FeaTures
n
LT3956
80V
IN
, 80V
OUT
Constant-Current,
Constant-Voltage Converter
DescripTion
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.
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.
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.
3000:1 True Color PWM
TM
Dimming
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
Programmable Undervoltage Lockout with Hysteresis
n
Constant-Voltage Loop Status Pin
n
PWM Disconnect Switch Driver
n
CTRL Pin Adjusts High Side Current Sense Threshold
n
Low Shutdown Current: <1µA
n
Programmable Soft-Start
n
Available in the 36-Lead (5mm
×
6mm) QFN Package
applicaTions
n
n
n
High Power LED
Battery Charger
Accurate Current Limited Voltage Regulator
Typical applicaTion
94% Efficient 25W White LED Headlamp Driver
V
IN
, 6V TO 60V
(80V TRANSIENT)
2.2µF
2
332k
100k
332k
INTV
CC
100k
VMODE
PWM
SS
RT
V
C
28.7k
375kHz
34k
4.7nF
40.2k
V
IN
EN/UVLO
V
REF
LT3956
CTRL
ISN
1M
FB
16.2k
PWMOUT
GND INTV
CC
INTV
CC
4.7µF
M1
3956 TA01a
Efficiency vs V
IN
100
22µH
SW
PGND
ISP
D1
2.2µF
5
96
0.68
370mA
EFFICIENCY (%)
25W LED STRING
92
88
84
80
0
20
47nF
40
V
IN
(V)
60
80
3956 TA01b
3956f
LT3956
absoluTe MaxiMuM raTings
(Note 1)
pin conFiguraTion
TOP VIEW
VMODE
CTRL
PWM
V
REF
V
IN
, ISP ISN ..............................................................80V
,
SW ............................................................................84V
EN/UVLO (Note 3) .....................................................80V
INTV
CC
...................................................... V
IN
+ 0.3V, 8V
PWMOUT ..................................................INTV
CC
+ 0.3V
CTRL, PWM,
VMODE
................................................12V
FB ...............................................................................8V
V
C
, V
REF
, SS ................................................................3V
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
SS
RT
36 35 34 33 32 31 30
NC 1
EN/UVLO 2
INTV
CC
3
GND 4
V
IN
6
SW 8
SW 9
NC 10
12 13 14 15 16 17
PGND
PGND
PGND
PGND
PGND
PGND
38
SW
37
GND
28 ISP
27 ISN
25 FB
24 GND
23 PWMOUT
21 SW
20 SW
UHE PACKAGE
36-LEAD (5mm 6mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W,
θ
JC
= 5°C/W
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
LT3956EUHE#TRPBF
LT3956IUHE#TRPBF
PART MARKING*
3956
3956
PACKAGE DESCRIPTION
36-Lead (5mm
×
6mm) Plastic QFN
36-Lead (5mm
×
6mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
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
PARAMETER
V
IN
Minimum Operating Voltage
V
IN
Shutdown I
Q
V
IN
Operating I
Q
(Not Switching)
V
REF
Voltage
V
REF
Line Regulation
The
l
denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at T
A
= 25°C. V
IN
= 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
CONDITIONS
V
IN
Tied to INTV
CC
EN/UVLO = 0V
EN/UVLO = 1.15V
PWM = 0V
–100µA ≤ I
VREF
≤ 0µA
4.5V ≤ V
IN
≤ 80V
l
l
MIN
V
C
TYP
0.1
1.4
MAX
4.5
1
5
1.7
2.045
UNITS
V
µA
µA
mA
V
%/V
1.965
2.00
0.006
3956f
LT3956
elecTrical characTerisTics
PARAMETER
SW Pin Leakage
SW Pin Current Limit
SW Pin Voltage Drop
SS Pull-Up Current
Error Amplifier
Full-Scale Current Sense Threshold ( V
(ISP–ISN)
)
Current Sense Threshold at CTRL = 1V ( V
(ISP–ISN)
)
Current Sense Threshold at CTRL = 0.5V ( V
(ISP–ISN)
)
Current Sense Threshold at CTRL = 0.1V ( V
(ISP–ISN)
)
CTRL Range for Current Sense Threshold Adjustment
CTRL Input Bias Current
Current Sense Amplifier Input Common Mode
Range ( V
ISN
)
ISP/ISN Short-Circuit Threshold ( V
(ISP–ISN)
)
ISP/ISN Short-Circuit Fault Sensing Common Mode
Range ( V
ISN
)
ISP/ISN Input Bias Current (Combined)
LED Current Sense Amplifier g
m
V
C
Output Impedance
V
C
Standby Input Bias Current
FB Regulation Voltage (V
FB
)
FB Amplifier g
m
FB Pin Input Bias Current
FB Voltage Loop Active Threshold
FB Overvoltage Threshold
Oscillator
Switching Frequency
SW Minimum Off-Time
SW Minimum On-Time
Linear Regulator
INTV
CC
Regulation Voltage
Dropout (V
IN
– INTV
CC
)
INTV
CC
Undervoltage Lockout
INTV
CC
Current Limit
INTV
CC
Current in Shutdown
EN/UVLO = 0V, INTV
CC
= 7V
I
INTVCC
= –10mA, V
IN
= 7V
l
The
l
denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at T
A
= 25°C. V
IN
= 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
CONDITIONS
SW = 48V
l
MIN
3.3
8
l
l
l
l
TYP
5
3.9
220
10
250
225
100
0
50
MAX
10
4.6
13
257
231
103
4.5
1.1
100
80
UNITS
µA
A
mV
µA
mV
mV
mV
mV
V
nA
V
mV
V
µA
µA
µS
kΩ
I(SW) = 2A
Current Out of Pin
FB = 0V, ISP = 48V, CTRL ≥ 1.2V
CTRL = 1V, FB = 0V, ISP = 48V
CTRL = 0.5V
CTRL = 0.1V, FB = 0V, ISP = 48V
Current Out of Pin, CTRL = 0V
2.9
ISN = 0V
300
0
PWM = 5V (Active), ISP = ISN = 48V
PWM = 0V (Standby), ISP = ISN = 48V
1V < V
C
< 2V
PWM = 0V
ISP = ISN = 0V, 48V
FB = V
FB
, ISP = ISN
Current Out of Pin, FB = 1V
VMODE
Falling
PWMOUT Falling
V
FB
–
65mV
V
FB
+ 50mV
l
240
217
96
–2.5
0
335
370
3
80
0
120
15000
–20
1.220
1.232
1.250
1.250
480
40
V
FB
–
50mV
V
FB
+
60mV
100
1000
170
200
7
7.15
1
4.1
14
17
8
0.1
20
1.270
1.265
100
V
FB
–
40mV
V
FB
+
80mV
125
1050
nA
V
V
µS
nA
V
V
R
T
= 100k
R
T
= 10k
l
90
925
kHz
kHz
ns
ns
7.3
4.4
25
12
V
V
V
mA
µA
3956f
LT3956
elecTrical characTerisTics
PARAMETER
Logic Inputs/Outputs
PWM Threshold Voltage
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
VMODE
Output Low (V
OL
)
VMODE
Pin Leakage
PWMOUT Driver
t
r
PWMOUT Driver Output Rise Time
t
f
PWMOUT Driver Output Fall Time
PWMOUT Output Low (V
OL
)
PWMOUT Output High (V
OH
)
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 LT3956E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
INTV
CC
–
0.05
C
L
= 560pF
C
L
= 560pF
35
35
0.05
ns
ns
V
V
I
VIN
Drops Below 1µA
EN/UVLO = 1.15V
EN/UVLO = 1.30V
I
VMODE
= 1mA
FB = 0V,
VMODE
= 12V
0.1
1.7
2.1
10
l
l
The
l
denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at T
A
= 25°C. V
IN
= 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted.
CONDITIONS
MIN
0.85
45
1.185
TYP
1.35
60
1.220
20
0.4
2.5
100
200
5
1.245
MAX
1.8
UNITS
V
kΩ
V
mV
V
µA
nA
mV
µA
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 3:
For V
IN
below 6V, the EN/UVLO pin must not exceed V
IN
for proper
operation.
Typical perForMance characTerisTics
V
(ISP–ISN)
Threshold vs V
CTRL
300
250
V
(ISP – ISN)
THRESHOLD (mV)
V
(ISP – ISN)
THRESHOLD (mV)
200
150
100
50
0
–50
0
0.5
1
1.5
CTRL VOLTAGE (V)
2
3956 G01
T
A
= 25°C, unless otherwise noted.
V
(ISP–ISN)
Full-Scale Threshold
vs Temperature
256
254
V
(ISP– ISN)
THRESHOLD (mV)
252
250
248
246
244
242
–50
50
25
0
75
TEMPERATURE (°C)
125
3956 G03
V
(ISP–ISN)
Threshold vs V
ISP
with Reduced CTRL Voltage
103
102
101
100
99
98
97
CTRL = 0.5V
CTRL = 2V
0
20
40
60
ISP VOLTAGE (V)
80
3956 G02
–25
100
3956f
LT3956
Typical perForMance characTerisTics
FB Regulation Voltage (V
FB
)
vs Temperature
1.28
1.27
1.26
V
REF
(V)
1.25
V
FB
(V)
1.24
1.23
1.22
1.21
1.20
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3956 G04
T
A
= 25°C, unless otherwise noted.
V
REF
Voltage vs Temperature
2.04
2.03
2.02
2.01
2.00
1.99
1.98
1.97
1.96
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3956 G05
V
REF
Voltage vs V
IN
2.04
2.03
2.02
2.01
V
REF
(V)
2.00
1.99
1.98
1.97
1.96
0
20
40
V
IN
(V)
60
80
3956 G06
Switching Frequency vs R
T
10000
500
Switching Frequency
vs Temperature
R
T
= 26.7k
2.4
EN/UVLO Hysteresis Current
vs Temperature
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (kHz)
450
I
EN/UVLO
(µA)
125
3956 G08
2.2
1000
400
2.0
100
350
1.8
10
10
R
T
(k)
100
3956 G07
300
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
1.6
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3956 G09
Quiescent Current vs V
IN
2.0
PWM = 0V
4.4
SW Pin Current Limit
vs Temperature
1.28
1.26
EN/UVLO VOLTAGE (V)
EN/UVLO Threshold
vs Temperature
V
IN
CURRENT (mA)
CURRENT LIMIT (A)
1.5
4.2
EN/UVLO RISING
1.24
1.22
1.20
1.18
–50
EN/UVLO FALLING
1.0
4.0
0.5
3.8
0
0
20
40
V
IN
(V)
60
80
3956 G10
3.6
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3956 G11
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3956 G12
3956f
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