LT3980
1
3980fa
For more information www.linear.com/LT3980
58V, 2A, 2.4MHz
Step-Down Switching Regulator
with 85µA Quiescent Current
The LT ®3980 is an adjustable frequency (100kHz to
2.4MHz) monolithic buck switching regulator that accepts
input voltages up to 58V (80V transient). A high efficiency
200mΩ switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Catch diode
current sense (DA pin) protects the circuit during input
voltage transients even when a high switching frequency
is used. 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
LT3980 can further enhance low output current efficiency
by drawing bias current from the output when VOUT 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 VOUT reaches 91% of
the programmed output voltage. The LT3980 is available
in 16-pin MSOP and 3mm × 4mm DFN packages with
exposed pads for low thermal resistance.
Automotive Battery Regulation
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
Wide Input Range: Operation from 3.6V to 58V
Overvoltage Lockout Protects Circuits Through 80V
Transients
■ 2A Maximum Output Current
■ Low Ripple (<15mVP-P) Burst Mode® Operation:
IQ = 85µA at 12VIN to 3.3VOUT
Adjustable Switching Frequency: 100kHz to 2.4MHz
Low Shutdown Current: IQ < 1µA
Catch Diode Current Sense Protects Circuit Through
Short-Circuit and Input Overvoltage
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Saturating Switch Design: 200mΩ On-Resistance
Thermal Protection
Soft-Start Capability
Small 16-Pin Thermally Enhanced MSOP and
3mm × 4mm DFN Packages
5V Step-Down Converter Efficiency, VOUT = 5V
Typical applicaTion
DescripTion
FeaTures
applicaTions
SW
FB
VC
PG
RT
VIN BD
VIN
6.5V TO 58V
TRANSIENT
TO 80V
V
OUT
5V
2A
10µF
0.47µF
1nF
47µF
100k
4.75k
97.6k
10µH
22pF
536k
GND
DA
OFF ON
LT3980
3980 TA01
RUN/SS BOOST
SYNC
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
ILOAD (A)
0 0.5
EFFICIENCY (%)
12
3980 TA01b
1.5
VIN = 12V
VIN = 48V
f = 400kHz
D = DIODES, INC. SBR3U100LP
VIN = 24V
85
70
60
55
50
95
90
80
75
65
LT3980
2
3980fa
For more information www.linear.com/LT3980
The denotes the specifications which apply over the full operating temper-
ature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)
elecTrical characTerisTics
VIN, RUN/SS Voltage (Note 5) ................................... 80V
BOOST Pin Voltage ...................................................75V
BOOST Pin Above SW Pin .........................................30V
FB, RT, VC Voltage .......................................................5V
PG, BD, SYNC Voltage ..............................................25V
(Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage 3 3.6 V
VIN Overvoltage Lockout 58 61.5 64 V
Quiescent Current from VIN VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
20
70
0.01
35
120
0.5
60
160
µA
µA
µA
Operating Junction Temperature Range (Note 2)
LT3980E ............................................. 40°C to 125°C
LT3980I .............................................. 40°C to 125°C
LT3980H ............................................ 40°C to 150°C
Storage Temperature Range ...................65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300°C
1
2
3
4
5
6
7
14
13
12
11
10
9
8
DA
NC
VIN
SW
BOOST
BD
RUN/SS
SYNC
NC
PG
FB
VC
RT
GND
TOP VIEW
DE14 PACKAGE
14-LEAD (4mm × 3mm) PLASTIC DFN
15
GND
θJA = 45°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
SYNC
PG
NC
FB
NC
VC
RT
GND
16
15
14
13
12
11
10
9
NC
DA
VIN
NC
SW
BOOST
BD
RUN/SS
TOP VIEW
MSE PACKAGE
16-LEAD PLASTIC MSOP
17
GND
θJA = 45°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
pin conFiGuraTion
orDer inFormaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3980EDE#PBF LT3980EDE#TRPBF 3980 14-Lead (3mm × 4mm) Plastic DFN 40°C to 125°C
LT3980IDE#PBF LT3980IDE#TRPBF 3980 14-Lead (3mm × 4mm) Plastic DFN 40°C to 125°C
LT3980EMSE#PBF LT3980EMSE#TRPBF 3980 16-Lead Plastic MSOP 40°C to 125°C
LT3980IMSE#PBF LT3980IMSE#TRPBF 3980 16-Lead Plastic MSOP 40°C to 125°C
LT3980HMSE#PBF LT3980HMSE#TRPBF 3980 16-Lead Plastic MSOP 40°C to 150°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/
absoluTe maximum raTinGs
LT3980
3
3980fa
For more information www.linear.com/LT3980
PARAMETER CONDITIONS MIN TYP MAX UNITS
Quiescent Current from BD VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
55
0.01
82
1
0.5
115
5
µA
µA
µA
Feedback Voltage
782
770
790
790
798
805
mV
mV
FB Pin Bias Current (Note 3) VFB = 0.8V, VC = 1.2V 10 40 nA
FB Voltage Line Regulation 4V < VIN < 56V 0.002 0.01 %/V
Error Amp gm500 µmho
Error Amp Gain 1800
VC Source Current 60 µA
VC Sink Current 60 µA
VC Pin to Switch Current Gain 3.87 A/V
VC Clamp Voltage 2 V
Switching Frequency RT = 8.66k
RT = 29.4k
RT = 187k
1.95
0.86
195
2.25
1.07
225
2.55
1.27
255
MHz
MHz
kHz
Minimum Switch Off-Time 140 230 nS
Switch Current Limit Duty Cycle = 5% 3.3 4 4.9 A
Switch VCESAT ISW = 2A 540 mV
DA Pin Current to Pause OSC 1.9 2.4 3 A
Boost Schottky Reverse Leakage VBD = 0V 0.02 2 µA
Minimum Boost Voltage (Note 4) 1.7 2.2 V
BOOST Pin Current ISW = 2A 40 55 mA
RUN/SS Pin Current VRUN/SS = 2.5V 6 10 µA
RUN/SS Input Voltage High 2.5 V
RUN/SS Input Voltage Low 0.4 V
PG Threshold Offset from Feedback Voltage VFB Rising 50 65 80 mV
PG Hysteresis 14 mV
PG Leakage VPG = 5V 0.1 1 µA
PG Sink Current VPG = 0.4V 200 700 µA
SYNC Threshold 0.575 0.675 0.775 V
SYNC Pin Bias Current VSYNC = 0V 0.1 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 LT3980E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3980I specifications are
guaranteed over the –40°C to 125°C temperature range. The LT3980H
specifications are guaranteed over the –40°C to 150°C operating
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 3: Bias current flows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5: Absolute Maximum at VIN and RUN/SS pins is 80V for non-
repetitive 1 minute transients, and 60V for continuous operation.
The denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
elecTrical characTerisTics
LT3980
4
3980fa
For more information www.linear.com/LT3980
INPUT VOLTAGE (V)
0
SUPPLY CURRENT (µA)
3980 G04
50
30
10 20
10
130
110
90
70
30 40 50 60
VOUT = 3.3V
DUTY CYCLE (%)
0
SWITCH CURRENT LIMIT (A)
40
3980 G08
3.5
20 60
2.5
2.0
5.0
4.5
4.0
3.0
80 100
TYPICAL
MINIMUM
TEMPERATURE (°C)
–50
SUPPLY CURRENT (µA)
350
25
3980 G05
200
100
–25 0 50
50
0
400
300
250
150
75 100 150125
VIN = 12V
VOUT = 3.3V
CATCH DIODE: DIODES, INC. PDS360
INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
ILOAD (A)
0 0.5
EFFICIENCY (%)
12
3980 G02
1.5
VIN = 12V
VIN = 48V
f = 400kHz
D = DIODES, INC. SBR3U100LP
VIN = 24V
85
70
60
55
50
95
90
80
75
65
INPUT VOLTAGE (V)
LOAD CURRENT (A)
3980 G07
2.0
3.5
3.0
2.5
TYPICAL
MINIMUM
5 10 20 30 40 5015 25 35 45 6055
INPUT VOLTAGE (V)
LOAD CURRENT (A)
3980 G06
2.0
3.5
3.0
2.5
TYPICAL
MINIMUM
5 10 20 30 40 5015 25 35 45 6055
TEMPERATURE (°C)
SWITCH CURRENT LIMIT (A)
4.0
4.5
5.5
5.0
3980 G09
3.5
3.0
2.0
2.5
6.5
6.0
DUTY CYCLE = 10%
–50 25–25 0 50 75 100 150125
Efficiency, VOUT = 5V
No-Load Supply Current
Maximum Load Current,
VOUT = 3.3V
Switch Current Limit Switch Current Limit
Maximum Load Current,
VOUT = 5V
No-Load Supply Current
TA = 25°C unless otherwise noted.
SWITCHING FREQUENCY (MHz)
0.20
EFFICIENCY (%)
POWER LOSS (W)
84
86
1.60
82
80
0.60 1.00
0.40 0.80 1.20 1.80
1.40 2.00
74
72
78
88
76
3.0
3.5
2.5
2.0
0.5
0
1.5
4.0
1.0
3980 G03
VIN = 12V
VOUT = 5V
IOUT = 2A
Typical perFormance characTerisTics
Efficiency, VOUT = 3.3V Efficiency vs Switching Frequency
ILOAD (A)
0 0.5
EFFICIENCY (%)
12
3980 G01
1.5
VIN = 12V
VIN = 48V
f = 400kHz
D = DIODES, INC. SBR3U100LP
VIN = 24V
85
70
60
55
50
90
80
75
65
LT3980
5
3980fa
For more information www.linear.com/LT3980
RUN/SS PIN VOLTAGE (V)
0
SWITCH CURRENT LIMIT (A)
1.5
3980 G17
4
2
0.5 1 2
1
0
7
6
5
3
2.5 3 3.5
FB PIN VOLTAGE (mV)
0
SWITCHING FREQUENCY (kHz)
800
1000
1200
600
3980 G14
600
400
200 400 800
500
100 300 700 900
200
0
TEMPERATURE (°C)
180
200
220
3980 G16
160
140
120
100
240
–50 25–25 0 50 75 100 150125
ILOAD = 1A
RUN/SS PIN VOLTAGE (V)
RUN/SS PIN CURRENT (µA)
16
20
24
3980 G18
12
8
4
0
0 10 20 30 40 50 60
SWITCH CURRENT (A)
0
BOOST PIN CURRENT (mA)
15
45
60
75
120
3980 G11
30
90
105
0 31 2
TEMPERATURE (°C)
FEEDBACK VOLTAGE (mV)
800
3980 G12
760
840
780
820
–50 25–25 0 50 75 100 150125
TEMPERATURE (°C)
FREQUENCY (MHz)
1.00
1.10
3880 G13
0.90
0.80
1.20
0.95
1.05
0.85
1.15
–50 25–25 0 50 75 100 150125
RF = 32.4k
SWITCH CURRENT (A)
0
400
500
700
3
3980 G10
300
200
1 2
100
0
600
VOLTAGE DROP (mV)
Boost Pin Current
vs Switch Current Feedback Voltage
Switching Frequency Frequency Foldback
Minimum Switch On-Time Soft-Start RUN/SS Pin Current
Switch Voltage Drop
TA = 25°C unless otherwise noted.
Typical perFormance characTerisTics
RT vs Frequency
RESISTANCE (kΩ)
1
2000
SWITCHING FREQUENCY (kHz)
2500
3000
10 100 1000
1500
1000
500
0
3500
3980 G15
LT3980
6
3980fa
For more information www.linear.com/LT3980
FB PIN ERROR VOLTAGE (mV)
–200
–50
VC PIN CURRENT (µA)
–20
0
20
0 200
50
3980 G20
–40
–100 100
40
10
–10
30
–30
Error Amp Output Current
TEMPERATURE (°C)
VC VOLTAGE (V)
1.50
2.00
2.50
3980 G23
1.00
0.50
0
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
–50 25–25 0 50 75 100 150125
3980 G25
IL
0.2A/DIV
VSW
5V/DIV
VOUT
10mV/DIV
5µs/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
TEMPERATURE (°C)
THRESHOLD VOLTAGE (%)
85
90
95
3980 G24
80
75
–50 25–25 0 50 75 100 150125
3980 G26
IL
0.2A/DIV
VSW
5V/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 110mA
1µs/DIV 3980 G27
IL
0.5A/DIV
VSW
5V/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 1A
1µs/DIV
Minimum Input Voltage,
VOUT = 3.3V
Minimum Input Voltage,
VOUT = 5V VC Voltages Power Good Threshold
Switching Waveforms:
Transition from Burst Mode
Operation to Full Frequency
Switching Waveforms: Full
Frequency Continuous Operation
Switching Waveforms:
Burst Mode Operation
TA = 25°C, unless otherwise noted.
Typical perFormance characTerisTics
BOOST DIODE CURRENT (A)
0
BOOST DIODE V
F
(V)
0.8
1.0
1.2
2.0
3980 G19
0.6
0.4
00.5 1.0 1.5
0.2
1.4
Boost Diode
1 10 100 1000 2000
f = 400kHz
ILOAD (mA)
INPUT VOLTAGE (V)
4
5
3980 G21
3
2
6
TO START
TO RUN
ILOAD (mA)
INPUT VOLTAGE (V)
6
7
3980 G22
5
4
8
1 10 100 1000 2000
TO START
TO RUN
f = 400kHz
LT3980
7
3980fa
For more information www.linear.com/LT3980
SYNC (Pin 1/Pin 1): This is the external clock synchro-
nization input. Ground this pin for low ripple Burst Mode
operation at low output loads. Tie to a voltage above 0.8V
to select pulse-skipping mode. Tie to a clock source for
synchronization. Clock edges should have rise and fall
times faster than 1µs. Tie pin to GND if not used. See the
Synchronization section in Applications Information.
NC (Pins 2, 13/Pins 3, 5, 13, 16): No Connect. These
pins are not connected to internal circuitry.
PG (Pin 3/Pin 2): The PG pin is the open collector output
of an internal comparator. PG remains low until the FB pin
is within 9% of the final regulation voltage. PG output is
valid when VIN is above 3.6V and RUN/SS is high.
FB (Pin 4/Pin 4): The LT3980 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
VC (Pin 5/Pin 6): The VC pin is the output of the internal
error amplifier. The voltage on this pin controls the peak
switch current. Tie an RC network from this pin to ground
to compensate the control loop.
RT (Pin 6/Pin 7): Oscillator Resistor Input. Connecting
a resistor to ground from this pin sets the switching
frequency.
GND (Pin 7, 15/Pin 8, 17): Ground. The exposed pads
must be soldered to the PCB.
RUN/SS (Pin 8/Pin 9): The RUN/SS pin is used to put the
LT3980 in shutdown mode. Tie to ground to shut down
the LT3980. Tie to 2.5V or more for normal operation. If
the shutdown feature is not used, tie this pin to the VIN
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
BD (Pin 9/Pin 10): This pin connects to the anode of the
boost Schottky diode. BD also supplies current to the
internal regulator.
BOOST (Pin 10/Pin 11): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 11/Pin 12): The SW pin is the output of the
internal power switch. Connect this pin to the inductor,
catch diode and boost capacitor.
VIN (Pin 12/Pin 14): The VIN pin supplies current to the
LT3980’s internal regulator and to the internal power switch.
This pin must be locally bypassed.
DA (Pin 14/Pin 15): This pin measures catch diode current
and pauses the oscillator during overcurrent conditions.
+
+
+
OSCILLATOR
100kHzTO2.4MHz
BurstMode
DETECT
VC CLAMP
SOFT-START
SLOPE COMP
R
VIN
VIN
RUN/SS
BOOST
SW
SWITCH
LATCH
VC
VOUT
C2
C3
CF
L1
D1
DISABLE
CC
RC
BD
RT
R2
GND
ERROR AMP
R1
FB
RT
C1
PG
0.725V
S
Q
Σ
DA
3680 BD
INTERNAL 0.79V REF
SYNC
block DiaGram
pin FuncTions
(DFN, MSOP)
LT3980
8
3980fa
For more information www.linear.com/LT3980
The LT3980 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables 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. An error amplifier measures the output
voltage through an external resistor divider tied to the FB
pin and servos the VC pin. If the error amplifiers 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 to
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the VIN pin,
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LT3980
in shutdown, disconnecting the output and reducing the
input current to less than 0.5µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
To further optimize efficiency, the LT3980 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 75µA in a typical application.
The oscillator reduces the LT3980’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. In addition, the LT3980 monitors the catch
diode current flowing through the DA pin and pauses the
oscillator during overcurrent conditions to keep inductor
current at safe levels.
The LT3980 contains a power good comparator which trips
when the FB pin is at 91% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3980 is
enabled and VIN is above 3.6V.
The LT3980 has an overvoltage protection feature which
disables switching action when the VIN goes above 61.5V
typical (58V minimum). When switching is disabled, the
LT3980 can safely sustain input voltages up to 62V.
operaTion
LT3980
9
3980fa
For more information www.linear.com/LT3980
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
RR V
V
OUT
12
0791=
.
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3980 uses a constant frequency PWM architecture
that can be programmed to switch from 100kHz to 2.4MHz
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 Figure 1.
SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
432
215
137
97.6
76.8
60.4
51.1
43.2
35.7
32.4
24.9
20
16.2
14
11
Figure 1. Switching Frequency vs RT Value
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, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (fSW(MAX)) for a
given application can be calculated as follows:
fVV
tVVV
SW MAXDOUT
ON MINDINSW
() ()
=+
+
()
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 (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high VIN/VOUT ratio. Also, as shown
in the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3980 switch has
finite minimum on and off times. The switch can turn on
for a minimum of ~200ns and turn off for a minimum of
~200ns. This means that the minimum and maximum
duty cycles are:
DC ft
DC ft
MINSWON MIN
MAXSWOFFMIN
=
=
()
()
1–
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on time (~200ns), and the tOFF(MIN) is
the minimum switch off time (~200ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
A good choice of switching frequency should allow ade-
quate input voltage range (see next section) and keep the
inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3980 applications de-
pends on switching frequency, Absolute Maximum Ratings
of the VIN and BOOST pins, and the operating mode.
The LT3980 can operate from input voltages of up to 58V,
and withstand voltages up to 80V. Note that while VIN is
above 61V typical (58V minimum and 64V maximum)
the part will keep the switch off and the output will not
be in regulation.
T
he switching frequency should be chosen according to
the following equation:
VVV
ft VV
IN MAXOUT D
SW ON MIN
DS
W
() ()
=++
where VIN(MAX) is the maximum typical operating input
voltage, VOUT is the output voltage, VD is the catch diode
applicaTions inFormaTion
LT3980
10
3980fa
For more information www.linear.com/LT3980
drop (~0.5V), VSW is the internal switch drop (~0.5V at
max load), fSW is the switching frequency (set by RT),
and tON(MIN) is the minimum switch on time (~200ns).
Note that a higher switching frequency will depress the
maximum operating input voltage. Conversely, a lower
switching frequency will be necessary to achieve safe
operation at high input voltages.
Input voltages up to 58V are acceptable regardless of the
switching frequency. In this mode, the LT3980 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain safe inductor current.
The minimum input voltage is determined by either the
LT3980’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
VVV
ft VV
IN MINOUT D
SW OFFMIN
DS
W
() ()
=++
1–
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch off time (200ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔIL increases with higher VIN or VOUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔIL = 0.4(IOUT(MAX))
where IOUT(MAX) is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT3980’s switch current limit (ILIM).
The peak inductor current is:
IL(PEAK) = IOUT(MAX) + ΔIL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ΔIL is the inductor
ripple current. The LT3980’s switch current limit (ILIM) is
4A at low duty cycles and decreases linearly to 3A at DC
= 0.8. The maximum output current is a function of the
inductor ripple current:
IOUT(MAX) = ILIMΔIL/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
LVV
fI
VV
V
OUT D
SW L
OUT D
IN MAX
=+
+
1–
()
where VD is the voltage drop of the catch diode (~0.4V),
VIN(MAX) is the maximum input voltage, VOUT is the output
voltage, fSW is the switching frequency (set by RT), and
L is in the inductor value.
The inductors RMS and saturation current rating must
be greater than the maximum load current. For robust
operation in fault conditions (start-up or short circuit) and
high input voltage (>40V), the saturation current should
be above 3.5A. 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 1 lists several vendors and suitable types.
Table 1. Inductor Vendors
VENDOR URL PART SERIES TYPE
Murata www.murata.com LQH55D Open
TDK www.component.tdk.com SLF10145 Shielded
Toko www.toko.com D75C
D75F
Shielded
Open
Sumida www.sumida.com CDRH74
CR75
CDRH8D43
Shielded
Open
Shielded
NEC www.nec-tokin.com MPLC073
MPBI0755
Shielded
Shielded
Vishay www.vishay.com IHLP2525CE01 Shielded
applicaTions inFormaTion
LT3980
11
3980fa
For more information www.linear.com/LT3980
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease 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. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details of
maximum output current and discontinuous mode oper-
ation, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (VOUT/VIN > 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3980 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 10µF to 22µF ceramic capacitor is
adequate to bypass the LT3980 and will easily handle the
ripple current. Note that larger input capacitance is required
when a lower switching frequency is used. 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 lower performance electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage rip-
ple at the LT3980 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 L
T3980 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3980. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3980 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3980’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 4.7µF ceramic ca-
pacitor can be used for local bypassing of the LT3980
input. However, the lower input capacitance will result in
increased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3980 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3980 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3980’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
CVf
OUT OUT SW
=100
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 the compensation network is also adjusted
to maintain the loop bandwidth. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer. See the Frequency Compensation
section to choose an appropriate compensation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
applicaTions inFormaTion
LT3980
12
3980fa
For more information www.linear.com/LT3980
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
Table 2. Capacitor Vendors
VENDOR URL PART SERIES COMMANDS
Panasonic www.panasonic.com Ceramic, Polymer,
Tantalum
EEF Series
Kemet www.kemet.com Ceramic, Tantalum T494, T495
Sanyo www.sanyovideo.com Ceramic, Polymer,
Tantalum
POSCAP
Murata www.murata.com Ceramic SOT-23
AVX www.avxcorp.com Ceramic, Tantalum SOT-23
Taiyo Yuden www.taiyo-yuden.com Ceramic TPS Series
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
ID(AVG) = IOUT (VIN – VOUT)/VIN
where IOUT is the output load current. The only reason to
consider a diode with a larger current rating than necessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3980 will keep the
switch off when VIN > 64V which allows the use of 64V
rated Schottky even when VIN ranges up to 80V.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3980 due to their piezoelectric nature.
When in Burst Mode operation, the LT3980’s switching
frequency depends on the load current, and at very light
loads the LT3980 can excite the ceramic capacitor at
audio frequencies, generating audible noise. Since the
LT3980 operates at a lower current limit during Burst
Mode operation, the noise is nearly silent to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Frequency Compensation
The LT3980 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3980 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequen-
cy compensation is provided by the components tied to
the VC pin, as shown in Figure 2. Generally a capacitor
(CC) and a resistor (RC) in series to ground are used. In
addition, there may be lower value capacitor in parallel.
This capacitor (CF) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
+
0.79V
SW
VC
gm =
500µmho
GND
3M
LT3980
3980 F02
R1
OUTPUT
ESR
CF
CC
RC
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
gm = 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
CPL
Figure 2. Model for Loop Response
applicaTions inFormaTion
LT3980
13
3980fa
For more information www.linear.com/LT3980
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature. The
LT1375 data sheet contains a more thorough discussion
of loop compensation and describes how to test the sta-
bility using a transient load. Figure 2 shows an equivalent
circuit for the LT3980 control loop. The error amplifier is a
transconductance amplifier with finite output impedance.
The power section, consisting of the modulator, power
switch and inductor, is modeled as a transconductance
amplifier generating an output current proportional to
the voltage at the VC pin. Note that the output capacitor
integrates this current, and that the capacitor on the VC pin
(CC) integrates the error amplifier output current, resulting
in two poles in the loop. In most cases a zero is required
and comes from either the output capacitor ESR or from
a resistor RC in series with CC. This simple model works
well as long as the value of the inductor is not too high
and the loop crossover frequency is much lower than the
switching frequency. A phase lead capacitor (CPL) across
the feedback divider may improve the transient response.
Figure 3 shows the transient response when the load
current is stepped from 0.5A to 1.5A and back to 0.5A.
Low Ripple Burst Mode Operation and Pulse-Skipping
Mode
The LT3980 is capable of operating in either low ripple
Burst Mode operation or pulse-skipping mode which are
selected using the SYNC pin. See the Synchronization
section for details.
To enhance efficiency at light loads, the LT3980 can be
operated in low ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst
Mode operation, the LT3980 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the
output capacitor. Because the LT3980 delivers power to
the output with single, low current pulses, the output ripple
is kept below 15mV for a typical application. In addition,
VIN and BD quiescent currents are reduced to typically
35µA and 82µA respectively during the sleep time. As
the load current decreases towards a no-load condition,
the percentage of time that the LT3980 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high efficiency even at very low loads.
See Figure 4. At higher output loads (above 140mA for
the front page application) the LT3980 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.
Figure 3. Transient Load Response of the LT3980 Front Page
Application as the Load Current Is Stepped from 0.5A to 1.5A
3980 F03
IL
0.5A/DIV
VOUT
100mV/DIV
50µs/DIV
VIN = 12V
VOUT = 5V
applicaTions inFormaTion
Figure 4. Burst Mode Operation
3980 F04
IL
0.2A/DIV
VSW
5V/DIV
VOUT
10mV/DIV
5µs/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
LT3980
14
3980fa
For more information www.linear.com/LT3980
If low quiescent current is not required the LT3980 can
operate in pulse-skipping mode. The benefit of this mode
is that the LT3980 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode operation. The front page application circuit
will switch at full frequency at output loads higher than
about 60mA.
BOOST and BIAS Pin Considerations
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
0.22µF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 5b). For lower output voltages the
boost diode can be tied to the input (Figure 5c), or to an-
other supply greater than 2.8V. Tying BD to VIN reduces
the maximum input voltage to 28V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
applicaTions inFormaTion
Figure 5. Three Circuits for Generating the Boost Voltage
VIN
BOOST
SW
BD
VIN
VOUT
4.7µF
C3
GND
LT3980 VIN
BOOST
SW
BD
VIN
VOUT
4.7µF
C3
D2
GND
LT3980
VIN
BOOST
SW
BD
VIN
VOUT
4.7µF
C3
GND
LT3980
3980 FO5
(5a) For VOUT > 2.8V (5b) For 2.5V < VOUT < 2.8V
(5c) For VOUT < 2.5V; VIN(MAX) = 30V
LT3980
15
3980fa
For more information www.linear.com/LT3980
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where VIN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to VIN; however, this restricts the
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinuous
and the effective duty cycle can be very high. This reduces
the minimum input voltage to approximately 300mV above
VOUT. At higher load currents, the inductor current is
continuous and the duty cycle is limited by the maximum
duty cycle of the LT3980, requiring a higher input voltage
to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3980,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
While operating with high boost voltages (>10V), it is
important to ensure that the power dissipation from the
boost circuit is not too high. See the Typical Performance
Characteristics section for the plot, BOOST Pin Current
vs Switch Current. Boost circuit power dissipation is
calculated as follows:
PBOOST = IBOOST VBOOST – SWDC
Where DC is the switch duty cycle, IBOOST is the boost pin
current, and VBOOST – VSW is the voltage between the boost
pin and switch pin. If the PBOOST > 0.5W, a Zener can be put
between the boost pin and the boost capacitor such that
the power is dissipated in the Zener instead of the LT3980.
The minimum operating voltage of an LT3980 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3980 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
applicaTions inFormaTion
1 10 100 1000 2000
f = 400kHz
ILOAD (mA)
INPUT VOLTAGE (V)
4
5
3980 F06a
3
2
6
TO START
TO RUN
ILOAD (mA)
INPUT VOLTAGE (V)
6
7
3980 F06b
5
4
8
1 10 100 1000 2000
TO START
TO RUN
f = 400kHz
LT3980
16
3980fa
For more information www.linear.com/LT3980
create a voltage ramp at this pin. Figure 7 shows the start-
up and shutdown waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20µA when the RUN/
SS pin reaches 2.5V.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.5V (this can be ground or a logic output). Tie
to a voltage above 0.8V to select pulse-skipping mode.
Synchronizing the LT3980 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
The LT3980 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will skip pulses to maintain regulation.
The LT3980 may be synchronized over a 150kHz to 2MHz
range. The RT resistor should be chosen to set the LT3980
switching frequency 25% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the RT should be chosen for 200kHz.
To assure reliable and safe operation the LT3980 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
the Inductor Selection section. It is also important to note
that slope compensation is set by the RT value: When the
sync frequency is much higher than the one set by RT, the
slope compensation will be significantly reduced which
may require a larger inductor value to prevent subharmonic
oscillation.
Shorted and Reversed Input Protection
If an inductor is chosen that will not saturate excessively,
an LT3980 buck regulator will tolerate a shorted output.
There is another situation to consider in systems where
the output will be held high when the input to the LT3980
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 LT3980’s output. If
the VIN pin is allowed to float and the RUN/SS pin is held
high (either by a logic signal or because it is tied to VIN),
then the LT3980’s internal circuitry will pull its quiescent
current through its SW pin. This is fine if your system can
tolerate a few mA in this state. If you ground the RUN/
SS pin, the SW pin current will drop to essentially zero.
However, if the VIN pin is grounded while the output is
held high, then parasitic diodes inside the LT3980 can pull
large currents from the output through the SW pin and the
applicaTions inFormaTion
Figure 7. To Soft-Start the LT3980, Add a
Resistor and Capacitor to the RUN/SS Pin
3680 F07
IL
1A/DIV
VRUN/SS
2V/DIV
VOUT
2V/DIV
RUN/SS
GND
RUN
15k
2ms/DIV
0.22µF
LT3980
17
3980fa
For more information www.linear.com/LT3980
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3980 Runs Only When the Input Is Present
VIN BOOST
GND FB
RUN/SS
VC
SW
D4
MBRS360
VIN
LT3980
3980 F08
VOUT
BACKUP
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
pacitors can cause problems if the LT3980 is plugged into
a live supply (see Linear Technology Application Note 88
for a complete discussion). The low loss ceramic capacitor,
combined with stray inductance in series with the power
source, forms an under damped tank circuit, and the
voltage at the VIN pin of the LT3980 can ring to twice the
nominal input voltage, possibly exceeding the LT3980’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3980 into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3980 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
first plot is the response with a 4.7µF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 10b. A 0.7Ω resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1µF capacitor improves high
frequency filtering. For high input voltages its impact on
efficiency is minor, reducing efficiency by 1.5 percent for
a 5V output at full load operating from 24V.
applicaTions inFormaTion
VIN pin. Figure 8 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
RPG
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO RUN/SS
VIAS TO PG
VIAS TO VIN
OUTLINE OF LOCAL
GROUND PLANE
3980 F09
L1 C2
RRT
RC
R2
R1
CC
VOUT
D1 C1
GND
VIAS TO SYNC
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 9 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT3980’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, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and VC nodes small so that the ground
traces will shield them 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 LT3980 to additional ground planes within the circuit
board and on the bottom side.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3980 circuits. However, these ca-
LT3980
18
3980fa
For more information www.linear.com/LT3980
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3980
cool. 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 LT3980. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
to ambient can be reduced to JA = 35°C/W or less. With
100 LFPM airflow, this resistance can fall by another 25%.
Further increases in airflow will lead to lower thermal re-
sistance. Because of the large output current capability of
the LT3980, it is possible to dissipate enough heat to raise
the junction temperature beyond the absolute maximum of
125°C. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches 125°C.
Power dissipation within the LT3980 can be estimated by
calculating the total power loss from an efficiency mea-
surement and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3980 power dissipation by the thermal resistance from
junction to ambient.
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 100
shows how to generate a bipolar output supply using a
buck regulator.
applicaTions inFormaTion
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3980 Is Connected to a Live Supply
+
LT3980
4.7µF
VIN
20V/DIV
IIN
10A/DIV
20µs/DIV
VIN
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
(10a)
(10b)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
LT3980
4.7µF0.1µF
0.7Ω VIN
20V/DIV
IIN
10A/DIV
20µs/DIV
DANGER
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM RATING
(10c)
+
LT3980
4.7µF
22µF
AI.EI.
3980 F10
VIN
20V/DIV
IIN
10A/DIV
20µs/DIV
+
LT3980
19
3980fa
For more information www.linear.com/LT3980
Typical applicaTions
5V Step-Down Converter
3.3V Step-Down Converter
SW
FB
VC
PG
RT
VIN BD
VIN
6.5V TO 58V
TRANSIENT
TO 80V
V
OUT
5V
2A
4.7µF
0.47µF
47µF
f = 400kHz
D
4.75k
97.6k
L
8.2µH
536k
GND
1nF
ON OFF
LT3980
3980 TA02
RUN/SS BOOST
SYNC
DA
100k
22pF
SW
FB
VC
PG
RT
VIN BD
VIN
4.3V TO 58V
TRANSIENT
TO 80V
V
OUT
3.3V
2A
4.7µF
0.47µF
47µF
f = 400kHz
D
4.75k
97.6k
L
6.8µH
GND
2.2nF
ON OFF
LT3980
3980 TA03
RUN/SS BOOST
SYNC 316k
DA
100k
22pF
2.5V Step-Down Converter
SW
FB
VC
PG
RT
VIN BD
VIN
4V TO 58V
TRANSIENT
TO 80V
V
OUT
2.5V
2A
4.7µF
F
47µF
f = 300kHz
D1
8.45k
137k
L
4.7µH
215k
GND
2.2nF
ON OFF
LT3980
D2
3980 TA04
RUN/SS BOOST
SYNC
DA
100k
22pF
LT3980
20
3980fa
For more information www.linear.com/LT3980
Typical applicaTions
1.8V Step-Down Converter
12V Step-Down Converter
SW
FB
VC
PG
RT
VIN BD
VIN
15V TO 58V
TRANSIENT
TO 80V
V
OUT
12V
2A
10µF
0.47µF
22µF
50k
f = 600kHz
D
12k
60.4k
L
15µH
GND
1nF
ON OFF
LT3980
3980 TA06
RUN/SS BOOST
SYNC 715k
DA
SW
FB
VC
PG
RT
VIN BD
VIN
3.5V TO 32V
VOUT
1.8V
2A
4.7µF
0.47µF
100µF
f = 400kHz
D
2.49k
97.6k
L
3.3µH
127k
GND
680pF
ON OFF
LT3980
3980 TA08
RUN/SS BOOST
SYNC
DA
100k
22pF
5V, 1.2MHz Step-Down Converter
SW
FB
VC
PG
RT
VIN BD
V
IN
8.6V TO 40V
TRANSIENT
TO 80V
V
OUT
5V
2A
4.7µF
0.47µF
22µF
f = 1.2MHz
D
4.75k
24.9k
L
4.7µH
GND
1nF
ON OFF
LT3980
3980 TA05
RUN/SS BOOST
SYNC
536k
DA
100k
22pF
LT3980
21
3980fa
For more information www.linear.com/LT3980
3.00 ±0.10
(2 SIDES)
4.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC
PACKAGE OUTLINE MO-229
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
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
1.70 ± 0.10
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
3.00 REF
1.70 ± 0.05
17
148
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DE14) DFN 0806 REV B
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
3.00 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
2.20 ±0.05
0.70 ±0.05
3.60
±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.25 ± 0.05
0.50 BSC
3.30 ±0.05
3.30 ±0.10
0.50 BSC
DE Package
14-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1708 Rev B)
packaGe DescripTion
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT3980
22
3980fa
For more information www.linear.com/LT3980
MSOP (MSE16) 0213 REV F
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
16
1615 141312 1110
12345678
9
9
18
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.
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ±0.038
(.0120 ±.0015)
TYP
0.50
(.0197)
BSC
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
0.1016 ±0.0508
(.004 ±.002)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0.280 ±0.076
(.011 ±.003)
REF
4.90 ±0.152
(.193 ±.006)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.12 REF
0.35
REF
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
packaGe DescripTion
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT3980
23
3980fa
For more information www.linear.com/LT3980
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 of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 10/13 Clarified Efficiency graph
Clarified graphs
Clarified SYNC pin description
Clarified graph
Clarified graph
Clarified Synchronization description
1
6
7
15
16
16
LT3980
24
3980fa
For more information www.linear.com/LT3980
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
LINEAR TECHNOLOGY CORPORATION 2009
LT 1013 REV A • PRINTED IN USA
(408) 432-1900 FAX: (408) 434-0507 www.linear.com/LT3980
PART NUMBER DESCRIPTION COMMENTS
LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency Micropower
Step-Down DC/DC Converter with POR Reset and Watchdog Timer
VIN: 3.6V to 36V (Transient to 60V), VOUT(MIN) = 0.8V,
IQ = 75µA, ISD <A, 3mm × 3mm QFN-16 Package
LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <A,
3mm × 3mm DFN-12
LT3970 40V, 350mA (IOUT), 2.2MHz High Efficiency Step-Down DC/DC
Converter with Only 2.5µA of Quiescent Current
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <A,
3mm × 3mm DFN-10 and MSOP-10 Packages
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, VOUT(MIN) = 0.78V, IQ = 70µA, ISD <A,
3mm × 3mm DFN-10 and MSOP-10E Packages
LT3685 36V with Transient Protection to 60V, Dual 2A (IOUT), 2.4MHz,
High Efficiency Step-Down DC/DC Converter
VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD <A,
3mm × 3mm DFN-10 and MSOP-10E Packages
LT3481 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
VIN: 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 50µA, ISD <A,
3mm × 3mm DFN-10 and MSOP-10E Packages
LT3684 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz,
High Efficiency Step-Down DC/DC Converter
VIN: 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 850µA, ISD <A,
3mm × 3mm DFN-10 and MSOP-10E Packages
LT3508 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz,
High Efficiency Step-Down DC/DC Converter
VIN: 3.7V to 37V, VOUT(MIN) = 0.8V, IQ = 4.6mA, ISD =A,
4mm × 4mm QFN-24 and TSSOP-16E Packages
LT3505 36V with Transient Protection to 40V, 1.4A (IOUT), 3MHz,
High Efficiency Step-Down DC/DC Converter
VIN: 3.6V to 34V, VOUT(MIN) = 0.78V, IQ = 2mA, ISD = 2µA,
3mm × 3mm DFN-8 and MSOP-8E Packages
LT3500 36V, 40VMAX, 2.5MHz High Efficiency Step-Down DC/DC Converter and
LDO Controller
VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA, ISD = 10µA,
3mm × 3mm DFN-10 Package
LT3507 36V, 2.5MHz, Triple (2.4A + 1.5A + 1.5A (IOUT)) with LDO Controller High
Efficiency Step-Down DC/DC Converter
VIN: 4V to 36V, VOUT(MIN) = 0.8V, IQ = 7mA, ISD <A,
5mm × 7mm QFN-38 Package
LT3437 60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter with Burst
Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD <A,
3mm × 3mm DFN-10 and TSSOP-16E Packages
LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC
Converters with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD <A,
TSSOP-16E Package
LT3434/LT3435 60V, 2.4A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC
Converters with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.2V, IQ = 100µA, ISD <A,
TSSOP-16E Package
LT1936 36V, 1.4A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1µA,
MS8E Package
LT3493 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down
DC/DC Converter
VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD <A,
2mm × 3mm DFN-6 Package
LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD < 25µA,
TSSOP-16 and TSSOP-16E Packages
SW
FB
VC
PG
RT
VIN BD
VIN
3.6V TO 32V
VOUT
1.2V
2A
4.7µF
0.47µF
100µF
f = 400kHz
D
14k
97.6k
L
3.3µH
GND
1nF
ON OFF
LT3980
3980 TA09
RUN/SS BOOST
SYNC
100k
52.3k
DA
1.2V Step-Down Converter
relaTeD parTs
Typical applicaTion