buck regulator and a 200mA low dropout linear regula-
tor (LDO). The wide input voltage range of 3.6V to 36V
makes the LT3645 suitable for regulating power from a
wide variety of sources, including 24V industrial supplies
and automotive batteries. Its high operating frequency
allows the use of tiny, low cost inductors and capacitors,
resulting in a very small solution.
Cycle-by-cycle current limit and frequency foldback pro-
vide protection against shorted outputs. Soft-start and
frequency foldback eliminate input current surge during
start-up.
The linear regulator operates from the V
CC2
pin at voltages
down to 1.2V. It supplies 200mA of output current with a
typical dropout voltage of 310mV.
Other features of the LT3645 include a <2μA shutdown,
short circuit protection, soft-start and thermal shutdown.
The LT3645 is available in the thermally enhanced 16-lead
(3mm × 3mm) QFN package, or a 12-lead MSE package.
L,
LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
n
n
n
n
n
n
n
n
Wide Input Range:
Operation from 3.6V to 36V
Overvoltage Lockout Protects Circuit Through
55V Transients on Input
500mA Output Current Switching Regulator
High Switching Frequency: 750kHz
200mA Low Dropout Linear Regulator
1.2V to 16V Input; 0.8V to 8V Output
310mV Dropout Voltage V
CC2
to OUT2
Precision Programmable Undervoltage Lockout
Short-Circuit Robust
Internal Soft-Start
<2μA Shutdown Current
Small Thermally Enhanced 16-Lead (3mm × 3mm)
QFN and 12-Lead MSE Packages
APPLICATIONS
n
n
Automotive CMOS Image Sensors
Industrial/Automotive Micro-Controller Supply
TYPICAL APPLICATION
3.3V/5V Step-Down Converter
90
BOOST
6.2V TO 36V
1μF
V
IN
LT3645
DA
ON OFF
EN/UVLO
FB
10k
EN2
PGOOD
NPG
V
CC2
OUT2
31.6k
FB2
GND
10k
3645 TA01a
Buck Regulator Efficiency
V
OUT
= 5V
5V
300mA
52.3k
10μF
EFFICIENCY (%)
80
V
OUT
= 3.3V
70
0.1μF
15μH
SW
60
3.3V
200mA
2.2μF
50
V
IN
= 12V
0
100
400
300
200
LOAD CURRENT (mA)
500
3645 TA01b
3645f
1
LT3645
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, EN/UVLO (Note 5).......................…….–0.3V to 55V
BOOST Voltage……………………………………….55V
BOOST Above SW Voltage ............................... …….25V
V
CC2
Voltage .............................................. –0.3V to 16V
V
OUT2
Voltage .............................................. –0.3V to 8V
FB, FB2 Voltages .......................................... –0.3V to 6V
EN2, NPG Voltages .................................... –0.3V to 16V
Operating Junction Temperature Range (Note 2)
LT3645E ............................................ –40°C to 125°C
LT3645I ............................................. –40°C to 125°C
LT3645H ............................................ –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)…… ........... 300°C
PIN CONFIGURATION
TOP VIEW
OUT2
V
CC2
FB2
NC
16 15 14 13
NC 1
NC 2
NPG 3
EN2 4
5
EN/UVLO
6
FB
7
GND
8
DA
17
GND
12 NC
11 V
IN
10 BOOST
9
SW
EN/UVLO
FB
GND
DA
BOOST
SW
1
2
3
4
5
6
TOP VIEW
12
11
10
9
8
7
NPG
EN2
FB2
OUT2
V
CC2
V
IN
13
GND
UD PACKAGE
16-LEAD (3mm
×
3mm) PLASTIC QFN
θ
JA
= 58.7°C/W,
θ
JC
= 7.1°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
MSE PACKAGE
12-LEAD PLASTIC MSOP
θ
JA
= 40°C/W,
θ
JC
= 5°C/W TO 10°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3645EUD#PBF
LT3645IUD#PBF
LT3645EMSE#PBF
LT3645IMSE#PBF
LT3645HMSE#PBF
TAPE AND REEL
LT3645EUD#TRPBF
LT3645IUD#TRPBF
LT3645EMSE#TRPBF
LT3645IMSE#TRPBF
LT3645HMSE#TRPBF
PART MARKING*
LFVS
LFVS
3645
3645
3645
PACKAGE DESCRIPTION
16-Lead Plastic QFN
16-Lead Plastic QFN
12-Lead Plastic MSOP
12-Lead Plastic MSOP
12-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–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/
3645f
2
LT3645
ELECTRICAL CHARACTERISTICS
PARAMETER
Undervoltage Lockout on V
IN
Overvoltage Lockout on V
IN
Overvoltage Lockout Hysteresis
Feedback Voltage FB
FB Pin Bias Current
Feedback Voltage Line Regulation
V
IN
Quiescent Current
V
IN
Quiescent Current in Shutdown
Switching Frequency
Maximum Duty Cycle
Switch Current Limit
DA Pin Current to Stop Osc
Switch V
CESAT
Switch Leakage Current
Minimum Boost Voltage Above Switch
BOOST Pin Current
BOOST Schottky Forward Drop
EN/UVLO Threshold High
EN/UVLO Threshold Hysteresis
EN/UVLO Input Current
Buck Soft-Start Time
LDO Minimum Input Voltage V
CC2
LDO Feedback Voltage FB2
LDO FB2 Bias Current
LDO Line Regulation
LDO Load Regulation
LDO Dropout Voltage (V
CC2
to V
OUT2
)
I
LOAD
= 10mA
I
LOAD
= 10mA
I
LOAD
= 200mA
I
LOAD
= 200mA
I
LOAD
= 200mA
l
l
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, BOOST = 15.3V, V
CC2
= 3.3V, OUT2 = 1.8V unless
otherwise noted. (Notes 2, 3)
CONDITIONS
Rising
Rising
l
l
MIN
3
36
0.79
0.785
TYP
3.4
38.5
1
0.8
0.8
20
0.015
1.4
0.01
MAX
3.6
40
0.81
0.813
300
3
2
825
1.25
1.25
2
UNITS
V
V
V
V
nA
%/V
mA
μA
kHz
%
A
A
mV
μA
V
mA
V
V
mV
μA
μA
ms
V
mV
nA
%/V
mV
Not Switching
V
EN/UVLO
= 0.3V, V
CC2
= 0V, V
OUT2
= 0V
675
100mA Load
Rising (Note 4)
I
SW
= 500mA
I
SW
= 500mA
I
SW
= 500mA
I
OUT
= 50mA
Rising
V
EN/UVLO
= 5V
V
EN/UVLO
= 0V
I
LOAD
= 200mA, V
OUT2
= 0.8V, V
IN
= 4.0V
l
l
l
750
87
1
1
400
1.6
10
0.7
83
0.8
0.6
2.2
18
0.9
1.29
50
1
1.8
1.38
810
300
1.17
1.23
50
25
0.9
1.1
782
797
20
0.020
–1
45
310
65
145
1.4
1.7
mV
mV
mV
V
V
mA
mA
LDO Dropout Voltage (V
IN
to V
OUT2
)
LDO Current Limit
EN2 Pin Threshold
LDO Soft-Start Time
NPG V
CESAT
NPG Leakage
FB2 NPG Threshold, % of Regulation Voltage
l
l
1.1
270
210
0.5
1.3
0.8
0.6
Rising
Falling
I
NPG
= 1mA, V
FB
= V
FB2
= 850mV
V
NPG
= 16V, V
FB
= V
FB2
= 750mV
V
FB
= 800mV, V
FB2
Rising
l
l
1.6
1.2
0.4
0.5
V
V
ms
V
μA
%
88
90
92
3645f
3
LT3645
ELECTRICAL CHARACTERISTICS
PARAMETER
FB NPG Threshold, % of Regulation Voltage
NPG Threshold Hysteresis
Note1:
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 LT3645E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3645I is guaranteed over the full
–40°C to 125°C operating temperature range. The LT3645H is guaranteed
over the full –40°C to 150°C operating temperature range. High junction
temperatures degrade operating lifetimes. Operating lifetime is derated at
junction temperatures greater than 125°C.
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, BOOST = 15.3V, V
CC2
= 3.3V, OUT2 = 1.8V unless
otherwise noted. (Notes 2, 3)
CONDITIONS
V
FB2
= 800mV, V
FB
Rising
MIN
88
TYP
90
25
MAX
92
UNITS
%
mV
Note 3:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum junction operating temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may result in
device degradation or failure.
Note 4:
Current Measurements are performed when the outputs are not
switching. Slope compensation reduces current limit at high duty cycles.
Note 5:
Absolute Maximum Voltage at V
IN
and EN/UVLO pins is 55V for
nonrepetitive one second transients, and 36V for continuous operation.
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