LT3976
40V, 5A, 2MHz Step-Down
Switching Regulator with
3.3µA Quiescent Current
Features
Description
Ultralow Quiescent Current:
3.3µA IQ at 12VIN to 3.3VOUT
n Low Ripple Burst Mode® Operation
Output Ripple < 15mVP-P
n Wide Input Range: Operation from 4.3V to 40V
n 5A Maximum Output Current
n Excellent Start-Up and Dropout Performance
n Adjustable Switching Frequency: 200kHz to 2MHz
n Synchronizable Between 250kHz to 2MHz
n Accurate Programmable Undervoltage Lockout
n Low Shutdown Current: I = 700nA
Q
n Power Good Flag
n Soft-Start Capability
n Thermal Shutdown Protection
n Current Limit Foldback with Soft-Start Override
n Saturating Switch Design: 75mΩ On-Resistance
n Small, Thermally Enhanced 16-Lead MSOP and
24-Lead 3mm × 5mm QFN Packages
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.
n
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 VOUT 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.
Applications
Automotive Battery Regulation
Portable Products
n Industrial Supplies
n
n
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
No-Load Supply Current
3.3V Step-Down Converter
8
VIN
4.3V TO 40V
IN REGULATION
VOUT = 3.3V
OFF ON
EN
VIN
BOOST
PG
10µF
SW
SS
10nF
RT
SYNC
130k
f = 400kHz
3.3µH
0.47µF
PDS540
LT3976
INPUT CURRENT (µA)
7
2Ω
470pF
OUT
FB
GND
VOUT
3.3V
5A
1M
10pF
576k
3976 TA01a
47µF
1210
×2
6
5
4
3
2
1
0
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
3976 G05
3976f
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1
LT3976
Absolute Maximum Ratings
(Note 1)
VIN, 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
GND
NC
FB
FB
TOP VIEW
24 23 22 21
SS 1
TOP VIEW
SYNC
PG
RT
EN
VIN
VIN
VIN
NC
19 PG
NC 3
18 RT
BST 4
MSE PACKAGE
16-LEAD PLASTIC MSOP
θJA = 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
17 NC
25
GND
NC 5
16 EN
SW 6
15 VIN
SW 7
14 VIN
SW 8
13 VIN
NC
9 10 11 12
NC
17
GND
16
15
14
13
12
11
10
9
NC
1
2
3
4
5
6
7
8
NC
FB
SS
OUT
BOOST
SW
SW
SW
NC
20 SYNC
OUT 2
UDD PACKAGE
24-LEAD (3mm × 5mm) PLASTIC QFN
θJA = 46°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3976EMSE#PBF
LT3976EMSE#TRPBF
3976
16-Lead Plastic MSOP
–40°C to 125°C
LT3976IMSE#PBF
LT3976IMSE#TRPBF
3976
16-Lead Plastic MSOP
–40°C to 125°C
LT3976HMSE#PBF
LT3976HMSE#TRPBF
3976
16-Lead Plastic MSOP
–40°C to 150°C
LT3976EUDD#PBF
LT3976EUDD#TRPBF
LGHV
24-Lead (3mm × 5mm) Plastic QFN
–40°C to 125°C
LT3976IUDD#PBF
LT3976IUDD#TRPBF
LGHV
24-Lead (3mm × 5mm) Plastic QFN
–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
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LT3976
Electrical
Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 2)
PARAMETER
CONDITIONS
Minimum Input Voltage
(Note 3)
Dropout Comparator Threshold
(VIN – OUT) Falling
MIN
l
430
Dropout Comparator Threshold Hysteresis
Quiescent Current from VIN
FB Pin Current
TYP
MAX
4
4.3
V
500
570
mV
25
VEN Low
VEN High, VSYNC Low
VEN High, VSYNC Low
0.7
1.6
l
VFB = 1.5V
l
Feedback Voltage
l
FB Voltage Line Regulation
4.3V < VIN < 40V (Note 3)
Switching Frequency
RT = 11.8k
RT = 41.2k
RT = 294k
1.183
1.173
1.8
0.8
160
UNITS
mV
1.3
2.7
30
µA
µA
µA
0.1
12
nA
1.197
1.197
1.212
1.222
V
V
0.0003
0.01
%/V
2.25
1
200
2.7
1.2
240
MHz
MHz
kHz
Minimum Switch On-Time
120
Minimum Switch Off-Time (Note 4)
150
200
ns
10
12.5
A
7.5
ns
Switch Current Limit
VFB = 1V
Foldback Switch Current Limit
VFB = 0V
4.8
A
Switch VCESAT
ISW = 1A
80
mV
Switch Leakage Current
0.02
1
μA
Boost Schottky Forward Voltage
ISH = 100mA
730
Boost Schottky Reverse Leakage
VREVERSE = 12V
0.02
2
1.3
1.8
V
20
32
mA
1.02
1.12
Minimum Boost Voltage (Note 5)
l
BOOST Pin Current
ISW = 1A, VBOOST – VSW = 3V
EN Voltage Threshold
EN Falling, VIN ≥ 4.3V
l
0.92
mV
EN Voltage Hysteresis
60
EN Pin Current
0.2
20
8.4
13
PG Threshold Offset from VFB
VFB Falling
5
PG Hysteresis as % of Output Voltage
VPG = 3V
PG Sink Current
VPG = 0.4V
0.02
l
SYNC Low Threshold
SS Source Current
VSS = 0.5V
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
%
%
1
µA
480
μA
0.6
1.0
V
1.18
VSYNC = 6V
nA
125
SYNC High Threshold
SYNC Pin Current
V
mV
1.7
PG Leakage
μA
1.5
0.1
0.9
1.8
V
nA
2.6
μA
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 (TJ, in °C) is
calculated from the ambient temperature (TA, in °C) and power dissipation
(PD, in Watts) according to the formula:
TJ = TA + (PD • θJA)
where θJA (in °C/W) is the package thermal impedance.
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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 3.3VOUT
Efficiency at 5VOUT
fSW = 800kHz
VOUT = 5V
95
90
90
85
85
80
75
70
65
60
50
0.5
0
1
1.5 2 2.5 3 3.5
LOAD CURRENT (A)
4
4.5
90
80
75
70
65
50
0
0.5
1
1.5 2 2.5 3 3.5
LOAD CURRENT (A)
3976 G01
INPUT CURRENT (µA)
EFFICIENCY (%)
70
60
50
40
30
12V
24V
36V
0
0.01
0.1
10
100
1
LOAD CURRENT (mA)
1000 10000
3976 G04
12V
24V
36V
0.1
10
100
1
LOAD CURRENT (mA)
1000 10000
3976 G03
No-Load Supply Current
10000
IN REGULATION
VOUT = 3.3V
7
80
10
0
0.01
5
No-Load Supply Current
8
FRONT PAGE APPLICATION
VOUT = 3.3V
20
30
10
6
INPUT CURRENT (µA)
90
4.5
50
40
3976 G02
Efficiency at 3.3VOUT
100
4
70
60
20
12V
24V
36V
55
5
fSW = 800kHz
VOUT = 5V
80
60
12V
24V
36V
55
Efficiency at 5VOUT
100
FRONT PAGE APPLICATION
VOUT = 3.3V
95
EFFICIENCY (%)
EFFICIENCY (%)
100
EFFICIENCY (%)
100
TA = 25°C, unless otherwise noted.
5
4
3
2
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 3.3V
1000
DUE TO
CATCH DIODE
LEAKAGE
100
10
1
0
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
3976 G05
1
–55
–25
5
35
95
65
TEMPERATURE (°C)
125
155
3876 G06
3976f
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LT3976
Typical Performance Characteristics
Reference Voltage
Load Regulation
1.230
0.5
1.225
0.4
1.220
1.210
1.205
1.200
1.195
1.190
1.185
Line Regulation
0.05
VIN = 12V
VOUT = 3.3V
0.3
0.03
0.2
0.1
0
–0.1
–0.2
1.175
–0.4
1.170
–55
–0.5
–25
65
35
5
95
TEMPERATURE (°C)
125
155
Thermal Derating
–0.03
–0.04
0
1
3
2
LOAD CURRENT (A)
4
2
4
I-GRADE
2
9.0
8.5
8.0
7.0
150
125
10
CYCLE
CURRENT LIMIT (A)
8
5
3
2
1
0
0.2
0.6
0.8
0.4
FB PIN VOLTAGE (V)
1.0
9
CURRENT LIMIT (A)
11
10
9
8
7
125
155
3976 G13
9
8
7
7
6
5
4
3
1.2
3976 G14
VFB = 1V
VFB = 0V
3
1
1.0
30% DUTY CYCLE
4
2
0.6
0.8
0.4
FB PIN VOLTAGE (V)
1.0
5
1
0.2
0.8
6
2
0
0.4
0.6
DUTY CYCLE
Soft-Start
10
8
0
0.2
3976 G12
30% DUTY CYCLE
VSS = 3V
1.2
3976 G14
0
3976 G11
Current Limit Foldback
30% DUTY
VSS = 3V
9
7
6
4
0
40
7.5
LIMITED BY MAXIMUM
JUNCTION TEMPERATURE
ΘJA = 40°C/W
3976 G10
10
65
35
95
5
TEMPERATURE (°C)
H-GRADE
3
Switch Current Limit
6
–55 –25
35
9.5
12V
24V
36V
1 VOUT = 5V
fSW = 400kHz
2.5in × 2.5in 4-LAYER BOARD
0
0
25
75
50
100
TEMPERATURE (°C)
150
30% DUTY CYCLE
25
20
15
30
INPUT VOLTAGE (V)
Switch Current Limit
CURRENT LIMIT (A)
LIMITED BY MAXIMUM
JUNCTION TEMPERATURE
ΘJA = 40°C/W
10
10.0
CURRENT LIMIT (A)
I-GRADE
125
5
3976 G09
5
H-GRADE
1 VOUT = 3.3V
fSW = 400kHz
2.5in × 2.5in 4-LAYER BOARD
0
0
25
75
50
100
TEMPERATURE (°C)
CURRENT LIMIT (A)
–0.05
Thermal Derating
LOAD CURRENT (A)
LOAD CURRENT (A)
3
12
5
6
4
0
3976 G08
6
12V
24V
36V
0.01
–0.02
3976 G07
5
0.02
–0.01
–0.3
1.180
VOUT = 5V
LOAD = 1A
0.04
CHANGE IN OUTPUT (%)
1.215
CHANGE IN OUTPUT (%)
REFERENCE VOLTAGE (V)
TA = 25°C, unless otherwise noted.
0
0
0.5
1.5
2.0
1.0
SS PIN VOLTAGE (V)
2.5
3976 G15
3976f
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LT3976
Typical Performance Characteristics
BOOST Pin Current
Switch VCESAT
BOOST PIN CURRENT (mA)
250
VCESAT (mV)
200
80
300
200
150
100
50
0
60
50
40
30
20
1
0
2
3
5
4
LOAD = 1A
120
100
0
1
3
2
SWITCH CURRENT (A)
200
LOAD = 5A
150
LOAD = 2.5A
125
LOAD = 1A
100
–55 –25
65
35
95
5
TEMPERATURE (°C)
125
300
720
660
600
540
480
250
200
150
100
50
65
35
95
5
TEMPERATURE (°C)
125
0
155
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
SWITCHING FREQUENCY (MHz)
3976 G20
3976 G19
3976 G21
Internal Undervoltage Lockout
(UVLO)
Frequency Foldback
700
EN Thresholds
1.09
6
600
155
350
420
–55 –25
155
125
RT Programmed Switching
Frequency
RT RESISTOR (kΩ)
SWITCHING FREQUENCY (kHz)
225
65
35
95
5
TEMPERATURE (°C)
3976 G18
780
VSYNC = 0V
fSW = 2MHz
175
5
4
LOAD = 5A
60
–55 –25
Switching Frequency
250
LOAD = 2.5A
3976 G17
Minimum Off-Time
MINIMUM OFF-TIME (ns)
140
80
3976 G16
EN RISING
1.08
5
400
300
200
EN THRESHOLD (V)
1.07
500
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
160
10
0
VSYNC = 0V
fSW = 2MHz
180
70
SWITCH CURRENT (A)
4
3
2
1.06
1.05
1.04
1.03
1
100
0
Minimum On-Time
90
MINIMUM ON-TIME (ns)
350
TA = 25°C, unless otherwise noted.
0
0.2
0.8
0.6
0.4
FB PIN VOLTAGE (V)
1
1.2
3976 G22
0
–55 –25
EN FALLING
1.02
65
35
95
5
TEMPERATURE (°C)
125
155
3976 G23
1.01
–55 –25
95
65
35
TEMPERATURE (°C)
5
125
155
3976 G24
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LT3976
Typical Performance Characteristics
Minimum Input Voltage,
VOUT = 3.3V
Minimum Input Voltage,
VOUT = 5V
PG Thresholds
6.5
1.12
5.0
VOUT = 5V
fSW = 800kHz
1.11
6.0
FB RISING
1.09
1.08
FB FALLING
1.07
1.06
VOUT = 3.3V
FRONT PAGE APPLICATION
4.5
TO RUN/TO START
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1.10
PG THRESHOLD (V)
TA = 25°C, unless otherwise noted.
5.5
5.0
4.5
TO RUN/TO START
4.0
3.5
3.0
1.05
1.04
–55 –25
95
65
35
TEMPERATURE (°C)
5
125
4.0
155
1
0
3
2
LOAD CURRENT (A)
4
SS Pin Current
Burst Frequency
2.4
140
2.2
120
VOUT = 3.3V
fSW = 600kHz
400
300
200
VSS = 0.5V
2.0
1.8
1.6
1.4
1.2
100
20
40 60 80 100 120 140 160
LOAD CURRENT (mA)
1.0
–55 –25
95
65
35
TEMPERATURE (°C)
5
VBST = VIN
100
80
60
40
125
155
0
0
2
4
8
10 12
6
OUT PIN VOLTAGE (V)
3976 G29
3976 G28
Boost Diode Forward Voltage
14
16
3976 G30
Dropout Comparator Thresholds
600
1.6
580
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
5
20
DROPOUT THRESHOLD (mV)
0
BOOST DIODE VOLTAGE (V)
0
160
OUT PIN CURRENT (mA)
500
SS PIN CURRENT (µA)
800
600
4
Boost Capacitor Charger
2.6
VOUT = 5V
fSW = 800kHz
3
2
LOAD CURRENT (A)
3976 G27
900
700
1
0
3976 G26
3976 G25
SWITCHING FREQUENCY (kHz)
2.5
5
560
540
VOUT RISING
520
500
480
VOUT FALLING
460
440
420
0
1
0.5
1.5
BOOST DIODE CURRENT (A)
2
400
–55
–25
3976 G31
65
35
5
95
TEMPERATURE (°C)
125
155
3976 G32
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LT3976
Typical Performance Characteristics
Start-Up/Dropout Performance
VIN
1V/DIV
Start-Up/Dropout Performance
VIN
1V/DIV
VIN
VOUT
TA = 25°C, unless otherwise noted.
VIN
VSW
5V/DIV
VOUT
IL
0.5A/DIV
VOUT
1V/DIV
VOUT
1V/DIV
Burst Mode Switching Waveforms
VOUT
10mV/DIV
2.5Ω LOAD
100ms/DIV
(2A IN REGULATION)
1kΩ LOAD
100ms/DIV
(5mA IN REGULATION)
3976 G33
Full Frequency Switching
Waveforms
VSW
2V/DIV
IL
1A/DIV
IL
1A/DIV
VOUT
20mV/DIV
VOUT
50mV/DIV
3976 G36
1µs/DIV
VIN = 5V
5µs/DIV
VOUT SET FOR 5V
ILOAD = 0.5A
COUT = 47µF
Load Transient: 0.5A to 4.5A
IL
2A/DIV
VOUT
200mV/DIV
VOUT
500mV/DIV
50µs/DIV
5µs/DIV
3976 G35
3976 G37
Load Transient: 10mA to 4A
IL
2A/DIV
12VIN
3.3VOUT
COUT = 2 × 47µF
VIN = 12V
VOUT = 3.3V
ILOAD = 20mA
COUT = 47µF
Dropout Switching Waveforms
VSW
5V/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 1A
COUT = 47µF
3976 G34
3976 G38
12VIN
3.3VOUT
COUT = 2 × 47µF
50µs/DIV
3976 G39
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LT3976
Pin Functions
(MSE/UDD)
FB (Pin 1/Pins 23, 24): The LT3976 regulates the FB pin
to 1.197V. Connect the feedback resistor divider tap to this
pin. Also, connect a phase lead capacitor between FB and
the output. Typically, this capacitor is 10pF.
SS (Pin 2/Pin 1): A capacitor is tied between SS and ground
to slowly ramp up the peak current limit of the LT3976 on
start-up. There is an internal 1.8μA pull-up on this pin.
The soft-start capacitor is actively discharged when the
EN pin goes low, during undervoltage lockout or thermal
shutdown. Float this pin to disable soft-start.
OUT (Pin 3/Pin 2): This pin is an input to the dropout
comparator which maintains a minimum dropout of
500mV between VIN and OUT. The OUT pin connects to
the anode of the internal boost diode. This pin also supplies the current to the LT3976’s internal regulator when
OUT is above 3.2V. Connect this pin to the output when
the programmed output voltage is less than 16V.
BOOST (Pin 4/Pin 4): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pins 5, 6, 7/Pins 6, 7, 8): The SW pin is the output of
an internal power switch. Connect these pins to the inductor, catch diode, and boost capacitor. An R-C snubber to
GND is needed to ensure robustness under all conditions.
Typical values are 2Ω and 470pF.
VIN (Pins 10, 11, 12/Pins 13, 14, 15): The VIN pin supplies current to the LT3976’s internal circuitry and to the
internal power switch. These pins must be locally bypassed.
EN (Pin 13/Pin 16): The part is in shutdown when this
pin is low and active when this pin is high. The hysteretic
threshold voltage is 1.08V going up and 1.02V going down.
The EN threshold is only accurate when VIN is above 4.3V.
If VIN is lower than 3.9V, internal UVLO will place the part
in shutdown. Tie to VIN if shutdown feature is not used.
RT (Pin 14/Pin 18): A resistor is tied between RT and
ground to set the switching frequency.
PG (Pin 15/Pin 19): The PG pin is the open-drain output of
an internal comparator. PGOOD remains low until the FB
pin is within 8.4% of the final regulation voltage. PGOOD
is valid when VIN is above 2V.
SYNC (Pin 16/Pin 20): This is the external clock synchronization input. Ground this pin for low ripple Burst Mode
operation at low output loads. Tie to a clock source for
synchronization, which will include pulse skipping at low
output loads. When in pulse-skipping mode, quiescent
current increases to 11µA in a typical application at no
load. Do not float this pin.
GND (Exposed Pad Pin 17/Pin 21, Exposed Pad Pin 25):
Ground. The exposed pad must be soldered to the PCB.
NC (Pins 8, 9/Pins 3, 5, 9-12, 17, 22): No Connects.
These pins are not connected to internal circuitry.
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9
LT3976
Block Diagram
OUT
VIN
C1
INTERNAL 1.197V REF
EN
1.02V
+
–
+
SHDN
RT
0.5V
–
+
SLOPE COMP
OSCILLATOR
200kHz TO 2MHz
RT
SYNC
+
–
+
–
VIN
SWITCH
LATCH
BOOST
R
S
C3
Q
Burst Mode
DETECT
PG
ERROR AMP
+
–
+
–
1.097V
VC
L1
SW
R3
C6
VOUT
D1 C2
VC CLAMP
1.8µA
SS
SHDN
C4
OPT
FB
GND
R2
R1
3976 BD
C5
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LT3976
Operation
The LT3976 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
between the VIN and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
VC (see Block Diagram). An error amplifier measures the
output voltage through an external resistor divider tied
to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the
output; if it decreases, less current is delivered. An active
clamp on the VC pin provides current limit. The VC pin is
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the VIN
pin, but if the OUT pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency.
If the EN pin is low, the LT3976 is shut down and draws
700nA from the input. When the EN pin falls below 1.02V,
the switching regulator will shut down, and when the EN
pin rises above 1.08V, the switching regulator will become
active. This accurate threshold allows programmable
undervoltage lockout.
The switch driver operates from either VIN or from the
BOOST pin. An external capacitor is 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.
To further optimize efficiency, the LT3976 automatically
switches 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 1.7μA. In a typical application, 3.3μA will be
consumed from the supply when regulating with no load.
The oscillator reduces the LT3976’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup and overload.
The LT3976 can provide up to 5A of output current. A
current limit foldback feature throttles back the current
limit during overload conditions to limit the power dissipation. When SS is below 2V, the LT3976 overrides
the current limit foldback circuit to avoid interfering with
start-up. Thermal shutdown further protects the part from
excessive power dissipation, especially in elevated ambient
temperature environments.
If the input voltage decreases towards the programmed
output voltage, the LT3976 will start to skip switch-off
times and decrease the switching frequency to maintain
output regulation. As the input voltage decreases below
the programmed output voltage, the output voltage will be
regulated 500mV below the input voltage. This enforced
minimum dropout voltage limits the duty cycle and keeps
the boost capacitor charged during dropout conditions.
Since sufficient boost voltage is maintained, the internal
switch can fully saturate yielding low dropout performance.
The LT3976 contains a power good comparator which
trips when the FB pin is at 91.6% of its regulated value.
The PG output is an open-drain 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 VIN is
above 2V. When the LT3976 is shut down the PG pin is
actively pulled low.
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LT3976
Applications Information
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3976 operates
in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while
minimizing the input quiescent current. In Burst Mode
operation the LT3976 delivers single pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. When in
sleep mode the LT3976 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3976
consumes several mA of input current in addition to the
switch current. Therefore, the total quiescent current will
be greater than 1.7μA when regulating.
As the output load decreases, the frequency of single current pulses decreases (see Figure 1) and the percentage
of time the LT3976 is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3976. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
900
SWITCHING FREQUENCY (kHz)
800
VOUT = 5V
fSW = 800kHz
700
600
VOUT = 3.3V
fSW = 600kHz
300
200
100
0
0
20
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
VSW
5V/DIV
IL
0.5A/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 20mA
COUT = 47µF
5µs/DIV
3976 F02
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple will
be inversely proportional to the output capacitance. In a
typical application with a 22µF output capacitor, the output
ripple is about 10mV, and with a 47µF output capacitor
the output ripple is about 5mV. The output voltage ripple
can continue to be decreased by increasing the output
capacitance, though care must be taken to minimize the
effects of output capacitor ESR and ESL.
At higher output loads (above 150mA for the front page
application) the LT3976 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
500
400
diode should have less than a few µA of typical reverse
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
40 60 80 100 120 140 160
LOAD CURRENT (mA)
3976 F01
Figure 1. Switching Frequency in Burst Mode Operation
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3976 will pulse skip at light loads. At very
3976f
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LT3976
Applications Information
light loads, the part will go to sleep between groups of
pulses, so the quiescent current of the part will still be low,
but not as low as in Burst Mode operation. The quiescent
current in a typical application when synchronized with an
external clock is 11µA at no load. Holding the SYNC pin
DC high yields no advantages in terms of output ripple or
minimum load to full frequency, so is not recommended.
Table 1. Switching Frequency vs RT Value
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
⎛ V
⎞
R1= R2 ⎜ OUT – 1⎟
⎝ 1.197V ⎠
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from VOUT to FB.
Setting the Switching Frequency
The LT3976 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Table 1.
To estimate the necessary RT value for a desired switching
frequency, use the equation:
RT =
51.1
1.09
( fSW )
– 9.27
where RT is in kΩ and fSW is in MHz.
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.2
294
0.3
182
0.4
130
0.6
78.7
0.8
54.9
1.0
41.2
1.2
32.4
1.4
26.1
1.6
21.5
1.8
17.8
2.0
14.7
2.2
12.4
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values
may be used. The disadvantages are lower efficiency, and
lower maximum input voltage. The highest acceptable
switching frequency (fSW(MAX)) for a given application
can be calculated as follows:
fSW(MAX) =
VOUT + VD
tON(MIN) ( VIN – VSW + VD )
where VIN is the typical input voltage, VOUT is the output
voltage, VD is the catch diode drop (~0.5V), and VSW is
the internal switch drop (~0.3V at max load). This equation shows that slower switching frequency is necessary
to safely accommodate high VIN/VOUT ratio. This is due
to the limitation on the LT3976’s minimum on-time. The
minimum on-time is a strong function of temperature.
Use the typical minimum on-time curve to design for an
application’s maximum temperature, while adding about
30% for part-to-part variation. The minimum duty cycle that
can be achieved taking minimum on time into account is:
DCMIN = fSW • tON(MIN)
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on-time.
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LT3976
Applications Information
A good choice of switching frequency should allow adequate input voltage range (see next two sections) and
keep the inductor and capacitor values small.
Maximum Input Voltage Range
The LT3976 can operate from input voltages of up to 40V.
Often the highest allowed VIN during normal operation
(VIN(OP-MAX)) is limited by the minimum duty cycle rather
than the absolute maximum ratings of the VIN pin. It can
be calculated using the following equation:
VIN(OP-MAX) =
VOUT + VD
– VD + VSW
fSW • tON(MIN)
where tON(MIN) is the minimum switch on-time. A lower
switching frequency can be used to extend normal operation to higher input voltages.
The circuit will tolerate inputs above the maximum operating input voltage and up to the absolute maximum
ratings of the VIN and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where VIN is higher than VIN(OP-MAX), the LT3976 will enter
pulse-skipping operation where some switching pulses are
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
typical operation. Do not overload when VIN is greater
than VIN(OP-MAX).
During start-up or overload, the switch node slews very
fast due to the 10A peak current limit. At high voltages
during these conditions, an R-C snubber on the switch node
is required to ensure robustness of the LT3976. Typical
values for the snubber are 2Ω and 470pF. See the Typical
Applications section to see how the snubber is connected.
Minimum Input Voltage Range
The minimum input voltage is determined by either the
LT3976’s minimum operating voltage of 4.3V, its maximum
duty cycle, or the enforced minimum dropout voltage.
See the Typical Performance Characteristics section for
the minimum input voltage across load for outputs of
3.3V and 5V.
The duty cycle is the fraction of time that the internal
switch is on during a clock cycle. Unlike many fixed frequency regulators, the LT3976 can extend its duty cycle
by remaining on for multiple clock cycles. The LT3976
will not switch off at the end of each clock cycle if there
is sufficient voltage across the boost capacitor (C3 in
the Block Diagram). Eventually, the voltage on the boost
capacitor falls and requires refreshing. When this occurs,
the switch will turn off, allowing the inductor current to
recharge the boost capacitor. This places a limitation on
the maximum duty cycle as follows:
DCMAX =
βSW
βSW + 1
where βSW is equal to the beta of the internal power switch.
The beta of the power switch is typically about 50, which
leads to a DCMAX of about 98%. This leads to a minimum
input voltage of approximately:
VIN(MIN1) =
VOUT + VD
– VD + VSW
DCMAX
where VOUT is the output voltage, VD is the catch diode
drop, VSW is the internal switch drop and DCMAX is the
maximum duty cycle.
The final factor affecting the minimum input voltage is
the minimum dropout voltage. When the OUT pin is tied
to the output, the LT3976 regulates the output such that
it stays 500mV below VIN. This enforced minimum dropout voltage is due to reasons that are covered in the next
section. This places a limitation on the minimum input
voltage as follows:
VIN(MIN2) = VOUT + VDROPOUT(MIN)
where VOUT is the programmed output voltage and
VDROPOUT(MIN) is the minimum dropout voltage of 500mV.
Combining these factors leads to the overall minimum
input voltage:
VIN(MIN) = Max (VIN(MIN1), VIN(MIN2), 4.3V)
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LT3976
Applications Information
Minimum Dropout Voltage
To achieve a low dropout voltage, the internal power switch
must always be able to fully saturate. This means that the
boost capacitor, which provides a base drive higher than
VIN, must always be able to charge up when the part starts
up and then must also stay charged during all operating
conditions.
During start-up if there is insufficient inductor current, such
as during light load situations, the boost capacitor will be
unable to charge. When the LT3976 detects that the boost
capacitor is not charged, it activates a 100mA (typical)
pull-down on the OUT pin. If the OUT pin is connected to
the output, the extra load will increase the inductor current
enough to sufficiently charge the boost capacitor. When
the boost capacitor is charged, the current source turns
off, and the part may re-enter Burst Mode operation.
To keep the boost capacitor charged regardless of load
during dropout conditions, a minimum dropout voltage
is enforced. When the OUT pin is tied to the output, the
LT3976 regulates the output such that:
VIN – VOUT > VDROPOUT(MIN)
where VDROPOUT(MIN) is 500mV. The 500mV dropout voltage limits the duty cycle and forces the switch to turn off
regularly to charge the boost capacitor. Since sufficient
voltage across the boost capacitor is maintained, the switch
is allowed to fully saturate and the internal switch drop
stays low for good dropout performance. Figure 3 shows
the overall VIN to VOUT performances during start-up and
dropout conditions.
VIN
1V/DIV
VIN
VOUT
VOUT
1V/DIV
1kΩ LOAD
100ms/DIV
(5mA IN REGULATION)
3976 F03
It is important to note that the 500mV dropout voltage
specified is the minimum difference between VIN and
VOUT. When measuring VIN to VOUT with a multimeter,
the measured value will be higher than 500mV because
you have to add half the ripple voltage on the input and
half the ripple voltage on the output. With the normal
ceramic capacitors specified in the data sheet, this measured dropout voltage can be as high as 650mV at high
load. If some bulk electrolytic capacitance is added to the
input and output the voltage ripple, and subsequently the
measured dropout voltage, can be significantly reduced.
Additionally, when operating in dropout at high currents,
high ripple voltage on the input and output can generate
audible noise. This noise can also be significantly reduced
by adding bulk capacitance to the input and output to
reduce the voltage ripple.
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher VIN or VOUT and
decreases with higher inductance and faster switching
frequency. A good first choice for the inductor value is:
L=
VOUT + VD
2fSW
where fSW is the switching frequency in MHz, VOUT is the
output voltage, VD is the catch diode drop (~0.5V) and L
is the inductor value is μH.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions (start-up or overload) and high input voltage
(>30V), the saturation current should be above 13A. To
keep the efficiency high, the series resistance (DCR)
should be less than 0.1Ω, and the core material should
be intended for high frequency applications. Table 2 lists
several inductor vendors.
Figure 3. VIN to VOUT Performance
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LT3976
Applications Information
Table 2. Inductor Vendors
VENDOR
URL
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Toko
www.tokoam.com
Würth Elektronik
www.we-online.com
Coiltronics
www.cooperet.com
Murata
www.murata.com
The inductor value must be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) = ILIM –
ΔIL
2
The LT3976 limits its peak switch current in order to protect
itself and the system from overload and short-circuit faults.
The LT3976’s switch current limit (ILIM) is typically 10A at
low duty cycles and decreases linearly to 8A at DC = 0.8.
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:
ΔIL =
(1– DC) • ( VOUT + VD )
L • fSW
where fSW is the switching frequency of the LT3976, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3976 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (IOUT(MAX))
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current and
reduces the output voltage ripple. If your load is lower than
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. 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
16
maximum load current will depend on the 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
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillations, see Application Note 19.
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 the equations above to check that the LT3976 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 IOUT
is less than ΔIL/2.
Current Limit Foldback and Thermal Protection
The LT3976 has a large peak current limit to ensure a 5A
max output current across duty cycle and current limit
distribution, as well as allowing a reasonable inductor
ripple current. During a short-circuit fault, having a large
current limit can lead to excessive power dissipation and
temperature rise in the LT3976, as well as the inductor and
catch diode. To limit this power dissipation, the LT3976
starts to fold back the current limit when the FB pin falls
below 0.8V. The LT3976 typically lowers the peak current
limit about 50% from 10A to 5A.
During start-up, when the output voltage and FB pin are low,
current limit foldback could hinder the LT3976’s ability to
start up into a large load. To avoid this potential problem,
the LT3976’s current limit foldback will be disabled until
the SS pin has charged above 2V. Therefore, the use of
a soft-start capacitor will keep the current limit foldback
feature out of the way while the LT3976 is starting up.
The LT3976 has thermal shutdown to further protect the
part during periods of high power dissipation, particularly
in high ambient temperature environments. The thermal
shutdown feature detects when the LT3976 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3976 cools, the part will
restart and resume switching. A thermal shutdown event
actively discharges the soft-start capacitor.
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3976f
LT3976
Applications Information
Input Capacitor
Bypass the input of the LT3976 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3976 and will easily handle the ripple current. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). If the input power source has high impedance, or
there is significant inductance due to long wires or cables,
additional bulk capacitance may be necessary. This can
be provided with a low performance electrolytic capacitor.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3976 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10μF capacitor is capable of this task, but only if it is
placed close to the LT3976 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3976.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3976 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, possibly exceeding the LT3976’s voltage rating. If the input
supply is poorly controlled or the user will be plugging
the LT3976 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
Technology Application Note 88 for a complete discussion.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3976 to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching
frequency) is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3976’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT =
where fSW is in MHz, and COUT is the recommended 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 combined with a phase lead capacitor (typically
10pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
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. Table 3 lists several capacitor
vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER
URL
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Ceramic Capacitors
When in dropout, the LT3976 can excite ceramic capacitors at audio frequencies. At high load, this could be
unacceptable. Simply adding bulk input capacitance to
the input and output will significantly reduce the voltage
ripple and the audible noise generated at these nodes to
acceptable levels.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3976. As previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3976 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3976’s rating. If the input
supply is poorly controlled or the user will be plugging
the LT3976 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
Technology Application Note 88 for a complete discussion.
300
VOUT • fSW
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LT3976
Applications Information
Catch Diode Selection
The catch diode (D1 from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
Table 4. Schottky Diodes. The Reverse Current Values Listed
Are Estimates Based Off of Typical Curves for Reverse Current
vs Reverse Voltage at 25°C
IR at
VR = 20V
25°C
(µA)
VR (V)
IAVE (A)
40
5
450
500
120
B540C
40
5
510
550
2
PDS540
40
5
480
520
4
PDS560
60
5
610
670
0.9
SBR8A45SP5
45
8
450
—
18
SBR8AU60P5
60
8
400
—
60
PART NUMBER
⎛V – V ⎞
ID(AVG) = IOUT ⎜ IN OUT ⎟
VIN
⎝
⎠
VF at
5A MAX
25°C
(mV)
VF at 5A
TYP 25°C
(mV)
On Semiconductor
MBRS540T3
where IOUT is the output load current. The current rating of
the diode should be selected to be greater than or equal to
the application’s output load current, so that the diode is
robust for a wide input voltage range. A diode with even
higher current rating can be selected for the worst-case
scenario of overload, where the max diode current can then
increase to the typical peak switch current. Short circuit is
not the worst-case condition due to current limit foldback.
Peak reverse voltage is equal to the regulator input voltage.
For inputs up to 40V, a 40V diode is adequate.
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
by the LT3976 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size. Table 4 lists several
Schottky diodes and their manufacturers.
Diodes Inc.
charge the boost capacitor. Above 16V, the OUT pin abs
max is violated. For outputs between 2.5V and 3.2V, an
external Schottky diode to the output is sufficient because
an external Schottky will have much lower forward voltage
drop than the internal boost diode.
BOOST and OUT Pin Considerations
For output voltages less than 2.5V, there are two options.
An external Schottky diode can charge the boost capacitor from the input (Figure 4c) or from an external voltage
source (Figure 4d). Using an external voltage source is
the better option because it is more efficient than charging the boost capacitor from the input. However, such
a voltage rail is not always available in all systems. For
output voltages greater than 16V, an external Schottky
diode from an external voltage source should be used to
charge the boost capacitor (Figure 4e). In applications
using an external voltage source, the supply should be
between 3.1V and 16V. When using the input, the input
voltage may not exceed 27V. In all cases, the maximum
voltage rating of the BOOST pin must not be exceeded.
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.47μF
capacitor will work well. The BOOST pin must be more
than 1.8V above the SW pin for best efficiency and more
than 2.6V above the SW pin to allow the LT3976 to skip
off times to achieve very high duty cycles. For outputs
between 3.2V and 16V, the standard circuit with the OUT
pin connected to the output (Figure 4a) is best. Below 3.2V
the internal Schottky diode may not be able to sufficiently
When the output is above 16V, the OUT pin can not be
tied to the output or the OUT pin abs max will be violated.
It should instead be tied to GND (Figure 4e). This is to
prevent the dropout circuitry from interfering with switching behavior and to prevent the 100mA active pull-down
from drawing power. It is important to note that when
the output is above 16V and the OUT pin is grounded,
the dropout circuitry is not connected, so the minimum
dropout will be about 1.5V, rather than 500mV. If the
output is less than 3.2V and an external Schottky is used
18
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3976f
LT3976
Applications Information
to charge the boost capacitor, the OUT pin should still be
tied to the output even though the minimum input voltage
of the LT3976 will be limited by the 4.3V minimum rather
than the minimum dropout voltage.
Enable and Undervoltage Lockout
The LT3976 is in shutdown when the EN pin is low and
active when the pin is high. The falling threshold of the
EN comparator is 1.02V, with 60mV of hysteresis. The EN
pin can be tied to VIN if the shutdown feature is not used.
With the OUT pin connected to the output, a 100mA active load will charge the boost capacitor during light load
start-up and an enforced 500mV minimum dropout voltage
will keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section). This
yields excellent start-up and dropout performance. Figure 5
shows the minimum input voltage for 3.3V and 5V outputs.
VIN
BOOST
SW
VIN
VIN
LT3976
GND
BOOST
SW
VIN
VIN
LT3976
OUT
VOUT
GND
(4a) For 3.2V ≤ VOUT ≤ 16V
BOOST
VIN
OUT
VOUT
GND
(4b) For 2.5V ≤ VOUT ≤ 3.2V
BOOST
VIN
OUT
VOUT
(4c) For VOUT < 2.5V, VIN < 27V
VS
SW
VIN
VIN
LT3976
GND
SW
LT3976
VS
BOOST
SW
LT3976
OUT
VOUT
GND
OUT
VOUT
3976 F04
(4d) For VOUT < 2.5V, 3.1V ≤ VS ≤ 16V
(4e) For VOUT > 16V, 3.1V ≤ VS ≤ 16V
Figure 4. Five Circuits for Generating the Boost Voltage
6.5
5.0
VOUT = 5V
fSW = 800kHz
6.0
5.5
5.0
4.5
4.0
VOUT = 3.3V
FRONT PAGE APPLICATION
4.5
TO RUN/TO START
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
VIN
Undervoltage lockout (UVLO) can be added to the LT3976
as shown in Figure 6. Typically, UVLO is used in situations where the input supply is current limited, or has a
relatively high source resistance. A switching regulator
draws constant power from the source, so source current increases as source voltage drops. This looks like a
TO RUN/TO START
4.0
3.5
3.0
0
1
3
2
LOAD CURRENT (A)
4
5
2.5
0
1
3
2
LOAD CURRENT (A)
4
3976 F05a
5
3976 F05b
Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current
3976f
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19
LT3976
Applications Information
VIN
R3
LT3976
1.02V
EN
+
–
IL
1A/DIV
SHDN
VOUT
1V/DIV
R4
LT3976 F06
VSS
0.5V/DIV
Figure 6. Undervoltage Lockout
negative resistance load to the source and can cause the
source to current limit or latch low under low source voltage
conditions. UVLO prevents the regulator from operating
at source voltages where the problems might occur. The
UVLO threshold can be adjusted by setting the values R3
and R4 such that they satisfy the following equation:
⎛ R3+R4 ⎞
VUVLO = VEN(THRESH) ⎜
⎟
⎝
⎠
R4
where VEN(THRESH) is the falling threshold of the EN pin,
which is approximately 1.02V, and where switching should
stop when VIN falls below VUVLO. Note that due to the
comparator’s hysteresis, switching will not start until the
input is about 6% above VUVLO.
When operating in Burst Mode operation for light load
currents, the current through the UVLO resistor network
can easily be greater than the supply current consumed
by the LT3976. Therefore, the UVLO resistors should be
large to minimize their effect on efficiency at low loads.
Soft-Start
The SS pin can be used to soft start the LT3976 by throttling the maximum input current during start-up and reset.
An internal 1.8μA current source charges an external
capacitor generating a voltage ramp on the SS pin. The
SS pin clamps the internal VC node, which slowly ramps
up the current limit. Maximum current limit is reached
when the SS pin is about 1.5V or higher. By selecting a
large enough capacitor, the output can reach regulation
without overshoot. Figure 7 shows start-up waveforms
for a typical application with a 10nF capacitor on SS for
a 1.65Ω load when the EN pin is pulsed high for 7ms.
The external SS capacitor is actively discharged when the
EN pin is low, or during thermal shutdown. The active
pull-down on the SS pin has a resistance of about 150Ω.
1ms/DIV
3976 F07
Figure 7. Soft-Start Waveforms for the Front-Page Application
with a 10nF Capacitor on SS. EN Is Pulsed High for About 7ms
with a 1.65Ω Load Resistor
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.5V (this can be ground or a logic output).
Synchronizing the LT3976 oscillator 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 0.5V
and peaks above 1.5V (up to 6V).
The LT3976 will pulse skip at low output loads while synchronized to an external clock to maintain regulation. At
very light loads, the part will go to sleep between groups of
pulses, so the quiescent current of the part will still be low,
but not as low as in Burst Mode operation. The quiescent
current in a typical application when synchronized with an
external clock is 11µA at no load. Holding the SYNC pin
DC high yields no advantages in terms of output ripple or
minimum load to full frequency, so is not recommended.
Never float the SYNC pin.
The LT3976 may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3976
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the RT should be selected for 200kHz.
To assure reliable and safe operation the LT3976 will only
synchronize when the output voltage is 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 RT resistor (see
Inductor Selection section). The slope compensation is set
by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
3976f
20
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LT3976
Applications Information
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, than the slope compensation will be
sufficient for all synchronization frequencies.
Power Good Flag
The PG pin is an open-drain output which is used to indicate
to the user when the output voltage is within regulation.
When the output is lower than the regulation voltage by
more than 8.4%, as determined from the FB pin voltage,
the PG pin will pull low to indicate the power is not good.
Otherwise, the PG pin will go high impedance and can
be pulled logic high with a resistor pull-up. The PG pin is
only comparing the output voltage to an accurate reference when the LT3976 is enabled and VIN is above 4.3V.
When the part is shutdown, the PG is actively pulled low to
indicate that the LT3976 is not regulating the output. The
input voltage must be greater than 1.4V to fully turn-on
the active pull-down device. Figure 8 shows the status of
the PG pin as the input voltage is increased.
Protection section). There is another situation to consider
in systems where the output will be held high when the
input to the LT3976 is 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
LT3976’s output. If the VIN pin is allowed to float and the
EN/UVLO pin is held high (either by a logic signal or because it is tied to VIN), then the LT3976’s internal circuitry
will pull its quiescent current through its SW pin. This is
fine if your system can tolerate a few μA in this state. If
you ground the EN pin, the SW pin current will drop to
essentially zero. However, if the VIN pin is grounded while
the output is held high, regardless of EN, parasitic diodes
inside the LT3976 can pull current from the output through
the SW pin and the VIN pin. Figure 9 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
D4
PDS540
VIN
EN
PG PIN VOLTAGE (V)
4
BOOST
VIN
LT3976
GND
VOUT
SW
OUT
FB
+
BACKUP
3
3976 F09
2
Figure 9. 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 LT3976 Runs Only When the Input
Is Present
1
0
PCB Layout
0
0.5 1 1.5 2 2.5 3 3.5
INPUT VOLTAGE (V)
4 4.5
5
3976 F08
Figure 8. PG Pin Voltage Versus Input Voltage when PG
Is Connected to 3V Through a 150k Resistor. The FB Pin
Voltage Is 1.15V
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, a LT3976 buck regulator will tolerate a shorted
output and the power dissipation will be limited by current
limit foldback (see Current Limit Foldback and Thermal
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 10 shows
a sample component placement with trace, ground plane
and via locations, which serves as a good PCB layout
example. Note that large, switched currents flow in the
LT3976’s VIN and SW pins, the catch diode (D1), and the
input capacitor (C1). The loop formed by these components should be as small as possible. These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit board, and their
connections should be made on that layer. Place a local,
3976f
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21
LT3976
Applications Information
VOUT
FB
OUT BST
17
•••
•••
•• •• ••
•• •• ••
•• •• ••
•••
•••
•••
SW
VOUT
PG
RT
VIN
EN
3976 F10
Figure 10. Layout Showing a Good PCB Design
unbroken ground plane below these components. The SW
and BOOST nodes should be as small as possible. Finally,
keep the FB and RT nodes small so that the ground traces
will shield it from the SW and BOOST nodes. The exposed
pad on the bottom of the package must be soldered to
ground so that the pad acts as a heat sink. To keep thermal
resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3976 to
additional ground planes within the circuit board and on
the bottom side.
High Temperature Considerations
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction temperature. When the power switch is off, the power Schottky
diode is in parallel with the power converter’s output
filter stage. As a result, an increase in a diode’s leakage
current results in an effective increase in the load, and a
corresponding increase in the input quiescent current.
Therefore, the catch Schottky diode must be selected
with care to avoid excessive increase in light load supply
current at high temperatures.
CHIP TEMPERATURE RISE (°C)
SYNC
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
VOUT = 3.3V
fSW = 400kHz
2.5in × 2.5in 4-LAYER BOARD
12V
24V
36V
1
3
4
2
OUTPUT CURRENT (A)
5
3976 F11a
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3976. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3976.
Placing additional vias can reduce the thermal resistance
further. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
(See Thermal Derating curve in the Typical Performance
Characteristics section.)
Power dissipation within the LT3976 can be estimated by
calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
Figure 11a. Temperature Rise of the LT3976 in
the Front Page Application
90
VOUT = 5V
fSW = 800kHz
2.5in × 2.5in 4-LAYER BOARD
80
CHIP TEMPERATURE RISE (°C)
SS
LT3976 power dissipation by the thermal resistance from
junction to ambient. The temperature rise of the LT3976 for
a 3.3V and 5V application was measured using a thermal
camera and is shown in Figure 11.
70
60
50
40
30
20
12V
24V
36V
10
0
1
3
4
2
OUTPUT CURRENT (A)
5
3976 F11b
Figure 11b. Temperature Rise of the LT3976 in a
5VOUT Application
3976f
22
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LT3976
Applications Information
Fault Tolerance of QFN Package
The QFN package is designed to tolerate single fault conditions. Shorting two adjacent pins together or leaving one
single pin floating does not raise the output voltage or cause
damage to the LT3976 regulator. However, the application
circuit must meet a few requirements discussed in this
section in order to achieve this fault tolerance.
Tables 5 and 6 show the effects that result from shorting
adjacent pins or from a floating pin, respectively.
There are three items which require consideration in terms
of the application circuit to achieve fault tolerance: SSOUT pin short, RT-PG pin short, and PG-SYNC pin short.
If the output voltage is less than 6V, then the application
circuit can be setup normally (see Figure 12a) because a
SS to OUT short will not violate the SS pin 6V absolute
maximum and a PG short to either RT or SYNC will not
violate the 6V absolute maximum on each of those pins.
If the output voltage is greater than 6V, the best way to
solve the problem of violating the SS absolute maximum
when shorted to OUT is to tie the OUT pin to GND. Note
that grounding the OUT pin will compromise the dropout
performance of the LT3976. When OUT is grounded, an
external Schottky diode to either the output, VIN, or another voltage source must be used to charge the boost
capacitor. The PG pull-up resistor must be increased
Table 5. Effects of Pin Shorts
PINS
EFFECT
SS-OUT
VOUT may fall below regulation voltage for VOUT less than or equal to 6V. For outputs above 6V, the absolute maximum of the SS pin
would be violated, so the OUT pin must be tied to GND (see discussion in the Fault Tolerance section)
VIN-EN
No effect. In most applications, EN is tied to VIN. If EN is driven with a logic signal, the customer must ensure that the circuit generating
that signal can withstand the maximum VIN
RT-PG
No effect if PG is floated. VOUT will fall below regulation if PG is connected to the output with a resistor pull-up as long as the resister
divider formed by the PG pin pull-up and the RT resistor prevents the RT pin absolute maximum from being violated (see discussion in
the Fault Tolerance section). In both cases, the switching frequency will be significantly increased if the output goes below regulation,
which may cause the LT3976 to go into pulse-skipping mode if the minimum on-time is violated.
PG-SYNC
No effect if PG is floated. No effect if PG is connected to the output with a resistor pull-up as long as there is a resistor to GND on the
SYNC pin or the SYNC pin is tied to GND. This is to ensure that the resistor divider formed by the PG pin pull-up and the SYNC pin
resistor to GND prevents the SYNC pin Absolute Maximum from being violated (see discussion in the Fault Tolerance section).
Table 6. Effects of Floating Pins
EFFECT
SS
No effect; soft-start feature will not function.
OUT
VOUT may fall below regulation voltage. With the OUT pin disconnected, the boost capacitor cannot be charged and thus the power
switch cannot fully saturate, which increases power dissipation.
BOOST
SW
700
SWITCHING FREQUENCY (kHz)
PIN
VOUT may fall below regulation voltage. With the BOOST pin disconnected, the boost capacitor cannot be charged and thus the power
switch cannot fully saturate, which increases power dissipation.
No effect; there are several SW pins.
No effect; there are several VIN pins.
VOUT may fall below regulation voltage. Part may work normally or be shutdown depending on how the application circuit couples to the
floating EN pin.
RT
VOUT may fall below regulation voltage.
PG
No effect.
FB
No effect. The LT3976 may be in Burst Mode operation or pulse-skipping mode depending on how the application circuit couples to the
floating SYNC pin.
No effect; there are two FB pins.
No effect; there are several GND connections. If Exposed Pad is floated, thermal performance will be degraded.
3976f
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400
300
200
100
0
0.2
0.8
0.6
0.4
FB PIN VOLTAGE (V)
1
1.2
3976 G22
EN
GND
500
0
VIN
SYNC
600
23
LT3976
Applications Information
and a SYNC pin resistor to GND added, so that a PG pin
short to either SYNC or RT will form resistor dividers to
keep the voltage on the SYNC and RT pins below their
rated absolute maximum. This application is shown in
Figure 12b. The external Schottky must be connected
such that the absolute maximum of the BOOST pin is not
violated. The SYNC pin resistor can be removed if the
SYNC pin is grounded or PG is left floating both of which
also result in fault tolerant circuits.
Other Linear Technology Publications
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.
VIN
BOOST
VIN
EN
EXTERNAL
INPUT
0.47µF
3.3µH
SW
2Ω
SYNC
470pF
LT3976
10µF
10nF
SS
PG
OUT
RT
FB
150k
PGOOD
1M
VOUT
GND
34.9k
316k
47µF
1210
×2
10pF
3976 F12a
f = 800kHz
Figure 12a. Fault Tolerant for VOUT < 6V
(Note: For VOUT < 3.3V External Boost Schottky Diode Is Needed)
VIN
BOOST
VIN
EN
EXTERNAL
INPUT
0.47µF
3.3µH
SW
2Ω
SYNC
470pF
LT3976
10µF
40.2k
10nF
SS
PG
OUT
RT
FB
249k
PGOOD
1M
VOUT
GND
54.9k
316k
47µF
1210
×2
10pF
3976 F12b
f = 800kHz
Figure 12b. Fault Tolerant for VOUT < 27V
(Note: For VOUT < 3V External Boost Schottky Diode
Should Be Connected to the Input)
Figure 12. Two Example Circuits to Achieve Fault Tolerance (FMEA) with the LT3976 QFN Package
3976f
24
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LT3976
Typical Applications
5V Step-Down Converter
12V Step-Down Converter
VIN
13.2V* TO 40V
VIN
6V* TO 40V
VIN
OFF ON
VIN
EN
BOOST
PG
SW
10µF
10nF
0.47µF 3.3µH
10pF
3976 TA02
f = 800kHz
D = PDS540
L = IHLP-2525CZ-01
* MINIMUM VIN CAN BE LOWERED WITH ADDITIONAL
INPUT AND OUTPUT CAPACITANCE.
10nF
OFF ON
EN
10nF
RT
SYNC
–
24V
VOUT
5V
5A
1M
10pF
CBULK
100µF
BOOST
10µF
×2
10nF
RT
SYNC
130k
f = 400kHz
D = PDS540
L = IHLP-2525CZ-01
VOUT
4V
5A
1M
FB
GND
10pF
432k
3976 TA05
47µF
1210
×2
VIN
EN
BOOST
PG
SW
10µF
VOUT
2.5V
5A
1M
4.7pF
909k
3976 TA06
47µF
1210
×4
SS
10nF
0.47µF 2.2µH
2Ω
LT3976
470pF
FB
GND
470pF
DFLS160
OFF ON
2Ω
OUT
2Ω
OUT
RT
SYNC
VIN
4.3V TO 27V
SW
SS
0.47µF 3.3µH
54.9k
0.47µF
LT3976
SW
LT3976
47nF
10µF
3.3µH
PG
EN
1.8V Step-Down Converter
DFLS160
VIN
3976 TA03
f = 800kHz
D = PDS540
L = IHLP-2525CZ-01
3976 TA04
EN
BOOST
SS
2.5V Step-Down Converter
OFF ON
PG
499k
47µF
1210
f = 2MHz
D = PDS540
L = IHLP-2525CZ-01
VIN
4.3V TO 40V
47µF
1210
VIN
5.49M
+
150k
316k
14.7k
10pF
110k
PGOOD
FB
GND
VOUT
12V
5A
1M
FB
GND
+
V
470pF
PG
OUT
SS
RT
SYNC
4V Step-Down Converter with a High Impedance Input Source
SW
LT3976
470pF
OUT
f = 800kHz
D = PDS540
L = IHLP-4040DZ-01
* MINIMUM VIN CAN BE LOWERED WITH ADDITIONAL
INPUT AND OUTPUT CAPACITANCE.
0.47µF
1.5µH
2Ω
4.7µF
2Ω
54.9k
5V, 2MHz Step-Down Converter with Power Good
BOOST
0.47µF 6.8µH
LT3976
47µF
1210
×2
316k
VIN
SW
SS
VOUT
5V
5A
1M
FB
GND
54.9k
VIN
5.9V TO 18V
(40V TRANSIENTS)
BOOST
PG
470pF
OUT
RT
SYNC
EN
10µF
2Ω
LT3976
SS
OFF ON
RT
SYNC
97.6k
f = 500kHz
D = PDS540
L = IHLP-2525CZ-01
470pF
OUT
VOUT
1.8V
5A
499k
FB
GND
10pF
1M
3976 TA07
47µF
1210
×4
3976f
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25
LT3976
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev E)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
5.23
(.206)
MIN
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
8
1
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102 3.20 – 3.45
(.065 ±.004) (.126 – .136)
0.305 ±0.038
(.0120 ±.0015)
TYP
16
0.50
(.0197)
BSC
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 ±0.076
(.011 ±.003)
REF
16151413121110 9
DETAIL “A”
0° – 6° TYP
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
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
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0911 REV E
3976f
26
For more information www.linear.com/3976
LT3976
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UDD Package
24-Lead Plastic QFN (3mm × 5mm)
(Reference LTC DWG # 05-08-1833 Rev Ø)
0.70 ±0.05
3.50 ±0.05
2.10 ±0.05
3.65 ±0.05
1.50 REF
1.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 ±0.10
0.75 ±0.05
1.50 REF
23
R = 0.05 TYP
PIN 1 NOTCH
R = 0.20 OR 0.25
× 45° CHAMFER
24
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
5.00 ±0.10
1
2
3.65 ±0.10
3.50 REF
1.65 ±0.10
(UDD24) QFN 0808 REV Ø
0.200 REF
0.00 – 0.05
R = 0.115
TYP
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3976f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
its circuits
as described
herein will not infringe on existing patent rights.
Forofmore
information
www.linear.com/3976
27
LT3976
Typical Application
1.2V Step-Down Converter
DFLS160
VIN
4.3V TO 27V
(40V TRANSIENT)
3.3V
VIN
OFF ON
EN
BOOST
PG
SW
10µF
10nF
2Ω
LT3976
SS
RT
SYNC
0.47µF 2.2µH
470pF
OUT
VOUT
1.2V
5A
FB
GND
130k
3976 TA08
f = 400kHz
D = PDS540
L = IHLP-2525CZ-01
47µF
1210
×4
Related Parts
PART NUMBER DESCRIPTION
COMMENTS
LT3480
36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode® Operation
VIN = 3.6V to 38V, Transients to 60V, VOUT(MIN) = 0.78V,
IQ = 70µA, ISD < 1µA, 3mm × 3mm DFN-10, MSOP-10E
LT3980
58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
VIN = 3.6V to 58V, Transients to 80V, VOUT(MIN) = 0.79V,
IQ = 75µA, ISD < 1µA, 3mm × 4mm DFN-16, MSOP-16E
LT3971
38V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down
DC/DC Converter with Only 2.8µA of Quiescent Current
VIN = 4.2V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA,
3mm × 3mm DFN-10, MSOP-10E
LT3991
55V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down
DC/DC Converter with Only 2.8µA of Quiescent Current
VIN = 4.2V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA,
3mm × 3mm DFN-10, MSOP-10E
LT3970
40V, 350mA (IOUT), 2MHz, High Efficiency Step-Down
DC/DC Converter with Only 2.5µA of Quiescent Current
VIN = 4.2V to 40V, VOUT(MIN) = 1.2V, IQ = 2.5µA, ISD < 0.7µA,
2mm × 3mm DFN-10, MSOP-10E
LT3990
62V, 350mA (IOUT), 2.2MHz, High Efficiency Step-Down
DC/DC Converter with Only 2.5µA of Quiescent Current
VIN = 4.2V to 62V, VOUT(MIN) = 1.2V, IQ = 2.5µA, ISD < 0.7µA,
3mm × 3mm DFN-10, MSOP-16E
3976f
28 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/3976
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/3976
LT 0113 • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2013
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