LT3976
40V, 5A, 2MHz Step-Down
Switching Regulator with
3.3µA Quiescent Current
FeaTures
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DescripTion
The
LT
®
3976
is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 40V. Low quiescent current design consumes only
3.3µA of supply current while regulating with no load. Low
ripple Burst Mode operation maintains high efficiency at
low output currents while keeping the output ripple below
15mV in a typical application. The LT3976 can supply up
to 5A of load current and has current limit foldback to
limit power dissipation during short-circuit. A low dropout
voltage of 500mV is maintained when the input voltage
drops below the programmed output voltage, such as
during automotive cold crank.
An internally compensated current mode topology is used
for fast transient response and good loop stability. A high
efficiency 75mΩ switch is included on the die along with a
boost Schottky diode. An accurate 1.02V threshold enable
pin can be driven directly from a microcontroller or used
as a programmable undervoltage lockout. A capacitor on
the SS pin provides a controlled inrush current (soft-start).
A power good flag signals when V
OUT
reaches 91.6% of
the programmed output voltage. The LT3976 is available
in small 16-lead MSOP and 24-lead 3mm × 5mm QFN
packages with exposed pad for low thermal resistance.
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Ultralow Quiescent Current:
3.3µA I
Q
at 12V
IN
to 3.3V
OUT
Low Ripple Burst Mode
®
Operation
Output Ripple < 15mV
P-P
Wide Input Range: Operation from 4.3V to 40V
5A Maximum Output Current
Excellent Start-Up and Dropout Performance
Adjustable Switching Frequency: 200kHz to 2MHz
Synchronizable Between 250kHz to 2MHz
Accurate Programmable Undervoltage Lockout
Low Shutdown Current: I
Q
= 700nA
Power Good Flag
Soft-Start Capability
Thermal Shutdown Protection
Current Limit Foldback with Soft-Start Override
Saturating Switch Design: 75mΩ On-Resistance
Small, Thermally Enhanced 16-Lead MSOP and
24-Lead 3mm × 5mm QFN Packages
Automotive Battery Regulation
Portable Products
Industrial Supplies
applicaTions
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L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Typical applicaTion
3.3V Step-Down Converter
V
IN
4.3V TO 40V
OFF ON
EN
PG
10µF
LT3976
SS
10nF
RT
SYNC
130k
f = 400kHz
OUT
FB
GND
BOOST
SW
INPUT CURRENT (µA)
8
7
No-Load Supply Current
IN REGULATION
V
OUT
= 3.3V
V
IN
6
5
4
3
2
1
0
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
0.47µF
PDS540
2
3.3µH
470pF
1M
10pF
576k
3976 TA01a
V
OUT
3.3V
5A
47µF
1210
×2
3976 G05
3976f
For more information
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1
LT3976
absoluTe MaxiMuM raTings
(Note 1)
V
IN
, EN Voltage (Note 3) ...........................................40V
BOOST Pin Voltage ...................................................55V
BOOST Pin Above SW Pin.........................................30V
FB, RT, SYNC, SS Voltage ...........................................6V
PG Voltage ................................................................30V
OUT Voltage ..............................................................16V
Operating Junction Temperature Range (Note 2)
LT3976E............................................. –40°C to 125°C
LT3976I ............................................. –40°C to 125°C
LT3976H ............................................ –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
pin conFiguraTion
TOP VIEW
GND
20 SYNC
19 PG
18 RT
25
GND
17 NC
16 EN
15 VIN
14 VIN
13 VIN
9 10 11 12
NC
NC
NC
NC
NC
FB
TOP VIEW
FB
SS
OUT
BOOST
SW
SW
SW
NC
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
SYNC
PG
RT
EN
V
IN
V
IN
V
IN
NC
SS 1
OUT 2
NC 3
BST 4
NC 5
SW 6
SW 7
SW 8
17
GND
MSE PACKAGE
16-LEAD PLASTIC MSOP
θ
JA
= 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
FB
24 23 22 21
UDD PACKAGE
24-LEAD (3mm
×
5mm) PLASTIC QFN
θ
JA
= 46°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
orDer inForMaTion
LEAD FREE FINISH
LT3976EMSE#PBF
LT3976IMSE#PBF
LT3976HMSE#PBF
LT3976EUDD#PBF
LT3976IUDD#PBF
TAPE AND REEL
LT3976EMSE#TRPBF
LT3976IMSE#TRPBF
LT3976HMSE#TRPBF
LT3976EUDD#TRPBF
LT3976IUDD#TRPBF
PART MARKING*
3976
3976
3976
LGHV
LGHV
PACKAGE DESCRIPTION
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
24-Lead (3mm × 5mm) Plastic QFN
24-Lead (3mm × 5mm) Plastic QFN
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
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/
3976f
2
For more information
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LT3976
elecTrical characTerisTics
PARAMETER
Minimum Input Voltage
Dropout Comparator Threshold
Dropout Comparator Threshold Hysteresis
Quiescent Current from V
IN
FB Pin Current
Feedback Voltage
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2)
CONDITIONS
(Note 3)
(V
IN
– OUT) Falling
V
EN
Low
V
EN
High, V
SYNC
Low
V
EN
High, V
SYNC
Low
V
FB
= 1.5V
l
MIN
430
TYP
4
500
25
0.7
1.6
MAX
4.3
570
1.3
2.7
30
12
1.212
1.222
0.01
2.7
1.2
240
200
12.5
UNITS
V
mV
mV
µA
µA
µA
nA
V
V
%/V
MHz
MHz
kHz
ns
ns
A
A
mV
l
l
0.1
1.183
1.173
1.8
0.8
160
1.197
1.197
0.0003
2.25
1
200
120
150
FB Voltage Line Regulation
Switching Frequency
4.3V < V
IN
< 40V (Note 3)
R
T
= 11.8k
R
T
= 41.2k
R
T
= 294k
Minimum Switch On-Time
Minimum Switch Off-Time (Note 4)
Switch Current Limit
Foldback Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 5)
BOOST Pin Current
EN Voltage Threshold
EN Voltage Hysteresis
EN Pin Current
PG Threshold Offset from V
FB
PG Hysteresis as % of Output Voltage
PG Leakage
PG Sink Current
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Current
SS Source Current
V
SYNC
= 6V
V
SS
= 0.5V
0.9
V
PG
= 3V
V
PG
= 0.4V
l
V
FB
= 1V
V
FB
= 0V
I
SW
= 1A
I
SH
= 100mA
V
REVERSE
= 12V
l
7.5
10
4.8
80
0.02
730
0.02
1.3
20
1
2
1.8
32
1.12
20
13
1
μA
mV
μA
V
mA
V
mV
nA
%
%
µA
μA
V
I
SW
= 1A, V
BOOST
– V
SW
= 3V
EN Falling, V
IN
≥ 4.3V
l
0.92
1.02
60
0.2
V
FB
Falling
5
8.4
1.7
0.02
125
0.6
480
1.0
1.18
0.1
1.8
2.6
1.5
V
nA
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 LT3976E is 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
LT3976I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3976H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C. The junction temperature (T
J
, in °C) is
calculated from the ambient temperature (T
A
, in °C) and power dissipation
(P
D
, in Watts) according to the formula:
T
J
= T
A
+ (P
D
•
θ
JA
)
where
θ
JA
(in °C/W) is the package thermal impedance.
3976f
For more information
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3
LT3976
elecTrical characTerisTics
Note 3:
Minimum input voltage depends on application circuit.
Note 4:
The LT3976 contains circuitry that extends the maximum duty
cycle if there is sufficient voltage across the boost capacitor. See the
Application Information section for more details.
Note 5:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 6:
This IC includes overtemperature protection that is intended
to protect the device 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 or permanently damage the device.
Typical perForMance characTerisTics
Efficiency at 5V
OUT
100
95
90
EFFICIENCY (%)
EFFICIENCY (%)
85
80
75
70
65
60
55
50
0
0.5
1
1.5 2 2.5 3 3.5
LOAD CURRENT (A)
4
12V
24V
36V
4.5
5
f
SW
= 800kHz
V
OUT
= 5V
100
95
90
T
A
= 25°C, unless otherwise noted.
Efficiency at 5V
OUT
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
5
0
0.01
0.1
10
100
1
LOAD CURRENT (mA)
12V
24V
36V
1000 10000
3976 G03
Efficiency at 3.3V
OUT
FRONT PAGE APPLICATION
V
OUT
= 3.3V
f
SW
= 800kHz
V
OUT
= 5V
85
80
75
70
65
60
55
50
0
0.5
1
1.5 2 2.5 3 3.5
LOAD CURRENT (A)
4
12V
24V
36V
4.5
3976 G01
3976 G02
Efficiency at 3.3V
OUT
100
90
80
INPUT CURRENT (µA)
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.01
0.1
10
100
1
LOAD CURRENT (mA)
12V
24V
36V
1000 10000
3976 G04
No-Load Supply Current
8
7
6
5
4
3
2
1
0
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
IN REGULATION
V
OUT
= 3.3V
INPUT CURRENT (µA)
10000
No-Load Supply Current
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 3.3V
DUE TO
CATCH DIODE
LEAKAGE
FRONT PAGE APPLICATION
V
OUT
= 3.3V
1000
100
10
1
–55
–25
5
35
95
65
TEMPERATURE (°C)
125
155
3976 G05
3876 G06
3976f
4
For more information
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LT3976
Typical perForMance characTerisTics
Reference Voltage
1.230
1.225
1.220
REFERENCE VOLTAGE (V)
CHANGE IN OUTPUT (%)
1.215
1.210
1.205
1.200
1.195
1.190
1.185
1.180
1.175
1.170
–55
–25
65
35
5
95
TEMPERATURE (°C)
125
155
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
0
1
3
2
LOAD CURRENT (A)
4
5
3976 G08
T
A
= 25°C, unless otherwise noted.
Load Regulation
V
IN
= 12V
V
OUT
= 3.3V
CHANGE IN OUTPUT (%)
0.05
0.04
0.03
0.02
0.01
0
Line Regulation
V
OUT
= 5V
LOAD = 1A
–0.01
–0.02
–0.03
–0.04
–0.05
5
10
25
20
15
30
INPUT VOLTAGE (V)
35
40
3976 G07
3976 G09
Thermal Derating
6
5
LOAD CURRENT (A)
4
3
2
1 V
OUT
= 3.3V
f
SW
= 400kHz
2.5in × 2.5in 4-LAYER BOARD
0
0
25
75
50
100
TEMPERATURE (°C)
LIMITED BY MAXIMUM
JUNCTION TEMPERATURE
Θ
JA
= 40°C/W
6
5
LOAD CURRENT (A)
Thermal Derating
10.0
9.5
CURRENT LIMIT (A)
12V
24V
36V
H-GRADE
9.0
8.5
8.0
7.5
7.0
Switch Current Limit
12V
24V
36V
H-GRADE
4
3
2
I-GRADE
I-GRADE
125
150
3976 G10
1 V
OUT
= 5V
f
SW
= 400kHz
2.5in × 2.5in 4-LAYER BOARD
0
0
25
75
50
100
TEMPERATURE (°C)
LIMITED BY MAXIMUM
JUNCTION TEMPERATURE
Θ
JA
= 40°C/W
125
150
3976 G11
0
0.2
0.4
0.6
DUTY CYCLE
0.8
1.0
3976 G12
Switch Current Limit
12
11
CURRENT LIMIT (A)
10
9
8
7
6
–55 –25
CURRENT LIMIT (A)
10
Current Limit Foldback
9
8
CURRENT LIMIT (A)
7
6
5
4
3
2
1
0
0
0.2
0.6
0.8
0.4
FB PIN VOLTAGE (V)
1.0
1.2
3976 G14
Soft-Start
10
9
8
CURRENT LIMIT (A)
7
6
5
4
3
2
1
30% DUTY CYCLE
V
FB
= 1V
V
FB
= 0V
30% DUTY CYCLE
30% DUTY
V
SS
= 3V
CYCLE
10
9
8
7
6
5
4
3
2
1
0
30% DUTY CYCLE
V
SS
= 3V
65
35
95
5
TEMPERATURE (°C)
125
155
0
0.2
0.6
0.8
0.4
FB PIN VOLTAGE (V)
1.0
1.2
3976 G14
0
0
0.5
1.5
2.0
1.0
SS PIN VOLTAGE (V)
2.5
3976 G15
3976 G13
3976f
For more information
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5
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