LT3681
36V, 2A, 2.8MHz Step-Down
Switching Regulator with
Integrated Power Schottky Diode
FEATURES
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DESCRIPTIO
■
■
■
■
■
■
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■
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Wide Input Voltage Range: 3.6V to 34V Operating,
36V Maximum
2A Maximum Output Current
Low Ripple Burst Mode
®
Operation
50µA I
Q
at 12V
IN
to 3.3V
OUT
Output Ripple < 15mV
P-P
Adjustable Switching Frequency: 300kHz to 2.8MHz
Low Shutdown Current: I
Q
< 1µA
Integrated Boost Diode
Integrated Power Schottky Diode
Power Good Flag
Saturating Switch Design: 0.18Ω On-Resistance
1.265V Feedback Reference Voltage
Output Voltage: 1.265V to 20V
Soft-Start Capability
Small 14-Pin Thermally Enhanced
(4mm x 3mm) DFN Package
APPLICATIO S
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Automotive Battery Regulation
Power for Portable Products
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
The LT
®
3681 is an adjustable frequency (300kHz to 2.8MHz)
monolithic buck switching regulator that accepts input
voltages up to 34V (36V maximum). A high efficiency
0.18Ω switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
logic circuitry. An undedicated power Schottky diode is
integrated into the LT3681 to minimize the solution size.
Current mode topology is used for fast transient response
and good loop stability. Low ripple Burst Mode operation
maintains high efficiency at low output currents while
keeping output ripple below 15mV in a typical application.
In addition, the LT3681 can further enhance low output
current efficiency by drawing bias current from the output
when V
OUT
is above 3V. 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). A power good flag signals when V
OUT
reaches
90% of the programmed output voltage. The LT3681
is available in 14-Pin 4mm x 3mm DFN package 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.
TYPICAL APPLICATIO
V
IN
6.3V TO
34V
OFF ON
20k
4.7
m
F
V
C
RT
330pF
PG
60.4k
V
IN
RUN/SS
5V Step-Down Converter
BD
BOOST
0.47
m
F
L
6.8
m
H
V
OUT
5V
2A
100
90
80
70
EFFICIENCY (%)
60
50
40
30
20
590k
GND
FB
200k
22
m
F
10
0
0.0001
0.001
LT3681
SW
DC
DA
BIAS
L: NEC PLC-0745-6R8
3681 TA01
U
Efficiency
10.0000
1.0000
POWER LOSS (W)
0.1000
0.0100
V
IN
= 12V
V
OUT
= 3.3V
L = 4.7
m
H
F = 800 kHz
0.01
0.1
I
LOAD
(A)
1
10
3681 TA01b
U
U
0.0010
0.0001
3681f
1
LT3681
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PG
BIAS
FB
GND
V
C
R
T
GND
1
2
3
4
5
6
7
16
15
14 RUN/SS
13 V
IN
12 SW
11 BOOST
10 BD
9 DC
8 DA
V
IN
, RUN/SS Voltage .................................................36V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V
C
Voltage .......................................................5V
BIAS, PG, BD Voltage ................................................30V
Maximum Junction Temperature .......................... 125°C
DC above DA .............................................................40V
Operating Temperature Range (Note 2)
LT3681E............................................... –40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
DE14MA PACKAGE
14-LEAD (4mm
´
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W
EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB
EXPOSED PAD PIN 16 IS DC
ORDER PART NUMBER
LT3681EDE
DE PART MARKING
3681
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
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
RUNS/SS
= 10V, V
BOOST
= 15V, V
BIAS
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
●
MIN
TYP
3
0.01
22
75
0.01
MAX
3.6
0.5
60
120
0.5
120
5
3
1.29
1.3
100
0.02
UNITS
V
μA
μA
μA
μA
μA
μA
V
V
V
nA
%/V
μMho
μA
μA
A/V
V
V
RUN/SS
= 0.2V
V
BIAS
= 3V, Not Switching
V
BIAS
= 0, Not Switching
●
Quiescent Current from BIAS
V
RUN/SS
= 0.2V
V
BIAS
= 3V, Not Switching
V
BIAS
= 0, Not Switching
●
50
0
2.7
1.25
1.24
1.265
1.265
30
0.002
330
800
65
85
3.5
2
Minimum Bias Voltage
Feedback Voltage
●
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
Error Amp GM
Error Amp Gain
V
C
Source Current
V
C
Sink Current
V
C
Pin to Switch Current Gain
V
C
Clamp Voltage
V
FB
= 1.25V, V
C
= 0.4V
4V < V
IN
< 34V
●
3681f
2
U
W
U
U
W W
W
LT3681
ELECTRICAL CHARACTERISTICS
PARAMETER
Power Schottky Diode Forward Voltage
Power Schottky Diode Leakage Current
Switching Frequency
I
DA
= 1A
I
DA
= 2A
V
DC-DA
= 40V
RT = 8.66k
RT = 29.4k
RT = 187k
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 10V, V
RUNS/SS
= 10V V
BOOST
= 15V, V
BIAS
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
MIN
TYP
0.50
0.56
100
2.5
1.25
250
3.2
2.8
1.4
300
130
3.8
360
0.02
●
MAX
UNITS
V
V
μA
MHz
MHz
kHz
nS
A
mV
µA
V
mA
μA
V
V
mV
mV
3.1
1.55
350
200
4.4
2
2.1
35
10
0.2
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
V
PG
= 5V
V
PG
= 3V
V
FB
Rising
I
SW
= 1A
V
RUN/SS
= 2.5V
Duty Cycle = 5%
I
SW
= 2A
V
SW
= 10V, V
BIAS
= 0V
1.5
18
5
2.5
122
5
0.1
1
μA
μA
●
100
600
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 LT3681E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Bias current flows into the FB pin.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
3681f
3
LT3681
TYPICAL PERFOR A CE CHARACTERISTICS
T
A
= 25°C unless otherwise noted.
Efficiency (V
OUT
= 5.0V)
100
90
80
70
EFFICIENCY (%)
60
50
40
30
20
10
0
0.0001
0.001
L: NEC PLC-0745-4R7
f: 800kHz
0.01
0.1
LOAD CURRENT (A)
1
10
3681 G01
V
IN
= 12V
V
IN
= 24V
EFFICIENCY (%)
60
50
40
30
20
10
0
0.0001
0.001
V
IN
= 24V
V
IN
= 12V
EFFICIENCY (%)
No Load Supply Current
90
80
90
SUPPLY CURRENT (µA)
SUPPLY CURRENT (µA)
70
60
50
40
30
20
10
0
0
FRONT PAGE APPLICATION
5
10
20
25
15
INPUT VOLTAGE (V)
30
35
3681 G04
LOAD CURRENT (A)
Maximum Load Current
4.0
3.5
LOAD CURRENT (A)
3.0
2.5
2.0
1.5
1.0
5
10
20
25
15
INPUT VOLTAGE (V)
30
3681 G07
SWITCH CURRENT LIMIT(A)
SWITCH CURRENT LIMIT (A)
V
OUT
= 5.0V
L = 4.7µH
f = 800 kHz
4
U W
Efficiency (V
OUT
= 3.3V)
100
90
80
70
V
IN
= 7V
90
85
80
Efficiency vs Switching Frequency
V
IN
= 12V
V
IN
= 24V
75
70
65
60
L: NEC PLC-0745-4R7
f: 800kHz
0.01
0.1
LOAD CURRENT (A)
1
10
3681 G02
55
50
0
0.5
V
OUT
= 3.3V
L = 10µH
LOAD = 1A
1
2
2.5
1.5
SWITCHING FREQUENCY (MHz)
3
3681 G03
No Load Supply Current vs
Temperature
100
V
IN
= 12V
V
OUT
= 3.3V
4.0
3.5
INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
3.0
2.5
2.0
1.5
1.0
–20
60
0
20
40
TEMPERATURE (°C)
80
3681 G05
Maximum Load Current
80
70
60
50
40
–40
V
OUT
= 3.3V
L = 4.7µH
f = 800 kHz
5
10
25
20
15
INPUT VOLTAGE (V)
30
3681 G06
Switch Current Limit
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
20
60
40
DUTY CYCLE (%)
80
100
3681 G08
Switch Current Limit
4.5
4.0
3.0
2.5
2.0
1.5
1.0
0.5
0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
DUTY CYCLE = 90 %
DUTY CYCLE = 10 %
3681 G09
3681f
LT3681
TYPICAL PERFOR A CE CHARACTERISTICS
T
A
= 25°C unless otherwise noted.
Switch Voltage Drop
700
600
BOOST PIN CURRENT (mA)
VOLTAGE DROP (mV)
500
400
300
200
100
0
0
500
1000 1500 2000 2500 3000 3500
SWITCH CURRENT (mA)
3681 G10
FEEDBACK VOLTAGE (V)
Switching Frequency
1.20
1.15
1.10
FREQUENCY (MHz)
1.05
1.00
0.95
0.90
0.85
0.80
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
SWITCHING FREQUENCY (kHz)
R
T
= 45.3kΩ
1200
1000
800
600
400
200
0
0
200
400 600 800 1000 1200 1400
FB PIN VOLTAGE (mV)
3681 G14
MINIMUM SWITCH ON TIME (ns)
Soft Start
4.0
3.5
SWITCH CURRENT LIMIT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
0
0.5
2
2.5
1.5
RUN/SS PIN VOLTAGE (V)
1
3
3.5
3681 G16
RUN/SS PIN CURRENT (µA)
8
6
4
2
BOOST DIODE V
f
(V)
U W
Boost Pin Current
90
80
70
60
50
40
30
20
10
0
0
500 1000 1500 2000 2500 3000 3500
SWITCH CURRENT (mA)
3681 G11
Feedback Voltage
1.290
1.285
1.280
1.275
1.270
1.265
1.260
1.255
1.250
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
4681 G12
Frequency Foldback
140
R
T
= 45.3kΩ
120
100
80
60
40
20
Minimum Switch On-Time
0
–50
–25
25
0
50
75
TEMPERATURE (˚C)
100
125
4681 G13
3681 G15
RUN/SS Pin Current
12
10
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
5
20
30
15
25
10
RUN/SS PIN VOLTAGE (V)
35
3681 G17
Boost Diode
0
0
1.0
0.5
1.5
BOOST DIODE CURRENT (A)
2.0
3681 G18
3681f
5
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