LT3972
33V, 3.5A, 2.4MHz
Step-Down Switching Regulator
with 75µA Quiescent Current
FEATURES
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DESCRIPTION
Wide Input Range:
Operation from 3.6V to 33V
Overvoltage Lockout Protects Circuits
Through 62V Transients
3.5A Maximum Output Current
Low Ripple (30V),
the saturation current should be above 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.
ΔIL = 0.4(IOUT(MAX))
3972fa
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LT3972
APPLICATIONS INFORMATION
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
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 LT3972 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 LT3972 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 LT3972 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 LT3972 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 LT3972. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3972 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3972’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 LT3972 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 LT3972 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
LT3972 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
LT3972’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
3972fa
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LT3972
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMANDS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
(864) 963-6300
www.kemet.com
Ceramic,
Tantalum
Sanyo
(408) 749-9714
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
(408) 436-1300
AVX
Taiyo Yuden
(864) 963-6300
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
Ceramic
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
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.
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
T494, T495
POSCAP
TPS Series
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 LT3972 will keep the
switch off when VIN > 35V which allows the use of 40V
rated Schottky even when VIN ranges up to 62V. Table 3
lists several Schottky diodes and their manufacturers.
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 LT3972 due to their piezoelectric
nature. When in Burst Mode operation, the LT3972’s
switching frequency depends on the load current, and at
very light loads the LT3972 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since
the LT3972 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.
3972fa
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LT3972
APPLICATIONS INFORMATION
LT3972
CURRENT MODE
POWER STAGE
gm = 5.3mho
SW
ERROR
AMPLIFIER
OUTPUT
R1
CPL
FB
gm =
500μmho
+
The LT3972 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3972 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 LT3972 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
3972 F02
Figure 2. Model for Loop Response
VOUT
100mV/DIV
IL
1A/DIV
VIN = 12V
VOUT = 3.3V
10μs/DIV
3972 F03
Figure 3. Transient Load Response of the LT3972 Front Page
Application as the Load Current is Stepped from 1A to 3A.
VOUT = 5V
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LT3972
APPLICATIONS INFORMATION
Low Ripple Burst Mode Operation
and Pulse-Skipping Mode
The LT3972 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 LT3972 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 LT3972 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 LT3972 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 LT3972 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 LT3972 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.
If low quiescent current is not required the LT3972 can
operate in pulse-skipping mode. The benefit of this mode
VSW
5V/DIV
IL
0.2A/DIV
VOUT
10mV/DIV
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
5μs/DIV
Figure 4. Burst Mode Operation
3972 F04
is that the LT3972 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. Select pulse-skipping mode by applying
a clock signal or a DC voltage higher than 0.9V 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 LT3972 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3972 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
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LT3972
APPLICATIONS INFORMATION
6.0
VOUT
BD
5.5
TO START
(WORST CASE)
VIN
VIN
LT3972
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
LT3972
TO START
(WORST CASE)
SW
(5b) For 2.5V < VOUT < 2.8V
VOUT
LT3972
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
3972 F06
4.7μF
GND
SW
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3972 FO5
(5c) For VOUT < 2.5V; VIN(MAX) = 30V
Figure 5. Three Circuits For Generating The Boost Voltage
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 LT3972, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3972,
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 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.
3972fa
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LT3972
APPLICATIONS INFORMATION
Shorted and Reversed-Input Protection
IL
1A/DIV
RUN
15k
RUN/SS
VRUN/SS
2V/DIV
GND
0.22μF
VOUT
2V/DIV
2ms/DIV
3972 F07
Figure 7. To Soft-Start the LT3972, add a Resisitor
and Capacitor to the RUN/SS Pin
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.5V (this can be ground or a logic output).
Synchronizing the LT3972 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 LT3972 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 LT3972 may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3972
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 LT3972 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.
If the inductor is chosen so that it won’t saturate excessively, an LT3972 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
LT3972 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 LT3972’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 LT3972’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
LT3972 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
MBRS340
VIN
VIN
BOOST
LT3972
RUN/SS
VOUT
SW
VC
GND FB
BACKUP
3972 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 LT3972
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 LT3972’s VIN and SW pins, the catch
3972fa
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LT3972
APPLICATIONS INFORMATION
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 LT3972 to additional ground planes within the circuit
board and on the bottom side.
L1
C2
VOUT
RRT
CC
RC
R2
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3972 circuits. However, these capacitors can cause problems if the LT3972 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 LT3972 can ring to twice the
nominal input voltage, possibly exceeding the LT3972’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3972 into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3972 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.
R1
High Temperature Considerations
D1
C1
GND
RPG
3972 F09
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO SYNC
VIAS TO RUN/SS
VIAS TO PG
VIAS TO VIN
OUTLINE OF LOCAL
GROUND PLANE
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
The PCB must provide heat sinking to keep the LT3972
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 LT3972. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
3972fa
17
LT3972
APPLICATIONS INFORMATION
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 LT3972, 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 LT3972 can be estimated by
calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
loss. The die temperature is calculated by multiplying the
LT3972 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.
DANGER
VIN
20V/DIV
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM RATING
LT3972
+
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
LT3972
VIN
20V/DIV
+
0.1μF
4.7μF
IIN
10A/DIV
(10b)
LT3972
+
22μF
35V
AI.EI.
20μs/DIV
VIN
20V/DIV
+
4.7μF
IIN
10A/DIV
(10c)
20μs/DIV
3972 F10
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3972 is Connected to a Live Supply
3972fa
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LT3972
TYPICAL APPLICATIONS
5V Step-Down Converter
VOUT
5V
3.5A
VIN
6.3V TO 33V
TRANSIENT
TO 62V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
10μF
SW
LT3972
D
RT
15k
L
4.7μH
PG
SYNC
63.4k
536k
FB
GND
680pF
47μF
100k
f = 600kHz
3972 TA02
D: ON SEMI MBRA340
L: NEC MPLC0730L4R7
3.3V Step-Down Converter
VOUT
3.3V
3.5A
VIN
4.4V TO 33V
TRANSIENT
TO 62V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3972
SW
D
RT
19k
L
3.3μH
PG
SYNC
63.4k
680pF
f = 600kHz
316k
GND
FB
22μF
100k
3972 TA03
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
3972fa
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LT3972
TYPICAL APPLICATIONS
2.5V Step-Down Converter
VOUT
2.5V
3.5A
VIN
4V TO 33V
TRANSIENT
TO 62V
BD
VIN
RUN/SS
ON OFF
D2
BOOST
1μF
VC
4.7μF
SW
LT3972
D1
RT
15.4k
L
3.3μH
PG
215k
SYNC
63.4k
FB
GND
680pF
47μF
100k
f = 600kHz
3972 TA04
D1: ON SEMI MBRA340
D2: MBR0540
L: NEC MPLC0730L3R3
5V, 2MHz Step-Down Converter
VOUT
5V
2.5A
VIN
8.6V TO 22V
TRANSIENT
TO 62V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3972
SW
D
RT
15k
L
2.2μH
PG
536k
SYNC
12.7k
680pF
f = 2MHz
GND
FB
22μF
100k
3972 TA05
D: ON SEMI MBRA340
L: NEC MPLC0730L2R2
3972fa
20
LT3972
TYPICAL APPLICATIONS
12V Step-Down Converter
VOUT
12V
3.5A
VIN
15V TO 33V
TRANSIENT
TO 62V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
10μF
SW
LT3972
D
RT
17.4k
L
8.2μH
PG
715k
SYNC
63.4k
FB
GND
680pF
47μF
50k
f = 600kHz
3972 TA06
D: ON SEMI MBRA340
L: NEC MBP107558R2P
1.8V Step-Down Converter
VOUT
1.8V
3.5A
VIN
3.5V TO 27V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3972
SW
D
RT
16.9k
L
3.3μH
PG
SYNC
78.7k
680pF
f = 500kHz
127k
GND
FB
47μF
100k
3972 TA08
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
3972fa
21
LT3972
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
3972fa
22
LT3972
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
3972fa
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
LT3972
TYPICAL APPLICATION
1.2V Step-Down Converter
VOUT
1.2V
3.5A
VIN
3.6V TO 27V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47μF
VC
4.7μF
LT3972
SW
D
RT
17k
L
3.3μH
PG
52.3k
SYNC
78.7k
GND
470pF
FB
100k
100μF
f = 500kHz
3972 TA09
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
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