LT3956IUHE-PBF [Linear]
80VIN, 80VOUT Constant-Current, Constant-Voltage Converter; 80VIN ， 80VOUT恒定电流，恒定电压转换器
| 型号： | LT3956IUHE-PBF |
| 厂家： | 
Linear |
| 描述： | 80VIN, 80VOUT Constant-Current, Constant-Voltage Converter |
| 文件： | 总20页 (文件大小：1423K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3956
80V , 80V
IN
OUT
Constant-Current,
Constant-Voltage Converter
DescripTion
FeaTures
n
3000:1 True Color PWMTM Dimming
The LT®3956 is a DC/DC converter designed to operate as
a constant-current source and constant-voltage regulator.
It is ideally suited for driving high current LEDs. It features
an internal low side N-channel power MOSFET rated for
84V at 3.3A and driven 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.
n
Wide Input Voltage Range: 4.5V to 80V
n
Output Voltage Up to 80V
n
Internal 3.3A/84V Switch
n
Constant-Current and Constant-Voltage Regulation
n
250mV 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
n
n
n
n
n
nꢀ
Programmable Undervoltage Lockout with Hysteresis
Constant-Voltage Loop Status Pin
PWM Disconnect Switch Driver
CTRL Pin Adjusts High Side Current Sense Threshold
Low Shutdown Current: <1µA
Programmable Soft-Start
Available in the 36-Lead (5mm × 6mm) QFN Package
The LT3956 senses 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
High Power LED
n
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 7199560 and 7321203.
Battery Charger
n
Accurate Current Limited Voltage Regulator
Typical applicaTion
Efficiency vs VIN
94% Efficient 25W White LED Headlamp Driver
V
, 6V TO 60V
IN
D1
22µH
(80V TRANSIENT)
100
2.2µF
s 2
2.2µF
s 5
332k
V
SW
IN
EN/UVLO
PGND
96
92
88
100k
V
ISP
REF
332k
LT3956
0.68Ω
370mA
CTRL
ISN
FB
INTV
CC
40.2k
1M
100k
VMODE
PWM
SS
25W LED STRING
16.2k
84
RT
PWMOUT
GND INTV
V
C
CC
80
0
20
60
80
40
(V)
28.7k
375kHz
34k
4.7nF
INTV
CC
V
IN
4.7µF
47nF
M1
3956 TA01b
3956 TA01a
3956f
ꢀ
LT3956
absoluTe MaxiMuM raTings
pin conFiguraTion
(Note 1)
TOP VIEW
V , ISP, ISN ..............................................................80V
IN
SW............................................................................84V
EN/UVLO (Note 3).....................................................80V
INTV ...................................................... V + 0.3V, 8V
36 35 34 33 32 31 30
NC
1
2
3
4
28 ISP
27 ISN
CC
IN
PWMOUT..................................................INTV + 0.3V
EN/UVLO
CC
INTV
CC
CTRL, PWM, VMODE................................................12V
37
GND
GND
FB
25
FB ...............................................................................8V
24 GND
V , V , SS................................................................3V
C
REF
V
PWMOUT
6
23
IN
RT ............................................................................1.5V
PGND to GND......................................................... 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
Lead Temperature (Soldering, 10 sec) .................. 300°C
38
SW
SW
8
9
21 SW
20 SW
SW
NC 10
12 13 14 15 16 17
UHE PACKAGE
36-LEAD (5mm s 6mm) PLASTIC QFN
T
= 125°C, θ = 43°C/W, θ = 5°C/W
JMAX
JA
JC
EXPOSED PAD (PIN 37) IS GND, MUST BE SOLDERED TO PCB
EXPOSED PAD (PIN 38) IS SW, MUST BE SOLDERED TO PCB
orDer inForMaTion
LEAD FREE FINISH
LT3956EUHE#PBF
LT3956IUHE#PBF
TAPE AND REEL
PART MARKING*
3956
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3956EUHE#TRPBF
LT3956IUHE#TRPBF
–40°C to 125°C
–40°C to 125°C
36-Lead (5mm × 6mm) Plastic QFN
36-Lead (5mm × 6mm) Plastic QFN
3956
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/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
4.5
V
IN
IN
IN
Shutdown I
EN/UVLO = 0V
EN/UVLO = 1.15V
0.1
1
5
µA
µA
Q
V
V
V
Operating I (Not Switching)
PWM = 0V
1.4
2.00
0.006
1.7
mA
V
IN
Q
l
Voltage
–100µA ≤ I
≤ 0µA
1.965
2.045
REF
REF
VREF
Line Regulation
4.5V ≤ V ≤ 80V
%/V
IN
3956f
ꢁ
LT3956
elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
3.3
8
TYP
5
MAX
10
UNITS
µA
SW Pin Leakage
SW = 48V
l
SW Pin Current Limit
SW Pin Voltage Drop
SS Pull-Up Current
Error Amplifier
3.9
220
10
4.6
A
I(SW) = 2A
mV
µA
Current Out of Pin
13
l
l
l
l
Full-Scale Current Sense Threshold (V
)
FB = 0V, ISP = 48V, CTRL ≥ 1.2V
CTRL = 1V, FB = 0V, ISP = 48V
CTRL = 0.5V
240
217
96
250
225
100
0
257
231
103
4.5
1.1
100
80
mV
mV
mV
mV
V
(ISP–ISN)
Current Sense Threshold at CTRL = 1V (V
)
(ISP–ISN)
Current Sense Threshold at CTRL = 0.5V (V
Current Sense Threshold at CTRL = 0.1V (V
)
)
(ISP–ISN)
(ISP–ISN)
CTRL = 0.1V, FB = 0V, ISP = 48V
–2.5
0
CTRL Range for Current Sense Threshold Adjustment
CTRL Input Bias Current
Current Out of Pin, CTRL = 0V
ISN = 0V
50
nA
V
Current Sense Amplifier Input Common Mode
2.9
Range ( V )
ISN
ISP/ISN Short-Circuit Threshold (V
)
300
0
335
370
3
mV
V
(ISP–ISN)
ISP/ISN Short-Circuit Fault Sensing Common Mode
Range ( V
)
ISN
ISP/ISN Input Bias Current (Combined)
PWM = 5V (Active), ISP = ISN = 48V
PWM = 0V (Standby), ISP = ISN = 48V
80
0
µA
µA
0.1
LED Current Sense Amplifier g
120
µS
kΩ
nA
m
V Output Impedance
C
1V < V < 2V
15000
C
V Standby Input Bias Current
C
PWM = 0V
–20
20
lꢀ
FB Regulation Voltage (V
)
FB
ISP = ISN = 0V, 48V
1.220
1.232
1.250
1.250
1.270
1.265
V
V
FB Amplifier g
FB = V , ISP = ISN
480
40
µS
nA
V
m
FB
FB Pin Input Bias Current
Current Out of Pin, FB = 1V
100
FB Voltage Loop Active Threshold
VMODE Falling
V
–
V
FB
–
V –
FB
40mV
FB
65mV
50mV
FB Overvoltage Threshold
PWMOUT Falling
V
FB
+ 50mV
V
+
V +
FB
80mV
V
FB
60mV
Oscillator
l
Switching Frequency
R = 100k
T
90
925
100
1000
125
1050
kHz
kHz
T
R = 10k
SW Minimum Off-Time
SW Minimum On-Time
Linear Regulator
170
200
ns
ns
INTV Regulation Voltage
7
7.15
1
7.3
V
V
CC
Dropout (V – INTV
)
CC
I
= –10mA, V = 7V
INTVCC IN
IN
l
INTV Undervoltage Lockout
4.1
17
8
4.4
25
12
V
CC
INTV Current Limit
14
mA
µA
CC
INTV Current in Shutdown
EN/UVLO = 0V, INTV = 7V
CC
CC
3956f
ꢂ
LT3956
elecTrical characTerisTics The l denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Logic Inputs/Outputs
l
l
PWM Threshold Voltage
0.85
45
1.35
60
1.8
V
kΩ
V
PWM Pin Resistance to GND
EN/UVLO Threshold Voltage Falling
EN/UVLO Rising Hysteresis
EN/UVLO Input Low Voltage
EN/UVLO Pin Bias Current Low
EN/UVLO Pin Bias Current High
1.185
1.220
20
1.245
mV
V
I
Drops Below 1µA
0.4
2.5
100
200
5
VIN
EN/UVLO = 1.15V
EN/UVLO = 1.30V
1.7
2.1
10
µA
nA
mV
µA
VMODE Output Low (V
VMODE Pin Leakage
PWMOUT Driver
)
I
= 1mA
VMODE
OL
FB = 0V, VMODE = 12V
0.1
t PWMOUT Driver Output Rise Time
C = 560pF
35
35
ns
ns
V
r
L
t PWMOUT Driver Output Fall Time
f
C = 560pF
L
PWMOUT Output Low (V
)
OL
0.05
PWMOUT Output High (V
)
OH
INTV
–
V
CC
0.05
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.
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3956I is guaranteed to meet performance specifications over the –40°C
to 125°C operating junction temperature range.
Note 2: The LT3956E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
Note 3: For V below 6V, the EN/UVLO pin must not exceed V for proper
operation.
IN
IN
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted.
V(ISP–ISN) Threshold vs VISP
with Reduced CTRL Voltage
V(ISP–ISN) Full-Scale Threshold
vs Temperature
V(ISP–ISN) Threshold vs VCTRL
256
254
252
250
248
246
244
242
300
250
200
150
100
50
103
102
101
100
99
CTRL = 2V
CTRL = 0.5V
98
0
–50
97
–50 –25
0
25
50
75 100 125
0
0.5
1
1.5
2
0
20
40
60
80
CTRL VOLTAGE (V)
ISP VOLTAGE (V)
TEMPERATURE (°C)
3956 G03
3956 G02
3956 G01
3956f
ꢃ
LT3956
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted.
FB Regulation Voltage (VFB
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
–25
50
75 100 125
0
20
40
(V)
60
80
–50
0
25
–25
TEMPERATURE (°C)
V
TEMPERATURE (°C)
IN
3956 G05
3956 G04
3956 G06
Switching Frequency
vs Temperature
EN/UVLO Hysteresis Current
vs Temperature
Switching Frequency vs RT
10000
1000
100
500
450
400
350
300
2.4
2.2
2.0
1.8
1.6
R
T
= 26.7k
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
3956 G08
3956 G09
3956 G07
SW Pin Current Limit
vs Temperature
EN/UVLO Threshold
vs Temperature
Quiescent Current vs VIN
4.4
4.2
4.0
3.8
3.6
1.28
1.26
1.24
1.22
1.20
1.18
2.0
1.5
1.0
0.5
0
PWM = 0V
EN/UVLO RISING
EN/UVLO FALLING
–50
0
25
50
75 100 125
–25
50
–50
0
25
75 100 125
–25
0
20
40
(V)
60
80
TEMPERATURE (°C)
TEMPERATURE (°C)
V
IN
3956 G12
3956 G11
3956 G10
3956f
ꢄ
LT3956
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted.
Quiescent Current
vs Switching Frequency
INTVCC Current Limit
vs Temperature
INTVCC Voltage vs Temperature
20
18
16
14
12
10
7.4
7.3
7.2
7.1
7.0
12
10
8
NOT SWITCHING
6
4
2
0
–50
0
25
50
75 100 125
0
400
600
800
1000
–25
–50
0
25
50
TEMPERATURE (°C)
75 100
125
200
–25
TEMPERATURE (°C)
SWITCHING FREQUENCY (kHz)
3956 G14
3956 G13
3956 G15
SW Pin Current Limit
vs Duty Cycle
LED Current Sense Threshold
vs FB Voltage
INTVCC Dropout Voltage
vs INTVCC Current
312.5
250.0
187.5
125.0
62.50
0
0
–0.5
–1.0
–1.5
–2.0
–2.5
4.5
4.0
3.5
3.0
2.5
V
CTRL
= 2V
–40°C
25°C
125°C
0
6
9
12
15
3
0
25
50
75
100
1.2
1.22
1.24
1.26
1.28
LDO CURRENT (mA)
DUTY CYCLE (%)
FB VOLTAGE (V)
3956 G18
3956 G17
3956 G16
ISP/ISN Input Bias Current
vs CTRL Voltage
Switch On-Resistance
vs Temperature
PWMOUT Waveform
180
160
140
120
100
80
80
60
40
20
0
C
= 2.2nF
PWMOUT
PWM
ISP
ISN
INPUT
PWMOUT
5V/DIV
60
40
3956 G21
200ns/DIV
20
0
–50 –25
0
25
50
75 100 125
0
0.5
1
1.5
2
TEMPERATURE (°C)
CTRL (V)
3956 G19
3956 G20
3956f
ꢅ
LT3956
pin FuncTions
NC:NoInternalConnection.Thesepinsmaybeleftfloating
or connected to an adjacent pin.
FB: Voltage Loop Feedback Pin. FB is intended for con-
stant-voltage regulation or for LED protection/open LED
detection. The internal transconductance amplifier with
EN/UVLO: Shutdown and Undervoltage Detect Pin. An
accurate 1.22V falling threshold with externally program-
mable 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 1.24V (nominal) threshold (but below
6V), EN/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 resis-
tor selection. An undervoltage condition resets soft-start.
output V will regulate FB to 1.25V (nominal) through the
C
DC/DC converter. If the FB input is regulating the loop, the
VMODE 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 VMODE
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.
Tie to 0.4V, or less, to disable the device and reduce V
quiescent current below 1µA.
ISN: Connection point for the negative terminal of the
current feedback resistor. If ISN is greater than 2.9V, the
IN
LED current can be programmed by I
= 250mV/R
LED
LED
)
LED
INTV : Regulated supply for internal loads, GATE driver
CC
when V
> 1.2V or I
= (V
–100mV)/(4 • R
CTRL
LED
CTRL
and PWMOUT driver. Supplied from V and regulates to
IN
if V
< 1V. Input bias current is typically 20µA. Below
CTRL
7.15V (typical). INTV must be bypassed with a 4.7µF
CC
3V, ISN is an input to the short-circuit protection feature
that forces GATE to 0V if ISP exceeds ISN by more than
350mV (typ).
capacitor placed close to the pin. Connect INTV directly
CC
to V if V is always less than or equal to 7V.
IN
IN
GND: Ground. The exposed pad, Pin 37, is ground and
must be soldered directly to the ground plane.
ISP:Connectionpointforthepositiveterminalofthecurrent
feedback resistor. Input bias current for this pin depends
on CTRL pin voltage, as shown in the Typical Performance
Characteristics. ISP is an input to the short-circuit protec-
tion feature when ISN is less than 3V.
V : Input Supply Pin. Must be locally bypassed with
IN
a 0.22µF (or larger) capacitor to PGND placed close to
the IC.
SW: The exposed pad, Pin 38, is the drain of the switch-
ing N-channel MOSFET and must be connected to the
external inductor.
V : Transconductance Error Amplifier Output Pin. This pin
C
is 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.
PGND: Source terminal of switch and the GND input to
the switch current comparator. Kelvin connect to the GND
plane close to the IC using Pin 12. Pins 13 to 17 should be
connectedexternallytothePGNDterminalsofcomponents
in the switching path. See the Board Layout section.
CTRL: Current Sense Threshold Adjustment Pin. Regula-
ting threshold V
is 0.25 • V
CTRL
plus an offset
(ISP – ISN)
< 1V. For V
CTRL
PWMOUT:BufferedVersionofthePWMSignal. Thispinis
used to drive the LED load disconnect N-channel MOSFET
or level shift. This pin also serves in a protection function
for the FB overvoltage condition—will toggle if the FB
for 0V < V
> 1.2V the current sense
CTRL
threshold is constant at the full-scale value of 250mV. For
1V < V < 1.2V, the dependence of the current sense
CTRL
threshold upon V
transitions from a linear function
CTRL
input is greater than the FB regulation voltage (V ) plus
FB
to a constant value, reaching 98% of full-scale value by
= 1.1V. Connect CTRL to V for the 250mV default
60mV (typical). The PWMOUT pin is driven from INTV .
CC
V
CTRL
REF
Use of a MOSFET with gate cut-off voltage higher than
threshold. Do not leave this pin open.
1V is recommended.
3956f
ꢆ
LT3956
pin FuncTions
V
: Voltage Reference Output Pin (typically 2V). This
high. When PWM input goes high again, the VMODE pin
will be updated. This pin may be used to report an open
LED fault. Use a pull-up current less than 1mA.
REF
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.
SS: Soft-Start Pin. This pin modulates oscillator fre-
PWM: A signal low turns off switcher, idles oscillator and
quency and compensation pin voltage (V ) clamp. The
C
disconnects V pin from all internal loads. PWMOUT pin
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 EN/UVLO pin)
or thermal limit.
C
follows PWM pin. PWM has an internal pull-down resistor.
If not used, connect to INTV .
CC
VMODE: An open-collector pull-down on VMODE asserts
if the FB input is greater than the FB regulation threshold
minus 50mV (typical). To function, the pin requires an
external pull-up resistor. When the PWM input is low
and the DC/DC converter is idle, the VMODE condition is
latched to the last valid state when the PWM input was
RT: Switching Frequency Adjustment Pin. Set the fre-
quency using a resistor to GND (for resistor values, see
the Typical Performance curve or Table 1). Do not leave
the RT pin open.
block DiagraM
EN/UVLO
–
+
A6
FB
VC
PWMOUT PWM
1.25V
V
IN
–
+
SHDN
1.22V
2.1µA
1.31V
LDO
–
+
OVFB
COMPARATOR
A8
INTV
7.15V
CC
A5
+
–
10µA AT
FB = 1.25V
g
m
1.25V
SW
SHORT-CIRCUIT
DETECT
SCILMB
10µA
+
SCILMB
A10
+
+
–
350mV
–
R
Q
–
g
A2
m
DRIVER
S
EAMP
ISN
ISP
PWM
COMPARATOR
+
–
10µA AT
5k
A1
I
+
–
PGND
GND
SENSE
A1 = A1
+
–
CTRL
A4
BUFFER
CTRL
+
–
1.1V
A3
Q2
+
RAMP
GENERATOR
VC
SSCLAMP
10µA
20k
100kHz TO 1MHz
OSCILLATOR
VMODE
FAULT
–
+
140µA
LOGIC
1.25V
+
+
–
1.2V
FB
1mA (MAX)
V
REF
TSD
165°C
FREQ
PROG
–
+
170k
A7
2V
SS
RT
3956f
ꢇ
LT3956
operaTion
tor. Likewise, at an ISP/ISN common mode voltage less
than 3V, the difference between ISP and ISN is monitored
to determine if the output is in a short-circuit condition. If
the difference between ISP and ISN is greater than 335mV
(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.
The LT3956 is a constant-frequency, current mode
converter with a low side N-channel MOSFET switch.
The switch and PWMOUT pin drivers, and other chip
loads, are powered from INTV , which is an internally
CC
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 power switch is
turned off and the PWMOUT pin is driven to GND, the
V pin is high impedance to store the previous switching
C
In voltage feedback mode, the operation is similar to that
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 internal power MOSFET
switch. A voltage input proportional to the switch current,
sensed by an internal current sense resistor, is added to
a stabilizing slope compensation ramp and the resulting
switch-current sense signal is fed into the positive termi-
nal 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
described above, except the voltage at the V pin is set by
C
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
output of the error amplifier, labeled V , the latch is reset
C
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 compensa-
tion return to their starting points and a new cycle begins
with the set pulse from the oscillator.
tied together and the CTRL input tied to V
.
REF
Two LED specific functions featured on the LT3956 are
controlled by the voltage feedback pin. First, when the
FB pin exceeds a voltage 50mV lower (–4%) than the FB
regulationvoltage, thepull-downdriverontheVMODEpin
is activated. 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.
WhentheFBpinexceedstheFBregulationvoltageby60mV
(5% typical), the PWMOUT pin is driven low, ignoring the
state of the PWM input. In the case where the PWMOUT
pin drives a disconnect NFET, this action isolates the
LED load from GND, preventing excessive current from
damaging the LEDs. If the FB input exceeds the overvolt-
age threshold (1.31V typical), then an externally driven
overvoltage event may have caused the FB pin to be too
high and the VMODE pull-down will be deactivated until
the FB pin drops below the overvoltage threshold.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current or
voltage in the load. The V signal is integrated over many
C
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 is not allowed to
exceed the current limit threshold of 3.9A (typical). If the
SW pin exceeds the current limit threshold, the SR latch is
reset regardless of the output state of the PWM compara-
3956f
ꢈ
LT3956
applicaTions inForMaTion
LED Current Programming
INTV Regulator Bypassing and Operation
CC
The LED current is programmed by placing an appropri-
The INTV pin requires a capacitor for stable operation
CC
ate value current sense resistor, R , between the ISP
and to store the charge for the switch driver and PWMOUT
pin 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
LED
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 250mV
(typical)thresholdacrossthesenseresistor. TheCTRLpin
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:
to the INTV pin and also to the IC ground.
CC
An internal current limit on the INTV output protects
CC
the LT3956 from excessive on-chip power dissipation.
The INTV pin has its own undervoltage disable (UVLO)
CC
set to 4.1V (typical) to protect the internal MOSFET from
excessive power dissipation caused by not being fully en-
hanced.IftheINTV pindropsbelowtheUVLOthreshold,
CC
the PWMOUT pin will be forced to 0V, the power switch
turned off and the soft-start pin will be reset.
VCTRL − 100mV
ILED
=
RLED • 4
Iftheinputvoltage, V , willnotexceed7V, thentheINTV
IN
CC
pin could be connected to the input supply. This action
allowstheLT3956tooperatefromaslowas4.5V. Beaware
When the CTRL pin voltage is between 1V and 1.2V
the LED current varies with CTRL, but departs from the
previous equation by an increasing amount as the CTRL
voltage increases. Ultimately, above CTRL = 1.2V, the LED
current no longer varies with CTRL. At CTRL = 1.1V, the
that a small current (less than 12μA) will load the INTV
CC
inshutdown.Otherwise,theminimumoperatingV value
IN
isdeterminedbythedropoutvoltageofthelinearregulator
and the 4.4V (4.1V typical) INTV undervoltage lockout
CC
actual value of I
is ~98% of the equation’s estimate.
LED
threshold mentioned above.
When V
is higher than 1.2V, the LED current is regu-
CTRL
Programming the Turn-On and Turn-Off Thresholds
With the EN/UVLO Pin
lated to:
250mV
RLED
ILED
=
ThefallingUVLOvaluecanbeaccuratelysetbytheresistor
divider. A small 2.1µA pull-down current is active when
EN/UVLO is below the falling 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:
The CTRL pin should not be left open (tie to V
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 V to reduce
output power and switching current when V is low.
if not
REF
IN
IN
R1+ R2
VIN,FALLING = 1.22 •
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
R2
VIN,RISING = 2.1µA •R1+ V
IN,FALLING
V
IN
LT3956
R1
R2
EN/UVLO
V pin filters the signal so the average difference between
C
ISP and ISN is regulated to the user-programmed value.
Ripple voltage amplitude (peak-to-peak) in excess of
3956 F01
Figure 1
3956f
ꢀ0
LT3956
applicaTions inForMaTion
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 or SEPIC application, the output voltage can
be set by selecting the values of R3 and R4 (see Figure 2)
according to the following equation:
Output Current Capability
An important consideration when using a switch with a
fixed current limit is whether the regulator will be able
to supply the load at the extremes of input and output
voltage range. Several equations are provided to help
determine this capability. Some margin to data sheet
limits is included.
R3 + R4
VOUT = 1.25 •
R4
ForaboosttypeLEDdriver,settheresistorfromtheoutput
to the FB pin such that the expected voltage level during
normal operation will not exceed 1.1V. For an LED driver
of buck mode or a buck-boost mode 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:
For boost converters:
V
IN(MIN)
IOUT(MAX) ≤ 2.5A
VOUT(MAX)
For buck mode converters:
≤ 2.5A
R3
VOUT = VBE + 1.25 •
R4
I
OUT(MAX)
+
For SEPIC and buck-boost mode converters:
R3
R
LED
C
OUT
V
V
OUT
IN(MIN)
LED
ARRAY
IOUT(MAX) ≤ 2.5A
100k
(VOUT(MAX) + V
)
–
LT3956
IN(MIN)
FB
These equations assume the inductor value and switch-
ing frequency have been selected so that inductor ripple
current is ~600mA. Ripple current higher than this value
will reduce available output current. Be aware that current
limited operation at high duty cycle can greatly increase
inductor ripple current, so additional margin may be
required at high duty cycle.
R4
3956 F03
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
ISP/ISN Short-Circuit Protection Feature for SEPIC
If some level of analog dimming is acceptable at minimum
supplylevels,thentheCTRLpincanbeusedwitharesistor
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 in a SEPIC converter. The action
threshold (335mV, 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 switch-current
limit—it prevents switch turn-on until the ISP/ISN differ-
divider to V (as shown on page 1) to provide a higher
IN
output current at nominal V levels.
IN
V
OUT
LT3956
R3
FB
R4
3956 F02
Figure 2. Feedback Resistor Connection
for Boost or SEPIC LED Drivers
ence falls below the threshold.
3956f
ꢀꢀ
LT3956
applicaTions inForMaTion
Dimming Control
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 mini-
mum/maximum duty cycle:
There are two methods to control the current source for
dimming using the LT3956. 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 zero and full current to achieve a precisely pro-
grammedaveragecurrent. Tomakethismethodofcurrent
controlmoreaccurate,theswitchdemandcurrentisstored
on the V node during the quiescent phase when PWM is
C
low. This feature minimizes recovery time when the PWM
signal goes high. To further improve the recovery time, a
disconnect switch may be used in the LED current path to
prevent the ISP node from discharging during the PWM
signal low phase. The minimum PWM on or off time will
depend on the choice of operating frequency through the
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 and
SW node rise/fall times.
R input.Forbestoverallperformance,theminimumPWM
T
low or high time should be at least six switching cycles
(6μs for f = 1MHz).
SW
Programming the Switching Frequency
300
250
The R frequency adjust pin allows the user to program
T
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 appropriate
MINIMUM ON-TIME
200
MINIMUM OFF-TIME
150
100
50
0
R resistor value see Table 1. An external resistor from the
T
RT pin to GND is required—do not leave this pin open.
–50
0
25
50
75 100 125
–25
TEMPERATURE (°C)
3956 F04
Table 1. Switching Frequency vs RT Value
f
(kHz)
R (k)
OSC
T
Figure 4. Typical Switch Minimum On
and Off Pulse Width vs Temperature
1000
10
900
800
700
600
500
400
300
200
100
11.8
13
Thermal Considerations
15.4
17.8
21
The LT3956 is rated to a maximum input voltage of 80V.
Careful attention must be paid to the internal power dis-
sipation of the IC at higher input voltages to ensure that a
junctiontemperatureof125°Cisnotexceeded.Thisjunction
limitisespeciallyimportantwhenoperatingathighambient
temperatures.IftheLT3956’sjunctiontemperaturereaches
165°C (typ), the power switch will be turned off and the
26.7
35.7
53.6
100
soft-start (SS) pin will be discharged to GND. Switching
3956f
ꢀꢁ
LT3956
applicaTions inForMaTion
will be enabled after the device temperature drops 10°C.
This function is intended to protect the device during
momentary overload conditions.
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 major contributors to internal power dissipation are
the current in the linear regulator to drive the switch, and
the ohmic losses in the switch. The linear regulator power
is proportional to V and switching frequency, so at high
IN
V
the switching frequency should be chosen carefully
IN
VOUT
1µF
A • µs
to ensure that the IC does not exceed a safe junction
temperature. The internal junction temperature of the IC
can be estimated by:
CIN(µF) = ILED(A)
•
• TSW(µs) •
V
IN
Therefore, a 4.7µF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 1A load.
2
T = T + [V • (I + f • 7nC) + I
• 0.14Ω • D
]
SW
J
A
IN
Q
SW
SW
• θ
JA
where T is the ambient temperature, I is the quiescent
WiththesameV voltagerippleof100mV,theinputcapaci-
A
Q
IN
current of the part (maximum 1.7mA) and θ is the
tor for a buck converter can be estimated as follows:
JA
package thermal impedance (43°C/W for the 5mm × 6mm
4.7µF
CIN(µF) = ILED(A) • TSW(µs)
•
QFN package). For example, an application with T
=
A(MAX)
A • µs
85°C,V
=60V,f =400kHz,andhavinganaverage
IN(MAX)
SW
switchingcurrentof2.5Aat70%dutycycle,themaximum
IC junction temperature will be approximately:
A 10µF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 1A load.
2
T = 85°C + [(2.5A) • 0.14Ω • 0.7 + 60V •
J
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 PGND
return of the switch. 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:
(1.7mA + 400kHz • 7nC)] • 43°C/W= 123°C
The Exposed Pads on the bottom of the package must be
solderedtoaplane.Theseshouldthenbeconnectedtointer-
nal copper planes with thermal vias placed directly under
the package to spread out the heat dissipated by the IC.
Open LED Detection
The LT3956 provides an open-drain status pin, VMODE,
that pulls low when the FB pin is within ~50mV of its 1.25V
regulated voltage. If the open LED clamp voltage is pro-
grammedcorrectlyusingtheFBpin,thentheFBpinshould
neverexceed1.1VwhenLEDsareconnected,therefore,the
onlywayfortheFBpintobewithin50mVofthe regulation
voltage is for an open LED event to have occurred.
IIN(RMS) = ILED
•
1–D •D
where D is the switch duty cycle.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
TDK
WEB SITE
www.tdk.com
Input Capacitor Selection
Kemet
www.kemet.com
www.murata.com
www.t-yuden.com
Theinputcapacitorsuppliesthetransientinputcurrentfor
the power inductor of the converter and must be placed
andsizedaccordingtothetransientcurrentrequirements.
Theswitchingfrequency,outputcurrentandtolerableinput
Murata
Taiyo Yuden
3956f
ꢀꢂ
LT3956
applicaTions inForMaTion
Output Capacitor Selection
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 3 has some recom-
mended component vendors.
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 an X7R type ceramic capacitor is
recommended.
Table 3. Schottky Rectifier Manufacturers
VENDOR
WEB SITE
On Semiconductor
Diodes, Inc.
www.onsemi.com
www.diodes.com
www.centralsemi.com
To achieve the same LED ripple current, the required filter
capacitor is larger in the boost and buck-boost mode ap-
plications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values.
Central Semiconductor
Inductor Selection
TheinductorusedwiththeLT3956shouldhaveasaturation
current rating appropriate to the maximum switch current
of 4.6A. Choose an inductor value based on operating
frequency, input and output voltage to provide a current
mode signal of approximately 0.6A magnitude. The fol-
lowing equations are useful to estimate the inductor value
(T = 1/f ):
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
voltageovershoot. Thesoft-startintervalissetbythesoft-
start capacitor selection according to the equation:
SW
OSC
TSW • VLED V – V
(
)
IN
LED
LBUCK
=
2V
10µA
V • 0.6A
TSS = CSS
•
TSW • VLED • V
IN
LBUCK-BOOST
=
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 EN/UVLO falls below its threshold,
V
LED + V • 0.6A
IN
TSW • V
VLED – VIN
(
)
IN
LBOOST
=
V• 0.6A
during an overtemperature event or during an INTV un-
CC
dervoltageevent.Duringstart-upwithEN/UVLO,charging
of the soft-start capacitor is enabled after the first PWM
high period.
Table 4 provides some recommended inductor vendors.
Table 4. Inductor Manufacturers
VENDOR
Sumida
WEB SITE
www.sumida.com
www.we-online.com
www.cooperet.com
www.rencousa.com
www.coilcraft.com
Schottky Rectifier Selection
Würth Elektronik
Coiltronics
Renco
The power Schottky diode conducts current during the
interval when the switch is turned off. Select a diode rated
for the maximum SW voltage of the application and the
RMSdiodecurrent. IfusingthePWMfeaturefordimming,
Coilcraft
3956f
ꢀꢃ
LT3956
applicaTions inForMaTion
Loop Compensation
interference (EMI), it is important to minimize the area of
the high dV/dt switching node between the inductor, SW
pin 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 to PGND, and
2) from the switch node through the Schottky rectifier and
filtercapacitortoPGND, shouldbeminimized. Theground
points of these two switching current traces should come
to a common point then connect to the ground plane at the
PGND pin of the LT3956 through a separate via to Pin 12,
as shown in the suggested layout (Figure 5). Likewise, the
TheLT3956usesaninternaltransconductanceerrorampli-
fier whose V output compensates the control loop. The
C
external inductor, output capacitor and the compensation
resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at V are selected to optimize control loop
C
responseandstability.FortypicalLEDapplications,a4.7nF
compensation capacitor at V is adequate, and a series re-
C
sistor should always be used to increase the slew rate on
theV pintomaintaintighterregulationofLEDcurrentdur-
C
ground terminal of the bypass capacitor for the INTV
CC
ing fast transients on the input supply to the converter.
regulator should be placed near the GND of the IC. The
groundforthecompensationnetworkandotherDCcontrol
signals should be star connected to the GND Exposed Pad
of the IC. Do not extensively route high impedance signals
Board Layout
The high speed operation of the LT3956 demands careful
attention to board layout and component placement. The
exposed pads of the package are important for thermal
managementoftheIC.Itiscrucialtoachieveagoodelectri-
calandthermalcontactbetweentheGNDexposedpadand
the ground plane of the board. To reduce electromagnetic
such as FB and V , as they may pick up switching noise.
C
Since there is a small variable DC input bias current to
the ISN and ISP inputs, resistance in series with these
pins should be minimized to avoid creating an offset in
the current sense threshold.
C
C
C
SS
R
R
C
T
VIA FROM V
OUT
36 35 34 33 32 31 30
LT3956
1
2
3
4
28
27
+
–
VIA FROM LED
LED
3
CV
CC
25
24
23
R1 R2
R4 R3
M1
VIAS TO GND PLANE
VIAS TO SW PLANE
GND
1
2
V
6
IN
VIAS FROM
PGND
8
9
21
20
SW
10
12 13 14 15 16 17
PGND
VIAS
D1
L1
C
C
OUT
OUT
R
S
+
LED
+
V
LED
OUT
VIA VIA
CV
IN
V
IN
PGND
3956 F05
Figure 5. Boost Converter Suggested Layout
3956f
ꢀꢄ
LT3956
Typical applicaTions
94% Efficient 25W White LED Headlamp Driver
V
IN
L1
22µH
6V TO 60V
D1
(80V TRANSIENT)
M1: VISHAY SILICONIX Si2328DS
D1: DIODES INC PDS5100
L1: COILTRONICS DR125-220
C1, C2: MURATA GRM42-2x7R225
C
C
OUT
R1
VIN
V
SW
2.2µF
2.2µF
IN
332k
s 2
s 5
PGND
EN/UVLO
R2
100k
V
ISP
REF
332k
R
S
LT3956
370mA
0.68Ω
CTRL
ISN
FB
INTV
CC
40.2k
R3
1M
100k
VMODE
PWM
SS
R4
16.2k
25W LED STRING
(CURRENT DERATED
FOR V < 11V)
IN
RT
PWMOUT
GND INTV
V
C
CC
R
T
28.7k
R
C
INTV
C
VCC
4.7µF
375kHz
CC
20k
C
C
SS
C
4.7nF
M1
47nF
3956 TA02a
SEE SUGGESTED LAYOUT (FIGURE 5)
PWM Waveforms for 25W Headlamp Driver
PWM
I
LED
200mA/DIV
I
LI
1A/DIV
3956 TA02b
5µs/DIV
V
V
= 68V
OUT
IN
= 15V
3956f
ꢀꢅ
LT3956
Typical applicaTions
Buck-Boost Mode LED Driver
Efficiency vs VIN
L1
68µH
D1
V
IN
100
96
92
88
84
80
V
9V TO
45V
OUT
C1
1µF
4.7µF
35V
V
SW
ISP
IN
4.7µF
1M
100V
EN/UVLO
V
IN
V
187k
REF
680mΩ
LT3956
ISN
CTRL
INTV
CC
619k
10k
24V
LED STRING
350mA
FB
100k
VMODE
PWM
SS
PGND
RT
PWMOUT
GND INTV
0
10
40
50
20
30
V
C
CC
V
(V)
750Ω
Q1
IN
35.7k
300kHz
3956 TA03b
3.4k
10nF
INTV
M1
CC
4.7µF
0.1µF
V
IN
L1: COILCRAFT MSS1038-683
1k
D1: ON SEMICONDUCTOR MBRS3100T3
M1: ZETEX ZXM6IP03F
3956 TA03a
Q1: ZETEX FMMT493
28VIN /0V to 28V SEPIC SuperCap Charger with Input Current Limit
Input and Output Current
vs Output Voltage
C4
10µF
L1A
33µH
D1
V
V
OUT
0V TO 28V
200mΩ
IN
28V
3.0
2.5
2.0
1.5
1.0
0.5
0
1:1
≤ 1.2A
C1
ISP
ISN
1µF
10µF
L1B
C3
10µF
SW
V
IN
PGND
FB
EN/UVLO
536k
OUTPUT
LT3956
25k
PWMOUT
1M
VMODE
CTRL
INPUT
1M
10k
40.2k INTV
CC
14k
INTV
V
REF
SS
CC
59k
PWM
2k
0
10
15
(V)
20
25
30
5
RT
GND
V
C
Q1
V
OUT
3956 TA04b
28.7k
375kHz
C2
4.7µF
30.1k
10nF
3956 TA04a
L1: WÜRTH ELEKTRONIK 744871330
D1: ON SEMI MBRS36OT
Q1: MMBTA42
C1, C3, C4: TAIYO-YUDEN GMK 3I6BJ106
3956f
ꢀꢆ
LT3956
Typical applicaTions
Efficiency vs VIN
90% Efficient, 20W SEPIC LED Driver
C4
L1A
100
96
92
88
84
80
2.2µF (50V)
33µH
D1
V
IN
8V TO
50V
1:1
C1
4.7µF
50V
C3
1M
250k
25k
10µF
s2
V
IN
L1B
EN/UVLO
SW
35V
185k
V
PGND
REF
CTRL
ISP
ISN
FB
0.25Ω
1A
INTV
LT3956
CC
100k
1M
VMODE
PWM
SS
20W
LED
0
10
40
50
20
30
STRING
56.2k
V
(V)
IN
RT
3956 TA05b
PWMOUT
GND INTV
V
C
CURRENT
DERATED
FOR V < 13V
CC
28.7k
375kHz
C2
4.7µF
10V
IN
15k
10nF
0.01µF
M1
3956 TA05a
L1: COILTRONICS DRQ127-330
D1: VISHAY PDS5100
M1: ZETEX ZXM61N03F
90W Buck Mode LED Driver, 80VIN/60VOUT
Efficiency vs VIN
V
IN
100
98
96
94
92
90
64V TO
80V
267k 200k
C2
2.2µF
s 3
1M
V
ISP
IN
EN/UVLO
470Ω
1.5A
Q1
0.1Ω
200k
20k
ISN
FB
INTV
CC
LT3956
100k
M1
10k
VMODE
V
PWMOUT
Q2
1k
REF
16 WHITE
LEDs, 90W
24.3k
13k
CTRL
L1
33µH
64
76
80
68
72
(V)
PWM
SS
RT
V
IN
SW
PGND
3956 TA05b
D1
V
C
GND INTV
CC
28.7k
375kHz
V
IN
INTV
4.7µF
CC
C1
0.1µF
0.01µF
2.2µF
s 4
3956 TA06a
D1: VISHAY 10MQ100N
L1: WÜRTH ELEKTRONIK 744066330
M1: VISHAY SILICONIX Si7113DN
Q1: ZETEX FMMT593
Q2: ZETEX FMMT493
C1, C2: MURATA GRM42-2x7R225
3956f
ꢀꢇ
LT3956
package DescripTion
UHE Package
Variation: UHE28MA
36-Lead Plastic QFN (5mm s 6mm)
(Reference LTC DWG # 05-08-1836 Rev C)
28 27
25 24 23
21 20
0.70 p0.05
17
16
15
14
30
1.88
p 0.05
1.53
31
p 0.05
5.50 p 0.05
3.00 p 0.05
3.00 p 0.05
32
4.10 p 0.05
33
0.12
p 0.05
PACKAGE OUTLINE
0.48 p 0.05
13
34
1.50 REF
35
12
36
1
2
3
4
6
8
9
10
0.25 p0.05
2.00 REF
0.50 BSC
5.10 p 0.05
6.50 p 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.75 p 0.05
PIN 1 NOTCH
R = 0.30 OR
0.35 s 45o
CHAMFER
R = 0.10
1.50 REF
33 34 35
5.00 p 0.10
TYP
30 31 32
36
PIN 1
TOP MARK
(NOTE 6)
28
1
2
3
4
27
1.88 p 0.10
2.00 REF
3.00 p 0.10
0.12
p 0.10
25
24
6.00 p 0.10
6
23
0.48 p 0.10
1.53 p 0.10
8
R = 0.125
TYP
21
20
3.00 p 0.10
9
10
0.40 p 0.10
17 16 15
0.25 p 0.05
0.50 BSC
14 13 12
0.200 REF
(UHE28MA) QFN 0110 REV C
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
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
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 0.20mm ON ANY SIDE
3956f
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.
ꢀꢈ
LT3956
Typical applicaTion
Buck Mode 1A LED Driver with High Dimming Ratio and Open LED Reporting
Efficiency vs VIN
V
IN
100
96
92
88
84
80
24V TO
80V
C2
4.7µF
s 5
1M
200k 200k
200k
V
ISP
IN
EN/UVLO
750Ω 0.1Ω
1A
61.9k
Q2
ISN
FB
20k
M1
V
REF
30.1k
10k
Q1
1k
PWMOUT
CTRL
6 WHITE LEDs
20W
LT3956
INTV
CC
L1
33µH
100k
20
50
(V)
60
70
80
30
40
VMODE
SW
V
IN
D1
PWM
SS
3956 TA06b
V
IN
C1
1µF
s 2
0.1µF
RT
V
C
GND INTV PGND
CC
28.7k
375kHz
INTV
CC
47k
D1: DIODES INC B1100/B
L1: WÜRTH 74456133
M1: VISHAY SILICONIX Si5435BDC
Q1: ZETEX FMMT493
2.2nF
4.7µF
3956 TA07a
Q2: ZETEX FMMT593
C1: TDKC3226X7R2A105K
C2: TDKC3225X7RIE475K
relaTeD parTs
PART NUMBER
DESCRIPTION
COMMENTS
V : 6V to 100V, V = 100V, True Color PWM Dimming = 3000:1,
OUT(MAX)
LT3756/LT3756-1/
LT3756-2
100V , 100V , Full Featured LED Controller
IN
OUT
IN
I
< 1µA, 3mm × 3mm QFN-16 and MS16E Packages
SD
LT3755/LT3755-1/
LT3755-2
40V , 75V , Full Featured LED Controller
V : 4.5V to 40V, V
SD
= 60V, True Color PWM Dimming = 3000:1,
OUT(MAX)
IN
OUT
IN
I
< 1µA, 3mm × 3mm QFN-16 and MS16E Packages
LT3474
36V, 1A (I ), 2MHz, Step-Down LED Driver
V : 4V to 36V, V
SD
= 13.5V, True Color PWM Dimming = 400:1,
LED
IN
OUT(MAX)
OUT(MAX)
I
< 1µA, TSSOP16E Package
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 V : 2.8V to 16V, V
= 36V, True Color PWM Dimming = 1000:1,
OUT(MAX)
IN
with 1000:1 Dimming
I
< 10µA, 5mm × 7mm QFN Package
SD
LT3477
3A, 42V, 3MHz Boost, Buck-Boost, Buck LED Driver
V : 2.5V to 25V, V
= 40V, Dimming = Analog/PWM, I < 1µA,
OUT(MAX) SD
IN
QFN and TSSOP20E Packages
LT3478/LT3478-1
4.5A, 42V, 2.5MHz High Current LED Driver with
3000:1 Dimming
V : 2.8V to 36V, V = 42V, True Color PWM Dimming = 3000:1,
SD
IN
OUT(MAX)
I
< 3µA, TSSOP16E Package
3956f
LT 0510 • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
ꢁ0
●
●
LINEAR TECHNOLOGY CORPORATION 2010
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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