LT3685
36V, 2A, 2.4MHz Step-Down
Switching Regulator
FEATURES
■
DESCRIPTION
The LT
®
3685 is an adjustable frequency (200kHz to 2.4MHz)
monolithic step-down switching regulator that accepts
input voltages up to 38V operating and 60V maximum. An
internal overvoltage protection circuit turns off the power
switch when V
IN
is above 38V typical (36V minimum) which
then allows the part to safely withstand 60V transients. A
high efficiency 0.25 switch is included on the die along
with a boost Schottky diode and the necessary oscillator,
control, and logic circuitry. Current mode topology is
used for fast transient response and good loop stability.
The LT3685’s high operating frequency allows the use of
small, low cost inductors and ceramic capacitors result-
ing in low output ripple while keeping total solution size
to a minimum. The low current shutdown mode 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). A power good flag signals when
V
OUT
reaches 89% of the programmed output voltage. The
LT3685 is available in 10-Pin MSOP and 3mm
×
3mm DFN
packages with exposed pads for low thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
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Wide Input Range:
Operation From 3.6V to 36V
Overvoltage Lockout Protects Circuit through
60V Transients
2A Maximum Output Current
Adjustable Switching Frequency: 200kHz to 2.4MHz
Low Shutdown Current: I
Q
< 1μA
Integrated Boost Diode
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Saturating Switch Design: 0.25 On-Resistance
0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 20V
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
(3mm
×
3mm) DFN Packages
APPLICATIONS
■
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Automotive Battery Regulation
Set Top Box
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
TYPICAL APPLICATION
3.3V Step-Down Converter
V
IN
4.5V TO 36V
TRANSIENT
TO 60V
OFF ON
14k
4.7μF
470pF
40.2k
V
C
R
T
PG
SYNC
316k
GND
FB
100k
22μF
50
0
3685 TA01
Efficiency
V
OUT
3.3V
2A
100
V
OUT
= 5V
90
EFFICIENCY (%)
V
OUT
= 3.3V
V
IN
RUN/SS
BD
BOOST
0.47μF 4.7μH
LT3685
SW
80
70
60
V
IN
= 12V
L = 5.6μH
F = 800 kHz
0.5
1.0
1.5
LOAD CURRENT (A)
2
3685 TA01b
3685fb
1
LT3685
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, RUN/SS Voltage (Note 5) ...................................60V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V
C
Voltage .......................................................5V
PG, BD, SYNC Voltage ..............................................30V
Operating Junction Temperature Range (Note 2)
LT3685E ............................................. –40°C to 125°C
LT3685I .............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
BD
BOOST
SW
V
IN
RUN/SS
1
2
3
4
5
11
10 R
T
9 V
C
8 FB
7 PG
6 SYNC
BD
BOOST
SW
V
IN
RUN/SS
1
2
3
4
5
10
9
8
7
6
R
T
V
C
FB
PG
SYNC
11
DD PACKAGE
10-LEAD (3mm 3mm) PLASTIC DFN
JA
= 45°C/W,
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
MSE PACKAGE
10-LEAD PLASTIC MSOP
JA
= 45°C/W,
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3685EDD#PBF
LT3685IDD#PBF
LT3685EMSE#PBF
LT3685IMSE#PBF
TAPE AND REEL
LT3685EDD#TRPBF
LT3685IDD#TRPBF
LT3685EMSE#TRPBF
LT3685IMSE#TRPBF
PART MARKING
LCYG
LCYG
LTCYF
LTCYF
PACKAGE DESCRIPTION
10-Lead (3mm
×
3mm) Plastic DFN
10-Lead (3mm
×
3mm) Plastic DFN
10-Lead Plastic MSOP
10-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
*For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
V
IN
Overvoltage Lockout
Quiescent Current from V
IN
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 10V, V
RUN/SS
= 10V, V
BOOST
= 15V, V
BD
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
●
●
MIN
36
TYP
3
38
0.01
450
1.3
MAX
3.6
40
0.5
600
1.7
UNITS
V
V
μA
μA
mA
V
RUN/SS
= 0.2V
V
BD
= 3V, Not Switching
V
BD
= 0, Not Switching
●
3685fb
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LT3685
ELECTRICAL CHARACTERISTICS
PARAMETER
Quiescent Current from BD
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 10V, V
RUN/SS
= 10V V
BOOST
= 15V, V
BD
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
V
RUN/SS
= 0.2V
V
BD
= 3V, Not Switching
V
BD
= 0, Not Switching
●
MIN
TYP
0.01
0.9
1
2.7
MAX
0.5
1.3
5
3
800
805
30
0.01
UNITS
μA
mA
μA
V
mV
mV
nA
%/V
μMho
μA
μA
A/V
V
Minimum Bias Voltage (BD Pin)
Feedback Voltage
●
780
775
790
790
7
0.002
500
1000
45
45
3.5
2
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
Error Amp g
m
Error Amp Gain
V
C
Source Current
V
C
Sink Current
V
C
Pin to Switch Current Gain
V
C
Clamp Voltage
Switching Frequency
V
C
= 1.2V
4V < V
IN
< 36V
●
R
T
= 8.66k
R
T
= 29.4k
R
T
= 187k
●
2.1
0.9
160
3.2
2.4
1
200
60
3.7
500
0.02
2.7
1.15
240
150
4.2
2
2.1
35
10
2.5
MHz
MHz
kHz
nS
A
mV
μA
V
mA
μA
V
V
mV
mV
Minimum Switch Off-Time
Switch Current Limit
Switch V
CESAT
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
RUN/SS Pin Current
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
PG Hysteresis
PG Leakage
PG Sink Current
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Bias Current
V
SYNC
= 0V
V
PG
= 5V
V
PG
= 0.4V
V
FB
Rising
I
SW
= 1A
V
RUN/SS
= 2.5V
Duty Cycle = 5%
I
SW
= 2A
V
SW
= 10V, V
BD
= 0V
●
1.5
22
5
0.2
90
12
0.1
1
μA
μA
V
●
100
0.5
600
0.7
0.1
V
μA
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LT3685E 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 LT3685I specifications are
guaranteed over the –40°C to 125°C temperature range.
Note 3:
Bias current flows out of the FB pin.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5:
Absolute Maximum Voltage at V
IN
and RUN/SS pins is 60V for
nonrepetitive 1 second transients, and 40V for continuous operation.
3685fb
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LT3685
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
100
V
IN
= 12V
90
85
80
EFFICIENCY (%)
EFFICIENCY (%)
80
V
IN
= 24V
70
V
IN
= 34V
V
IN
= 34V
75
70
65
60
L: NEC PLC-0745-5R6
f: 800kHz
55
50
0
V
OUT
= 3.3V
1.5
L: NEC PLC-0745-5R6
f: 800kHz
1.0
5
10
V
OUT
= 3.3V
L = 4.7μH
f = 800 kHz
15
20
INPUT VOLTAGE (V)
25
30
3685 G03
T
A
= 25°C unless otherwise noted.
Efficiency
V
IN
= 7V
V
IN
= 12V
LOAD CURRENT (A)
4.0
3.5
3.0
2.5
Maximum Load Current
90
TYPICAL
V
IN
= 24V
MINIMUM
2.0
60
V
OUT
= 5V
0
50
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
3685 G01
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
3685 G02
Maximum Load Current
3.5
TYPICAL
SWITCH CURRENT LIMIT(A)
3.0
LOAD CURRENT (A)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Switch Current Limit
4.5
4.0
SWITCH CURRENT LIMIT (A)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
20
60
40
DUTY CYCLE (%)
80
100
3685 G05
Switch Current Limit
DUTY CYCLE = 10 %
2.5
MINIMUM
2.0
DUTY CYCLE = 90 %
1.5
1.0
5
10
20
15
INPUT VOLTAGE (V)
V
OUT
= 5V
L = 4.7μH
f = 800kHz
25
30
3685 G04
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G06
Switch Voltage Drop
700
600
BOOST PIN CURRENT (mA)
VOLTAGE DROP (mV)
500
400
300
200
100
0
0
500
1000
2000
1500
SWITCH CURRENT (mA)
2500
3685 G07
Boost Pin Current
80
70
60
50
40
30
20
10
0
0
500
1000
1500
2000
SWITCH CURRENT (mA)
2500
3685 G08
3685fb
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LT3685
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage
840
1.20
1.15
820
FREQUENCY (MHz)
1.10
1.05
1.00
0.95
0.90
0.85
760
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G09
T
A
= 25°C unless otherwise noted.
Switching Frequency
R
T
= 29.4k
SWITCHING FREQUENCY (kHz)
1200
1000
800
600
400
200
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G10
Frequency Foldback
R
T
= 29.4k
FEEDBACK VOLTAGE (mV)
800
780
0.80
–50 –25
0
100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
3685 G11
Minimum Switch On-Time
140
MINIMUM SWITCH ON TIME (ns)
120
SWITCH CURRENT LIMIT (A)
100
80
60
40
20
0
–50 –25
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
25 50 75 100 125 150
TEMPERATURE (˚C)
3685 G12
Soft-Start
12
10
8
6
4
2
0
RUN/SS Pin Current
0
0.5
2.5
2
1.5
RUN/SS PIN VOLTAGE (V)
1
3
3.5
3685 G13
RUN/SS PIN CURRENT (μA)
0
5
20
30
15
25
10
RUN/SS PIN VOLTAGE (V)
35
3685 G14
Boost Diode
1.4
1.2
BOOST DIODE V
f
(V)
V
C
PIN CURRENT (μA)
0
0.5
1.0
1.5
BOOST DIODE CURRENT (A)
2.0
3685 G15
Error Amp Output Current
50
40
30
1.0
0.8
0.6
0.4
0.2
0
20
10
0
–10
–20
–30
–40
–50
–200
–100
100
0
FB PIN ERROR VOLTAGE (V)
200
3685 G16
3685fb
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