FEATURES
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LT3845
High Voltage Synchronous
Current Mode Step-Down
Controller with Adjustable
Operating Frequency
DESCRIPTION
The LT
®
3845 is a high voltage, synchronous, current
mode controller used for medium to high power, high
efficiency supplies. It offers a wide 4V to 60V input range
(7.5V minimum start-up voltage). An onboard regulator
simplifies the biasing requirements by providing IC power
directly from V
IN
.
Burst Mode
®
operation maintains high efficiency at light
loads by reducing IC quiescent current to 120μA. Light
load efficiency is also improved with the reverse inductor
current inhibit function which supports discontinuous
operation.
Additional features include adjustable fixed operating
frequency that can be synchronized to an external clock
for noise sensitive applications, gate drivers capable of
driving large N-channel MOSFETs, a precision undervoltage
lockout, 10μA shutdown current, short-circuit protection
and a programmable soft-start.
The LT3845 is available in a 16-lead thermally enhanced
TSSOP package.
, 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 5481178, 6611131, 6304066, 6498466, 6580258.
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High Voltage Operation: Up to 60V
Synchronizable Up to 600kHz
Adjustable Constant Frequency: 100kHz to 500kHz
Output Voltages Up to 36V
Adaptive Nonoverlap Circuitry Prevents Switch
Shoot-Through
Reverse Inductor Current Inhibit for Discontinuous
Operation Improves Efficiency with Light Loads
Programmable Soft-Start
120μA No Load Quiescent Current
10μA Shutdown Supply Current
1% Regulation Accuracy
Standard Gate N-Channel Power MOSFETs
Current Limit Unaffected by Duty Cycle
Reverse Overcurrent Protection
16-Lead Thermally Enhanced TSSOP Package
APPLICATIONS
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12V and 42V Automotive and Heavy Equipment
48V Telecom Power Supplies
Avionics and Industrial Control Systems
Distributed Power Converters
TYPICAL APPLICATION
High Voltage Step-Down Regulator 48V to 12V at 75W
V
IN
20V TO 55V
100
2.2μF
100V
47μF
63V
82.5k
1500pF
0.1μF
1M
V
IN
SHDN
C
SS
LT3845
BOOST
TG
SW
V
CC
BG
PGND
SENSE
+
SENSE
–
SGND
1μF
1N4148
3845 TA01a
Efficiency and Power Loss vs
Load Current
V
IN
= 48V
7
6
5
4
3
2
LOSS
70
65
0.1
1
0
1
LOAD CURENT (A)
10
3845 TA01b
95
Si7370DP
0.01Ω
BAS521
Si7370DP
15μH
B160
EFFICIENCY(%)
V
OUT
12V
75W
33μF
×3
90
85
80
75
POWER LOSS (W)
BURST_EN
20k
16.2k
2200pF
143k
49.9k
100pF
SYNC
f
SET
V
FB
V
C
3845fc
1
LT3845
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
V
IN
SHDN
C
SS
BURST_EN
V
FB
V
C
SYNC
f
SET
1
2
3
4
5
6
7
8
17
16 BOOST
15 TG
14 SW
13 V
CC
12 BG
11 PGND
10 SENSE
+
9
SENSE
–
Input Supply Voltage (V
IN
) ........................................65V
Boosted Supply Voltage (BOOST) .............................80V
Switch Voltage (SW) (Note 8) ....................... 65V to –2V
Differential Boost Voltage
(BOOST to SW).....................................................24V
Bias Supply Voltage (V
CC
) .........................................24V
SENSE
+
and SENSE
–
Voltages ..................................40V
Differential Sense Voltage
(SENSE
+
to SENSE
–
)................................... 1V to –1V
BURST_EN Voltage ...................................................24V
SYNC, V
C
, V
FB
, C
SS
, and
SHDN
Voltages ....................5V
SHDN
Pin Currents ..................................................1mA
Operating Junction Temperature Range (Note 2)
LT3845E (Note 3) ............................... –40°C to 125°C
LT3845I .............................................. –40°C to 125°C
Storage Temperature.............................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3845EFE#PBF
LT3845IFE#PBF
TAPE AND REEL
LT3845EFE#TRPBF
LT3845IFE#TRPBF
PART MARKING
3845FE
3845FE
PACKAGE DESCRIPTION
16-Lead Plastic TSSOP
16-Lead Plastic TSSOP
TEMPERATURE RANGE
–40°C to 125°C
–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/
3845fc
2
LT3845
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Operating Voltage Range (Note 4)
V
IN
Minimum Start Voltage
V
IN
UVLO Threshold (Falling)
V
IN
UVLO Threshold Hysteresis
V
IN
Supply Current
V
IN
Burst Mode Current
V
IN
Shutdown Current
BOOST Operating Voltage Range
BOOST Operating Voltage Range (Note 5)
BOOST UVLO Threshold (Rising)
BOOST UVLO Threshold Hysteresis
BOOST Supply Current (Note 6)
BOOST Burst Mode Current
BOOST Shutdown Current
V
CC
Operating Voltage Range (Note 5)
V
CC
Output Voltage
V
CC
UVLO Threshold (Rising)
V
CC
UVLO Threshold Hysteresis
V
CC
Supply Current (Note 6)
V
CC
Burst Mode Current
V
CC
Shutdown Current
V
CC
Current Limit
Error Amp Reference Voltage
V
FB
Pin Input Current
SHDN
Enable Threshold (Rising)
SHDN
Threshold Hysteresis
Sense Pins Common Mode Range
Current Limit Sense Voltage
Reverse Protect Sense Voltage
Reverse Current Inhibit Offset
Input Current (I
SENSE+
+ I
SENSE–
)
V
SENSE+
– V
SENSE–
V
SENSE+
– V
SENSE–
, V
BURST_EN
= V
CC
V
BURST_EN
= 0V or V
BURST_EN
= V
FB
V
SENSE(CM)
= 0V
V
SENSE(CM)
= 2V
V
SENSE(CM)
> 4V
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
CC
= BOOST = BURST_EN = 10V,
SHDN
= 2V, R
SET
= 49.9kΩ,
SENSE
–
= SENSE
+
= 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted.
CONDITIONS
●
●
●
MIN
4
3.6
TYP
MAX
60
7.5
4
UNITS
V
V
V
mV
μA
μA
μA
V
V
V
mV
mA
μA
μA
3.8
670
20
20
9
V
CC
> 9V
V
BURST_EN
= 0V, V
FB
= 1.35V
V
SHDN
= 0V
V
BOOST
– V
SW
V
BOOST
– V
SW
V
BOOST
– V
SW
V
BURST_EN
= 0V
V
SHDN
= 0V
Over Full Line and Load Range
●
●
●
15
75
20
5
400
1.4
0.1
0.1
●
●
8
6.25
500
3
100
20
–150
1.231
25
20
8.3
V
V
V
mV
mA
μA
μA
mA
V
V
nA
V
mV
V
mV
mV
mV
μA
μA
μA
●
3.7
V
BURST_EN
= 0V
V
SHDN
= 0V
Measured at V
FB
Pin
V
FB
= 1.231V
●
●
–40
1.224
1.215
1.3
0
90
1.238
1.245
1.4
36
115
●
●
●
1.35
120
100
–100
10
800
–20
–300
Operating Frequency
Minimum Programmable Frequency
Maximum Programmable Frequency
External Sync Frequency Range
SYNC Input Resistance
SYNC Voltage Threshold
Soft-Start Capacitor Control Current
Error Amp Transconductance
Error Amp DC Voltage Gain
Error Amp Sink/Source Current
●
●
●
●
●
290
270
500
100
300
310
330
100
600
kHz
kHz
kHz
kHz
kHz
kΩ
V
μA
μS
dB
μA
40
1.4
2
270
340
62
±30
410
2
3845fc
3
LT3845
ELECTRICAL CHARACTERISTICS
PARAMETER
TG, BG Drive On Voltage (Note 7)
TG, BG Drive Off Voltage
TG, BG Drive Rise/Fall Time
Minimum TG Off Time
Minimum TG On Time
Gate Drive Nonoverlap Time
TG Fall to BG Rise
BG Fall to TG Rise
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
CC
= BOOST = BURST_EN = 10V,
SHDN
= 2V, R
SET
= 49.9kΩ,
SENSE
–
= SENSE
+
= 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted.
CONDITIONS
C
LOAD
= 3300pF
C
LOAD
= 3300pF
10% to 90% or 90% to 10%, C
LOAD
= 3300pF
●
●
MIN
TYP
9.8
0.1
50
350
250
200
150
MAX
UNITS
V
V
ns
650
400
ns
ns
ns
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 LT3845 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 LT3845E 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
LT3845I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 4:
V
IN
voltages below the start-up threshold (7.5V) are only
supported when the V
CC
is externally driven above 6.5V.
Note 5:
Operating range is dictated by MOSFET absolute maximum V
GS
.
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 –2V absolute maximum on the SW pin is a transient condition.
It is guaranteed by design and not subject to test.
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Threshold (Rising)
vs Temperature
1.38
SHUTDOWN THRESHOLD, FALLING (V)
SHUTDOWN THRESHOLD, RISING (V)
1.37
1.36
1.35
1.34
1.33
1.32
–50 –25
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25
V
CC
(V)
Shutdown Threshold (Falling)
vs Temperature
8.2
8.1
8.0
7.9
7.8
7.7
7.6
V
CC
vs Temperature
I
CC
= 20mA
0
25
50
75
TEMPERATURE (°C)
100
125
0
25
50
75
TEMPERATURE (°C)
100
125
7.5
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
3845 G01
3845 G02
3845 G03
3845fc
4
LT3845
TYPICAL PERFORMANCE CHARACTERISTICS
V
CC
vs I
CC(LOAD)
8.05
T
A
= 25°C
9
8
8.00
7
V
CC
(V)
7.95
V
CC
(V)
6
5
7.90
4
7.85
3
I
CC
CURRENT LIMIT (mA)
V
CC
vs V
IN
I
CC
= 20mA
T
A
= 25°C
225
200
175
150
125
100
75
I
CC
Current Limit vs Temperature
0
5
10
15 20 25
I
CC(LOAD)
(mA)
30
35
40
4
5
6
7
8
V
IN
(V)
9
10
11
12
50
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3845 G04
3845 G05
3845 G06
V
CC
UVLO Threshold (Rising)
vs Temperature
6.5
V
CC
UVLO THRESHOLD, RISING (V)
25
I
CC
vs V
CC
(SHDN = 0V)
ERROR AMP TRANSCONDUCTANCE (μS)
T
A
= 25°C
350
345
340
335
330
325
Error Amp Transconductance
vs Temperature
6.4
20
6.2
I
CC
(μA)
6.3
15
10
6.1
5
6.0
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
0
0
2
4
6
8
10 12 14 16 18 20
V
CC
(V)
3845 G08
320
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3845 G07
3845 G09
I
(SENSE + SENSE–)
vs V
SENSE(CM)
800
600
I
(SENSE+ + SENSE
–
)
(μA)
400
200
0
–200
–400
T
A
= 25°C
OPERATING FREQUENCY (kHz)
308
306
304
302
300
298
296
294
292
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
V
SENSE(CM)
(V)
3845 G10
+
Operating Frequency
vs Temperature
1.234
1.233
ERROR AMP REFERENCE (V)
1.232
1.231
1.230
1.229
1.228
Error Amp Reference
vs Temperature
290
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1.227
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3845 G11
3845 G12
3845fc
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