LT3695IMSE-3.3#PBF [Linear]
LT3695 Series - 1A Fault Tolerant Micropower Step-Down Regulator; Package: MSOP; Pins: 16; Temperature Range: -40°C to 85°C;
| 型号： | LT3695IMSE-3.3#PBF |
| 厂家： | 
Linear |
| 描述： | LT3695 Series - 1A Fault Tolerant Micropower Step-Down Regulator; Package: MSOP; Pins: 16; Temperature Range: -40°C to 85°C 开关 光电二极管 |
| 文件： | 总30页 (文件大小：487K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3695 Series
1A Fault Tolerant Micropower
Step-Down Regulator
FEATURES
DESCRIPTION
The LT®3695 series are adjustable frequency (250kHz to
2.2MHz)monolithicbuckswitchingregulatorsthataccept
input voltages up to 36V and can safely sustain transient
voltages up to 60V. The devices include a high efficiency
switch, aboostdiode, andthenecessaryoscillator, control
and logic circuitry. Current mode topology is used for fast
transient response and good loop stability. A SYNC pin
allowstheusertosynchronizetheparttoanexternalclock,
and to choose between low ripple Burst Mode operation
and standard PWM operation.
n
Wide Input Range:
Operation from 3.6V to 36V
Overvoltage Lockout Protects Circuits Through
60V Transients
n
FMEA Fault Tolerant:
Output Stays at or Below Regulation Voltage
During Adjacent Pin Short or When a Pin Is Left
Floating
n
1A Output Current
Low Ripple (< 15mV ) Burst Mode® Operation
n
P-P
IN
I = 75μA for 12V to 3.3V with No Load
Q
OUT
The LT3695 regulators tolerate adjacent pin shorts or
an open pin without raising the output voltage above its
programmed value.
n
n
n
n
n
n
n
Adjustable Switching Frequency: 250kHz to 2.2MHz
Short-Circuit Protected
Synchronizable Between 300kHz and 2.2MHz
Output Voltage: 0.8V to 20V
Low ripple Burst Mode operation maintains high effi-
ciency at low output currents while keeping output ripple
below 15mV in a typical application. Shutdown reduces
input supply current to less than 1μA while a resistor and
capacitor on the RUN/SS pin provide a controlled output
voltage ramp (soft-start). Protection circuitry senses the
current in the power switch and external Schottky catch
diodetoprotecttheLT3695regulatorsagainstshort-circuit
conditions. Frequency foldback and thermal shutdown
provide additional protection.
Power Good Flag
Fixed Output Voltage Versions for 3.3V and 5V Available
Small, Thermally Enhanced 16-Pin MSOP Package
APPLICATIONS
n
Automotive Battery Regulation
n
Automotive Entertainment Systems
n
Distributed Supply Regulation
Industrial Supplies
n
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
The LT3695 series is available in a thermally enhanced
16-pin MSOP package.
TYPICAL APPLICATION
Efficiency
100
5V Step-Down Converter
V
V
IN
V
= 5V
OUT
5V
OUT
6.9V TO 36V
90
80
TRANSIENT TO 60V
0.9A, V > 6.9V
1A, V > 12V
IN
IN
V
BD
BOOST
IN
RUN/SS
V
OUT
= 3.3V
2.2μF
ON OFF
40.2k
0.22μF
10μH
V
C
SW
LT3695
RT
70
PG
DA
FB
536k
16.2k
470pF
SYNC
GND PGND
60
50
V
= 12V
IN
102k
10μF
L = 10μH
f = 800kHz
3695 TA01a
f = 800kHz
0
0.2
0.4
0.6
0.8
1
LOAD CURRENT (A)
3695 TA01b
3695fa
1
LT3695 Series
ABSOLUTE MAXIMUM RATINGS (Notes 1, 2)
V , RUN/SS Voltage (Note 3)...................................60V
OUT1, OUT2 Voltage (LT3695-3.3, LT3695-5)...........16V
PG Voltage ................................................................30V
Operating Junction Temperature Range (Notes 4, 5)
LT3695E............................................. –40°C to 125°C
LT3695I.............................................. –40°C to 125°C
LT3695H ............................................ –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
IN
BOOST Pin Voltage ...................................................50V
BOOST Pin Above SW Pin.........................................30V
BD Voltage (LT3695).................................................30V
RT, V Voltage ............................................................5V
C
RT Pin Current .........................................................1mA
SYNC Voltage............................................................20V
FB Voltage (LT3695)....................................................5V
PIN CONFIGURATION
LT3695
LT3695-3.3, LT3695-5
TOP VIEW
TOP VIEW
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
PGND
DA
16 BOOST
15 BD
PGND
DA
16 BOOST
15 NC
NC
14 GND
13 PG
NC
14 OUT1
13 OUT2
12 GND
11 PG
17
PGND
SW
SW
17
PGND
RUN/SS
RT
12 NC
RUN/SS
RT
11 FB
SYNC
10 NC
SYNC
10 NC
V
9
V
C
V
9
V
C
IN
IN
MSE PACKAGE
16-LEAD PLASTIC MSOP
MSE PACKAGE
16-LEAD PLASTIC MSOP
θ
JA
= 40°C/W WITH EXPOSED PAD SOLDERED
θ
= 40°C/W WITH EXPOSED PAD SOLDERED
JA
JA
θ
= 110°C/W WITHOUT EXPOSED PAD SOLDERED
θ
= 110°C/W WITHOUT EXPOSED PAD SOLDERED
JA
ORDER INFORMATION
LEAD FREE FINISH
LT3695EMSE#PBF
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
LT3695EMSE#TRPBF
LT3695IMSE#TRPBF
LT3695HMSE#TRPBF
3695
3695
3695
LT3695IMSE#PBF
LT3695HMSE#PBF
LT3695EMSE-3.3#PBF
LT3695IMSE-3.3#PBF
LT3695HMSE-3.3#PBF
LT3695EMSE-5#PBF
LT3695IMSE-5#PBF
LT3695HMSE-5#PBF
LT3695EMSE-3.3#TRPBF 369533
LT3695IMSE-3.3#TRPBF 369533
LT3695HMSE-3.3#TRPBF 369533
LT3695EMSE-5#TRPBF
LT3695IMSE-5#TRPBF
LT3695HMSE-5#TRPBF
36955
36955
36955
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/
3695fa
2
LT3695 Series
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, unless otherwise noted. (Note 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Operating Voltage (Note 6)
LT3695
l
l
V
BD
V
BD
= 3.3V
< 3V
3.4
3.4
3.6
4.3
V
V
l
l
l
LT3695-3.3
LT3695-5
V
V
= 3.3V
= 5V
3.4
3.4
38
3.6
3.6
V
V
V
OUT1,2
OUT1,2
V
IN
Overvoltage Lockout
36
39.9
Quiescent Current from V
V
V
V
= 0.2V V = 3.3V
0.01
35
90
0.5
60
160
μA
μA
μA
IN
RUN/SS
RUN/SS
RUN/SS
BD
l
LT3695
= 10V, V = 3.3V, Not Switching
BD
= 10V, V = 0V, Not Switching
BD
l
l
LT3695-3.3
LT3695-5
V
V
= 10V, V
= 10V, V
= 3.3V, Not Switching
= 5V, Not Switching
35
35
60
60
μA
μA
RUN/SS
RUN/SS
OUT1,2
OUT1,2
Quiescent Current from BD Pin
LT3695
V
V
V
= 0.2V, V = 3.3V
0.01
55
0
0.5
100
–5
RUN/SS
RUN/SS
RUN/SS
BD
μA
μA
μA
l
= 10V, V = 3.3V, Not Switching
35
BD
= 10V, V = 0V, Not Switching
BD
Quiescent Current from OUT1,2 Pins
LT3695-3.3
V
V
= 0.2V
5
10
65
15
μA
μA
RUN/SS
RUN/SS
l
l
= 10V, V
= 3.3V, Not Switching
= 5V, Not Switching
43
112
OUT1,2
LT3695-5
Minimum BD Pin Voltage: LT3695
V
V
= 0.2V
= 10V, V
5
43
10
65
15
112
μA
μA
RUN/SS
RUN/SS
OUT1,2
2.8
3
V
Feedback Voltage: LT3695
792
785
800
800
808
815
mV
mV
l
l
FB Pin Bias Current: LT3695
Reference Voltage Line Regulation
Output Voltage
FB Pin Voltage = 800mV
3.6V < V < 36V
–5
–40
nA
0.001
0.005
%/V
IN
LT3695-3.3
3.27
3.25
3.3
3.3
3.33
3.35
V
V
l
l
LT3695-5
4.95
4.925
5
5
5.05
5.075
V
V
Error Amp g
I
=
VC
1.5μA
430
1300
50
μS
V/V
μA
μA
A/V
V
m
Error Amp Voltage Gain
V Source Current
C
V Sink Current
C
50
V Pin to Switch Current Gain
C
1.25
0.6
2
V Switching Threshold
C
0.4
0.8
V Clamp Voltage
C
V
Switching Frequency
R
R
R
= 8.06k
= 29.4k
= 158k
1.98
0.9
225
2.2
1.0
250
2.42
1.1
275
MHz
MHz
kHz
RT
RT
RT
l
l
Minimum Switch Off-Time
E- and I-Grades
H-Grade
130
130
210
250
ns
ns
Switch Current Limit (Note 7)
SYNC = 0V
SYNC = 3.3V or Clocked
1.45
1.18
1.7
1.4
2
1.66
A
A
3695fa
3
LT3695 Series
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, unless otherwise noted. (Note 4)
PARAMETER
Switch V
CONDITIONS
= 1A
MIN
TYP
350
1.6
MAX
UNITS
mV
A
I
CESAT
SW
DA Pin Current to Stop OSC
Switch Leakage Current
1.25
1.95
1
V
= 0V, V = 36V
0.01
720
0.1
μA
SW
IN
Boost Schottky Diode Voltage Drop
Boost Schottky Diode Reverse Leakage
Minimum Boost Voltage (Note 8)
BOOST Pin Current
I
= 50mA
900
1
mV
μA
BSD
V
= 10V, V = 0V
SW
BD
l
l
1.7
2.3
17.5
V
I
= 0.5A
10.5
mA
SW
RUN/SS Pin Current
V
V
= 2.5V
= 10V
4.5
12
7.5
20
μA
μA
RUN/SS
RUN/SS
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Leakage Current
2.5
V
V
0.2
1
V
V
= 5V
0.1
1000
90
μA
μA
%
PG
l
PG Sink Current
= 0.4V
100
88
PG
PG Threshold as % of V (LT3695) or Measured at FB (LT3695) or OUT1,2 (LT3695-3.3, LT3695-5)
92
FB
V
(LT3695-3.3, LT3695-5)
Pins (Pin Voltage Rising)
OUT
PG Threshold Hysteresis
LT3695
Measured at FB Pin
12
50
mV
mV
LT3695-3.3 Measured at OUT1,2, Pins
LT3695-5 Measured at OUT1,2, Pins
75
mV
SYNC Threshold Voltage
SYNC Input Frequency
300
0.3
550
800
2.2
mV
MHz
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 the device
reliability and lifetime.
Note 2: Positive currents flow into pins, negative currents flow out of pins.
Minimum and maximum values refer to absolute values.
Note 5: These ICs include overtemperature protection that is intended
to protect the devices during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair device
reliability.
Note 6: This is the voltage necessary to keep the internal bias circuitry in
regulation.
Note 3: Absolute maximum voltage at V and RUN/SS pins is 60V for
nonrepetitive 1 second transients, and 36V for continuous operation.
IN
Note 7: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycles.
Note 8: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 4: The LT3695E regulators are 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 LT3695I regulators are guaranteed over the full –40°C to
125°C operating junction temperature range. The LT3695H regulators are
guaranteed over the full –40°C to 150°C operating junction temperature
range.
3695fa
4
LT3695 Series
T = 25°C, unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (VOUT = 3.3V, SYNC = 0V)
Efficiency (VOUT = 5V, SYNC = 0V)
Efficiency (VOUT = 3.3V, SYNC = 0V)
100
90
1
90
80
90
80
L = 10μH
f = 800kHz
V
V
= 12V
L = 10μH
f = 800kHz
IN
OUT
= 3.3V
L = 10μH
V
= 12V
= 24V
IN
V
= 12V
IN
V
80
70
60
f = 800kHz
70
60
50
70
60
50
V
= 34V
IN
V
= 34V
IN
0.1
= 24V
50
40
30
20
V
IN
IN
40
30
20
40
30
20
0.01
10
0.001
10
10
10
100
10
100
10
LOAD CURRENT (mA)
0.1
1000
0.1
1000
0.1
100
1000
1
1
1
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3695 G03
3695 G01
3695 G02
No-Load Supply Current
No-Load Supply Current
Maximum Load Current
140
120
100
80
1300
1.75
V
= 3.3V
CATCH DIODE: DIODES, INC. B140
OUT
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
TYPICAL
V
V
= 12V
IN
OUT
1.50
1.25
1.00
0.75
0.50
0.25
= 3.3V
60
INCREASED SUPPLY CURRENT
DUE TO CATCH DIODE LEAKAGE
AT HIGH TEMPERATURE
MINIMUM
40
V
= 3.3V
OUT
20
SYNC = 0V
SYNC = 3.3V
L = 10μH
f = 800kHz
0
0
5
10 15 20 25 30 35 40
–50 –25
0
25 50 75 100 125 150
0
5
15
20 25 30 35
40
10
INPUT VOLTAGE (V)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3695 G04
3695 G05
3695 G06
Maximum Load Current
Maximum Load Current
Maximum Load Current
1.50
1.25
1.00
0.75
0.50
0.25
1.75
1.50
1.25
1.00
0.75
1.50
1.25
TYPICAL
TYPICAL
TYPICAL
1.00
0.75
0.50
MINIMUM
MINIMUM
MINIMUM
V
= 1.8V
V
= 5V
OUT
OUT
V
= 5V
0.50
0.25
OUT
L = 10μH
L = 10μH
SYNC = 0V
SYNC = 5V
SYNC = 0V
SYNC = 3.3V
SYNC = 0V
SYNC = 5V
L = 4.7μH
f = 2MHz
f = 500kHz
f = 800kHz
0.25
12
14
16
18
10 15 20 25 30 35
INPUT VOLTAGE (V)
15
20
25
30
35
8
10
20
0
5
40
5
40
10
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3695 G08
3695 G09
3682 G07
3695fa
5
LT3695 Series
T = 25°C, unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Current Limit
Switch Current Limit
(SYNC Pin Grounded)
Switch Voltage Drop
1.9
400
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
DC = 10%
1.7
1.5
1.3
1.1
0.9
0.7
0.5
SYNC < 0.3V
300
200
100
0
DC = 90%
SYNC > 0.8V
OR CLOCKED
40
60
80
0
100
–50 –25
0
25 50 75 100 125 150
0.50
SWITCH CURRENT (A)
20
0
0.75
1.00
1.25
0.25
TEMPERATURE (°C)
DUTY CYCLE (%)
3695 G11
3695 G10
3695 G12
Output Voltage:
LT3695-3.3, LT3695-5
BOOST Pin Current
Feedback Voltage
35
30
25
20
15
10
5
3.35
3.30
3.25
3.20
3.15
3.10
5.15
5.10
5.05
5.00
4.95
4.90
810
800
790
780
770
LT3695-3.3
LT3695-5
0
0.50
0.75
1.00
–50 –25
0
25 50 75 100 125 150
–50 –25
0
25 50 75 100 125 150
0
1.25
0.25
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH CURRENT (A)
3695 G15
3695 G14
3695 G13
Switching Frequency
Frequency Foldback: LT3695
Frequency Foldback: LT3695-3.3
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
1200
1000
800
1200
1000
800
600
400
200
0
R
= 29.4k
R = 29.4k
RT
T
R
= 29.4k
RT
600
400
200
0
–50 –25
0
25 50 75 100 125 150
0
100 200 300 400 500 600 700 800 900
0
1
1.5
2
2.5
3
3.5
0.5
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)
FB PIN VOLTAGE (mV)
3695 G16
3695 G17
3695 G18
3695fa
6
LT3695 Series
T = 25°C, unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start
Minimum Switch On-Time
Frequency Foldback: LT3695-5
1200
1000
800
600
400
200
0
120
100
80
60
40
20
0
2.0
1.8
1.6
1.4
R
= 29.4k
SYNC < 0.3V
I
= 1A
RT
OUT
1.2
1.0
0.8
0.6
0.4
0.2
0
0
2
3
4
5
1
–50 –25
0
25 50 75 100 125 150
0
0.5 1.0 1.5
2.0 2.5
3.0
3.5
OUTPUT VOLTAGE (V)
TEMPERATURE (°C)
RUN/SS PIN VOLTAGE (V)
3695 G20
3695 G21
3695 G19
Error Amplifier Output Current:
LT3695
RUN/SS Pin Current
Boost Diode Forward Voltage
12
10
8
1.4
60
50
40
30
20
10
1.2
1.0
0.8
0.6
0.4
0.2
0
6
0
–10
4
–20
–30
–40
–50
2
0
–60
–200
0
5
10 15 20 25 30 35 40
0
0.25
0.5
0.75
1
–100
0
100
200
RUN/SS PIN VOLTAGE (V)
BOOST DIODE CURRENT (A)
FB PIN ERROR VOLTAGE (mV)
3695 G22
3695 G23
3659 G24
Error Amplifier Output Current:
LT3695-3.3
Error Amplifier Output Current:
LT3695-5
Minimum Input Voltage
60
50
40
30
20
10
60
50
40
30
20
10
5.0
V
= 3.3V
OUT
L = 10μH
f = 800kHz
4.5
4.0
3.5
3.0
2.5
0
–10
0
–10
–20
–30
–40
–50
–20
–30
–40
–50
–60
–60
2.0
–750
–250
0
250
500
750
1
10
100
1000
–500
–900
–300
0
300
600
900
–600
OUTPUT ERROR VOLTAGE (mV)
OUTPUT ERROR VOLTAGE (mV)
LOAD CURRENT (mA)
3695 G27
3695 G25
3695 G26
3695fa
7
LT3695 Series
T = 25°C, unless otherwise noted.
A
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
Maximum VIN for Full Frequency
Maximum VIN for Full Frequency
6.5
6.0
5.5
5.0
4.5
4
40
35
40
35
V
= 5V
OUT
L = 10μH
f = 800kHz
T
= 25˚C
A
T
= 25˚C
A
30
25
20
15
30
25
20
15
T
= 85˚C
T
= 85˚C
A
A
V
= 3.3V
V
= 5V
OUT
OUT
L = 10μH
L = 10μH
10
5
10
5
f = 800kHz
SYNC = 3.3V
f = 800kHz
SYNC = 5V
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT(A)
0.2
0.1
1
10
100
1000
0
0.1
1
0
0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT(A)
1
LOAD CURRENT (mA)
3695 G28
3695 G29
3695 G30
Switching Waveforms,
60V Input Voltage Transient
Maximum VIN for Full Frequency
VC Voltages
40
35
2.5
2.0
1.5
1.0
0.5
0
V
T
= 25˚C
SW
A
10V/DIV
CURRENT LIMIT CLAMP
30
25
20
15
T
= 85˚C
A
V
IN
20V/DIV
V
OUT
5V/DIV
3695 G33
5ms/DIV
SWITCHING THRESHOLD
V
= 5V
V
LOAD
= 12V, FRONT PAGE APPLICATION
OUT
IN
L = 4.7μH
f = 2MHz
I
= 500mA
10
5
SYNC = 5V
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT(A)
0
1
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
0.1
3695 G32
3695 G31
Switching Waveforms,
Transition from Burst Mode
Operation to Full Frequency
Switching Waveforms,
Full Frequency Continuous
Operation
Switching Waveforms,
Burst Mode Operation
V
V
V
SW
SW
SW
5V/DIV
5V/DIV
5V/DIV
I
L
I
L
I
0.5A/DIV
L
0.2A/DIV
0.2A/DIV
V
OUT
20mV/DIV
V
V
OUT
20mV/DIV
OUT
20mV/DIV
3695 G34
3695 G35
3695 G36
5μs/DIV
1μs/DIV
1μs/DIV
V
LOAD
= 12V, FRONT PAGE APPLICATION
V
LOAD
= 12V, FRONT PAGE APPLICATION
V
LOAD
= 12V, FRONT PAGE APPLICATION
IN
IN
IN
I
= 5mA
I
= 55mA
I
= 500mA
3695fa
8
LT3695 Series
(LT3695/LT3695-3.3, LT3695-5)
PIN FUNCTIONS
PGND (Pin 1, Exposed Pad Pin 17/Pin 1, Exposed Pad
Pin 17): This is the power ground used by the catch diode
(D1 in the Block Diagram) when its anode is connected to
the DA pin. The exposed pad may be soldered to the PCB
in order to lower the thermal resistance.
V
(Pin 8/Pin 8): The V pin supplies current to the
IN IN
internal regulator and to the internal power switch. This
pin must be locally bypassed.
V (Pin 9/Pin 9): The V pin is the output of the internal
C
C
error amplifier. The voltage on this pin controls the peak
switch current. Tie an RC network from this pin to ground
to compensate the control loop.
DA (Pin 2/Pin 2): Connect the anode of the catch diode
(D1) to this pin. Internal circuitry senses the current
through the catch diode providing frequency foldback in
extreme situations.
FB(Pin11)LT3695:TheLT3695regulatestheFBpinto0.8V.
Connect the feedback resistor divider tap to this pin.
NC (Pins 3, 10, 12/Pins 3, 10, 15): No Connects. These
pins are not connected to internal circuitry and must be
left floating to ensure fault tolerance.
PG(Pin13/Pin11):ThePGpinistheopen-collectoroutput
of an internal comparator. PG remains low until the FB pin
(LT3695) or the OUT1,2 pins (LT3695-3.3, LT3695-5) are
within 10% of the final regulation voltage. PG output is
SW (Pin 4/Pin 4): The SW pin is the output of the internal
powerswitch. Connectthispintotheinductor, catchdiode
and boost capacitor.
valid when V is above the minimum input voltage and
IN
RUN/SS is high.
RUN/SS (Pin 5/Pin 5): The RUN/SS pin is used to put
the LT3695 regulators in shutdown mode. Tie to ground
to shut down the LT3695 regulators. Tie to 2.5V or more
for normal operation. RUN/SS also provides a soft-start
function; see the Applications Information section for
more information.
GND (Pin 14/Pin 12): The GND pin is the ground of all the
internal circuitry. Tie directly to the local GND plane.
OUT1, OUT2, (Pins14, 13)LT3695-3.3, LT3695-5:These
pins connect to the anode of the boost Schottky diode and
also supply current to the internal regulator. They also
connect to the internal feedback resistors and must be
connected to the output.
RT (Pin 6/Pin 6): Oscillator Resistor Input. Connect a
resistor from this pin to ground to set the switching
frequency.
BD (Pin 15) LT3695: This pin connects to the anode of
the boost Schottky diode and also supplies current to the
LT3695’s internal regulator.
SYNC (Pin 7/Pin 7): This is the external clock synchroni-
zation input. Ground this pin with a 100k resistor for low
ripple Burst Mode operation at low output loads. Tie to
0.8V or more for pulse-skipping mode operation. Tie to a
clocksourceforsynchronization.Clockedgesshouldhave
rise and fall times faster than 1μs. Note that the maximum
load current depends on which mode is chosen. See the
Applications Information section for more information.
BOOST (Pin 16/Pin 16): This pin is used to provide a
drive voltage, higher than the input voltage, to the internal
bipolar NPN power switch. Connect a capacitor (typically
0.22μF) between BOOST and SW.
3695fa
9
LT3695 Series
BLOCK DIAGRAM
LT3695
V
IN
V
IN
8
–
+
C1
OVLO
THERMAL
SHUTDOWN
BD
15
16
INTERNAL 0.8V REF
SLOPE COMP
BOOST
R
S
Q
OUT
RT
C3
L1
OSCILLATOR
250kHz TO 2.2MHz
OUTB
6
7
SW
DA
R
4
2
V
OUT
T
C2
D1
SYNC
+
–
SYNC
DISABLE
Burst Mode
DETECT
RUN/SS
PG
SOFT-START
5
ERROR AMP
13
V
C
CLAMP
+
–
+
–
V
C
0.720V
9
C
C
C
F
R
C
GND
14
FB
11
PGND
PGND
17
1
R2
R1
3695 BDa
LT3695-3.3/LT3695-5
V
IN
V
IN
8
–
+
OUT2
C1
13
14
16
OVLO
THERMAL
OUT1
SHUTDOWN
INTERNAL 0.8V REF
SLOPE COMP
BOOST
R
Q
S
OUT
RT
C3
L1
OSCILLATOR
250kHz TO 2.2MHz
OUTB
6
7
SW
DA
R
4
2
V
T
OUT
C2
D1
SYNC
+
–
SYNC
DISABLE
Burst Mode
DETECT
RUN/SS
PG
SOFT-START
5
ERROR AMP
11
V
CLAMP
C
+
–
+
–
V
C
0.720V
9
C
C
R2
R1
C
F
R
C
GND
12
PGND
PGND
17
1
3695 BD
3695fa
10
LT3695 Series
OPERATION
The LT3695 series are constant-frequency, current mode
To further optimize efficiency, the LT3695 regulators au-
tomatically switch to Burst Mode operation in light load
situations. Between bursts, all circuitry associated with
controlling the output switch is shut down, reducing the
input supply current to 75μA in a typical application.
step-down regulators. An oscillator, with frequency set by
R , enables an RS flip-flop, turning on the internal power
T
switch. An amplifier and comparator monitor the current
flowing between the V and SW pins, turning the switch
IN
off when this current reaches a level determined by the
The oscillator reduces the LT3695 regulators’ operating
frequency when the voltage at the FB pin (LT3695) or the
OUT1,2pins(LT3695-3.3,LT3695-5)islow.Thisfrequency
foldbackhelpstocontroltheoutputcurrentduringstart-up
and overload conditions.
voltage at V . An error amplifier measures the output
C
voltage through an external resistor divider tied to the FB
pin (LT3695) or through an internal resistor divider con-
nected to the output voltage (LT3695-3.3, LT3695-5), and
servos the V pin. If the error amplifier’s output increases,
C
Internal circuitry monitors the current flowing through the
catch diode via the DA pin and delays the generation of
new switch pulses if this current is too high (above 1.6A
nominal). This mechanism also protects the part during
short-circuit and overload conditions by keeping the cur-
rent through the inductor under control.
more current is delivered to the output; if it decreases,
less current is delivered. An active clamp on the V pin
C
provides current limit. The V pin is also clamped to the
C
voltage on the RUN/SS pin; soft-start is implemented by
generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
The LT3695 regulators contain a power good comparator
which trips when the FB pin (LT3695) or the OUT1,2 pins
(LT3695-3.3, LT3695-5) are at 90% of their regulated
value. The PG output is an open-collector transistor that
is off when the output is in regulation, allowing an external
resistor to pull the PG pin high. Power good is valid when
Aninternalregulatorprovidespowertothecontrolcircuitry.
The bias regulator normally draws power from the V pin,
IN
but if the BD pin is connected to an external voltage higher
than 3V (LT3695) or if the output voltage connected to the
OUT1andOUT2pinsexceeds3V(LT3695-3.3,LT3695-5),
bias power will be drawn from the external source. This
improves efficiency. The RUN/SS pin is used to place the
LT3695 regulators in shutdown, disconnecting the output
and reducing the input current to less than 1μA.
the LT3695 regulators are enabled and V is above the
IN
minimum input voltage.
TheLT3695regulatorshaveanovervoltageprotectionfea-
ture which disables switching action when V goes above
The switch driver operates from either the input or from
theBOOSTpin.Anexternalcapacitorandtheinternalboost
diode are used to generate a voltage at the BOOST pin that
is higher than the input supply. This allows the driver to
fully saturate the internal bipolar NPN power switch for
efficient operation.
IN
38V(typical)duringtransients.TheLT3695regulatorscan
then safely sustain transient input voltages up to 60V.
3695fa
11
LT3695 Series
APPLICATIONS INFORMATION
FB Resistor Network (LT3695)
Operating Frequency Trade-Offs
The output voltage of the LT3695 is programmed with a
resistordividerbetweentheoutputandtheFBpin. Choose
the resistor values according to:
Selectionoftheoperatingfrequencyisatrade-offbetween
efficiency, componentsize, minimumdropoutvoltageand
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage and higher dropout voltage. The
⎛
⎞
V
0.8V
R1= R2 OUT −1
⎜
⎟
⎝
⎠
highest acceptable switching frequency (f
) for a
SW(MAX)
Reference designators refer to the Block Diagram of the
LT3695.1%resistorsarerecommendedtomaintainoutput
voltage accuracy.
given application can be calculated as follows:
VOUT + VD
tON(MIN)(VIN − VSW + VD)
fSW(MAX)
=
Setting the Switching Frequency
where V is the typical input voltage, V
is the output
OUT
IN
The LT3695 regulators use a constant-frequency PWM
architecture that can be programmed to switch from
250kHz to 2.2MHz by using a resistor tied from the RT
voltage, V is the catch diode drop (~0.5V) and V is the
D
SW
internal switch drop (~0.5V at max load). This equation
shows that lower switching frequency is necessary to
pin to ground. A table showing the necessary R value for
T
safely accommodate high V /V
ratio. Also, as shown
IN OUT
a desired switching frequency is in Table 1.
intheInputVoltageRangesection,lowerfrequencyallows
a lower dropout voltage. Input voltage range depends on
the switching frequency because the LT3695 regulators’
switch has finite minimum on and off times. An internal
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
0.25
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
158
127
timer forces the switch to be off for at least t
per
OFF(MIN)
cycle; this timer has a maximum value of 210ns (250ns
90.9
71.5
57.6
47.5
40.2
34
for T > 125°C). On the other hand, delays associated with
J
turningoffthepowerswitchdictatetheminimumon-time,
t
,beforetheswitchcanbeturnedoff;t
hasa
ON(MIN)
ON(MIN)
maximumvalueof150nsovertemperature.Theminimum
and maximum duty cycles that can be achieved taking
minimum on and off times into account are:
29.4
22.6
18.2
14.7
12.1
9.76
8.06
DC
DC
= f
t
MIN
SW ON(MIN)
= 1 – f
t
MAX
SW OFF(MIN)
where f
is the switching frequency, t
is the
is the
SW
ON(MIN)
OFF(MIN)
minimum switch on time (150ns), and t
minimum switch off time (210ns, 250ns for T > 125°C).
J
These equations show that the duty cycle range increases
when the switching frequency is decreased.
3695fa
12
LT3695 Series
APPLICATIONS INFORMATION
A good choice of switching frequency should allow an
adequateinputvoltagerange(seeInputVoltageRangesec-
tion) and keep the inductor and capacitor values small.
input voltage. Conversely, a lower switching frequency
will be necessary to achieve optimum operation at high
input voltages.
Special attention must be paid when the output is in
start-up, short-circuit or other overload conditions. Dur-
ing these events, the inductor peak current might easily
reach and even exceed the maximum current limit of
the LT3695 regulators, especially in those cases where
the switch already operates at minimum on-time. The
circuitry monitoring the current through the catch diode
via the DA pin prevents the switch from turning on again
if the inductor valley current is above 1.6A nominal. In
these cases, the inductor peak current is therefore the
maximum current limit of the LT3695 regulators plus the
additional current overshoot during the turn off delay due
to minimum on time:
Input Voltage Range
The minimum input voltage is determined by either the
LT3695 regulators’ minimum operating voltage of ~3.6V
(V > 3V) or by their maximum duty cycle (see equation
BD
inOperatingFrequencyTrade-Offssection). Theminimum
input voltage due to duty cycle is:
VOUT + VD
VIN(MIN)
=
− VD + VSW
1− fSW OFF(MIN)
t
whereV
istheminimuminputvoltage,andt
IN(MIN)
OFF(MIN)
is the minimum switch off time. Note that a higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
VIN(MAX)− VOUT(OL)
IL(PEAK) = 2A +
where I
• tON(MIN)
L
The maximum input voltage for LT3695 regulator applica-
tions depends on switching frequency, the absolute maxi-
is the peak inductor current, V
is
L(PEAK)
IN(MAX)
the maximum expected input voltage, L is the inductor
value, t is the minimum on time and V is the
mum ratings of the V and BOOST pins and the operating
IN
ON(MIN)
OUT(OL)
mode.TheLT3695regulatorscanoperatefromcontinuous
input voltages up to 36V. Input voltage transients of up to
60V are also safely withstood. However, note that while
output voltage under the overload condition. The parts are
robustenoughtosurviveprolongedoperationunderthese
conditions as long as the peak inductor current does not
exceed 3.5A. Inductor current saturation and excessive
junction temperature may further limit performance.
V >V
(overvoltagelockout, 38Vtypical), theLT3695
IN
OVLO
regulators will stop switching, allowing the output to fall
out of regulation.
Input voltage transients of up to V
are acceptable
OVLO
For a given application where the switching frequency
and the output voltage are already fixed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
regardless of the switching frequency. In this case, the
LT3695 regulators may enter pulse-skipping operation
where some switching pulses are skipped to maintain
output regulation. In this mode the output voltage ripple
and inductor current ripple will be higher than in normal
operation.
VOUT + VD
VIN(MAX)
=
− VD + VSW
fSW ON(MIN)
t
Input voltage transients above V
and up to 60V can
OVLO
be tolerated. However, since the parts will stop switching
duringthesetransients,theoutputwillfalloutofregulation
and the output capacitor may eventually be completely
discharged. This case must be treated then as a start-up
where V
OUT
is the maximum operating input voltage,
IN(MAX)
V
is the output voltage, V is the catch diode drop
D
(~0.5V),V istheinternalswitchdrop(~0.5Vatmaxload),
SW
f
is the switching frequency (set by R ) and t
is
SW
T
ON(MIN)
condition as soon as V returns to values below V
IN
OVLO
the minimum switch on time (~150ns). Note that a higher
switching frequency will reduce the maximum operating
and the part starts switching again.
3695fa
13
LT3695 Series
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
The current in the inductor is a triangle wave with an av-
erage value equal to the load current. The peak inductor
and switch current is:
1.8
L = (VOUT + VD)•
fSW
ΔIL
2
ISW(PEAK) = IL(PEAK) = IOUT(MAX)
where I
+
is the peak inductor current, I
is
where f is the switching frequency in MHz, V
is the
L(PEAK)
OUT(MAX)
SW
OUT
the maximum output load current and ΔI is the inductor
output voltage, V is the catch diode drop (~0.5V) and L
L
D
ripple current. The LT3695 regulators limit their switch
current in order to protect themselves and the system
from overload faults. Therefore, the maximum output
current that the LT3695 regulators will deliver depends on
the switch current limit, the inductor value and the input
and output voltages.
is the inductor value in μH.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. To keep the efficiency high, the series
resistance (DCR) should be less than 0.1Ω, and the core
materialshouldbeintendedforhighfrequencyapplications.
Table 2 lists several vendors and suitable types.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
For robust operation in fault conditions (start-up or short-
circuit) and high input voltage (>30V), the saturation
current should be chosen high enough to ensure that the
inductor peak current does not exceed 3.5A. For example,
an application running from an input voltage of 36V
using a 10μH inductor with a saturation current of 2.5A
will tolerate the mentioned fault conditions.
(1−DC)•(VOUT + VD)
ΔIL =
L • fSW
where f
is the switching frequency of the LT3695
SW
regulators, DC is the duty cycle and L is the value of the
inductor.
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. If your load is lower
than the maximum load current, then you can relax the
value of the inductor and operate with higher ripple cur-
rent. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency.
Be aware that if the inductance differs from the simple
rule above, then the maximum load current will depend
on input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
maximum load current. For details of maximum output
current and discontinuous mode operation, see Linear
Technology’s Application Note 44. Finally, for duty cycles
To maintain output regulation, the inductor peak current
must be less than the LT3695 regulators’ switch current
limit, I . If the SYNC pin is grounded, I
is at least
LIM
LIM
1.45A at low duty cycles and decreases to 1.1A at DC =
90%. If the SYNC pin is tied to 0.8V or more or if it is
tied to a clock source for synchronization, I is at least
LIM
1.18A at low duty cycles and decreases to 0.85A at DC =
90%. The maximum output current is also a function of
the chosen inductor value and can be approximated by
the following expressions depending on the SYNC pin
configuration:
For the SYNC pin grounded:
ΔIL
2
ΔIL
2
IOUT(MAX) = ILIM
−
= 1.45A •(1−0.24•DC)−
greaterthan50%(V /V >0.5), aminimuminductance
OUT IN
is required to avoid sub-harmonic oscillations:
For the SYNC pin tied to 0.8V or more, or tied to a clock
source for synchronization:
1.2
LMIN = (VOUT + VD)•
fSW
ΔIL
2
ΔIL
2
IOUT(MAX) = ILIM
−
= 1.18A •(1−0.29•DC)−
3695fa
14
LT3695 Series
APPLICATIONS INFORMATION
Choosing an inductor value so that the ripple current is
smallwillallowamaximumoutputcurrentneartheswitch
current limit.
EMI. A 2.2μF capacitor is capable of this task, but only if
it is placed close to the LT3695 regulators (see the PCB
Layout section for more information). A second precau-
tion regarding the ceramic input capacitor concerns the
maximum input voltage rating of the LT3695 regulators.
A ceramic input capacitor combined with trace or cable
inductance forms a high-Q (underdamped) tank circuit.
If the LT3695 regulators circuit is plugged into a live sup-
ply, the input voltage can ring to twice its nominal value,
possibly exceeding the LT3695 regulators’ voltage rating.
For details see Application Note 88.
Table 2. Inductor Vendors
VENDOR
Murata
TDk
URL
PART SERIES
TYPE
www.murata.com
www.componenttdk.com
LQH55D
Open
SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D73C
Shielded
Shielded
Shielded
Open
D75F
Coilcraft
Sumida
www.coilcraft.com
www.sumida.com
MSS7341
MSS1038
Shielded
Shielded
Output Capacitor and Output Ripple
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3695 regulators to produce the DC output. In this
role it determines the output ripple, and low impedance
at the switching frequency is important. The second func-
tion is to store energy in order to satisfy transient loads
and stabilize the LT3695 regulators’ control loop. Ceramic
capacitors have very low equivalent series resistance
(ESR) and provide the best ripple performance. A good
starting value is:
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT3695 regulators will
be able to deliver the required output current. Note again
that these equations assume that the inductor current is
continuous. Discontinuous operation occurs when I
is less than ΔI /2.
OUT
50
VOUT
L
COUT
=
fSW
Input Capacitor
where f
is in MHz, and C
is the recommended
OUT
SW
Bypass the input of the LT3695 regulators’ circuit with a
ceramiccapacitorofX7RorX5Rtype.Y5Vtypeshavepoor
performance over temperature and applied voltage, and
should not be used. A 2.2μF to 10μF ceramic capacitor is
adequate to bypass the LT3695 regulators and will easily
handletheripplecurrent.Notethatlargerinputcapacitance
is required when a lower switching frequency is used. If
the input power source has high impedance, or there is
significantinductanceduetolongwiresorcables,additional
bulk capacitance may be necessary. This can be provided
with a lower performance electrolytic capacitor.
output capacitance in μF. Use X5R or X7R types. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
higher value capacitor if the compensation network is also
adjusted to maintain the loop bandwidth. A lower value
of output capacitor can be used to save space and cost
but transient performance will suffer. See the Frequency
Compensation section to choose an appropriate compen-
sation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolyticcapacitorscanbeusedfortheoutputcapacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3695 regulators and to force this very high fre-
quencyswitchingcurrentintoatightlocalloop,minimizing
3695fa
15
LT3695 Series
APPLICATIONS INFORMATION
Table 3. Capacitor Vendors
VENDOR
Panasonic
Kemet
PHONE
URL
PART SERIES
Ceramic, Polymer, Tantalum EEF Series
Ceramic, Tantalum T494, T495
COMMANDS
(714) 373-7366
(864) 963-6300
(408) 749-9714
(408) 436-1300
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
Sanyo
Ceramic, Polymer, Tantalum POSCAP
Ceramic
Murata
AVX
Ceramic, Tantalum
Ceramic
TPS Series
Taiyo Yuden
(864) 963-6300
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 0.05Ω 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. Table 3 lists several capacitor
vendors.
Table 4. Schottky Diodes
PART NUMBER V (V)
I
(A) V at 1A (mV) V at 2A (mV)
AVE F F
R
On-Semiconducor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B0540W
B120
20
40
20
40
0.5
0.5
1
620
530
550
595
1
Diode Selection
30
40
20
30
40
20
30
40
40
40
40
0.5
0.5
1
The catch diode (D1 from Block Diagram) conducts cur-
rent only during switch off time. Average forward current
in normal operation can be calculated from:
620
500
500
500
B130
1
B140
1
I
= I
• (1 – DC)
OUT
D(AVG)
B220
2
500
500
where DC is the duty cycle. The only reason to consider a
diodewithlargercurrentratingthannecessaryfornominal
operation is for the case of shorted or overloaded output
conditions. For the worst case of shorted output the diode
average current will then increase to a value that depends
on the following internal parameters: switch current limit,
catch diode (DA pin) current threshold and minimum
on-time. The worst case (taking maximum values for the
above mentioned parameters) is given by the following
expression:
B230
2
B140HB
DFLS240L
DFLS140
B240
1
530
510
2
500
500
1.1
2
Central Semiconductor
CMSH1-40M
CMSH1-40ML
CMSH2-40M
CMSH2-40L
CMSH2-40
40
40
40
40
40
1
1
2
2
2
500
400
550
400
500
V
L
1
2
ID(AVG)MAX = 2A + • IN •150ns
than the input voltage. If transients at the input of up to
60V are expected, use a diode with a reverse voltage rat-
ing of 40V. Table 4 lists several Schottky diodes and their
manufacturers.Ifoperatingathighambienttemperatures,
consider using a Schottky with low reverse leakage.
Peakreversevoltageisequaltotheregulatorinputvoltage
if it is below the overvoltage protection threshold. This
feature keeps the switch off for V > V
(39.9V maxi-
IN
OVLO
mum). For inputs up to the maximum operating voltage
of 36V, use a diode with a reverse voltage rating greater
3695fa
16
LT3695 Series
APPLICATIONS INFORMATION
Audible Noise
Loop compensation determines the stability and transient
performance. Optimizing the design of the compensation
network depends on the application and type of output
capacitor. A practical approach is to start with one of the
circuits in this data sheet that is similar to your applica-
tion and tune the compensation network to optimize the
performance. Stability should then be checked across all
operatingconditions, includingloadcurrent, inputvoltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Figure 1
shows an equivalent circuit for the LT3695 regulators
control loop. The error amplifier is a transconductance
amplifierwithfiniteoutputimpedance. Thepowersection,
consisting of the modulator, power switch and inductor, is
modeled as a transconductance amplifier generating an
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can sometimes cause
problems when used with the LT3695 regulators due to
their piezoelectric nature. When in Burst Mode operation,
theLT3695regulators’switchingfrequencydependsonthe
load current, and at very light loads the LT3695 regulators
canexcitetheceramiccapacitorataudiofrequencies,gen-
eratingaudiblenoise. SincetheLT3695regulatorsoperate
at a lower current limit during Burst Mode operation, the
noise is typically very quiet. If this is unacceptable, use
a high performance tantalum or electrolytic capacitor at
the output.
Frequency Compensation
The LT3695 regulators use current mode control to
regulate the output. This simplifies loop compensation.
In particular, the LT3695 regulators do not require the
ESR of the output capacitor for stability, so you are free
to use ceramic capacitors to achieve low output ripple and
small circuit size. Frequency compensation is provided by
output current proportional to the voltage at the V pin.
C
Note that the output capacitor integrates this current, and
that the capacitor on the V pin (C ) integrates the error
C
C
amplifier output current, resulting in two poles in the loop.
In most cases a zero is required and comes from either the
output capacitor ESR or from a resistor R in series with
C
the components tied to the V pin, as shown in Figure 1.
C
C .Thissimplemodelworkswellaslongasthevalueofthe
C
Generally a capacitor (C ) and a resistor (R ) in series to
C
C
inductor is not too high and the loop crossover frequency
ground are used. In addition, there may be a lower value
is much lower than the switching frequency. A phase lead
capacitor in parallel. This capacitor (C ) is used to filter
F
capacitor (C , LT3695 only) across the feedback divider
PL
noise at the switching frequency, and is required only if a
may improve the transient response. Figure 2 shows the
transient response when the load current is stepped from
300mA to 650mA and back to 300mA.
phase-lead capacitor (C , LT3695 only) is used or if the
PL
output capacitor has high ESR.
LT3695
CURRENT MODE
POWER STAGE
SW
FB
OUTPUT
g
= 1.25S
m
R1
C
PL
V
OUT
–
+
100mV/DIV
g
= 430μS
ESR
m
C1
+
0.8V
I
LOAD
C1
0.5A/DIV
3M
CERAMIC
POLYMER
OR
3695 F02
V
GND
C
20μs/DIV
TANTALUM
OR
ELECTROLITIC
R2
R
C
C
F
Figure 2. Transient Load Response of the LT3695
Regulators. A 3.3VOUT Typical Application with VIN = 12V
as the Load Current Is Stepped from 300mA to 650mA
C
C
3695 F01
Figure 1. Model for Loop Response. Note That R1 and R2 Are
Integrated in the LT3695-3.3 and LT3695-5
3695fa
17
LT3695 Series
APPLICATIONS INFORMATION
Low Ripple Burst Mode Operation
PWM operation at a lower output load current than when
in Burst Mode operation. With the SYNC pin tied low, the
front page application circuit will switch at full frequency
at output loads higher than about 100mA. With the SYNC
pin tied high, the front page application circuit will switch
at full frequency at output loads higher than about 30mA.
The maximum load current that the LT3695 regulators can
supply is reduced when SYNC is high.
The LT3695 regulators are capable of operating in either
low ripple Burst Mode operation or pulse-skipping mode
which are selected using the SYNC pin. See the Synchro-
nization section for more information.
To enhance efficiency at light loads, the LT3695 regulators
can be operated in low ripple Burst Mode operation which
keeps the output capacitor charged to the proper voltage
whileminimizingtheinputquiescentcurrent.DuringBurst
Mode operation, the LT3695 regulators deliver single
cycle bursts of current to the output capacitor followed by
sleep periods where the output power is delivered to the
load by the output capacitor. Because the LT3695 regula-
tors deliver power to the output with single, low current
pulses, the output ripple is kept below 15mV for a typical
BOOST Pin Considerations
CapacitorC3andtheinternalboostSchottkydiode(seethe
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.22μF
capacitor will work well. Figure 4 shows three ways to
arrange the boost circuit for the LT3695 regulators. The
BOOST pin must be more than 2.3V above the SW pin
for best efficiency. For outputs of between 3V and 8V, the
standard circuit (Figure 4a) is best. For outputs between
2.8V and 3V, use a 1μF boost capacitor. A 2.5V output
presents a special case because it is marginally adequate
to support the boosted drive stage while using the internal
boost diode. For reliable BOOST pin operation with 2.5V
outputs use a good external Schottky diode (such as the
ONSemiMBR0540),anda1μFboostcapacitor(seeFigure
4b). For lower output voltages the boost diode can be tied
to the input (Figure 4c), or to another supply greater than
2.8V. Keep in mind that a minimum input voltage of 4.3V
is required if the voltage at the BD pin is smaller than 3V.
application. In addition, V and BD (LT3695), and OUT1,2
IN
(LT3695-3.3, LT3695-5) quiescent currents are reduced
to typically 35μA, 55μA and 65μA, respectively, during
the sleep time. As the load current decreases towards a
no-load condition, the percentage of time that the LT3695
regulatorsoperateinsleepmodeincreasesandtheaverage
inputcurrentisgreatlyreducedresultinginhighefficiency
even at very low loads (see Figure 3). At higher output
loads (above about 70mA for the front page application)
the LT3695 regulators will be running at the frequency
programmed by the R resistor, and will be operating in
T
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
Tying BD to V reduces the maximum input voltage to
IN
25V. The circuit in Figure 4a is more efficient because the
BOOST pin current and BD pin quiescent current come
from a lower voltage source. You must also be sure that
the maximum voltage ratings of the BOOST and BD pins
are not exceeded.
If low quiescent current is not required, tie SYNC high to
select pulse-skipping mode. The benefit of this mode is
thattheLT3695regulatorswillenterfullfrequencystandard
As mentioned, a minimum of 2.5V across the BOOST
capacitor is required for proper operation of the internal
BOOST circuitry to provide the base current for the power
NPNswitch.ForBDpinvoltageshigherthan3V,theexcess
voltage across the BOOST capacitor does not bring an
increaseinperformancebutdissipatesadditionalpowerin
the internal BOOST circuitry instead. The BOOST circuitry
toleratesreasonableamountsofpower,howeverexcessive
powerdissipationonthiscircuitrymayimpairreliability.For
V
SW
5V/DIV
I
L
0.2A/DIV
V
OUT
20mV/DIV
3695 F03
5μs/DIV
V
I
= 12V, FRONT PAGE APPLICATION
= 5mA
IN
LOAD
Figure 3. Switching Waveforms, Burst Mode Operation
reliable operation, use no more than 8V on the BD pin for
3695fa
18
LT3695 Series
APPLICATIONS INFORMATION
V
theoutputisalreadyinregulation,thentheboostcapacitor
may 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 will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 5 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
OUT
BD
V
IN
V
BOOST
IN
C3
D1
SW
LT3695
DA
GND PGND
3695 F04a
(4a) For VOUT > 2.8V, VIN(MIN) = 4.3V if VOUT < 3V
V
OUT
D2
start. Theplotsshowtheworst-casesituationwhereV is
BD
BOOST
IN
V
IN
V
IN
ramping very slowly. For lower start-up voltage, the boost
C3
D1
diode can be tied to V ; however, this restricts the input
SW
LT3695
IN
range to one-half of the absolute maximum rating of the
BOOST pin. At light loads, the inductor current becomes
discontinuousandtheeffectivedutycyclecanbeveryhigh.
DA
GND PGND
3695 F04b
6.0
(4b) For 2.5V < VOUT < 2.8V, VIN(MIN) = 4.3V
5.5
5.0
4.5
4.0
TO START
(WORST CASE)
BD
V
IN
V
BOOST
IN
C3
D1
V
OUT
SW
LT3695
3.5
3.0
TO RUN
DA
V
A
= 3.3V
OUT
GND PGND
T
= 25˚C
2.5
2.0
L = 10μH
f = 800kHz
3695 F04c
1
10
100
1000
LOAD CURRENT (mA)
(4c) For VOUT < 2.5V, VIN(MAX) = 25V
8.0
Figure 4. Three Circuits for Generating
the Boost Voltage for the LT3695
7.5
7.0
6.5
6.0
TO START
(WORST CASE)
the circuit in Figure 4a. For higher output voltages, make
sure that there is no more than 8V at the BD pin either by
connecting it to another available supply higher than 3V or
5.5
5.0
4.5
TO RUN
by using a Zener diode between V
and BD to maintain
4.0
3.5
3.0
2.5
OUT
the BD pin voltage between 3V and 8V.
V
A
= 5V
OUT
T
= 25˚C
L = 10μH
The minimum operating voltage of the LT3695 regulators
applicationislimitedbytheminimuminputvoltageandby
themaximumdutycycleasoutlinedpreviously.Forproper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3695regulatorsareturnedonwiththeirRUN/SSpinwhen
f = 800kHz
2.0
1
10
100
1000
LOAD CURRENT(mA)
3695 F05
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3695fa
19
LT3695 Series
APPLICATIONS INFORMATION
This reduces the minimum input voltage to approximately
The maximum load current that the part can supply is
reduced when a clock signal is applied to SYNC.
300mV above V . At higher load currents, the inductor
OUT
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3695 regulators, requiring
a higher input voltage to maintain regulation.
TheLT3695regulatorsmaybesynchronizedovera300kHz
to 2.2MHz range. The R resistor should be chosen to set
T
theLT3695regulatorsswitchingfrequency20%belowthe
lowest synchronization input. For example, if the synchro-
Soft-Start
nization signal is 360kHz, the R should be chosen for
T
The RUN/SS pin can be used to soft-start the LT3695
regulators, reducing the maximum input current during
start-up. The RUN/SS pin is driven through an external
RC network to create a voltage ramp at this pin. Figure 6
shows the start-up and shutdown waveforms with the
soft-start circuit. By choosing a large RC time constant,
the peak start-up current can be reduced to the current
that is required to regulate the output, with no overshoot.
Choosethevalueoftheresistorsothatitcansupply7.5μA
when the RUN/SS pin reaches 2.5V. For fault tolerant ap-
plications, see the discussion of the RUN/SS resistor in
the Fault Tolerance section.
300kHz. To assure reliable and safe operation the LT3695
regulatorswillonlysynchronizewhentheoutputvoltageis
near regulation as indicated by the PG flag. It is therefore
necessary to choose a large enough inductor value to
supply the required output current at the frequency set
by the R resistor. See the Inductor Selection section for
T
more information. It is also important to note that slope
compensationissetbytheR value;toavoidsubharmonic
T
oscillations, calculate the minimum inductor value using
the frequency determined by R .
T
Shorted and Reversed Input Protection
If the inductor is chosen so that it will not saturate exces-
sively,theLT3695regulatorswilltolerateashortedoutput.
When operating in short-circuit condition, the LT3695
regulators will reduce their frequency until the valley cur-
rent is at a typical value of 1.6A (see Figure 7). There is
another situation to consider in systems where the output
willbeheldhighwhentheinputtotheLT3695regulatorsis
absent. This may occur in battery charging applications or
in battery backup systems where a battery or some other
supply is diode ORed with the LT3695 regulators’ output.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
Synchronizing the oscillator of the LT3695 regulators to
an external frequency can be done by connecting a square
wave (with 20% to 80% duty cycle) to the SYNC pin. The
square wave amplitude should have valleys that are below
0.3V and peaks that are above 0.8V (up to 6V).
TheLT3695regulatorswillnotenterBurstModeoperation
atlowoutputloadswhilesynchronizedtoanexternalclock,
but instead will skip pulses to maintain regulation.
If the V pin is allowed to float and the RUN/SS pin is held
IN
high (either by a logic signal or because it is tied to V ),
IN
V
RUN
V
SW
5V/DIV
20V/DIV
RUN
15k
0V
V
RUN/SS
5V/DIV
RUN/SS
GND
I
L
0.22μF
V
OUT
500mA/DIV
5V/DIV
I
L
1A/DIV
3695 F05
5ms/DIV
0A
3695 F07
2μs/DIV
Figure 6. To Soft-Start the LT3695 Regulators,
Add a Resistor and Capacitor to the RUN/SS Pin
Figure 7. The LT3695 Regulators Reduce Their Frequency
to Protect Against Shorted Output with 36V Input
3695fa
20
LT3695 Series
APPLICATIONS INFORMATION
then the LT3695 regulators’ internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the RUN/SS pin, the SW pin current will drop to essen-
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figures 9 and
10 show the recommended component placement with
trace, ground plane and via locations. Note that large,
tially zero. However, if the V pin is grounded while the
IN
outputisheldhigh,thenparasiticdiodesinsidetheLT3695
switched currents flow in the LT3695 regulators’ V , SW
IN
regulators can pull large currents from the output through
and PGND pins, the catch diode and the input capacitor
the SW pin and the V pin. Figure 8 shows a circuit that
IN
(C ). The loop formed by these components should be
IN
will run only when the input voltage is present and that
as small as possible. These components, along with the
protects against a shorted or reversed input.
inductor and output capacitor (C ), should be placed on
OUT
the same side of the circuit board, and their connections
shouldbemadeonthatlayer.AllconnectionstoGNDshould
be made at a common star ground point or directly to a
local, unbroken ground plane below these components.
The SW and BOOST nodes should be laid out carefully to
D4
MBRS140
BD
V
V
IN
BOOST
IN
LT3695
V
RUN/SS
SW
DA
OUT
avoid interference. Finally, keep the FB, R and V nodes
T
C
V
C
small so that the ground traces will shield them from the
SW and BOOST nodes. To keep thermal resistance low,
extend the ground plane as much as possible and add
thermal vias under and near the LT3695 regulators to any
additionalgroundplaneswithinthecircuitboardandonthe
bottom side. Keep in mind that the thermal design must
keep the junctions of the IC below the specified absolute
maximum temperature.
BACKUP
GND PGND FB
3695 F09
Figure 8. 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 Regulator Runs Only When the Input
Is Present
GND
V
GND
V
OUT
L
OUT
L
C2
D1
C2
D1
C3
C3
R2
R1
C
C
R
V
R
V
T
T
R
C
R
C
C1
C1
C
C
GND
GND
IN
IN
3695 F09
3695 F10
Figure 9. A Good PCB Layout Ensures Proper,
Low EMI Operation (LT3695)
Figure 10. A Good PCB Layout Ensures Proper,
Low EMI Operation (LT3695-3.3, LT3695-5)
3695fa
21
LT3695 Series
APPLICATIONS INFORMATION
High Temperature Considerations
800kHz). For a 12V input to 5V output the die temperature
elevation above ambient was 22°C with the exposed pad
soldered and 44°C without the exposed pad soldered.
The PCB must provide heat sinking to keep the LT3695
regulators cool. The exposed pad on the bottom of the
packagemaybesolderedtoacopperareawhichshouldbe
tied to large copper layers below with thermal vias; these
layers will spread the heat dissipated by the LT3695 regu-
lators. Place additional vias to reduce thermal resistance
further. With these steps, the thermal resistance from die
Fault Tolerance
The LT3695 regulators are designed to tolerate single fault
conditions. Shorting two adjacent pins together or leaving
onesinglepinfloatingdoesnotraiseV orcausedamage
OUT
to the LT3695 regulators. However, the application circuit
must meet the requirements discussed in this section in
order to achieve this tolerance level.
(or junction) to ambient can be reduced to θ = 40°C/W
JA
or less. With 100 LFPM airflow, this resistance can fall
by another 25%. Further increases in airflow will lead
to lower thermal resistance. Because of the large output
current capability of the LT3695 regulators, it is possible
to dissipate enough heat to raise the junction temperature
beyond the absolute maximum. When operating at high
ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches these
maximums. If the junction temperature reaches the ther-
mal shutdown threshold, the parts will stop switching to
prevent internal damage due to overheating.
Tables 5 and 6 show the effects that result from shorting
adjacent pins or from a floating pin, respectively.
For the best fault tolerance to inadvertent adjacent pin
shorts, the RUN/SS pin must not be directly connected to
either ground or V . If there was a short between RUN/SS
IN
and SW then connecting RUN/SS to V would tie SW
IN
to V and would thus raise V . Likewise, grounding
IN
OUT
RUN/SS would tie SW to ground and would damage the
power switch if this is done when the power switch is on.
A short between RT and a RUN/SS pin that is connected
PowerdissipationwithintheLT3695regulatorscanbeesti-
matedbycalculatingthetotalpowerlossfromanefficiency
measurement. The die temperature rise is calculated by
multiplying the power dissipation of the LT3695 regula-
tors by the thermal resistance from junction to ambient.
Die temperature rise was measured on a 2-layer, 10cm ×
to V would violate the absolute maximum ratings of the
IN
RT pin. Therefore, the current supplying the RUN/SS pin
must be limited, for example, with resistor R3 in Figures
11 and 12. In case of a short between RUN/SS and SW this
resistor charges C2 through the inductor L1 if the current
10cm circuit board in still air at a load current of 1A (f
=
it supplies from V is not completely drawn by R
, R1
LOAD
SW
IN
V
IN
V
IN
R3
V
IN
D2
R3
R
LT3695-3.3
LT3695-5
RUN/SS BOOST
V
BD
IN
RUN/SS BOOST
C3
D1
L1
C3
D1
L1
V
SW
LT3695
OUT
V
OUT
SW
SS
R
SS
47Ω
47Ω
RT
DA
FB
R1
C
R
RT
DA
OUT1
OUT2
SS
LOAD
C
R
SS
LOAD
220nF
220nF
R
R2
C2
T
R
C2
R4
T
3695 F11
3695 F12
Figure 11. LT3695: The Dashed Lines Show Where a Connection
Would Occur if There Were an Inadvertent Short from RUN/SS
to an Adjacent Pin or from BOOST to BD. In These Cases, R3
Protects Circuitry Tied to the RT or SW Pins, and D2 Shields
BOOST from VOUT. If CSS Is Used for Soft Start, RSS Isolates It
from SW
Figure 12. LT3695-3.3, LT3695-5: The Dashed Lines Show
Where a Connection Would Occur if There Were an Inadvertent
Short from RUN/SS to an Adjacent Pin. In These Cases, R3
Protects Circuitry Tied to the RT or SW Pins. R4 Provides an
Additional Load and May Be Necessary in Certain Situations
(See Text). If CSS Is Used for Soft Start, RSS Isolates It from SW
3695fa
22
LT3695 Series
APPLICATIONS INFORMATION
Table 5: Effects of Pin Shorts
PINS
EFFECT
No effect if V < V
PGND-DA
SW-RUN/SS
RUN/SS-RT
RT-SYNC
. See Input Voltage Range section for description of V
.
IN
IN(MAX)
IN(MAX)
The result of this short depends on the load resistance and on R3 (Figure 10). See the following discussion.
No effect or V
No effect or V
will fall below regulation voltage if I (Figure 10) < 1mA.
OUT
OUT
R3
will fall below regulation voltage if the current into the RT pin is less than 1mA.
SYNC-V
No effect if V does not exceed the absolute maximum voltage of SYNC (20V).
IN
IN
PG-GND
No effect.
GND-BD (LT3695)
V
may fall below regulation voltage, power dissipation of the power switch will be increased. Note that this short also
OUT
grounds the voltage source supplying BD. Make sure it is safe to short the supply for BD to ground. For this reason BD should
not be connected to V , but it is safe to connect it to V
.
OUT
IN
BD-BOOST (LT3695)
GND-OUT2
If diode D2 (see Figure 10) is used, no effect or V
may fall below regulation voltage. Otherwise the device may be damaged.
OUT
V
OUT
will fall below regulation voltage, because this shorts the output to ground. As a result, the power dissipation of the part
(LT3695-3.3, LT3695-5) may increase.
Table 6: Effects of Floating Pins
PIN
EFFECT
PGND
No effect if the Exposed Pad is soldered.
Otherwise: V
may fall below regulation voltage. Make sure that V < V
(see Input Voltage Range section for details)
IN(MAX)
OUT
IN
and provide a bypass resistor at the DA pin. See the following discussion.
V may fall below regulation voltage. Make sure that V < V (see Input Voltage Range section for details) and provide
OUT
DA
IN
IN(MAX)
a bypass resistor. See the following discussion.
SW
V
V
V
V
will fall below regulation voltage.
will fall below regulation voltage.
will fall below regulation voltage.
OUT
OUT
OUT
OUT
RUN/SS
RT
SYNC
may fall below regulation voltage. A floating SYNC pin configures the LT3695 for pulse-skipping mode. However, a
floating SYNC pin is sensitive to noise which can degrade device performance.
V
V
V
V
will fall below regulation voltage.
IN
OUT
may fall below regulation voltage. Disconnecting the V pin alters the loop compensation and potentially degrades device
C
OUT
C
performance. The output voltage ripple will increase if the part becomes unstable.
V will fall below regulation voltage.
OUT
FB (LT3695)
PG
No effect.
GND
Output maintains regulation, but potential degradation of device performance.
BD (LT3695)
V
may fall below regulation voltage. If BD is not connected, the boost capacitor cannot be charged and thus the power
OUT
switch cannot saturate properly, which increases its power dissipation.
OUT1, OUT2
(LT3695-3.3, LT3695-5)
No effect.
BOOST
V
may fall below regulation voltage. If BOOST is not connected, the boost capacitor cannot be charged and thus the power
OUT
switch cannot saturate properly, which increases its power dissipation.
3695fa
23
LT3695 Series
APPLICATIONS INFORMATION
+ R2, and the BD pin (if connected to V ) in the case of
Without load (R
= ∞) and assuming the minimum
OUT
LOAD
the LT3695, or by R
, R4 and the OUT1,2 pins in the
current of 43ꢀA into the OUT1,2 pins, this leads to:
LOAD
case of the LT3695-3.3 and LT3695-5. Since this causes
VOUT
V
OUT
to rise, the LT3695 regulators stop switching. The
R4 ≤
VIN(MAX) – VOUT
resistive divider formed by R3, R
and R1 + R2 and
OUT
LOAD
– 43µA
R4, respectively, must be adjusted for V
not to exceed
R3
its nominal value at the required maximum input voltage
as upper limit for R4. Depending on the required input
voltage range, R4 may be omitted.
V
. R3 must supply sufficient current into RUN/SS
IN(MAX)
at the required minimum input voltage V
for normal
IN(MIN)
non-fault situations. Based on the maximum RUN/SS cur-
rent of 7.5μA at V = 2.5V this gives
Tables 7 and 8 show example values for common appli-
cations. R must be included as the switch node would
RUN/SS
SS
otherwisehavetochargeC iftheSWpinandtheRUN/SS
SS
V
IN(MIN) – 2.5V
pin are shorted, which may damage the power switch.
R3≤
7.5µA
IfRUN/SSiscontrolledbyanexternalcircuitry, thecurrent
this circuitry can supply must be limited. This can be done
as discussed above. In addition, it may be necessary to
protect this external circuitry from the voltage at SW, for
example by using a diode.
ThecurrentthroughR3ismaximalatV
shorted to SW:
withRUN/SS
IN(MAX)
V
IN(MAX) – VOUT
IR3 =
R3
Table 7. LT3695: Example Values for R1, R2 and R3 for Common
Combinations of VIN and VOUT. IR1+R2 is the Current Drawn by
R1 + R2 in Normal Operation
For the LT3695, this current must be drawn by R
,
LOAD
R1 + R2, and the BD pin, if connected to V
:
OUT
V
V
V
R3
R1
R2
I
R1+R2
IN(MAX)
IN(MIN)
OUT
(V)
(V)
(V)
1.8
1.8
2.5
2.5
3.3
3.3
5
(kΩ)
(kΩ)
(kΩ)
(μA)
VOUT
IR3
≤
+ IBD
16
36
16
36
16
36
16
36
16
36
27
36
3.8
3.8
4.5
4.5
5.3
5.3
7
169
169
261
261
365
365
274
590
200
475
301
442
11.5
4.75
93.1
16.9
432
43.2
536
221
562
280
511
511
9.09
3.74
43.2
7.87
137
87
212
18
101
6
RLOAD || R1+ R2
(
)
Without load (R
= ∞) and assuming the minimum
LOAD
current of 35μA into the BD pin, this leads to
VOUT
IN(MAX) – VOUT
13.7
102
58
8
R1+ R2≤
V
– 35µA
R3
7
5
42.2
61.9
30.9
36.5
36.5
19
13
26
22
22
10
10
14
14
8
as upper limit for the feedback resistors. For V
< 2.5V
OUT
8
assume no current drawn by the BD pin, which gives
12
12
VOUT •R3
IN(MAX) – VOUT
R1+ R2≤
V
For the LT3695-3.3 and LT3695-5, the current through R3
must be drawn by R , R4 and the OUT1,2 pins:
LOAD
VOUT
RLOAD ||R4
IR3
≤
+ IOUT1,2
3695fa
24
LT3695 Series
APPLICATIONS INFORMATION
Table 8. LT3695-3.3, LT3695-5: Example Values for R3 and R4
for Common Combinations of VIN and VOUT. IR4 is the Current
Drawn by R4 in Normal Operation
The recommended connection for SYNC is shown in
Figure 13. If SYNC is to be driven by an external circuitry,
R may be used to isolate this circuitry from V . C must
S
IN
S
V
V
V
R3
R4
I
R4
IN(MAX)
IN(MIN)
OUT
be used in this case to provide a low impedance path
(V)
(V)
(V)
3.3
3.3
3.3
5
(kΩ)
(kΩ)
(μA)
for the synchronization signal. If SYNC is pulled low, R
S
16
24
36
16
24
36
5.3
5.3
5.3
7
309
365
365
267
464
590
None
215
prevents V from being shorted to ground in case of
IN
15
50
an inadvertent short between SYNC and V . If SYNC is
IN
66.5
None
None
442
pulled high to V , then R protects the RT pin during an
IN
S
inadvertent short between SYNC and RT.
7
5
If the DA pin or the PGND pin are inadvertently left float-
ing, the current path of the catch diode is interrupted
unless a bypass resistor is connected from DA to ground.
Use a 360mΩ (5% tolerance) resistor rated for a power
dissipation of:
7
5
11
The BOOST pin must not be shorted to a low impedance
node like V that clamps its voltage. For best fault toler-
OUT
anceoftheLT3695, supplycurrentintotheBDpinthrough
the Schottky diode D2 as shown in Figure 10. Note that
this diode must be able to handle the maximum output
current in case there is a short between the BD pin and
the GND pin.
2
P = I
• 0.36 • (1 – DC
)
LOAD(MAX)
MIN
where I
is the maximum load current and DC
MIN
LOAD(MAX)
istheminimumdutycycle.Forexample,thiswouldrequire
a power rating of at least 219mW for an output current of
800mA and a minimum duty cycle of 5%. Make sure not
A short between RUN/SS and SW may also increase the
output ripple. To suppress this, connect the soft-start
to exceed V
(see Input Voltage Range section for
IN(MAX)
network consisting of R and C to RUN/SS as shown
SS
SS
details) during start-up or overload conditions.
in Figure 10. C should not be smaller than 0.22μF.
SS
Other Linear Technology Publications
The SYNC pin must not be directly connected to either
ground or V . A short between RT and a SYNC pin that
IN
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
is connected to V could violate the absolute maximum
IN
ratings of the RT pin. A short between the SYNC pin and
the V pin could damage an external driver circuit which
IN
may be connected to SYNC or would short V to ground
IN
if SYNC is grounded.
V
IN
R
S
V
IN
100k
SYNC
SYNC
C
S
LT3695
LT3695-3.3
LT3695-5
100pF
RT
R
T
3695 F13
Figure 13. The Dashed Lines Show Where a Connection Would Occur
if There Were an Inadvertent Short from SYNC to an Adjacent Pin. In
This Case, RS Protects Circuitry Connecting to SYNC
3695fa
25
LT3695 Series
TYPICAL APPLICATIONS
Fully Tolerant 3.3V Step-Down Converter with Soft-Start
V
V
OUT
IN
5V TO 28.5V
3.3V
D2
TRANSIENT TO 36V
0.9A, V > 5V
1A, V > 6.5V
324k
IN
IN
V
BD
BOOST
IN
B140
L
RUN/SS
10μH
0.22μF
V
C
SW
D1
B140
LT3695
RT
2.2μF
PG
DA
FB
56.2k
47Ω
14k
40.2k
SYNC
0.22μF
470pF 100k
0.36Ω
17.8k
10μF
GND PGND
3695 TA02
f = 800kHz
1.8V Step-Down Converter
V
IN
3.6V TO 25V
V
BD
IN
4.7μF
L1
ON OFF
71.5k
RUN/SS
BOOST
6.8μH
0.22μF
V
1.8V
1A
OUT
V
C
SW
D1
B140
LT3695
RT
PG
DA
FB
127k
17.4k
330pF
SYNC
GND PGND
102k
22μF
3695 TA03
f = 500kHz
3695fa
26
LT3695 Series
TYPICAL APPLICATIONS
Fully Tolerant 5V Step-Down Converter with Soft-Start
V
IN
10V TO 16.5V
TRANSIENT TO 36V
365k
V
IN
L
RUN/SS
BOOST
SW
4.7μH
0.22μF
V
OUT
5V
V
C
0.9A
D1
B140
LT3695-5
RT
2.2μF
PG
DA
OUT1
0UT2
56.2k
47Ω
13.3k
680pF
9.76k
SYNC
100k
0.22μF
0.36Ω
10μF
GND PGND
3695 TA04
f = 2MHz
3695fa
27
LT3695 Series
PACKAGE DESCRIPTION
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev A)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
2.845 p 0.102
(.112 p .004)
0.889 p 0.127
(.035 p .005)
1
8
0.35
REF
5.23
(.206)
MIN
1.651 p 0.102
(.065 p .004)
1.651 p 0.102
(.065 p .004)
3.20 – 3.45
(.126 – .136)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
16
9
0.305 p 0.038
0.50
(.0197)
BSC
NO MEASUREMENT PURPOSE
4.039 p 0.102
(.159 p .004)
(NOTE 3)
(.0120 p .0015)
TYP
0.280 p 0.076
(.011 p .003)
RECOMMENDED SOLDER PAD LAYOUT
16151413121110
9
REF
DETAIL “A”
0o – 6o TYP
0.254
(.010)
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
1 2 3 4 5 6 7 8
DETAIL “A”
0.86
(.034)
REF
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 p 0.0508
(.004 p .002)
MSOP (MSE16) 0608 REV A
0.50
(.0197)
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
3695fa
28
LT3695 Series
REVISION HISTORY
REV
DATE
DESCRIPTION
PAGE NUMBER
A
11/09 All Sections Revised to Include LT3695-3.3 and LT3695-5
1-30
3695fa
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
29
LT3695 Series
TYPICAL APPLICATION
5V Step-Down Converter
V
V
IN
OUT
6.9V TO 36V
5V
0.9A, V > 6.9V
TRANSIENT TO 60V
IN
IN
V
BD
BOOST
IN
1A, V > 12V
2.2μF
ON OFF
40.2k
RUN/SS
0.22μF
10μH
V
SW
C
D1
B140
LT3695
RT
PG
DA
FB
536k
16.2k
470pF
SYNC
GND PGND
102k
10μF
3695 TA05
f = 800kHz
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
V : 4V to 40V Transient to 60V, V
LT3970
LT3689
LT3685
LT3684
LT3682
LT3508
LT3507
LT3505
LT3500
LT3493
LT3481
LT3480
LT3437
40V, 350mA, 2MHz High Efficiency MicroPower Step-Down DC/DC
Converter
= 1.21V, I = 2μA,
OUT(MAX) Q
IN
I
< 1μA, 3mm × 2mm DFN-10, MSOP-10 Packages
SD
36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency MicroPower V : 3.6V to 36V Transient to 60V, V
Step-Down DC/DC Converter with POR Reset and Watchdog Timer
= 0.8V,
IN
OUT(MAX)
I = 75μA, I < 1μA, 3mm × 3mm QFN-16 Package
Q SD
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High Efficiency
Step-Down DC/DC Converter
V : 3.6V to 38V, V
= 0.78V, I = 70μA, I < 1μA,
OUT
IN
OUT(MAX) Q SD
3mm × 3mm DFN-10, MSOP-10E Packages
34V with Transient Protection to 36V, 2A (I ), 2.8MHz, High Efficiency
Step-Down DC/DC Converter
V : 3.6V to 34V, V = 1.26V, I = 850μA, I < 1μA,
OUT
IN
OUT(MAX)
Q
SD
3mm × 3mm DFN-10, MSOP-10E Packages
36V, 60V
, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V : 3.6V to 36V, V = 0.8V, I = 75μA, I < 1μA,
MAX
IN
OUT(MAX)
Q
SD
Converter
3mm × 3mm DFN-12 Package
36V with Transient Protection to 40V, Dual 1.4A (I ), 3MHz, High
Efficiency Step-Down DC/DC Converter
V : 3.7V to 36V, V = 0.8V, I = 4.6mA, I = 1μA,
OUT
IN
OUT(MAX)
Q
SD
4mm × 4mm QFN-24, TSSOP-16E Packages
36V 2.5MHz, Triple (2.4A + 1.5A + 1.5A (I )) with LDO Controller High
Efficiency Step-Down DC/DC Converter
V : 4V to 36V, V = 0.8V, I = 7mA, I = 1μA,
OUT
IN
OUT(MAX)
Q
SD
5mm × 7mm QFN-38 Package
36V with Transient Protection to 40V, 1.4A (I ), 3MHz, High Efficiency
Step-Down DC/DC Converter
V : 3.6V to 34V, V = 0.78V, I = 2mA, I = 2μA,
OUT
IN
OUT(MAX)
Q
SD
3mm × 3mm DFN-8, MSOP-8E Packages
36V, 40V
, 2A, 2.5MHz High Efficiency Step-Down DC/DC Converter and V : 3.6V to 36V, V
= 0.8V, I = 2.5mA, I < 10μA,
OUT(MAX) Q SD
MAX
IN
LDO Controller
3mm × 3mm DFN-10 Package
36V, 1.4A (I ), 750kHz High Efficiency Step-Down DC/DC Converter
V : 3.6V to 36V, V
= 0.8V, I = 1.9mA, I < 1μA,
Q SD
OUT
IN
OUT(MAX)
2mm × 3mm DFN-6 Package
34V with Transient Protection to 36V, 2A (I ), 2.8MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
V : 3.6V to 34V, V = 1.26V, I = 50μA, I < 1μA,
OUT
IN
OUT(MAX)
Q
SD
3mm × 3mm DFN-10, MSOP-10E Packages
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
V : 3.6V to 38V, V = 0.78V, I = 70μA, I < 1μA,
OUT
IN
OUT(MAX)
Q
SD
3mm × 3mm DFN-10, MSOP-10E Packages
60V, 400mA (I ), MicroPower Step-Down DC/DC Converter with Burst
V : 3.3V to 60V, V = 1.25V, I = 100μA, I < 1μA,
OUT
IN
OUT(MAX)
Q
SD
Mode Operation
3mm × 3mm DFN-10, TSSOP-16E Package
LT3434/LT3435 60V, 2.4A (I ), 200kHz/500kHz, High Efficiency Step-Down DC/DC
V : 3.3V to 60V, V = 1.2V, I = 100μA, I < 1μA,
OUT
IN
OUT(MAX)
Q
SD
Converter with Burst Mode Operation
TSSOP-16E Package
LT1976/LT1977 60V, 1.2A (I ), 200kHz/500kHz, High Efficiency Step-Down DC/DC
V : 3.3V to 60V, V
= 1.2V, I = 100μA, I < 1μA,
Q SD
OUT
IN
OUT(MAX)
Converter with Burst Mode Operation
TSSOP-16E Package
LT1936
LT1766
36V, 1.4A (I ), 500kHz High Efficiency Step-Down DC/DC Converter
V : 3.6V to 36V, V
= 1.2V, I = 1.9mA, I < 1μA,
Q SD
OUT
IN
OUT(MAX)
MS8E Package
60V, 1.2A (I ), 200kHz, High Efficiency Step-Down DC/DC Converter
V : 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 25μA,
Q SD
OUT
IN
OUT(MAX)
TSSOP-16/E Package
3695fa
LT 1109 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
30
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●
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(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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