LT3680 - 36V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current

LT3680
36V, 3.5A, 2.4MHz
Step-Down Switching Regulator
with 75µA Quiescent Current
FEATURES
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DESCRIPTION
The LT®3680 is an adjustable frequency (200kHz to
2.4MHz) monolithic buck switching regulator that accepts
input voltages up to 36V. A high efficiency 95m 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. 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 LT3680 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 LT3680 is
available in 10-Pin MSOP and 3mm × 3mm DFN packages
with exposed pads for low thermal resistance.
Wide Input Voltage Range: 3.6V to 36V
3.5A Maximum Output Current
Low Ripple (<15mVP-P) Burst Mode® Operation:
IQ = 75μA at 12VIN to 3.3VOUT
Adjustable Switching Frequency: 200kHz to 2.4MHz
Low Shutdown Current: IQ < 0.5μA
Integrated Boost Diode
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Saturating Switch Design: 95m On-Resistance
0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 30V
Thermal Protection
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
APPLICATIONS
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L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Automotive Battery Regulation
Power for Portable Products
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
TYPICAL APPLICATION
5V Step-Down Converter
Efficiency
VOUT
5V
3.5A
VIN
6.3V TO 36V
90
BOOST
0.47μF 4.7μH
15k
10μF
EFFICIENCY (%)
RUN/SS
VC
LT3680
SW
RT
680pF
PG
63.4k
SYNC
VIN = 12V
BD
VIN
OFF ON
100
VIN = 24V
70
60
536k
GND
VIN = 34V
80
VOUT = 5V
L = 4.7μH
f = 600kHz
FB
47μF
100k
3680 TA01a
50
0
0.5
2
1.5
1
2.5
OUTPUT CURRENT (A)
3
3.5
3680 G01
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LT3680
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, RUN/SS Voltage .................................................36V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, VC Voltage .......................................................5V
PG, BD, SYNC Voltage ..............................................30V
Operating Junction Temperature Range (Note 2)
LT3680E............................................. –40°C to 125°C
LT3680I.............................................. –40°C to 125°C
LT3680H ............................................ –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300°C
PIN CONFIGURATION
TOP VIEW
BD
1
10 RT
BOOST
2
9 VC
SW
3
VIN
4
7 PG
RUN/SS
5
6 SYNC
11
TOP VIEW
BD
BOOST
SW
VIN
RUN/SS
8 FB
DD PACKAGE
10-LEAD (3mm s 3mm) PLASTIC DFN
θJA = 45°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
11
10
9
8
7
6
RT
VC
FB
PG
SYNC
MSE PACKAGE
10-LEAD PLASTIC MSOP
θJA = 45°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3680EDD#PBF
LT3680EDD#TRPBF
LCYK
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3680IDD#PBF
LT3680IDD#TRPBF
LCYK
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3680EMSE#PBF
LT3680EMSE#TRPBF
LTCYM
10-Lead Plastic MSOP
–40°C to 125°C
LT3680IMSE#PBF
LT3680IMSE#TRPBF
LTCYM
10-Lead Plastic MSOP
–40°C to 125°C
LT3680HMSE#PBF
LT3680HMSE#TRPBF
LTCYM
10-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/
3680fb
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LT3680
ELECTRICAL CHARACTERISTICS
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)
PARAMETER
Minimum Input Voltage
Quiescent Current from VIN
Quiescent Current from BD
CONDITIONS
MIN
●
VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
●
●
Minimum Bias Voltage (BD Pin)
Feedback Voltage
●
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
Error Amp gm
Error Amp Gain
VC Source Current
VC Sink Current
VC Pin to Switch Current Gain
VC Clamp Voltage
Switching Frequency
Minimum Switch Off-Time
Switch Current Limit
Switch VCESAT
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
RUN/SS Pin Current
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
PG Hysteresis
PG Leakage
PG Sink Current
SYNC Low Threshold
VFB = 0.8V, VC = 0.4V
4V < VIN < 36V
780
775
●
RT = 8.66k
RT = 29.4k
RT = 187k
2.2
1.0
200
●
Duty Cycle = 5%
ISW = 3.5A
VBOOST = 10V, VBD = 0V
4.6
●
ISW = 1A
VRUN/SS = 2.5V
TYP
3
0.01
30
120
0.01
90
1
2.7
790
790
MAX
3.6
0.5
65
160
0.5
130
5
3
800
805
UNITS
V
μA
μA
μA
μA
μA
μA
V
mV
mV
10
0.002
500
2000
60
60
5.3
2.0
2.45
1.1
230
60
5.4
335
0.02
1.5
35
5
40
0.01
nA
%/V
μMho
2
2.0
50
8
2.5
0.2
VFB Rising
VPG = 5V
VPG = 0.4V
●
200
0.5
65
10
0.1
800
SYNC High Threshold
SYNC Pin Bias Current
2.7
1.25
260
150
6.0
1
0.7
VSYNC = 0V
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 LT3680E 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 LT3680I specifications are
0.1
μA
μA
A/V
V
MHz
MHz
kHz
nS
A
mV
μA
V
mA
μA
V
V
mV
mV
μA
μA
V
V
μA
guaranteed over the –40°C to 125°C temperature range. The LT3680H
specifications are guaranteed over the –40°C to 150°C operating
temerature 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.
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LT3680
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
Efficiency
Efficiency
Efficiency
100
100
100
3.0
90
2.5
80
2.0
70
1.5
VIN = 12V
EFFICIENCY (%)
VIN = 24V
70
60
50
0.5
VIN = 24V
70
60
VOUT = 5V
L = 4.7μH
f = 600kHz
0
VIN = 34V
80
2
1.5
1
2.5
OUTPUT CURRENT (A)
3
50
3.5
0
0.5
50
2
1.5
1
2.5
OUTPUT CURRENT (A)
3
3680 G01
No Load Supply Current
VOUT = 3.3V
50
30
10
15
10
20
25
INPUT VOLTAGE (V)
5
30
300
INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
250
200
150
5
25 50 75 100 125 150
TEMPERATURE (°C)
4.0
VOUT = 5V
TA = 25°C
L = 4.7μH
f = 600kHz
20
25
15
INPUT VOLTAGE (V)
30
Switch Current Limit
6.5
6.0
5.5
SWITCH CURRENT LIMIT (A)
SWITCH CURRENT LIMIT(A)
MINIMUM
10
3680 G06
Switch Current Limit
4.5
VOUT = 3.3V
TA = 25°C
L = 4.7μH
f = 600kHz
2.5
0
6.0
5.0
4.5
4.0
DUTY CYCLE = 10 %
5.5
5.0
DUTY CYCLE = 90 %
4.5
4.0
3.5
3.0
3.5
2.5
3.0
3.0
10
3.5
3.0
Maximum Load Current
5
MINIMUM
4.0
3680 G05
5.5
3.5
4.5
100
0
–50 –25
35
TYPICAL
0.5
TYPICAL
5.0
VIN = 12V
VOUT = 3.3V
3680 G04
5.0
3.5
Maximum Load Current
50
0
3
CATCH DIODE: DIODES, INC. PDS360
SUPPLY CURRENT (μA)
SUPPLY CURRENT (μA)
70
2
1.5
1
2.5
OUTPUT CURRENT (A)
5.5
350
90
0.5
3680 G03
400
110
LOAD CURRENT (A)
0
3.5
1.0
3680 G02
No Load Supply Current
130
VIN = 12V
VOUT = 5V
L = 4.7μH
f = 600kHz
60
VOUT = 3.3V
L = 3.3μH
f = 600kHz
LOAD CURRENT (A)
EFFICIENCY (%)
VIN = 34V
80
TOTAL POWER LOSS (W)
VIN = 12V
90
EFFICIENCY (%)
90
20
25
15
INPUT VOLTAGE (V)
30
3680 G07
0
20
60
40
DUTY CYCLE (%)
80
100
3680 G08
2.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G09
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LT3680
TYPICAL PERFORMANCE CHARACTERISTICS
Boost Pin Current
120
600
105
500
400
300
200
90
75
60
45
30
100
0
1
0
2
4
3
SWITCH CURRENT (A)
5
0
1
2
3
SWITCH CURRENT (A)
800
780
4
760
–50 –25
5
Switching Frequency
Minimum Switch On-Time
Frequency Foldback
140
1.00
0.95
0.90
1000
0.85
0.80
–50 –25
MINIMUM SWITCH ON TIME (ns)
SWITCHING FREQUENCY (kHz)
1.15
1.05
800
600
400
200
25 50 75 100 125 150
TEMPERATURE (°C)
0
100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
3680 G13
5
4
3
2
40
20
0
3
3.5
3680 G16
0
25 50 75 100 125 150
TEMPERATURE (°C)
Boost Diode
12
1.4
10
1.2
8
6
4
2
1
2.5
2
1.5
RUN/SS PIN VOLTAGE (V)
60
3680 G15
BOOST DIODE VF (V)
RUN/SS PIN CURRENT (μA)
6
1
80
RUN/SS Pin Current
Soft-Start
0.5
100
3680 G14
7
0
120
0
–50 –25
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G12
1200
1.20
1.10
0
3680 G11
3680 G10
SWITCH CURRENT LIMIT (A)
820
15
0
FREQUENCY (MHz)
Feedback Voltage
840
FEEDBACK VOLTAGE (mV)
700
BOOST PIN CURRENT (mA)
VOLTAGE DROP (mV)
Switch Voltage Drop
TA = 25°C unless otherwise noted.
1.0
0.8
0.6
0.4
0.2
0
0
5
20
30
15
25
10
RUN/SS PIN VOLTAGE (V)
35
3680 G17
0
0
0.5
1.0
1.5
BOOST DIODE CURRENT (A)
2.0
3680 G18
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LT3680
TYPICAL PERFORMANCE CHARACTERISTICS
Error Amp Output Current
TA = 25°C unless otherwise noted.
Minimum Input Voltage
50
Minimum Input Voltage
5.0
6.5
40
4.5
20
10
0
–10
–20
–30
4.0
3.5
3.0
2.5
–40
–50
–200
2.0
–100
100
0
FB PIN ERROR VOLTAGE (mV)
6.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
VC PIN CURRENT (μA)
30
VOUT = 3.3V
TA = 25°C
L = 4.7μH
f = 800kHz
1
200
VC Voltages
4.0
10000
10
100
1000
LOAD CURRENT (mA)
1
10
100
1000
LOAD CURRENT (mA)
10000
3680 G21
Power Good Threshold
Switching Waveforms; Burst Mode
95
THRESHOLD VOLTAGE (%)
2.00
CURRENT LIMIT CLAMP
VC VOLTAGE (V)
VOUT = 5V
TA = 25 °C
L = 4.7μH
f = 800kHz
3680 G20
2.50
1.50
SWITCHING THRESHOLD
0.50
0
–50 –25
5.0
4.5
3680 G19
1.00
5.5
0
25 50 75 100 125 150
TEMPERATURE (°C)
VSW
5V/DIV
90
85
IL
0.2A/DIV
80
VOUT
10mV/DIV
75
–50 –25
3680 G22
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G23
Switching Waveforms; Transition
from Burst Mode to Full Frequency
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
5μs/DIV
3680 G24
Switching Waveforms; Full
Frequency Continuous Operation
VSW
5V/DIV
VSW
5V/DIV
IL
0.2A/DIV
IL
0.5A/DIV
VOUT
10mV/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 110mA
1μs/DIV
3680 G25
VIN = 12V
VOUT = 3.3V
ILOAD = 1A
1μs/DIV
3680 G26
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LT3680
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator.
BOOST (Pin 2): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
VIN (Pin 4): The VIN pin supplies current to the LT3680’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3680 in shutdown mode. Tie to ground to shut down
the LT3680. 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.
SYNC (Pin 6): 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.
Clock edges should have rise and fall times faster than 1μs.
Do not leave pin floating. See Synchronization section in
Applications Information.
PG (Pin 7): 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 8): The LT3680 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
VC (Pin 9): 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 10): Oscillator Resistor Input. Connecting a resistor
to ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
VIN
4
VIN
–
+
C1
INTERNAL 0.79V REF
5
10
RUN/SS
3
SLOPE COMP
BD
SWITCH
LATCH
BOOST
RT
OSCILLATOR
200kHzTO2.4MHz
C3
Q
S
SW
DISABLE
SYNC
L1
VOUT
3
C2
D1
BurstMode
DETECT
SOFT-START
7
2
R
RT
6
1
PG
ERROR AMP
+
–
+
–
0.725V
GND
11
FB
VC CLAMP
VC
9
CC
RC
CF
8
R2
R1
3680 BD
3680fb
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LT3680
OPERATION
The LT3680 is a constant frequency, current mode stepdown 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 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 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 LT3680
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 LT3680 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 LT3680’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 LT3680 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 LT3680 is
enabled and VIN is above 3.6V.
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LT3680
APPLICATIONS INFORMATION
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors according to:
⎛ V
⎞
R1= R2 ⎜ OUT – 1⎟
⎝ 0.79 V ⎠
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3680 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz 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)
0.2
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
2.2
2.4
215
140
100
78.7
63.4
53.6
45.3
39.2
34
26.7
22.1
18.2
15
12.7
10.7
9.09
Figure 1. Switching Frequency vs. RT Value
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff 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:
fSW(MAX ) =
VD + VOUT
tON(MIN) ( VD + VIN – VSW )
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 LT3680 switch has finite
minimum on and off times. The switch can turn on for a
minimum of ~150ns and turn off for a minimum of ~150ns.
Typical minimum on time at 25°C is 80ns. This means that
the minimum and maximum duty cycles are:
DCMIN = fSW tON(MIN)
DCMAX = 1– fSW tOFF(MIN)
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on time (~150ns), and the tOFF(MIN) is
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
A good choice of switching frequency should allow adequate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3680 applications
depends on switching frequency and Absolute Maximum Ratings of the VIN and BOOST pins (36V and 56V
respectively).
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following equation:
VIN(MAX ) =
VOUT + VD
–V +V
fSW tON(MIN) D SW
where VIN(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VD is the catch diode 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 (~100ns). Note that
a higher switching frequency will depress the maximum
3680fb
9
LT3680
APPLICATIONS INFORMATION
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage
transients of up to 36V are acceptable regardless of the
switching frequency. In this mode, the LT3680 may enter
pulse skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
The minimum input voltage is determined by either the
LT3680’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:
VIN(MIN) =
VOUT + VD
–V +V
1– fSW tOFF(MIN) D SW
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch off time (150ns). 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 LT3680’s switch current limit (ILIM).
The peak inductor current is:
IL(PEAK) = IOUT(MAX) + ΔIL/2
ripple current. The LT3680’s switch current limit (ILIM) is
5.5A at low duty cycles and decreases linearly to 4.5A at
DC = 0.8. The maximum output current is a function of
the inductor ripple current:
IOUT(MAX) = ILIM – ΔIL/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:
⎛V +V ⎞⎛ V +V ⎞
L = ⎜ OUT D ⎟ ⎜ 1– OUT D ⎟
VIN(MAX ) ⎠
⎝ fSWΔ IL ⎠ ⎝
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 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 short circuit) and high input voltage (>30V),
the saturation current should be above 5A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1 , 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.componenttdk.com
SLF10145
Shielded
Toko
www.toko.com
D75C
D75F
Shielded
Open
Sumida
www.sumida.com
CDRH74
CR75
CDRH8D43
Shielded
Open
Shielded
NEC
www.nec.com
MPLC073
MPBI0755
Shielded
Shielded
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ΔIL is the inductor
3680fb
10
LT3680
APPLICATIONS INFORMATION
Of course, such a simple design guide will not always result 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 3.5A, 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 operation, 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 LT3680 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 LT3680 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 supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3680 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 LT3680 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 LT3680. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3680 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3680’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 4.7μF ceramic capacitor can be used for local bypassing of the LT3680 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 LT3680 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
LT3680 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
LT3680’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT =
100
VOUT fSW
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
3680fb
11
LT3680
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
COMMANDS
Polymer,
EEF Series
Tantalum
Kemet
(864) 963-6300
www.kemet.com
Ceramic,
Tantalum
Sanyo
(408) 749-9714
www.sanyovideo.com
T494, T495
Ceramic,
Polymer,
POSCAP
Tantalum
Murata
(408) 436-1300
AVX
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
Taiyo Yuden
(864) 963-6300
www.taiyo-yuden.com
capacitor. Low ESR is important, so choose one that is
intended for use in switching regulators. The ESR should
be specified by the supplier, and should be 0.05 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.
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. Table
3 lists several Schottky diodes and their manufacturers.
TPS Series
Ceramic
Table 3. Diode Vendors
PART NUMBER
VR
(V)
IAVE
(A)
VF AT 3A
(mV)
On Semiconductor
MBRA340
40
3
500
Diodes Inc.
PDS340
B340A
B340LA
40
40
40
3
3
3
500
500
450
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3680 due to their piezoelectric
nature. When in Burst Mode operation, the LT3680’s
switching frequency depends on the load current, and at
very light loads the LT3680 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since
the LT3680 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.
3680fb
12
LT3680
APPLICATIONS INFORMATION
LT3680
CURRENT MODE
POWER STAGE
gm = 5.3mho
SW
ERROR
AMPLIFIER
OUTPUT
R1
CPL
FB
gm =
500μmho
+
The LT3680 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3680 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. Frequency
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.
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 1A to 3A and back to 1A.
–
Frequency Compensation
ESR
0.8V
C1
+
3M
C1
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 compensation 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 stability using a transient load. Figure 2 shows an equivalent
circuit for the LT3680 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
VC
CF
POLYMER
OR
TANTALUM
GND
RC
CERAMIC
R2
CC
3680 F02
Figure 2. Model for Loop Response
VOUT
100mV/DIV
IL
1A/DIV
VIN = 12V
VOUT = 3.3V
10μs/DIV
3680 F03
Figure 3. Transient Load Response of the LT3680 Front Page
Application as the Load Current is Stepped from 1A to 3A.
VOUT = 5V
3680fb
13
LT3680
APPLICATIONS INFORMATION
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3680 is capable of operating in either Low-Ripple Burst
Mode or pulse-skipping mode which are selected using the
SYNC pin. See the Synchronization section for details.
To enhance efficiency at light loads, the LT3680 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 LT3680 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 LT3680 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 30μA and
90μA respectively during the sleep time. As the load current
decreases towards a no load condition, the percentage of
time that the LT3680 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 LT3680 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 is seamless, and will not disturb
the output voltage.
If low quiescent current is not required the LT3680 can
operate in Pulse-Skip mode. The benefit of this mode is
that the LT3680 will enter full frequency standard PWM
VSW
5V/DIV
IL
0.2A/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
5μs/DIV
Figure 4. Burst Mode Operation
3680 F04
operation at a lower output load current than when in Burst
Mode. The front page application circuit will switch at full
frequency at output loads higher than about 60mA. Select
pulse-skipping mode by applying a clock signal or a DC
voltage higher than 0.8V to the SYNC pin.
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
another 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.
The minimum operating voltage of an LT3680 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 LT3680 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
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
3680fb
14
LT3680
APPLICATIONS INFORMATION
6.0
VOUT
BD
5.5
TO START
(WORST CASE)
VIN
VIN
LT3680
GND
4.7μF
INPUT VOLTAGE (V)
BOOST
C3
SW
5.0
4.5
4.0
TO RUN
3.5
3.0
(5a) For VOUT > 2.8V
VOUT = 3.3V
TA = 25°C
L = 8.2μH
f = 700kHz
2.5
2.0
VOUT
BD
BOOST
VIN
VIN
LT3680
TO START
(WORST CASE)
SW
(5b) For 2.5V < VOUT < 2.8V
VOUT
LT3680
5.0
TO RUN
4.0
VOUT = 5V
TA = 25°C
L = 8.2μH
f = 700kHz
2.0
BOOST
VIN
6.0
3.0
BD
VIN
10000
8.0
INPUT VOLTAGE (V)
GND
100
1000
LOAD CURRENT (mA)
C3
7.0
4.7μF
10
1
D2
1
C3
10
100
1000
LOAD CURRENT (mA)
10000
3680 F06
4.7μF
GND
SW
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3680 FO5
(5c) For VOUT < 2.5V; VIN(MAX) = 30V
Figure 5. Three Circuits For Generating The Boost Voltage
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 LT3680, requiring a higher input voltage
to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3680,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 7 shows the startup and shut-down 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 0.3V (this can be ground or a logic output).
3680fb
15
LT3680
APPLICATIONS INFORMATION
IL
1A/DIV
RUN
15k
RUN/SS
0.22μF
VRUN/SS
2V/DIV
GND
VOUT
2V/DIV
2ms/DIV
3680 F07
Figure 7. To Soft-Start the LT3680, Add a Resisitor
and Capacitor to the RUN/SS Pin
Synchronizing the LT3680 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 0.3V
and peaks that are above 0.8V (up to 6V).
The LT3680 will not enter Burst Mode at low output loads
while synchronized to an external clock, but instead will
skip pulses to maintain regulation.
The LT3680 may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3680
switching frequency 20% 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 LT3680 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. 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 the inductor is chosen so that it won’t saturate excessively, an LT3680 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
LT3680 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3680’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 LT3680’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
LT3680 can pull large currents from the output through
the SW pin and the 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.
D4
MBRS140
VIN
VIN
BOOST
LT3680
RUN/SS
VOUT
SW
VC
GND FB
BACKUP
3680 F08
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 LT3680
Runs Only When the Input is Present
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 LT3680’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
3680fb
16
LT3680
APPLICATIONS INFORMATION
L1
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 LT3680 to additional ground planes within the circuit
board and on the bottom side.
C2
VOUT
CC
RRT
RC
Hot Plugging Safely
R2
R1
D1
C1
RPG
GND
3680 F09
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO VIN
VIAS TO RUN/SS
OUTLINE OF LOCAL
GROUND PLANE
VIAS TO PG
VIAS TO SYNC
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3680 circuits. However, these capacitors can cause problems if the LT3680 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
DANGER
VIN
20V/DIV
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM RATING
LT3680
+
4.7μF
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
IIN
10A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(10a)
0.7W
LT3680
VIN
20V/DIV
+
0.1μF
4.7μF
IIN
10A/DIV
(10b)
LT3680
+
22μF
35V
AI.EI.
20μs/DIV
VIN
20V/DIV
+
4.7μF
IIN
10A/DIV
(10c)
20μs/DIV
3680 F10
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3680 is Connected to a Live Supply
3680fb
17
LT3680
APPLICATIONS INFORMATION
voltage at the VIN pin of the LT3680 can ring to twice the
nominal input voltage, possibly exceeding the LT3680’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3680 into an
energized supply, the input network should be designed to
prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3680 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 0.7 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.
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 resistance. Because of the large output current capability of
the LT3680, 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.
High Temperature Considerations
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.
The PCB must provide heat sinking to keep the LT3680
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 LT3680. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
Power dissipation within the LT3680 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
LT3680 power dissipation by the thermal resistance from
junction to ambient.
Other Linear Technology Publications
TYPICAL APPLICATIONS
5V Step-Down Converter
VOUT
5V
3.5A
VIN
6.3V TO 36V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
10μF
LT3680
SW
D
RT
15k
L
4.7μH
PG
SYNC
63.4k
680pF
f = 600kHz
536k
GND
FB
47μF
100k
3680 TA02
D: ON SEMI MBRA340
L: NEC MPLC0730L4R7
3680fb
18
LT3680
TYPICAL APPLICATIONS
3.3V Step-Down Converter
VOUT
3.3V
3.5A
VIN
4.4V TO 36V
VIN
BD
RUN/SS
ON OFF
BOOST
L
3.3μH
0.47μF
VC
4.7μF
SW
LT3680
D
RT
19k
PG
SYNC
63.4k
316k
FB
GND
680pF
22μF
100k
f = 600kHz
3680 TA03
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
2.5V Step-Down Converter
VOUT
2.5V
3.5A
VIN
4V TO 36V
VIN
BD
RUN/SS
ON OFF
D2
BOOST
1μF
VC
4.7μF
LT3680
SW
D1
RT
15.4k
L
3.3μH
PG
215k
SYNC
63.4k
680pF
f = 600kHz
GND
FB
47μF
100k
3680 TA04
D1: ON SEMI MBRA340
D2: MBR0540
L: NEC MPLC0730L3R3
3680fb
19
LT3680
TYPICAL APPLICATIONS
5V, 2MHz Step-Down Converter
VOUT
5V
2.5A
VIN
8.6V TO 22V
TRANSIENT TO 36V
VIN
BD
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
SW
LT3680
D
RT
15k
L
2.2μH
PG
536k
SYNC
12.7k
FB
GND
680pF
22μF
100k
f = 2MHz
3680 TA05
D: ON SEMI MBRA340
L: NEC MPLC0730L2R2
12V Step-Down Converter
VOUT
12V
3.5A
VIN
15V TO 36V
VIN
BD
RUN/SS
ON OFF
BOOST
0.47μF
VC
10μF
LT3680
SW
D
RT
17.4k
L
8.2μH
PG
715k
SYNC
63.4k
GND
680pF
f = 600kHz
FB
47μF
50k
3680 TA06
D: ON SEMI MBRA340
L: NEC MBP107558R2P
3680fb
20
LT3680
TYPICAL APPLICATIONS
1.8V Step-Down Converter
VOUT
1.8V
3.5A
VIN
3.5V TO 27V
VIN
BD
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3680
SW
D
RT
16.9k
L
3.3μH
PG
SYNC
78.7k
680pF
f = 500kHz
127k
GND
FB
47μF
100k
3680 TA08
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
3680fb
21
LT3680
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
0.675 p0.05
3.50 p0.05
1.65 p0.05
2.15 p0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 p 0.05
0.50
BSC
2.38 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 p0.10
(4 SIDES)
R = 0.115
TYP
6
0.38 p 0.10
10
1.65 p 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
0.200 REF
1
0.25 p 0.05
0.50 BSC
0.75 p0.05
0.00 – 0.05
2.38 p0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
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
3680fb
22
LT3680
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev B)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.794 p 0.102
(.110 p .004)
5.23
(.206)
MIN
0.889 p 0.127
(.035 p .005)
1
2.06 p 0.102
(.081 p .004)
1.83 p 0.102
(.072 p .004)
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
10
0.50
0.305 p 0.038
(.0197)
(.0120 p .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
3.00 p 0.102
(.118 p .004)
(NOTE 3)
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
0.254
(.010)
DETAIL “A”
0o – 6o TYP
1 2 3 4 5
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
DETAIL “A”
0.18
(.007)
0.497 p 0.076
(.0196 p .003)
REF
10 9 8 7 6
SEATING
PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
0.1016 p 0.0508
(.004 p .002)
MSOP (MSE) 0307 REV B
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
3680fb
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.
23
LT3680
TYPICAL APPLICATION
1.2V Step-Down Converter
VOUT
1.2V
3.5A
VIN
3.6V TO 27V
VIN
BD
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3680
SW
D
RT
17k
L
3.3μH
PG
52.3k
SYNC
78.7k
GND
470pF
FB
100k
100μF
f = 500kHz
3680 TA09
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1766
60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC
Converter
VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25μA,
TSSOP16/E Package
LT1767
25V, 1.2A (IOUT), 1.2MHz, High Efficiency Step-Down DC/DC
Converter
VIN: 3V to 25V, VOUT(MIN) = 1.2V, IQ = 1mA, ISD < 6μA, MS8E
Package
LT1933
500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23
VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.6mA, ISD < 1μA,
ThinSOTTM 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
LT1940
Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC
Converter
VIN: 3.6V to 25V, VOUT(MIN) = 1.2V, IQ = 3.8mA, ISD < 30μA,
TSSOP16E Package
LT1976/LT1967
60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down
DC/DC Converters with Burst Mode Operation
VIN: 3.3V to 60V, VOUT(MIN) = 1.2V, IQ = 100μA, ISD < 1μA,
TSSOP16E 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 < 1μA,
TSSOP16 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 < 1μA,
3mm × 3mm DFN10 and TSSOP16E 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 < 1μA,
3mm × 3mm DFN10 and MSOP10E 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 < 1μA,
3mm × 3mm DFN10 and MSOP10E Packa ges
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 < 1μA,
2mm x 3mm DFN6 Package
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 DFN8 and MSOP8E 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 = 1μA,
4mm × 4mm QFN24 and TSSOP16E 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 < 1μA,
3mm × 3mm DFN10 and MSOP10E 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 < 1μA,
3mm × 3mm DFN10 and MSOP10E Packages
3680fb
24 Linear Technology Corporation
LT 1009 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007