QFN and TSSOP16E Packages
LT3756/LT3756-1/
LT3756-2
100VIN, 100VOUT LED Controller
VIN: 6V to 100V, VOUT(MAX) = 100V, True Color PWM Dimming = 3000:1,
ISD < 1µA, 3mm × 3mm QFN-16 and MS16E Packages
LTC®3783
High Current LED Controller
VIN: 3V to 36V, VOUT(MAX) = Ext FET, True Color PWM Dimming = 3000:1,
ISD < 20µA, 5mm × 4mm QFN10 and TSSOP16E Packages
37551fd
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LT 0310 REV D • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2008