LT3844
High Voltage, Current Mode
Switching Regulator Controller with
Programmable Operating Frequency
Features
Description
High Voltage Operation: Up to 60V
n Output Voltages Up to 36V (Step Down)
n Programmable Constant Frequency: 100kHz to
500kHz
n Synchronizable up to 600kHz
n Burst Mode® Operation: 120µA Supply Current
n 10µA Shutdown Supply Current
n ±1.3% Reference Accuracy
n Drives N-Channel MOSFET
n Programmable Soft-Start
n Programmable Undervoltage Lockout
n Internal High Voltage Regulator for Gate Drive
n Thermal Shutdown
n Current Limit Unaffected by Duty Cycle
n 16-Pin Thermally Enhanced TSSOP Package
The LT®3844 is a DC/DC controller used for medium power,
low part count, high efficiency supplies. It offers a wide
4V to 60V input range (7.5V minimum start-up voltage)
and can implement step-down, step-up, inverting and
SEPIC topologies.
n
The LT3844 includes Burst Mode operation, which reduces
quiescent current below 120µA and maintains high efficiency at light loads. An internal high voltage bias regulator
allows for simple biasing.
Additional features include current mode control for fast
line and load transient response; programmable fixed
operating frequency that can be synchronized to an external clock for noise sensitive applications; a gate driver
capable of driving large N-channel MOSFETs; a precision
undervoltage lockout function; 10µA shutdown current;
short-circuit protection and a programmable soft-start
function.
Applications
Industrial Power Distribution
12V and 42V Automotive and Heavy Equipment
n High Voltage Single Board Systems
n Distributed Power Systems
n Avionics
n Telecom Power
n
The LT3844 is available in a 16-lead thermally enhanced
TSSOP package.
n
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5731694, 6498466, 6611131
Typical Application
High Voltage Step-Down Regulator 48V to 12V, 50W
68µF
22µF
14.7k
VIN
1000pF
680p
SHDN
TG
CSS
SW
BURST_EN
VCC
VFB
10k
120pF
130k
R7
49.9k
BOOST
LT3844
PGND
VC
SENSE+
SYNC
SENSE–
fSET
7
VIN = 48V
88
Si7850DP
86
10Ω
1µF
10µH
PDS5100H
VOUT
12V
50W
0.015Ω
68µF
33µF
5
EFFICIENCY
84
4
82
3
LOSS
80
2
78
1
76
0.1
SGND
6
1
LOAD CURRENT (A)
POWER LOSS (W)
82.5k
90
0.22µF
1M
EFFICIENCY (%)
VIN
36V TO 60V
Efficiency and Power Loss
vs Load Current
0
10
3844 TA01b
3844 TA01
3844fc
For more information www.linear.com/LT3844
1
LT3844
Absolute Maximum Ratings
(Note 1)
Pin Configuration
Input Supply Voltage (VIN).......................... 65V to –0.3V
Boosted Supply Voltage (BOOST)............... 80V to –0.3V
Switch Voltage (SW) (Note 8).........................65V to –1V
Differential Boost Voltage
BOOST to SW......................................... 24V to –0.3V
Bias Supply Voltage (VCC)........................... 24V to –0.3V
SENSE+ and SENSE– Voltages.................... 40V to –0.3V
Differential Sense Voltage
SENSE+ to SENSE–.......................................1V to –1V
BURST_EN Voltage..................................... 24V to –0.3V
SYNC, VC, VFB, CSS and SHDN Voltages....... 5V to –0.3V
SHDN Pin Currents...................................................1mA
Operating Junction Temperature Range (Note 2)
LT3844E (Note 3)............................... –40°C to 125°C
LT3844I.............................................. –40°C to 125°C
Storage Temperature.............................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................... 300°C
Order Information
TOP VIEW
VIN
1
16 BOOST
SHDN
2
15 TG
CSS
3
14 SW
BURST_EN
4
VFB
5
VC
6
11 SENSE+
SYNC
7
10 SENSE–
fSET
8
9
17
13 VCC
12 PGND
SGND
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB
http://www.linear.com/product/LT3844#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3844EFE#PBF
LT3844EFE#TRPBF
3844EFE
16-Lead Plastic TSSOP
–40°C to 125°C
LT3844IFE#PBF
LT3844IFE#TRPBF
3844IFE
16-Lead Plastic TSSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9k,
SENSE– = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted.
PARAMETER
CONDITIONS
VIN Operating Voltage Range (Note 4)
VIN Minimum Start Voltage
VIN UVLO Threshold (Falling)
VIN UVLO Threshold Hysteresis
VIN Supply Current
VIN Burst Mode Current
VIN Shutdown Current
VCC > 9V
VBURST_EN = 0V, VFB = 1.35V
VSHDN = 0V
BOOST Operating Voltage Range
BOOST Operating Voltage Range (Note 5)
BOOST UVLO Threshold (Rising)
BOOST UVLO Threshold Hysteresis
VBOOST – VSW
VBOOST – VSW
VBOOST – VSW
BOOST Supply Current (Note 6)
BOOST Burst Mode Current
BOOST Shutdown Current
VBURST_EN = 0V
VSHDN = 0V
2
MIN
l
l
l
TYP
4
3.6
3.8
670
20
20
10
l
l
5
400
1.4
0.1
0.1
MAX
60
7.5
4
15
75
20
UNITS
V
V
V
mV
µA
µA
µA
V
V
V
mV
mA
µA
µA
3844fc
For more information www.linear.com/LT3844
LT3844
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9k,
SENSE– = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted.
PARAMETER
VCC Operating Voltage Range (Note 5)
VCC Output Voltage
VCC UVLO Threshold (Rising)
VCC UVLO Threshold Hysteresis
CONDITIONS
Over Full Line and Load Range
VCC Supply Current (Note 6)
VCC Burst Mode Current
VCC Shutdown Current
VCC Current Limit
VBURST_EN = 0V
VSHDN = 0V
Error Amp Reference Voltage
Measured at VFB Pin
VFB Pin Input Current
MIN
Input Current
(ISENSE+ + ISENSE–)
l
–40
l
1.224
1.215
VFB = 1.231V
MAX
V
V
V
mV
1.7
95
20
–120
2.1
mA
µA
µA
mA
1.231
1.238
1.245
25
VSENSE+ – VSENSE–
l
1.3
l
l
0
90
VSENSE(CM) = 0V
VSENSE(CM) = 2V
VSENSE(CM) > 4V
1.35
120
100
l
290
270
Minimum Programmable Frequency
Maximum Programmable Frequency
l
l
500
External Sync Frequency Range
l
100
SYNC Input Resistance
300
1.4
V
mV
36
115
V
mV
µA
µA
µA
310
330
kHz
kHz
100
kHz
kHz
600
kHz
40
SYNC Voltage Threshold
1.4
l
Soft-Start Capacitor Control Current
kΩ
2
V
410
µS
2
Error Amp Transconductance
l
Error Amp DC Voltage Gain
Error Amp Sink/Source Current
270
340
V
V
nA
350
–25
–170
Operating Frequency
UNITS
20
8.3
8
6.25
500
l
SHDN Enable Threshold (Rising)
SHDN Threshold Hysteresis
Sense Pins Common Mode Range
Current Limit Sense Voltage
l
l
TYP
µA
62
dB
±30
µA
TG Drive On Voltage (Note 7)
TG Drive Off Voltage
CLOAD = 2200pF
CLOAD = 2200pF
9.8
0.1
V
V
TG Drive Rise/Fall Time
10% to 90% or 90% to 10%, CLOAD = 2200pF
40
ns
Minimum TG Off Time
l
350
500
ns
Minimum TG On Time
l
250
350
ns
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 LT3844 includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: The LT3844E is 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 LT3844I is guaranteed
over the full –40°C to 125°C operating junction temperature range.
Note 4: VIN voltages below the start-up threshold (7.5V) are only
supported when the VCC is externally driven above 6.5V.
Note 5: Operating range is dictated by MOSFET absolute maximum VGS.
Note 6: Supply current specification does not include switch drive
currents. Actual supply currents will be higher.
Note 7: DC measurement of gate drive output “ON” voltage is typically
8.6V. Internal dynamic bootstrap operation yields typical gate “ON”
voltages of 9.8V during standard switching operation. Standard operation
gate “ON” voltage is not tested but guaranteed by design.
Note 8: The –1V absolute maximum on the SW pin is a transient condition.
It is guaranteed by design and not subject to test.
3844fc
For more information www.linear.com/LT3844
3
LT3844
Typical Performance Characteristics
Shutdown Threshold (Falling)
vs Temperature
Shutdown Threshold (Rising)
vs Temperature
1.37
1.36
1.35
1.34
1.33
1.32
–50 –25
0
50
25
75
TEMPERATURE (°C)
100
125
8.2
1.25
8.1
1.23
1.22
1.20
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
200
8
175
ICC CURRENT LIMIT (mA)
VCC (V)
6
5
15 20 25
ICC(LOAD) (mA)
30
35
3
40
4
5
8
9
VIN (V)
7
6
6.4
20
6.3
15
6.1
6.0
–50 –25
11
0
25
50
75
TEMPERATURE (°C)
100
125
125
100
50
–50
12
–25
0
25
50
75
TEMPERATURE (°C)
100
Error Amp Transconductance
vs Temperature
350
TA = 25°C
10
0
0
2
4
6
8
125
3844 G06
ICC vs VCC (SHDN = 0V)
5
3844 G07
4
10
ERROR AMP TRANSCONDUCTANCE (µS)
25
ICC (mA)
VCC UVLO THRESHOLD, RISING (V)
6.5
6.2
150
3844 G05
3844 G04
VCC UVLO Threshold (Rising)
vs Temperature
125
75
4
10
100
ICC Current Limit vs Temperature
VCC vs VIN
7.90
5
0
25
50
75
TEMPERATURE (°C)
3844 G03
9 I = 20mA
CC
TA = 25°C
7
VCC (V)
7.5
–50 –25
125
3844 G02
TA = 25°C
0
7.8
7.6
8.00
7.85
7.9
7.7
1.21
VCC vs ICC(LOAD)
7.95
ICC = 20mA
8.0
1.24
3844 G01
8.05
VCC vs Temperature
1.26
VCC (V)
SHUTDOWN THRESHOLD, FALLING (V)
SHUTDOWN THRESHOLD, RISING (V)
1.38
10 12 14 16 18 20
VCC (V)
3844 G08
345
340
335
330
325
320
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3844 G09
3844fc
For more information www.linear.com/LT3844
LT3844
Typical Performance Characteristics
I(SENSE+ + SENSE–) vs
VSENSE(CM)
Operating Frequency
vs Temperature
400
308
TA = 25°C
200
100
0
–100
1.233
ERROR AMP REFERENCE (V)
OPERATING FREQUENCY (kHz)
I(SENSE+ + SENSE–) (µA)
1.234
306
300
–200
Error Amp Reference
vs Temperature
304
302
300
298
296
294
290
–50
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VSENSE (CM) (V)
–25
0
25
50
75
TEMPERATURE (°C)
100
3844 G10
125
1.227
–50 –25
104
4.52
102
100
98
96
100
125
3844 G16
50
25
75
0
TEMPERATURE (°C)
100
3844 G12
VIN UVLO Threshold (Falling)
vs Temperature
3.86
3.84
4.50
3.82
4.48
4.46
3.80
4.44
3.78
4.42
4.40
–50 –25
125
VIN UVLO THRESHOLD, FALLING (V)
4.54
VIN UVLO THRESHOLD, RISING (V)
CURRENT SENSE THRESHOLD (mV)
1.229
VIN UVLO Threshold (Rising)
vs Temperature
106
50
25
75
0
TEMPERATURE (°C)
1.230
3844 G17
Maximum Current Sense
Threshold vs Temperature
94
–50 –25
1.231
1.228
292
0
1.232
50
25
75
0
TEMPERATURE (°C)
100
125
3844 G14
3.76
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3844 G15
3844fc
For more information www.linear.com/LT3844
5
LT3844
Pin Functions
VIN (Pin 1): The VIN pin is the main supply pin and should
be decoupled to SGND with a low ESR capacitor located
close to the pin.
voltage. Therefore, this pin cannot be pulled low with a
low impedance source. If the VC pin must be externally
manipulated, do so through a 1k series resistance.
SHDN (Pin 2): The SHDN pin has a precision IC enable
threshold of 1.35V (rising) with 120mV of hysteresis. It is
used to implement an undervoltage lockout (UVLO) circuit.
See Applications Information section for implementing
a UVLO function. When the SHDN pin is pulled below
a transistor VBE (0.7V), a low current shutdown mode
is entered, all internal circuitry is disabled and the VIN
supply current is reduced to approximately 9µA. Typical
pin input bias current is
