LINER LT3694 36v 500ma step-down regulator and 200ma ldo Datasheet

LT3645
36V 500mA Step-Down
Regulator and 200mA LDO
FEATURES
DESCRIPTION
n
The LT®3645 is a dual output regulator combining a 500mA
buck regulator and a 200mA low dropout linear regulator (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.
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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 VCC2 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
n
The linear regulator operates from the VCC2 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.
APPLICATIONS
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Cycle-by-cycle current limit and frequency foldback provide protection against shorted outputs. Soft-start and
frequency foldback eliminate input current surge during
start-up.
Automotive CMOS Image Sensors
Industrial/Automotive Micro-Controller Supply
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.
TYPICAL APPLICATION
Buck Regulator Efficiency
3.3V/5V Step-Down Converter
90
0.1μF
6.2V TO 36V
5V
300mA
SW
VIN
1μF
VOUT = 5V
15μH
LT3645
52.3k
DA
ON OFF
EN/UVLO
10μF
FB
10k
VOUT = 3.3V
70
60
EN2
PGOOD
80
EFFICIENCY (%)
BOOST
VCC2
NPG
3.3V
200mA
OUT2
31.6k
2.2μF
FB2
GND
50
VIN = 12V
0
100
400
300
200
LOAD CURRENT (mA)
500
3645 TA01b
10k
3645 TA01a
3645f
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LT3645
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, EN/UVLO (Note 5).......................…….–0.3V to 55V
BOOST Voltage……………………………………….55V
BOOST Above SW Voltage ............................... …….25V
VCC2 Voltage .............................................. –0.3V to 16V
VOUT2 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
NC
VCC2
OUT2
FB2
TOP VIEW
16 15 14 13
NC 1
TOP VIEW
12 NC
NC 2
NPG 3
EN/UVLO
FB
GND
DA
BOOST
SW
11 VIN
17
GND
10 BOOST
EN2 4
5
6
7
8
EN/UVLO
FB
GND
DA
9
SW
1
2
3
4
5
6
13
GND
12
11
10
9
8
7
NPG
EN2
FB2
OUT2
VCC2
VIN
MSE PACKAGE
12-LEAD PLASTIC MSOP
θJA = 40°C/W, θJC = 5°C/W TO 10°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
UD PACKAGE
16-LEAD (3mm × 3mm) PLASTIC QFN
θJA = 58.7°C/W, θJC = 7.1°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3645EUD#PBF
LT3645EUD#TRPBF
LFVS
16-Lead Plastic QFN
–40°C to 125°C
LT3645IUD#PBF
LT3645IUD#TRPBF
LFVS
16-Lead Plastic QFN
–40°C to 125°C
LT3645EMSE#PBF
LT3645EMSE#TRPBF
3645
12-Lead Plastic MSOP
–40°C to 125°C
LT3645IMSE#PBF
LT3645IMSE#TRPBF
3645
12-Lead Plastic MSOP
–40°C to 125°C
LT3645HMSE#PBF
LT3645HMSE#TRPBF
3645
12-Lead Plastic MSOP
–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
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless
otherwise noted. (Notes 2, 3)
PARAMETER
CONDITIONS
MIN
TYP
MAX
Undervoltage Lockout on VIN
Rising
l
3
3.4
3.6
V
Overvoltage Lockout on VIN
Rising
l
36
38.5
40
V
l
0.79
0.785
0.8
0.8
0.81
0.813
V
l
20
300
nA
l
0.015
Overvoltage Lockout Hysteresis
1
Feedback Voltage FB
FB Pin Bias Current
Feedback Voltage Line Regulation
VIN Quiescent Current
Not Switching
VIN Quiescent Current in Shutdown
VEN/UVLO = 0.3V, VCC2 = 0V, VOUT2 = 0V
1.4
Switching Frequency
Maximum Duty Cycle
100mA Load
Switch Current Limit
Rising (Note 4)
l
DA Pin Current to Stop Osc
Switch VCESAT
UNITS
V
%/V
3
mA
0.01
2
μA
675
750
825
kHz
83
87
0.8
1
1.25
1
1.25
0.6
ISW = 500mA
%
400
Switch Leakage Current
A
A
mV
2
μA
Minimum Boost Voltage Above Switch
ISW = 500mA
1.6
2.2
V
BOOST Pin Current
ISW = 500mA
10
18
mA
BOOST Schottky Forward Drop
IOUT = 50mA
0.7
0.9
V
EN/UVLO Threshold High
Rising
1.23
1.29
1.17
EN/UVLO Threshold Hysteresis
EN/UVLO Input Current
50
VEN/UVLO = 5V
VEN/UVLO = 0V
25
Buck Soft-Start Time
LDO Minimum Input Voltage VCC2
ILOAD = 200mA, VOUT2 = 0.8V, VIN = 4.0V
LDO Feedback Voltage FB2
l
LDO FB2 Bias Current
l
782
V
mV
50
1
μA
μA
0.9
1.8
ms
1.1
1.38
V
797
810
mV
20
300
nA
LDO Line Regulation
0.020
%/V
LDO Load Regulation
–1
mV
LDO Dropout Voltage (VCC2 to VOUT2)
LDO Dropout Voltage (VIN to VOUT2)
ILOAD = 10mA
ILOAD = 10mA
ILOAD = 200mA
l
ILOAD = 200mA
ILOAD = 200mA
l
LDO Current Limit
EN2 Pin Threshold
Rising
Falling
45
65
145
mV
mV
mV
1.4
1.7
V
V
310
1.1
270
l
210
l
l
0.5
LDO Soft-Start Time
mA
mA
1.3
0.8
1.6
V
V
0.6
1.2
ms
NPG VCESAT
INPG = 1mA, VFB = VFB2 = 850mV
0.4
V
NPG Leakage
VNPG = 16V, VFB = VFB2 = 750mV
0.5
μA
FB2 NPG Threshold, % of Regulation Voltage
VFB = 800mV, VFB2 Rising
92
%
88
90
3645f
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LT3645
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, BOOST = 15.3V, VCC2 = 3.3V, OUT2 = 1.8V unless
otherwise noted. (Notes 2, 3)
PARAMETER
CONDITIONS
MIN
FB NPG Threshold, % of Regulation Voltage
VFB2 = 800mV, VFB Rising
88
NPG Threshold Hysteresis
TYP
MAX
90
92
UNITS
%
25
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.
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 VIN and EN/UVLO pins is 55V for
nonrepetitive one second transients, and 36V for continuous operation.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency VOUT = 5V
Buck Minimum Input Voltage,
VOUT = 5V
Efficiency VOUT = 3.3V
90
90
80
80
8.0
VIN TO RUN
VIN TO START
70
70
VIN = 7V
VIN = 12V
VIN = 24V
0
100
200
300
400
OUTPUT CURRENT (mA)
50
500
VIN = 7V
VIN = 12V
VIN = 24V
0
100
200
300
400
LOAD CURRENT (mA)
6.5
6.0
Buck Minimum Input Voltage,
VOUT = 3.3V
5.0
500
803
799
4.0
801
FB VOLTAGE (mV)
FB VOLTAGE (mV)
4.5
1000
800
802
5.0
100
10
OUTPUT CURRENT (mA)
FB2 Voltage
804
VIN TO RUN
VIN TO START
5.5
1
3645 G03
FB Voltage
6.5
6.0
5.5
3645 G02
3645 G01
INPUT VOLTAGE (V)
7.0
60
60
50
INPUT VOLTAGE (V)
EFFICIENCY (%)
EFFICIENCY (%)
7.5
800
799
798
797
796
798
797
796
795
3.5
795
3.0
1
100
10
OUTPUT CURRENT (mA)
1000
3645 G04
794
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
3645 G05
794
–50
0
100
50
TEMPERATURE (°C)
150
3645 G06
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LT3645
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Power Switch Current Limit
350
950
300
900
250
200
150
100
50
0
3.7
850
800
750
700
0
600
–50
50 100 150 200 250 300 350 400 450 500
SWITCH CURRENT (mA)
0
100
50
TEMPERATURE (°C)
3.2
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
150
3645 G09
LDO Power Transistor Current
Limit
800
RISING
FALLING
340
790
38.5
38.0
37.5
37.0
320
780
CURRENT LIMIT (mA)
SWITCHING FREQUENCY (kHz)
39.0
770
760
750
740
730
100
50
TEMPERATURE (°C)
280
260
240
220
700
–50
150
300
720
710
0
100
50
TEMPERATURE (°C)
200
–50
150
LDO Dropout Voltage to VCC2
LDO Dropout Voltage
300
–0.02
100
50
0
OUT2 = 0.8V
OUT2 = 3.3V
0
50
150
100
LOAD CURRENT (mA)
200
3645 G13
OUTPUT VOLTAGE CHANGE (%)
300
DROPOUT VOLTAGE (mV)
0
150
150
LDO Load Regulation
350
200
100
50
TEMPERATURE (°C)
3645 G12
350
250
0
3645 G11
3645 G10
DROPOUT VOLTAGE (mV)
3.4
Switching Frequency
39.5
0
3.5
3645 G08
Overvoltage Lockout
36.5
–50
3.6
3.3
650
3645 G07
THRESHOLD (V)
Undervoltage Lockout
3.8
RISING THRESHOLD (V)
1000
CURRENT LIMIT (mA)
BUCK POWER SWITCH VCE (mV)
Buck Power Switch Voltage Drop
400
250
200
150
100
50
–0.04
–0.06
–0.08
–0.10
–0.12
–0.14
0
–60 –40 –20 0 20 40 60 80 100 120 140 160
TEMPERATURE (°C)
3645 G14
–0.16
0
50
100
150
OUTPUT CURRENT (mA)
200
3645 G15
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LT3645
TYPICAL PERFORMANCE CHARACTERISTICS
EN/UVLO Pin Current
EN/UVLO PIN Threshold Voltage
1.26
EN/UVLO PIN THRESHOLD VOLTAGE (V)
CURRENT INTO EN/UVLO PIN (μA)
120
100
80
60
40
20
0
0
5
10 15 20 25 30
EN/UVLO PIN VOLTAGE
35
1.25
RISING
1.24
1.23
1.22
1.21
1.20
FALLING
1.19
1.18
–50
40
0
50
100
TEMPERATURE (°C)
3645 G16
3645 G17
EN2 Threshold Voltage
NPG Threshold Voltage, FB = 0.8V
1.5
750
740
RISING
1.3
730
1.2
FB2 VOLTAGE (mV)
EN2 THRESHOLD VOLTAGE (V)
1.4
1.1
1.0
0.9
FALLING
0.8
720
700
680
670
660
100
50
TEMPERATURE (°C)
150
3645 G18
FALLING
690
0.6
0
RISING
710
0.7
0.5
–50
150
650
–50
0
100
50
TEMPERATURE (°C)
150
3645 G19
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LT3645
PIN FUNCTIONS
(MSOP/QFN)
EN/UVLO (Pin 1/Pin 5): The EN/UVLO pin is used to enable
the buck switching regulator and the low dropout linear
regulator (LDO). An accurate threshold of 1.23V allows the
user to set the undervoltage lockout point with a simple
resistor divider, see Precision Undervoltage Lockout section for more information. The EN/UVLO pin can be tied
directly to VIN if the UVLO or shutdown is not used.
FB (Pin 2/Pin 6): The FB pin programs the buck output
voltage. The LT3645 regulates the FB pin to 0.8V. The
feedback resistor divider tap should be connected to this
pin. The output voltage is programmed according to the
following equation:
⎞
⎛V
R1= R2 • ⎜ OUT – 1⎟
⎠
⎝ 0.8
where R1 connects between OUT and FB and R2 connects
between FB and GND. A good value for R2 is 10k.
GND (Pin 3, Exposed Pad Pin 13/Pin 7, Exposed Pad Pin 17):
The GND pin should be tied to a local ground plane below the
LT3645 and the circuit components. Return the feedback
dividers from FB and FB2 to this pin. The exposed pad
must be soldered to the PCB and electrically connected
to ground. Use a large ground plane and thermal vias to
optimize thermal performance.
DA (Pin 4/Pin 8): The DA pin senses the external catch
diode current and prevents the buck regulator from switching if the sensed current is too high. Connect the anode
of the external Schottky catch diode to this pin.
BOOST (Pin 5/Pin 10): The BOOST pin provides a drive
voltage to the internal bipolar NPN power switch. Tie a
0.1μF capacitor between the BOOST and SW pins.
SW (Pin 6/Pin 9): The SW pin is the output of the internal
buck power switch. Connect the inductor and the cathode
of the external catch Schottky diode to this pin.
VCC2 (Pin 8/Pin 14): The VCC2 pin supplies current to the
linear regulator’s output device. The VCC2 pin is also the
anode of an internal Schottky diode used to generate the
BOOST voltage. The VCC2 pin must be tied to a voltage
source greater than 2.5V to utilize the internal Schottky
boost diode. If the VCC2 pin is tied to a voltage lower than
2.5V, then an external Schottky diode must be connected
between a power supply greater than 2.5V (anode) and
the BOOST pin (cathode). Bypass this pin to ground with
a 0.1μF capacitor close to the part.
OUT2 (Pin 9/Pin 15): The OUT2 pin is the output of the
LDO. Connect a capacitor of at least 0.47μF from this pin
to ground. See Frequency Compensation (LDO) section
for more details.
FB2 (Pin 10/Pin 16): The FB2 pin programs the LDO output
voltage. The LT3645 regulates the FB2 pin to 0.797V. The
feedback resistor divider tap should be connected to this
pin. The output voltage is programmed according to the
following equation:
⎛V
⎞
R3 = R4 t ⎜ OUT2 – 1⎟
⎝ 0.797
⎠
where R3 connects between OUT2 and FB2 and R4
connects between FB2 and GND. A good value for R4 is
10k.
EN2 (Pin 11/Pin 4): The EN2 pin is used to enable the linear
regulator. Pull this pin above 1.6V to enable the LDO. Pull
EN2 below 0.5V to disable the LDO.
NPG (Pin 12/Pin 3): The NPG pin is an open-collector
output used to indicate that both buck and LDO output
voltages are in regulation. The NPG pin pulls low when
FB and FB2 both exceed 720mV.
NC (Pins 1, 2, 12, 13, QFN Only): No Connect Pins. Tie
these to ground.
VIN (Pin 7/ Pin 11): The VIN pin supplies current to the
LT3645’s internal circuitry, to the internal buck power
switch, and to the LDO. The VIN pin must be locally
bypassed.
3645f
7
8
PGOOD
LDO ON
ON OFF
C1
NPG
EN2
GND
EN/UVLO
START
1.3V
1.23V
REFERENCE
–
+
–
+
–
+
+
–
VIN
0.72V
START
BUCK
SOFT-START
ERROR
AMP
SOFT-START
OSCILLATOR
+
+
–
START
LDO
0.8V
–
+
Q
+
+
–
LOGIC
0.797V
R
S
ERROR
AMP
SLOPE COMPENSATION
CURRENT
COMPARATOR
VC
LDO DRIVER
VIN
–40mV
BUCK
DRIVER
Q1
Q2
FB2
OUT2
VCC2
FB
GND
DA
SW
BOOST
C4
D1
C2
L1
R4
R3
C3
R2
R1
LT3645
BLOCK DIAGRAM
3645f
LT3645
OPERATION
The LT3645 includes a constant frequency, current mode
step-down buck switching regulator together with a lowdropout regulator (LDO).
If EN/UVLO is less than ~0.7V, both the buck and LDO
are off, the output is disconnected and the input current
is less than 2μA. The buck turns on when EN/UVLO is
greater than 1.23V. An undervoltage lockout (UVLO)
turns the buck and LDO off when VIN is less than 3.4V.
An overvoltage lockout (OVLO) turns the buck and LDO
off when VIN is greater than 38.5V. The part will withstand
nonrepetitive one second input voltage transients up to
55V. An internal thermal shutdown circuit monitors the die
temperature and shuts both the buck and LDO off if the
die temperature exceeds ~160°C. The thermal shutdown
has 10 degrees of hysteresis.
An internal regulator provides power to the control circuitry
and produces the 0.8V feedback voltage for the buck and
LDO error amplifiers.
An internal, fixed-frequency oscillator in the step-down
regulator enables an RS flip-flop, turning on the internal
power switch Q1. A comparator monitors the current
flowing between the VIN and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at VC and the internal slope-compensation. An error
amplifier servos the VC node. The output of an external
resistor divider between OUT and ground is tied to the
VFB pin and presented to the negative error amp input.
The positive input to the error amp is a 0.8V reference, so
the voltage loop forces the VFB pin to 0.8V. The reference
voltage of the buck error amplifier is ramped over 900μs
during the soft-start period. When VC rises, it results in an
increase in output current, and when VC falls, it results in
less output current. Current limit is provided by an active
clamp on the VC node.
The buck power switch (Q1) is driven from the BOOST
pin. An external capacitor and internal diode are used to
generate a voltage at the BOOST pin that is higher than the
input supply, which allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
An external diode can be used to make the BOOST drive
more effective at low output voltages.
The oscillator reduces the LT3645’s operating frequency
during the soft-start period. This frequency foldback helps
to control the output current during startup.
The current in the external catch diode (D1) is sensed
through the DA pin. If the catch diode current exceeds
0.9A, the oscillator frequency is decreased. This prevents
current runaway during startup or overload.
The LDO only operates if EN/UVLO is greater than 1.23V
and EN2 is greater than 1.3V. If EN/UVLO is low and EN2
is high, the LDO will not start. When EN2 > 1.3V and EN/
UVLO > 1.23V, the LDO power transistor will turn on and
regulate the output at the OUT2 pin. An error amplifier
driving Q2 has its positive input at the 0.797V reference.
The output of an external resistor divider between OUT2
and ground is tied to the VFB2 pin and presented to the
negative error amp input, forcing the VFB2 pin to 0.797V.
The reference voltage of the LDO error amplifier is ramped
over 600μs during the soft-start period. The LDO power
transistor (Q2) is driven from the VIN pin. Q2 is a bipolar
NPN which draws its collector current from the VCC2 pin.
The NPG pin is an open-collector output that indicates
when both buck and LDO outputs are in at least 90% in
regulation. When FB and FB2 rise above 720mV, the NPG
pin is pulled low.
3645f
9
LT3645
APPLICATIONS INFORMATION
FB Resistor Networks
The output voltages are programmed with resistor dividers
between the outputs and the VFB and VFB2 pins. Choose
the resistors according to
©V
¹
R1" R2 t ª OUT – 1º
« 0.8
»
©V
¹
R3 " R4 t ª OUT2 – 1º
« 0.797 »
R2 and R4 should be 20k or less to avoid bias current
errors. In the step-down converter, an optional phase
lead capacitor of 22pf between VOUT and VFB reduces
light-load ripple.
Input Voltage Range
The maximum operating input voltage for the LT3645 is
36V. The minimum input voltage is determined by either
the LT3645’s minimum operating voltage of 3.6V or by
its maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
voltages up to 55V, but once the input voltage exceeds
36V, the power switch will shut off and stop regulating
the output voltage until the input voltage falls below 36V.
Minimum On Time
The LT3645 will operate at the correct frequency while
the input voltage is below VIN(MAX). At input voltages
that exceed VIN(MAX), the LT3645 will still regulate the
output properly (up to 38.5V); however, the LT3645 will
skip pulses to regulate the output voltage resulting in
increased output voltage ripple.
Figure 1 illustrates switching waveforms for a LT3645
application with VOUT = 1.2V near VIN(MAX) = 21.3V.
SWITCH
VOLTAGE
10V/DIV
INDUCTOR
CURRENT
0.5A/DIV
3645 F01
VIN = 18V
VOUT = 1.2V
IOUT = 500mA
COUT = 10μF
L = 10μH
DC = (VOUT + VD)/(VIN – VSW + VD)
where VD is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
VIN(MIN) = ((VOUT + VD)/DCMAX) – VD + VSW
with DCMAX = 0.83 for the LT3645.
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins. For fixed
frequency operation, the maximum input voltage is determined by the minimum duty cycle, which is:
VIN(MAX) = ((VOUT + VD)/DCMIN) – VD + VSW
with DCMIN = 0.075 for the LT3645.
Note that this is a restriction on the operating input voltage
for continuous mode operation. The circuit will continue
to regulate the output up until the overvoltage lockout
input voltage (38.5V). The part will tolerate transient input
Figure 1.
As the input voltage is increased, the part is required
to switch for shorter periods of time. Delays associated
with turning off the power switch dictate the minimum on
time of the part. The minimum on time for the LT3645 is
100ns. Figure 2 illustrates the switching waveforms when
the input voltage is increased to VIN = 22V.
SWITCH
VOLTAGE
10V/DIV
INDUCTOR
CURRENT
0.5A/DIV
3645 F02
VIN = 22V
VOUT = 1.2V
IOUT = 500mA
COUT = 10μF
L = 10μH
Figure 2.
3645f
10
LT3645
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
Vendor
URL
Part Series
Inductance Range (μH)
Size (mm)
Sumida
www.sumida.com
CDRH4D28
CDRH5D28
CDRH8D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko
www.toko.com
A916CY
D585LC
2 to 12
1.1 to 39
6.3 × 6.2
8.1 × 8.0
Würth Elektronik
www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
Now the required on time has decreased below the minimum on time of 100ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the part skips a few pulses so that the average inductor current meets and does not exceed the load
current requirement.
The LT3645 is robust enough to survive prolonged operation under these conditions as long as the peak inductor
current does not exceed 1.2A. Inductor saturation due
to high current may further limit performance in this
operating region.
Inductor Selection and Maximum Output Current
Choose the inductor value according to:
L = 2.2 •(VOUT + VD)/ƒ
where VD is the forward voltage drop of the catch diode
(~0.4V), f is the switching frequency in MHz and L is in
μH. With this value, there will be no subharmonic oscillation for applications with 50% or greater duty cycle. For
robust operation in fault conditions, the saturation current
should be above 1.5A. To keep efficiency high, the series
resistance (DCR) should be less than 0.1Ω. Table 1 lists
several inductor vendors. If the buck load current is less
than 500mA, then a lower valued inductor can be used.
Catch Diode
Depending on load current, a 500mA to 1A Schottky diode
is recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
overvoltage lockout voltage (38.5V). The ON Semiconduc-
tor MBRA140T3 and Central Semiconductor CMMSH1-40
are good choices, as they are rated for 1A continuous
forward current and a maximum reverse voltage of 40V.
Input Filter Network
Bypass VIN with a 1μF or higher ceramic capacitor of X7R
or X5R type. Y5V types have poor performance over temperature and applied voltage and should not be used. A 1μF
ceramic capacitor is adequate to bypass the LT3645 and
will easily handle the ripple current. However, if the input
power source has high impedance, or there is significant
inductance due to long wires or cables, additional bulk
capacitance might be necessary. This can be provided
with a low performance (high ESR) electrolytic capacitor
in parallel with the ceramic device. Step-down regulators
draw current from the input supply in pulses with very
fast rise and fall times. The input capacitor is required to
reduce the resulting voltage ripple at the LT3645 input
and to force this very high frequency switching current
into a tight local loop, minimizing EMI. A 1μF capacitor
is capable of this task, but only if it is placed close to the
LT3645 and catch diode (see the PCB layout section). A
second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3645.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (underdamped) tank circuit. If the LT3645 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3645’s voltage rating. This situation can
easily be avoided. For more details, see Linear Technology
Application Note 88.
3645f
11
LT3645
APPLICATIONS INFORMATION
Output Capacitor
BOOST Pin Considerations
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT3645 to produce the DC output. In this role it
determines the output ripple so low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT3645’s control loop.
The external capacitor C2 and an internal Schottky diode
connected between the VCC2 and BOOST pins form a
charge pump circuit which is used to generate a boost
voltage that is higher than the input voltage (VIN). In most
application circuits where the duty cycle is less than 50%,
use C2 = 0.1μF. If the duty cycle is higher than 50% then
use C2 = 0.22μF.
Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance.
A good value is:
The BOOST pin must be at least 2.2V above the SW pin
to fully saturate the NPN power switch (Q1). The forward
drop of the internal Schottky diode is 0.8V. This means
that VCC2 must be tied to a supply greater than 2.6V.
COUT = 26.4/(VOUT • ƒ)
where f is the switching frequency in MHz and COUT is in
μF. This choice will provide low output ripple and good
transient response. COUT = 10μF is a good choice for
output voltages above 2.5V. For lower output voltages
use 22μF or higher.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor R1 may be required to get the full benefit (see the
Compensation section). Using a small output capacitor
results in an increased loop crossover frequency.
Use X5R or X7R types and keep in mind that a ceramic
capacitor biased with VOUT will have less than its nominal
capacitance. High performance electrolytic capacitors can
be used for the output capacitor. Low ESR is important, so
choose one that is intended for use in switching regulators.
The ESR should be specified by the supplier and should be
0.1Ω or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR.
VCC2 may be tied to a supply between 2.2V and 2.6V if an
external Schottky diode (such as a BAS70) is connected
from VCC2 (anode) to BOOST (cathode).
If no voltage supply greater than 2.6V is available, then
an external boost Schottky diode can be tied from the
VIN pin (anode) to the BOOST pin (cathode) as shown in
Figure 3. In this configuration, the BOOST capacitor will be
charged to approximately the VIN voltage, and will change
if VIN changes. In this configuration the maximum operating VIN is 25V, because when VIN = 25V, then when the
power switch Q1 turns on, VSW ~ 25V, and since the boost
capacitor is charged to 25V, the BOOST pin will be at 50V.
This connection is not as efficient as the others because
the BOOST pin current comes from a higher voltage.
The minimum operating voltage of an LT3645 application
is limited by the undervoltage lockout (~3.4V) and by
the maximum duty cycle as outlined above. For proper
startup, the minimum input voltage is also limited by the
D2
Table 2 lists several capacitor vendors.
C3
BOOST
Table 2. Capacitor Vendors
AVX
LT3645
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
VIN
VIN
SW
VOUT
GND
3645 F03
VBOOST – VSW % VIN
MAX VBOOST % 2VIN
Figure 3.
3645f
12
LT3645
APPLICATIONS INFORMATION
boost circuit. If the input voltage is ramped slowly, or if
the LT3645 is turned on with the EN/UVLO pin when the
output is already in regulation, then the boost capacitor
might not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load generally
goes to zero once the circuit has started. The worst case
situation is when VIN is ramping very slowly. Figure 4a
shows the minimum input voltage needed to start a 5V
application versus output current. Figure 4b shows the
minimum input voltage needed to start a 3.3V application
versus output current.
Soft-Start
The LT3645 includes a 500μs internal soft-start for the
buck converter and a 500μs soft-start for the LDO regulator. Both soft-starts are reset if the EN/UVLO pin is low, if
VIN drops below 3.4V (undervoltage), if VIN exceeds 36V
(overvoltage), or when the die temperature exceeds 160°C
(thermal shutdown). The soft-start for the LDO can also be
reset by pulling the EN2 pin low. The soft-start functions
act to reduce the maximum input current during startup.
Soft-start can not be disabled in the LT3645.
Reversed Input Protection
In some systems, the output will be held high when the
input to the LT3645 is absent. This may occur in battery charging applications or in battery backup systems
where a battery or some other supply is diode OR’d with
the LT3645’s output. If the VIN pin is allowed to float and
the EN/UVLO pin is held high (either by a logic signal
or because it is tied to VIN), then the LT3645’s internal
circuitry will draw its quiescent current through its SW
pin. This is fine if the system can tolerate a few mA in this
state. You can reduce this current by grounding the EN/
UVLO pin, then the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT3645 can
8.0
7.0
6.5
6.0
5.5
5.0
VIN TO RUN
VIN TO START
6.0
INPUT VOLTAGE (V)
7.5
INPUT VOLTAGE (V)
6.5
VIN TO RUN
VIN TO START
5.5
5.0
4.5
4.0
3.5
1
100
10
OUTPUT CURRENT (mA)
3.0
1000
3645 F04a
1
100
10
OUTPUT CURRENT (mA)
1000
3645 F04b
(4b) Typical Minimum Input Voltage,
VOUT = 3.3V
(4a) Typical Minimum Input Voltage,
VOUT = 5V
Figure 4.
3645f
13
LT3645
APPLICATIONS INFORMATION
pull large currents from the output through the SW pin
and the VIN pin. Figure 5 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
G
OUT
COUT
CERAMIC
R1
CPL
BOOST
EN/UVLO
gm
SW
RC
LT3645
1M
ESR
0.8V
+
R2
CC
DA
D4
VIN
ELECTROLYTIC
VCC2
VIN
FB
BACKUP
3645 F06
gm = 100μA/V
G = 1A/V
RC = 150k
CC = 60pF
OUT2
EN2
FB2
Figure 6. Model for Loop Response
NPG
GND
3645 F05
Figure 5. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit from
a Reversed Input. The LT3645 Runs Only When the Input Is Present
Frequency Compensation (Buck)
The LT3645 uses current mode control to regulate the
loop. This simplifies loop compensation. In particular, the
LT3645 does not require the ESR of the output capacitor
for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. A low
ESR output capacitor will typically provide for a greater
margin of circuit stability than an otherwise equivalent
capacitor with higher ESR, although the higher ESR will
tend to provide a faster loop response. Figure 6 shows an
equivalent circuit for the LT3645 control loop.
The error amplifier (gm) is a transconductance type with
finite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance amplifier (G) generating an output
current proportional to the voltage at the VC node. Note
that the output capacitor integrates this current, and that
the capacitor on the VC node (CC) integrates the error
amplifier output current, resulting in two poles in the
loop. RC provides a zero. With the recommended output
capacitor, the loop crossover occurs above the RCCC zero.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. With a larger
ceramic capacitor that will have lower ESR, crossover may
be lower and a phase lead capacitor connected across
R1 in the feedback divider may improve the transient
response. Large electrolytic capacitors may have an ESR
3645f
14
LT3645
APPLICATIONS INFORMATION
large enough to create an additional zero, and the phase
lead might not be necessary. If the output capacitor is
different than the recommended capacitor, stability should
be checked across all operating conditions, including input
voltage and temperature.
Figure 7 shows the transient response of the LT3645 with a
few output capacitor choices. The output is 3.3V. The load
current is stepped from 0.25A to 0.5A and back to 0.25A,
and the oscilloscope traces show the output voltage. The
upper photo shows the recommended value. The second
photo shows the improved response (faster recovery)
resulting from a phase lead capacitor.
No Phase Lead Capacitor
pin and the FB2 pin. Capacitors up to 1nF can be used. This
bypass capacitor reduces system noise as well.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are specified with EIA temperature characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coefficients as shown in Figures 8 and 9.
When used with a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an effective value as low as 1μF to 2μF
for the DC bias voltage applied and over the operating
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
With Phase Lead Capacitor
CHANGE IN VALUE (%)
0
X5R
–20
–40
–60
Y5V
–80
–100
0
2
4
14
8
6
10 12
DC BIAS VOLTAGE (V)
16
3645 F08
Figure 8. Ceramic Capacitor DC Bias Characteristics
Figure 7.
40
Frequency Compensation (LDO)
CHANGE IN VALUE (%)
20
The LT3645 LDO requires an output capacitor for stability.
It is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2μF with an ESR of 0.5Ω
or less is recommended to prevent oscillations. Larger
values of output capacitance decrease peak deviations
and provide improved transient response for larger load
current changes. Bypass capacitors, used to decouple
individual components powered by the LT3645, increase
the effective output capacitor value. For improvement in
transient performance, place a capacitor across the OUT2
X5R
0
–20
–40
Y5V
–60
–80
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–100
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3645 F09
Figure 9. Ceramic Capacitor Temperature Characteristics
3645f
15
LT3645
APPLICATIONS INFORMATION
temperature range. The X5R and X7R dielectrics result in
more stable characteristics and are more suitable for use
as the output capacitor. The X7R type has better stability
across temperature, while the X5R is less expensive and
is available in higher values. Care still must be exercised
when using X5R and X7R capacitors; the X5R and X7R
codes only specify operating temperature range and maximum capacitance change over temperature. Capacitance
change due to DC bias with X5R and X7R capacitors
is better than Y5V and Z5U capacitors, but can still be
significant enough to drop capacitor values below appropriate levels. Capacitor DC bias characteristics tend to
improve as component case size increases, but expected
capacitance at operating voltage should be verified. Voltage
and temperature coefficients are not the only sources of
problems. Some ceramic capacitors have a piezoelectric
response. A piezoelectric device generates voltage across
its terminals due to mechanical stress, similar to the way
a piezoelectric microphone works. For a ceramic capacitor
the stress can be induced by vibrations in the system or
thermal transients.
With the resistor divider connected, the part will only
operate at input voltages greater than VIN(MIN). Note that
the resistor divider will always draw current from VIN. To
reduce this current, the user might use large value resistors for R7 and R8. This is acceptable as long as R7 and
R8 are selected such that they can supply 10μA to the
EN/UVLO pin. A good value for R8 is 100k.
Output Voltage Sequencing
There are a few applications available for sequencing the
buck and LDO output voltages. In Figures 11 and 12, the
buck output (OUT1) is programmed to 3.3V, while the LDO
output (OUT2) is programmed to 1.8V.
Figure 11 shows a standard configuration where OUT1 and
OUT2 come up as soon as possible. In this configuration,
4.7μH
OUT1
SW
LT3645
31.6K
DA
10μF
FB
Precision Undervoltage Lockout
10K
The EN/UVLO pin has an accurate 1.23V threshold that
can be used to shutdown the part when the input voltage
drops below a specified level. To perform this function, a
resistor divider between the EN/UVLO pin and the VIN pin
can be tied as shown in Figure 10. The resistor values can
be determined from the following equation:
VCC2
EN2
12.4k
10k
3645 F11
EN/UVLO
20V/DIV
VIN
R7
LT3645
OUT1
5V/DIV
OUT2
2V/DIV
EN/UVLO
R8
2.2μF
FB2
©V
¹
R7 " R8 t ª IN(MIN) – 1º
« 1.23V
»
VIN
OUT2
OUT2
NPG
5V/DIV
GND
500μs/DIV
3645 F10
Figure 11. OUT1 and OUT2 Come Up as Soon as Possible
Figure 10. Precision UVLO Circuit
3645f
16
LT3645
APPLICATIONS INFORMATION
there is a small delay before OUT2 begins ramping up as
OUT2 has to wait until VCC2 is above 2V before power can
be supplied to OUT2.
When both OUT2 and the buck output are in regulation,
the NPG pin will pull low, turning on PFET P1 and supplying power to OUT1.
Figure 12 utilizes the NPG pin to sequence the outputs such
that OUT1 comes into regulation after OUT2 is already in
regulation. When the part is off, the buck output, OUT1
and OUT2 will be 0V. The NPG pin will be high impedance,
PFET P1 will be off and OUT1 will be disconnected from
the buck output. When the part is turned on, first the buck
output will come up to 3.3V. Once the Buck output is in
regulation, the LDO output, OUT2 will come up to 1.8V.
The NPG pin is capable of sinking 1mA and will pull the
gate of P1 down to 300mV. Therefore R9 should be chosen
such that:
R9 < (VOUT1 – 300mV)/1mA
Where R7 is in Ω. For a 3.3V buck output application,
PFET P1 must be able to source 300mA to OUT1 from
the buck output with ~3V of gate drive. Note that PFET
4.7μH
BUCK OUTPUT
P1
SW
LT3645
R9
31.6K
31.6K
DA
OUT1
0.1μF
10μF
FB
10K
VCC2
EN2
NPG
OUT2
OUT2
12.4k
2.2μF
FB2
10k
3645 F12
EN/UVLO, 20V/DIV
BUCK OUTPUT, 5V/DIV
OUT1, 5V/DIV
OUT2
2V/DIV
NPG
5V/DIV
500μs/DIV
Figure 12. OUT2 Comes Up Before OUT1
3645f
17
LT3645
APPLICATIONS INFORMATION
P1 has a finite on-resistance which will result in power
dissipation and some loss in efficiency. For higher buck
output voltage applications, a smaller PFET may be used
since the gate drive will be higher.
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 13 shows the
recommended component placement with trace, ground
plane, and via locations.
Note that large, switched currents flow in the LT3645’s VIN
and SW pins, the catch diode (D1), and the input capacitor
(C1). The loop formed by these components should be as
small as possible and tied to system ground in only one
place. These components, along with the inductor and
output capacitor, should be placed on the same side of the
circuit board, and their connections should be made on
that layer. Place a local, unbroken ground system ground in
only one place. These components, along with the inductor
and output capacitor, should be placed on the same side
of the circuit board, and their connections should be made
OUT1
EN/UVLO
C3
OUT2
NPG EN2
R2
C4
R4
FB1
FB2
R1
R3
C2
C5
BOOST
SW
C1
VIN
VCC2
on that layer. Place a local, unbroken ground plane below
these components, and tie this ground plane to system
ground at one location (ideally at the ground terminal of
the output capacitor C1). The SW and BOOST nodes should
be kept as small as possible. Finally, keep the FB nodes
small so that the ground pin and ground traces will shield
them from the SW and BOOST nodes. Include vias near
the exposed GND pad of the LT3645 to help remove heat
from the LT3645 to the ground plane.
High Temperature Considerations
The die temperature of the LT3645 must be lower than
the maximum rating of 125°C (150°C for H-grade). This
is generally not a concern unless the ambient temperature is above 85°C. For higher temperatures, extra care
should be taken in the layout of the circuit to ensure good
heat sinking at the LT3645. The maximum load current
should be derated as the ambient temperature approaches
125°C. The die temperature is calculated by multiplying the
LT3645 power dissipation by the thermal resistance from
junction to ambient. Power dissipation within the LT3645
can be estimated by calculating the total power loss from
an efficiency measurement and subtracting the catch diode
loss. The resulting temperature rise at full load is nearly
independent of input voltage. Thermal resistance depends
upon the layout of the circuit board, but 68°C/W is typical
for the QFN (UD) package, and 40°C/W is typical for the
MSE package. Thermal shutdown will turn off the Buck
and LDO when the die temperature exceeds 160°C, but
it is not a warrant to allow operation at die temperatures
exceeding 125°C (150°C for H-grade).
Other Linear Technology Publications
D1
Application Notes 19, 35, and 44 contain more detailed
descriptions and design information for step-down regulators and other switching regulators. The LT1376 data
sheet has an extensive discussion of output ripple, loop
compensation, and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
step-down regulator.
DA
VIN
L1
MAIN PCB
BOARD
POWER
+
3645 F13
VIA TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
Figure 13.
3645f
18
LT3645
TYPICAL APPLICATIONS
5V Step-Down Converter with 3.3V Logic Rail
0.1μF
BOOST
VIN
12V
1μF
15μH
LT3645
EN/UVLO
52.3k
MBRM140
DA
ON OFF
5V
300mA
SW
10μF
FB
10k
EN2
NPG
PGOOD
VCC2
3.3V
200mA
OUT2
31.6k
2.2μF
FB2
GND
10k
3645 TA02
3.3V Step-Down Converter with 1.8V Logic Rail
0.1μF
BOOST
VIN
12V
1μF
10μH
LT3645
DA
ON OFF
3.3V
300mA
SW
EN/UVLO
MBRM140
31.6k
10μF
FB
10k
EN2
PGOOD
NPG
VCC2
1.8V
200mA
OUT2
12.4k
2.2μF
FB2
GND
10k
3645 TA03
3645f
19
LT3645
TYPICAL APPLICATIONS
3.3V Step-Down Converter with 1.8V Core Rail
0.1μF
L1
10μH
BOOST
VIN
12V
1μF
OUT1
3.3V
300mA
SW
LT3645
31.6k
DA
0.1μF
10μF
FB
10k
EN/UVLO
ON OFF
31.6K
EN2
VCC2
NPG
OUT2
1.8V
200mA
OUT2
12.4k
2.2μF
FB2
GND
10k
3645 TA04
2.5V Step-Down Converter with 1.2V Logic Rail
BAT85
0.1μF
BOOST
VIN
12V
1μF
4.7μH
LT3645
DA
ON OFF
2.5V
300mA
SW
EN/UVLO
MBRM140
21.5k
10μF
FB
10k
EN2
PGOOD
NPG
VCC2
1.2V
200mA
OUT2
4.99k
2.2μF
FB2
GND
10k
3645 TA05
3645f
20
LT3645
TYPICAL APPLICATIONS
3.3V Step-Down Converter with 5V Logic Rail
0.1μF
BOOST
VIN
12V
1μF
6.8μH
LT3645
DA
ON OFF
3.3V
450mA
SW
EN/UVLO
31.6k
MBRM140
10μF
FB
10k
PGOOD
EN2
NPG
VCC2
5.5V
5V
50mA
OUT2
0.1μF
52.3k
2.2μF
FB2
GND
10k
3645 TA06
3645f
21
LT3645
PACKAGE DESCRIPTION
MSE Package
12-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1666 Rev D)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 t 0.102
(.112 t .004)
5.23
(.206)
MIN
2.845 t 0.102
(.112 t .004)
0.889 t 0.127
(.035 t .005)
6
1
1.651 t 0.102 3.20 – 3.45
(.065 t .004) (.126 – .136)
0.12 REF
12
0.65
0.42 t 0.038
(.0256)
(.0165 t .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
4.039 t 0.102
(.159 t .004)
(NOTE 3)
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
7
NO MEASUREMENT PURPOSE
0.406 t 0.076
(.016 t .003)
REF
12 11 10 9 8 7
DETAIL “A”
0s – 6s TYP
3.00 t 0.102
(.118 t .004)
(NOTE 4)
4.90 t 0.152
(.193 t .006)
GAUGE PLANE
0.53 t 0.152
(.021 t .006)
1 2 3 4 5 6
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
TYP
0.650
(.0256)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.86
(.034)
REF
0.1016 t 0.0508
(.004 t .002)
MSOP (MSE12) 0910 REV D
3645f
22
LT3645
PACKAGE DESCRIPTION
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 p0.05
3.50 p 0.05
1.45 p 0.05
2.10 p 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 p0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 p 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 s 45o CHAMFER
R = 0.115
TYP
0.75 p 0.05
15
16
PIN 1
TOP MARK
(NOTE 6)
0.40 p 0.10
1
1.45 p 0.10
(4-SIDES)
2
(UD16) QFN 0904
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)
2. DRAWING NOT TO SCALE
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 0.15mm ON ANY SIDE
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
0.25 p 0.05
0.50 BSC
3645f
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3645
TYPICAL APPLICATION
1.8V Step-Down Converter with 0.8V Logic Rail
0.1μF
BOOST
4.7μH
VIN
12V
1μF
LT3645
DA
ON OFF
1.8V
500mA
SW
MBRM140
12.4k
10μF
FB
EN/UVLO
10k
EN2
PGOOD
NPG
VCC2
+
0.8V
200mA
OUT2
FB2
2.2μF
GND
0.1μF
V 3V
–
ALTERNATE
POWER SOURCE
3645 TA07
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LT3694
36V, 70V Transient Protection, 2.6A, 2.5MHz
High Efficiency Step-Down DC/DC Converter
with Dual LDO Controllers
VIN: 3.6V to 36V, Transient to 70V, VOUT(MIN) = 0.75V, IQ = 1mA, ISD < 1μA,
4mm × 5mm QFN-28, TSSOP-20E
LT3509
36V, 60V Transient Protection, Dual 700mA,
2.2MHz High Efficiency Step-Down DC/DC
Converter
VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1μA,
3mm × 4mm DFN-14, MSOP-16E
LT3689
36V, 60V Transient Protection, 800mA, 2.2MHz VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD < 1μA,
High Efficiency MicroPower Step-Down DC/DC 3mm × 3mm QFN-16
Converter with POR Reset and Watchdog Timer
LT3682
36V, 60VMax, 1A, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter
LT3970
40V, 350mA (IOUT), 2.2MHz, High Efficiency
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10,
Step-Down DC/DC Converter with Only 2.5μA of MSOP-10
Quiescent Current
LT3990
62V, 350mA (IOUT), 2.2MHz, High Efficiency
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD < 1μA, 3mm × 3mm DFN-10,
Step-Down DC/DC Converter with Only 2.5μA of MSOP-10
Quiescent Current
LT3791
38V, 1.2A, 2.2MHz High Efficiency MicroPower
Step-Down DC/DC Converter with IQ = 2.8μA
VIN: 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10,
MSOP-10E
LT3991
55V, 1.2A, 2.2MHz High Efficiency MicroPower
Step-Down DC/DC Converter with IQ = 2.8μA
VIN: 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8mA, ISD < 1μA, 3mm × 3mm DFN-10,
MSOP-10E
LT3480
36V with Transient Protection to 60V, 2A (IOUT),
2.4MHz, High Efficiency Step-Down DC/DC
Converter with Burst Mode® Operation
VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10,
MSOP-10E
LT3685
36V with Transient Protection to 60V, 2A (IOUT),
2.4MHz, High Efficiency Step-Down DC/DC
Converter
VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10,
MSOP-10E
VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD < 1μA,
3mm × 3mm QFN-12
3645f
24 Linear Technology Corporation
LT 0511 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2011