NSC LM2678SD-ADJ

LM2678
SIMPLE SWITCHER ® High Efficiency 5A Step-Down
Voltage Regulator
General Description
Features
The LM2678 series of regulators are monolithic integrated
circuits which provide all of the active functions for a stepdown (buck) switching regulator capable of driving up to 5A
loads with excellent line and load regulation characteristics.
High efficiency ( > 90%) is obtained through the use of a low
ON-resistance DMOS power switch. The series consists of
fixed output voltages of 3.3V, 5V and 12V and an adjustable
output version.
The SIMPLE SWITCHER concept provides for a complete
design using a minimum number of external components. A
high fixed frequency oscillator (260KHz) allows the use of
physically smaller sized components. A family of standard
inductors for use with the LM2678 are available from several
manufacturers to greatly simplify the design process.
The LM2678 series also has built in thermal shutdown,
current limiting and an ON/OFF control input that can power
down the regulator to a low 50µA quiescent current standby
condition. The output voltage is guaranteed to a ± 2% tolerance. The clock frequency is controlled to within a ± 11%
tolerance.
n Efficiency up to 92%
n Simple and easy to design with (using off-the-shelf
external components)
n 120 mΩ DMOS output switch
n 3.3V, 5V and 12V fixed output and adjustable (1.2V to
37V ) versions
n 50µA standby current when switched OFF
n ± 2%maximum output tolerance over full line and load
conditions
n Wide input voltage range: 8V to 40V
n 260 KHz fixed frequency internal oscillator
n −40 to +125˚C operating junction temperature range
Applications
n Simple to design, high efficiency ( > 90%) step-down
switching regulators
n Efficient system pre-regulator for linear voltage
regulators
n Battery chargers
Typical Application
10088603
SIMPLE SWITCHER ® is a registered trademark of National Semiconductor Corporation.
© 2005 National Semiconductor Corporation
DS100886
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LM2678 SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator
April 2005
LM2678
Connection Diagrams and Ordering Information
TO-263 Package
Top View
TO-220 Package
Top View
10088601
10088602
Order Number
LM2678S-3.3, LM2678S-5.0,
LM2678S-12 or LM2678S-ADJ
See NSC Package Number TS7B
Order Number
LM2678T-3.3, LM2678T-5.0,
LM2678T-12 or LM2678T-ADJ
See NSC Package Number TA07B
Top View
10088641
LLP-14
See NS package Number SRC14A
Ordering Information for LLP Package
Output Voltage
Order Information
Package Marking
Supplied As
12
LM2678SD-12
S0003BB
250 Units on Tape and Reel
12
LM2678SDX-12
S0003BB
2500 Units on Tape and Reel
3.3
LM2678SD-3.3
S0003CB
250 Units on Tape and Reel
3.3
LM2678SDX-3.3
S0003CB
2500 Units on Tape and Reel
5.0
LM2678SD-5.0
S0003DB
250 Units on Tape and Reel
5.0
LM2678SDX-5.0
S0003DB
2500 Units on Tape and Reel
ADJ
LM2678SD-ADJ
S0003EB
250 Units on Tape and Reel
ADJ
LM2678SDX-ADJ
S0003EB
2500 Units on Tape and Reel
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2
Storage Temperature Range
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Soldering Temperature
Input Supply Voltage
45V
ON/OFF Pin Voltage
−0.1V to 6V
Switch Voltage to Ground
−1V to VIN
Boost Pin Voltage
10 sec, 240˚C
Vapor Phase
75 sec, 219˚C
Supply Voltage
−0.3V to 14V
8V to 40V
Junction Temperature Range (TJ)
Internally Limited
ESD (Note 2)
4 sec, 260˚C
Infrared
Operating Ratings
VSW + 8V
Feedback Pin Voltage
Power Dissipation
Wave
−65˚C to 150˚C
−40˚C to 125˚C
2 kV
Electrical Characteristics
Limits appearing in bold type face apply over the entire junction temperature
range of operation, −40˚C to 125˚C. Specifications appearing in normal type apply for TA = TJ = 25˚C.
LM2678-3.3
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
3.234/3.201
3.366/3.399
VOUT
Output Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
3.3
η
Efficiency
VIN = 12V, ILOAD = 5A
82
Units
V
%
LM2678-5.0
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
4.900/4.850
5.100/5.150
VOUT
Output Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
5.0
η
Efficiency
VIN = 12V, ILOAD = 5A
84
Units
V
%
LM2678-12
Symbol
Parameter
Conditions
Typical
Min
Max
(Note 3)
(Note 4)
(Note 4)
11.76/11.64
12.24/12.36
VOUT
Output Voltage
VIN = 15V to 40V, 100mA ≤ IOUT ≤ 5A
12
η
Efficiency
VIN = 24V, ILOAD = 5A
92
Units
V
%
LM2678-ADJ
Symbol
Parameter
Conditions
VFB
Feedback
Voltage
VIN = 8V to 40V, 100mA ≤ IOUT ≤ 5A
VOUT Programmed for 5V
η
Efficiency
VIN = 12V, ILOAD = 5A
Typ
Min
Max
(Note 3)
(Note 4)
(Note 4)
1.21
1.186/1.174
1.234/1.246
84
3
Units
V
%
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LM2678
Absolute Maximum Ratings (Note 1)
LM2678
All Output Voltage Versions
Electrical Characteristics
Limits appearing in bold type face apply over the entire junction temperature range of operation, −40˚C to 125˚C. Specifications appearing in normal type apply for TA = TJ = 25˚C. Unless otherwise specified VIN =12V for the 3.3V, 5V and Adjustable
versions and VIN =24V for the 12V version.
Symbol
Parameter
Conditions
Typ
Min
Max
Units
4.2
6
mA
50
100/150
µA
8.3/8.75
A
DEVICE PARAMETERS
IQ
Quiescent
Current
VFEEDBACK = 8V
For 3.3V, 5.0V, and ADJ Versions
VFEEDBACK = 15V
For 12V Versions
ISTBY
Standby
Quiescent
Current
ON/OFF Pin = 0V
ICL
Current Limit
IL
Output Leakage
Current
VIN = 40V, ON/OFF Pin = 0V
VSWITCH = 0V
VSWITCH = −1V
16
200
15
RDS(ON)
Switch
On-Resistance
ISWITCH = 5A
0.12
0.14/0.225
Ω
fO
Oscillator
Frequency
Measured at Switch Pin
260
280
kHz
D
Duty Cycle
Maximum Duty Cycle
91
Minimum Duty Cycle
0
%
VFEEDBACK = 1.3V
ADJ Version Only
85
nA
IBIAS
Feedback Bias
Current
VON/OFF
ON/OFF
Threshold
Voltage
7
1.4
ION/OFF
ON/OFF Input
Current
ON/OFF Input = 0V
θJA
Thermal
Resistance
T Package, Junction to Ambient
θJA
6.1/5.75
225
µA
mA
%
0.8
20
2.0
V
45
µA
65
(Note 5)
T Package, Junction to Ambient
45
(Note 6)
θJC
T Package, Junction to Case
2
θJA
S Package, Junction to Ambient
56
˚C/W
(Note 7)
θJA
S Package, Junction to Ambient
35
(Note 8)
θJA
S Package, Junction to Ambient
26
(Note 9)
θJC
S Package, Junction to Case
2
θJA
SD Package, Junction to Ambient
55
(Note 10)
θJA
SD Package, Junction to Ambient
29
++
˚C/W
(Note 11)
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions under which of the device is
guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test condition, see the electrical
Characteristics tables.
Note 2: ESD was applied using the human-body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: Typical values are determined with TA = TJ = 25˚C and represent the most likely norm.
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LM2678
All Output Voltage Versions
Electrical Characteristics (Continued)
Note 4: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
tested during production with TA = TJ = 25˚C. All limits at temperature extremes are guaranteed via correlation using standard standard Quality Control (SQC)
methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a PC
board with minimum copper area.
Note 6: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with 1⁄2 inch leads soldered to a PC board
containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.
Note 7: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the
TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 8: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area
of the TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 9: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times the
area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Additional copper area will reduce thermal resistance further. See the thermal model in Switchers
Made Simple ® software.
Note 10: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area equal to the die attach paddle.
Note 11: Junction to ambient thermal resistance for the 14-lead LLP mounted on a PC board copper area using 12 vias to a second layer of copper equal to die
attach paddle. Additional copper area will reduce thermal resistance further. For layout recommendations, refer to Application Note AN-1187.
5
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LM2678
Typical Performance Characteristics
Normalized
Output Voltage
Line Regulation
10088609
10088610
Efficiency vs Input Voltage
Efficiency vs ILOAD
10088611
10088612
Switch Current Limit
Operating Quiescent Current
10088605
10088604
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LM2678
Typical Performance Characteristics
(Continued)
Standby Quiescent Current
ON/OFF Threshold Voltage
10088640
10088613
ON/OFF Pin Current (Sourcing)
Switching Frequency
10088614
10088615
Feedback Pin Bias Current
10088616
7
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LM2678
Typical Performance Characteristics
(Continued)
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 5A
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
10088617
10088618
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 2 A/div
A: VSW Pin Voltage, 10 V/div.
B: Inductor Current, 1 A/div
C: Output Ripple Voltage, 20 mV/div AC-Coupled
C: Output Ripple Voltage, 20 mV/div AC-Coupled
Horizontal Time Base: 1 µs/div
Horizontal Time Base: 1 µs//iv
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V,
L = 10 µH, COUT = 400 µF, COUTESR = 13 mΩ
10088619
10088620
A: Output Voltage, 100 mV//div, AC-Coupled.
A: Output Voltage, 100 mV/div, AC-Coupled.
B: Load Current: 500 mA to 5A Load Pulse
B: Load Current: 200 mA to 3A Load Pulse
Horizontal Time Base: 100 µs/div
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Horizontal Time Base: 200 µs/div
8
LM2678
Block Diagram
10088606
* Active Inductor Patent Number 5,514,947
† Active Capacitor Patent Number 5,382,918
9
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LM2678
GROUND
Application Hints
This is the ground reference connection for all components
in the power supply. In fast-switching, high-current applications such as those implemented with the LM2678, it is
recommended that a broad ground plane be used to minimize signal coupling throughout the circuit
The LM2678 provides all of the active functions required for
a step-down (buck) switching regulator. The internal power
switch is a DMOS power MOSFET to provide power supply
designs with high current capability, up to 5A, and highly
efficient operation.
The LM2678 is part of the SIMPLE SWITCHER family of
power converters. A complete design uses a minimum number of external components, which have been predetermined from a variety of manufacturers. Using either this
data sheet or a design software program called LM267X
Made Simple (version 2.0) a complete switching power
supply can be designed quickly. The software is provided
free of charge and can be downloaded from National Semiconductor’s Internet site located at http://www.national.com.
FEEDBACK
This is the input to a two-stage high gain amplifier, which
drives the PWM controller. It is necessary to connect pin 6 to
the actual output of the power supply to set the dc output
voltage. For the fixed output devices (3.3V, 5V and 12V
outputs), a direct wire connection to the output is all that is
required as internal gain setting resistors are provided inside
the LM2678. For the adjustable output version two external
resistors are required to set the dc output voltage. For stable
operation of the power supply it is important to prevent
coupling of any inductor flux to the feedback input.
SWITCH OUTPUT
This is the output of a power MOSFET switch connected
directly to the input voltage. The switch provides energy to
an inductor, an output capacitor and the load circuitry under
control of an internal pulse-width-modulator (PWM). The
PWM controller is internally clocked by a fixed 260KHz
oscillator. In a standard step-down application the duty cycle
(Time ON/Time OFF) of the power switch is proportional to
the ratio of the power supply output voltage to the input
voltage. The voltage on pin 1 switches between Vin (switch
ON) and below ground by the voltage drop of the external
Schottky diode (switch OFF).
ON/OFF
This input provides an electrical ON/OFF control of the
power supply. Connecting this pin to ground or to any voltage less than 0.8V will completely turn OFF the regulator.
The current drain from the input supply when OFF is only
50µA. Pin 7 has an internal pull-up current source of approximately 20µA and a protection clamp zener diode of 7V to
ground. When electrically driving the ON/OFF pin the high
voltage level for the ON condition should not exceed the 6V
absolute maximum limit. When ON/OFF control is not required pin 7 should be left open circuited.
INPUT
The input voltage for the power supply is connected to pin 2.
In addition to providing energy to the load the input voltage
also provides bias for the internal circuitry of the LM2678.
For guaranteed performance the input voltage must be in the
range of 8V to 40V. For best performance of the power
supply the input pin should always be bypassed with an input
capacitor located close to pin 2.
DAP (LLP PACKAGE)
The Die Attach Pad (DAP) can and should be connected to
PCB Ground plane/island. For CAD and assembly guidelines refer to Application Note AN-1187 at http://
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C BOOST
A capacitor must be connected from pin 3 to the switch
output, pin 1. This capacitor boosts the gate drive to the
internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high
efficiency. The recommended value for C Boost is 0.01µF.
DESIGN CONSIDERATIONS
10088607
FIGURE 1. Basic circuit for fixed output voltage applications.
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LM2678
Application Hints
(Continued)
10088608
FIGURE 2. Basic circuit for adjustable output voltage applications
Power supply design using the LM2678 is greatly simplified
by using recommended external components. A wide range
of inductors, capacitors and Schottky diodes from several
manufacturers have been evaluated for use in designs that
cover the full range of capabilities (input voltage, output
voltage and load current) of the LM2678. A simple design
procedure using nomographs and component tables provided in this data sheet leads to a working design with very
little effort. Alternatively, the design software, LM267X Made
Simple (version 6.0), can also be used to provide instant
component selection, circuit performance calculations for
evaluation, a bill of materials component list and a circuit
schematic.
The individual components from the various manufacturers
called out for use are still just a small sample of the vast
array of components available in the industry. While these
components are recommended, they are not exclusively the
only components for use in a design. After a close comparison of component specifications, equivalent devices from
other manufacturers could be substituted for use in an application.
Important considerations for each external component and
an explanation of how the nomographs and selection tables
were developed follows.
input and output voltage and load current. Several part types
are offered for a given amount of inductance. Both surface
mount and through-hole devices are available. The inductors
from each of the three manufacturers have unique characteristics.
Renco: ferrite stick core inductors; benefits are typically
lowest cost and can withstand ripple and transient peak
currents above the rated value. These inductors have an
external magnetic field, which may generate EMI.
Pulse Engineering: powdered iron toroid core inductors;
these also can withstand higher than rated currents and,
being toroid inductors, will have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors and are available only as surface
mount components. These inductors also generate EMI but
less than stick inductors.
OUTPUT CAPACITOR
The output capacitor acts to smooth the dc output voltage
and also provides energy storage. Selection of an output
capacitor, with an associated equivalent series resistance
(ESR), impacts both the amount of output ripple voltage and
stability of the control loop.
The output ripple voltage of the power supply is the product
of the capacitor ESR and the inductor ripple current. The
capacitor types recommended in the tables were selected
for having low ESR ratings.
In addition, both surface mount tantalum capacitors and
through-hole aluminum electrolytic capacitors are offered as
solutions.
Impacting frequency stability of the overall control loop, the
output capacitance, in conjunction with the inductor, creates
a double pole inside the feedback loop. In addition the
capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency
compensation circuitry of the LM2678 modify the gain and
phase shift of the closed loop system.
As a general rule for stable switching regulator circuits it is
desired to have the unity gain bandwidth of the circuit to be
limited to no more than one-sixth of the controller switching
INDUCTOR
The inductor is the key component in a switching regulator.
For efficiency the inductor stores energy during the switch
ON time and then transfers energy to the load while the
switch is OFF.
Nomographs are used to select the inductance value required for a given set of operating conditions. The nomographs assume that the circuit is operating in continuous
mode (the current flowing through the inductor never falls to
zero). The magnitude of inductance is selected to maintain a
maximum ripple current of 30% of the maximum load current. If the ripple current exceeds this 30% limit the next
larger value is selected.
The inductors offered have been specifically manufactured
to provide proper operation under all operating conditions of
11
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LM2678
Application Hints
The average current through the catch diode is dependent
on the switch duty cycle (D) and is equal to the load current
times (1-D). Use of a diode rated for much higher current
than is required by the actual application helps to minimize
the voltage drop and power loss in the diode.
(Continued)
frequency. With the fixed 260KHz switching frequency of the
LM2678, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended
capacitor value has been chosen to achieve this result.
In some cases multiple capacitors are required either to
reduce the ESR of the output capacitor, to minimize output
ripple (a ripple voltage of 1% of Vout or less is the assumed
performance condition), or to increase the output capacitance to reduce the closed loop unity gain bandwidth (to less
than 40KHz). When parallel combinations of capacitors are
required it has been assumed that each capacitor is the
exact same part type.
The RMS current and working voltage (WV) ratings of the
output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current
(set to be no more than 30% of the maximum load current by
the inductor selection) is the current that flows through the
output capacitor. The capacitor RMS current rating must be
greater than this ripple current. The voltage rating of the
output capacitor should be greater than 1.3 times the maximum output voltage of the power supply. If operation of the
system at elevated temperatures is required, the capacitor
voltage rating may be de-rated to less than the nominal room
temperature rating. Careful inspection of the manufacturer’s
specification for de-rating of working voltage with temperature is important.
INPUT CAPACITOR
Fast changing currents in high current switching regulators
place a significant dynamic load on the unregulated power
source. An input capacitor helps to provide additional current
to the power supply as well as smooth out input voltage
variations.
Like the output capacitor, the key specifications for the input
capacitor are RMS current rating and working voltage. The
RMS current flowing through the input capacitor is equal to
one-half of the maximum dc load current so the capacitor
should be rated to handle this. Paralleling multiple capacitors
proportionally increases the current rating of the total capacitance. The voltage rating should also be selected to be 1.3
times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the
maximum input voltage could be significantly higher than
normal operation and should be considered when selecting
an input capacitor.
The input capacitor should be placed very close to the input
pin of the LM2678. Due to relative high current operation
with fast transient changes, the series inductance of input
connecting wires or PCB traces can create ringing signals at
the input terminal which could possibly propagate to the
output or other parts of the circuitry. It may be necessary in
some designs to add a small valued (0.1µF to 0.47µF)
ceramic type capacitor in parallel with the input capacitor to
prevent or minimize any ringing.
CATCH DIODE
When the power switch in the LM2678 turns OFF, the current
through the inductor continues to flow. The path for this
current is through the diode connected between the switch
output and ground. This forward biased diode clamps the
switch output to a voltage less than ground. This negative
voltage must be greater than −1V so a low voltage drop
(particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode.
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During the switch ON time the diode will be reversed biased
by the input voltage. The reverse voltage rating of the diode
should be at least 1.3 times greater than the maximum input
voltage.
BOOST CAPACITOR
The boost capacitor creates a voltage used to overdrive the
gate of the internal power MOSFET. This improves efficiency
by minimizing the on resistance of the switch and associated
power loss. For all applications it is recommended to use a
0.01µF/50V ceramic capacitor.
ADDITIONAL APPLICATION INFORMATION
When the output voltage is greater than approximately 6V,
and the duty cycle at minimum input voltage is greater than
approximately 50%, the designer should exercise caution in
selection of the output filter components. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to
observe a large hysteresis in the current limit. This can affect
the output voltage of the device until the load current is
reduced sufficiently to allow the current limit protection circuit
to reset itself.
Under current limiting conditions, the LM267x is designed to
respond in the following manner:
1. At the moment when the inductor current reaches the
current limit threshold, the ON-pulse is immediately terminated. This happens for any application condition.
2. However, the current limit block is also designed to
momentarily reduce the duty cycle to below 50% to
avoid subharmonic oscillations, which could cause the
inductor to saturate.
3. Thereafter, once the inductor current falls below the
current limit threshold, there is a small relaxation time
during which the duty cycle progressively rises back
above 50% to the value required to achieve regulation.
If the output capacitance is sufficiently ‘large’, it may be
possible that as the output tries to recover, the output capacitor charging current is large enough to repeatedly retrigger the current limit circuit before the output has fully
settled. This condition is exacerbated with higher output
voltage settings because the energy requirement of the output capacitor varies as the square of the output voltage
(1⁄2CV2), thus requiring an increased charging current.
A simple test to determine if this condition might exist for a
suspect application is to apply a short circuit across the
output of the converter, and then remove the shorted output
condition. In an application with properly selected external
components, the output will recover smoothly.
Practical values of external components that have been
experimentally found to work well under these specific operating conditions are COUT = 47µF, L = 22µH. It should be
noted that even with these components, for a device’s current limit of ICLIM, the maximum load current under which the
possibility of the large current limit hysteresis can be minimized is ICLIM/2. For example, if the input is 24V and the set
output voltage is 18V, then for a desired maximum current of
1.5A, the current limit of the chosen switcher must be confirmed to be at least 3A.
SIMPLE DESIGN PROCEDURE
12
2 x 560µF/35V Panasonic HFQ (code C5)
Step 5: Use Table 4 to select an input capacitor. With 3.3V
output and 15µH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 2A (1/2 Iload max). Again
using Table 2 for specific component characteristics the
following choices are suitable:
(Continued)
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few
simple steps.
Step 1: Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
2 x 680µF/63V Sanyo MV-GX (code C13)
1 x 1200µF/63V Nichicon PL (code C25)
1 x 1500µF/63V Panasonic HFQ (code C16)
Maximum output load current
Step 2: Set the output voltage by selecting a fixed output
LM2678 (3.3V, 5V or 12V applications) or determine the
required feedback resistors for use with the adjustable
LM2678−ADJ
Step 6: From Table 5 a 5A or more Schottky diode must be
selected. For through-hole components only 40V rated diodes are indicated and 4 part types are suitable:
1N5825
MBR745
80SQ045
Step 3: Determine the inductor required by using one of the
four nomographs, Figure 3 through Figure 6. Table 1 provides a specific manufacturer and part number for the inductor.
Step 4: Using Table 3 (fixed output voltage) or Table 6
(adjustable output voltage), determine the output capacitance required for stable operation. Table 2 provides the
specific capacitor type from the manufacturer of choice.
6TQ045
Step 7: A 0.01µF capacitor will be used for Cboost.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alternator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required is 3.5A maximum. It is also desired to implement the
power supply with all surface mount components.
Step 1: Operating conditions are:
Step 5: Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capacitor from Table 2 with a sufficient working voltage (WV) rating
greater than Vin max, and an rms current rating greater than
one-half the maximum load current (2 or more capacitors in
parallel may be required).
Step 6: Select a diode from Table 5. The current rating of the
diode must be greater than I load max and the Reverse
Voltage rating must be greater than Vin max.
Step 7: Include a 0.01µF/50V capacitor for Cboost in the
design.
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
A system logic power supply bus of 3.3V is to be generated
from a wall adapter which provides an unregulated DC voltage of 13V to 16V. The maximum load current is 4A.
Through-hole components are preferred.
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 4A
Step 2: Select an LM2678T-3.3. The output voltage will have
a tolerance of
± 2% at room temperature and ± 3% over the full operating
temperature range.
Step 3: Use the nomograph for the 3.3V device ,Figure 3.
The intersection of the 16V horizontal line (Vin max) and the
4A vertical line (Iload max) indicates that L46, a 15µH inductor, is required.
From Table 1, L46 in a through-hole component is available
from Renco with part number RL-1283-15-43.
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 15µH inductor there are four through-hole
output capacitor solutions with the number of same type
capacitors to be paralleled and an identifying capacitor code
given. Table 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
2 x 220µF/10V Sanyo OS-CON (code C5)
2 x 820µF/16V Sanyo MV-GX (code C5)
1 x 3900µF/10V Nichicon PL (code C7)
Vout = 14.8V
Vin max = 28V
Iload max = 3.5A
Step 2: Select an LM2678S-ADJ. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in
Figure 2). For the adjustable device the output voltage is set
by the following relationship:
Where VFB is the feedback voltage of typically 1.21V.
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
Step 3: To use the nomograph for the adjustable device,
Figure 6, requires a calculation of the inductor
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where VSAT is the voltage drop across the internal power
switch which is Rds(ON) times Iload. In this example this would
be typically 0.12Ω x 3.5A or 0.42V and VD is the voltage drop
across the forward bisased Schottky diode, typically 0.5V.
13
www.national.com
LM2678
Application Hints
LM2678
Application Hints
Step 4: Use Table 6 to determine an output capacitor. With a
14.8V output the 12.5 to 15V row is used and with a 47µH
inductor there are three surface mount output capacitor solutions. Table 2 provides the actual capacitor characteristics
based on the C Code number. Any of the following choices
can be used:
(Continued)
The switching frequency of 260KHz is the nominal value to
use to estimate the ON time of the switch during which
energy is stored in the inductor.
For this example E • T is found to be:
1 x 33µF/20V AVX TPS (code C6)
1 x 47µF/20V Sprague 594 (code C8)
1 x 47µF/20V Kemet T495 (code C8)
Important Note: When using the adjustable device in low
voltage applications (less than 3V output), if the nomograph,
Figure 6, selects an inductance of 22µH or less, Table 6 does
not provide an output capacitor solution. With these conditions the number of output capacitors required for stable
operation becomes impractical. It is recommended to use
either a 33µH or 47µH inductor and the output capacitors
from Table 6.
Using Figure 6, the intersection of 27V • µS horizontally and
the 3.5A vertical line (Iload max) indicates that L48 , a 47µH
inductor, or L49, a 33µH inductor could be used. Either
inductor will be suitable, but for this example selecting the
larger inductance will result in lower ripple current.
From Table 1, L48 in a surface mount component is available
from Pulse Engineering with part number P0848.
www.national.com
14
MBRD1545CT
6TQ045S
(Continued)
Step 5: An input capacitor for this example will require at
least a 35V WV rating with an rms current rating of 1.75A
(1/2 Iout max). From Table 2 it can be seen that C12, a
33µF/35V capacitor from Sprague, has the highest voltage/
current rating of the surface mount components and that two
of these capacitor in parallel will be adquate.
Step 7: A 0.01µF capacitor will be used for Cboost.
LLP PACKAGE DEVICES
The LM2678 is offered in the 14 lead LLP surface mount
package to allow for a significantly decreased footprint with
equivalent power dissipation compared to the TO-263. For
details on mounting and soldering specifications, refer to
Application Note AN-1187.
Step 6: From Table 5 a 5A or more Schottky diode must be
selected. For surface mount diodes with a margin of safety
on the voltage rating one of two diodes can be used:
15
www.national.com
LM2678
Application Hints
LM2678
Inductor Selection Guides
For Continuous Mode Operation
10088621
10088622
FIGURE 3. LM2678-3.3
FIGURE 4. LM2678-5.0
10088624
10088623
FIGURE 6. LM2678-ADJ
FIGURE 5. LM2678-12
www.national.com
16
LM2678
Inductor Selection Guides For Continuous Mode Operation
(Continued)
TABLE 1. Inductor Manufacturer Part Numbers
Inductor
Inductance
Reference
(µH)
Number
Renco
Pulse Engineering
Current
(A)
Through Hole
Surface
Mount
Through
Hole
Surface
Mount
Coilcraft
Surface Mount
L23
33
1.35
RL-5471-7
RL1500-33
PE-53823
PE-53823S
DO3316-333
L24
22
1.65
RL-1283-22-43
RL1500-22
PE-53824
PE-53824S
DO3316-223
L25
15
2.00
RL-1283-15-43
RL1500-15
PE-53825
PE-53825S
DO3316-153
L29
100
1.41
RL-5471-4
RL-6050-100 PE-53829
PE-53829S
DO5022P-104
L30
68
1.71
RL-5471-5
RL6050-68
PE-53830
PE-53830S
DO5022P-683
L31
47
2.06
RL-5471-6
RL6050-47
PE-53831
PE-53831S
DO5022P-473
L32
33
2.46
RL-5471-7
RL6050-33
PE-53932
PE-53932S
DO5022P-333
L33
22
3.02
RL-1283-22-43
RL6050-22
PE-53933
PE-53933S
DO5022P-223
L34
15
3.65
RL-1283-15-43
—
PE-53934
PE-53934S
DO5022P-153
L38
68
2.97
RL-5472-2
—
PE-54038
PE-54038S
L39
47
3.57
RL-5472-3
—
PE-54039
PE-54039S
—
L40
33
4.26
RL-1283-33-43
—
PE-54040
PE-54040S
—
L41
22
5.22
RL-1283-22-43
—
PE-54041
P0841
—
L44
68
3.45
RL-5473-3
—
PE-54044
L45
10
4.47
RL-1283-10-43
—
—
P0845
DO5022P-103HC
L46
15
5.60
RL-1283-15-43
—
—
P0846
DO5022P-153HC
L47
10
5.66
RL-1283-10-43
—
—
P0847
DO5022P-103HC
L48
47
5.61
RL-1282-47-43
—
—
P0848
—
L49
33
5.61
RL-1282-33-43
—
—
P0849
—
—
—
—
Inductor Manufacturer Contact Numbers
Coilcraft
Coilcraft, Europe
Pulse Engineering
Phone
(800) 322-2645
FAX
(708) 639-1469
Phone
+44 1236 730 595
FAX
+44 1236 730 627
Phone
(619) 674-8100
FAX
(619) 674-8262
Pulse Engineering,
Phone
+353 93 24 107
Europe
FAX
+353 93 24 459
Renco Electronics
Phone
(800) 645-5828
FAX
(516) 586-5562
17
www.national.com
LM2678
Capacitor Selection Guides
TABLE 2. Input and Output Capacitor Codes
Capacitor
Reference
Code
www.national.com
Surface Mount
AVX TPS Series
C (µF) WV (V)
C1
330
6.3
C2
100
C3
220
C4
Irms
(A)
Sprague 594D Series
C (µF) WV (V)
1.15
120
6.3
10
1.1
220
10
1.15
68
47
16
0.89
C5
100
16
C6
33
C7
68
C8
22
Irms
(A)
Kemet T495 Series
C (µF) WV (V)
Irms
(A)
1.1
100
6.3
0.82
6.3
1.4
220
6.3
1.1
10
1.05
330
6.3
1.1
150
10
1.35
100
10
1.1
1.15
47
16
1
150
10
1.1
20
0.77
100
16
1.3
220
10
1.1
20
0.94
180
16
1.95
33
20
0.78
25
0.77
47
20
1.15
47
20
0.94
C9
10
35
0.63
33
25
1.05
68
20
0.94
C10
22
35
0.66
68
25
1.6
10
35
0.63
C11
15
35
0.75
22
35
0.63
C12
33
35
1
4.7
50
0.66
C13
15
50
0.9
18
LM2678
Capacitor Selection Guides
(Continued)
Input and Output Capacitor Codes (continued)
Through Hole
Capacitor
Sanyo OS-CON SA Series Sanyo MV-GX Series
Nichicon PL Series
Reference
Irms
Irms
Irms
Code
C (µF) WV (V)
(A)
C (µF) WV (V)
(A)
C (µF) WV (V)
(A)
Panasonic HFQ Series
C (µF) WV (V)
Irms
(A)
C1
47
6.3
1
1000
6.3
0.8
680
10
0.8
82
35
0.4
C2
150
6.3
1.95
270
16
0.6
820
10
0.98
120
35
0.44
C3
330
6.3
2.45
470
16
0.75
1000
10
1.06
220
35
0.76
C4
100
10
1.87
560
16
0.95
1200
10
1.28
330
35
1.01
C5
220
10
2.36
820
16
1.25
2200
10
1.71
560
35
1.4
C6
33
16
0.96
1000
16
1.3
3300
10
2.18
820
35
1.62
C7
100
16
1.92
150
35
0.65
3900
10
2.36
1000
35
1.73
C8
150
16
2.28
470
35
1.3
6800
10
2.68
2200
35
2.8
C9
100
20
2.25
680
35
1.4
180
16
0.41
56
50
0.36
C10
47
25
2.09
1000
35
1.7
270
16
0.55
100
50
0.5
C11
220
63
0.76
470
16
0.77
220
50
0.92
C12
470
63
1.2
680
16
1.02
470
50
1.44
C13
680
63
1.5
820
16
1.22
560
50
1.68
C14
1000
63
1.75
1800
16
1.88
1200
50
2.22
C15
220
25
0.63
330
63
1.42
C16
220
35
0.79
1500
63
2.51
C17
560
35
1.43
C18
2200
35
2.68
C19
150
50
0.82
C20
220
50
1.04
C21
330
50
1.3
C22
100
63
0.75
C23
390
63
1.62
C24
820
63
2.22
C25
1200
63
2.51
Capacitor Manufacturer Contact Numbers
Nichicon
Panasonic
AVX
Sprague/Vishay
Sanyo
Kemet
Phone
(847) 843-7500
FAX
(847) 843-2798
Phone
(714) 373-7857
FAX
(714) 373-7102
Phone
(845) 448-9411
FAX
(845) 448-1943
Phone
(207) 324-4140
FAX
(207) 324-7223
Phone
(619) 661-6322
FAX
(619) 661-1055
Phone
(864) 963-6300
FAX
(864) 963-6521
19
www.national.com
LM2678
Capacitor Selection Guides
(Continued)
TABLE 3. Output Capacitors for Fixed Output Voltage Application
Surface Mount
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
10
5
C1
5
C1
5
C2
15
4
C1
4
C1
4
C3
22
3
C2
2
C7
3
C4
33
1
C1
2
C7
3
C4
10
4
C2
4
C6
4
C4
15
3
C3
2
C7
3
C5
22
3
C2
2
C7
3
C4
33
2
C2
2
C3
2
C4
47
2
C2
1
C7
2
C4
10
4
C5
3
C6
5
C9
15
3
C5
2
C7
4
C9
22
2
C5
2
C6
3
C8
33
2
C5
1
C7
3
C8
47
2
C4
1
C6
2
C8
68
1
C5
1
C5
2
C7
100
1
C4
1
C5
1
C8
Through Hole
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
Sanyo OS-CON SA
Series
Sanyo MV-GX Series
Nichicon PL Series
No.
C Code
No.
C Code
No.
C Code
No.
C Code
10
2
C5
2
C6
1
C8
2
C6
15
2
C5
2
C5
1
C7
2
C5
22
1
C5
1
C10
1
C5
1
C7
33
1
C5
1
C10
1
C5
1
C7
10
2
C4
2
C5
1
C6
2
C5
15
1
C5
1
C10
1
C5
1
C7
22
1
C5
1
C9
1
C5
1
C5
33
1
C4
1
C5
1
C4
1
C4
47
1
C4
1
C4
1
C2
2
C4
10
2
C7
1
C10
1
C14
2
C4
15
1
C8
1
C6
1
C17
1
C5
22
1
C7
1
C5
1
C13
1
C5
33
1
C7
1
C4
1
C12
1
C4
47
1
C7
1
C3
1
C11
1
C3
68
1
C6
1
C2
1
C10
1
C3
100
1
C6
1
C2
1
C9
1
C1
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
www.national.com
Panasonic HFQ
Series
20
LM2678
Capacitor Selection Guides
(Continued)
TABLE 4. Input Capacitors for Fixed Output Voltage Application
(Assumes worst case maximum input voltage and load current for a given inductance value)
Surface Mount
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
10
3
C7
2
C10
3
C Code
C9
15
*
*
3
C13
4
C12
22
*
*
2
C13
3
C12
33
*
*
2
C13
3
C12
10
3
C4
2
C6
3
C9
15
4
C9
3
C12
4
C10
22
*
*
3
C13
4
C12
33
*
*
2
C13
3
C12
47
*
*
1
C13
2
C12
10
4
C9
2
C10
4
C10
15
4
C8
2
C10
4
C10
22
4
C9
3
C12
4
C10
33
*
*
3
C13
4
C12
47
*
*
2
C13
3
C12
68
*
*
2
C13
2
C12
100
*
*
1
C13
2
C12
Through Hole
Output
Inductance
Voltage (V)
(µH)
3.3
5
12
Sanyo OS-CON SA
Series
Sanyo MV-GX Series
Nichicon PL Series
Panasonic HFQ
Series
No.
C Code
No.
C Code
No.
C Code
No.
C Code
10
2
C9
2
C8
1
C18
1
C8
15
*
*
2
C13
1
C25
1
C16
22
*
*
1
C14
1
C24
1
C16
33
*
*
1
C14
1
C24
1
C16
10
2
C7
2
C8
1
C25
1
C8
15
*
*
2
C8
1
C25
1
C8
22
*
*
2
C13
1
C25
1
C16
33
*
*
1
C14
1
C23
1
C13
47
*
*
1
C12
1
C19
1
C11
10
2
C10
2
C8
1
C18
1
C8
15
2
C10
2
C8
1
C18
1
C8
22
*
*
2
C8
1
C18
1
C8
33
*
*
2
C12
1
C24
1
C14
47
*
*
1
C14
1
C23
1
C13
68
*
*
1
C13
1
C21
1
C15
100
*
*
1
C11
1
C22
1
C11
* Check voltage rating of capacitors to be greater than application input voltage.
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
21
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LM2678
Capacitor Selection Guides
(Continued)
TABLE 5. Schottky Diode Selection Table
Reverse
Voltage
(V)
3A
20V
SK32
30V
SK33
Surface Mount
5A or More
Through Hole
3A
5A or
More
1N5820
SR302
MBRD835L
30WQ03F
40V
1N5821
31DQ03
SK34
MBRD1545CT
1N5822
30BQ040
6TQ045S
1N5825
MBR340
MBR745
30WQ04F
31DQ04
80SQ045
MBRS340
SR403
6TQ045
MBRD340
50V or
More
SK35
MBR350
30WQ05F
31DQ05
SR305
Diode Manufacturer Contact Numbers
International Rectifier
Motorola
General
Semiconductor
Diodes, Inc.
www.national.com
Phone
(310) 322-3331
FAX
(310) 322-3332
Phone
(800) 521-6274
FAX
(602) 244-6609
Phone
(516) 847-3000
FAX
(516) 847-3236
Phone
(805) 446-4800
FAX
(805) 446-4850
22
LM2678
Capacitor Selection Guides
(Continued)
TABLE 6. Output Capacitors for Adjustable Output Voltage Applications
Surface Mount
Output Voltage
(V)
1.21 to 2.50
2.5 to 3.75
3.75 to 5
5 to 6.25
6.25 to 7.5
7.5 to 10
10 to 12.5
12.5 to 15
15 to 20
20 to 30
30 to 37
Inductance
(µH)
AVX TPS Series
Sprague 594D
Series
Kemet T495 Series
No.
C Code
No.
C Code
No.
C Code
33*
7
C1
6
C2
7
C3
47*
5
C1
4
C2
5
C3
33*
4
C1
3
C2
4
C3
47*
3
C1
2
C2
3
C3
22
4
C1
3
C2
4
C3
33
3
C1
2
C2
3
C3
47
2
C1
2
C2
2
C3
22
3
C2
3
C3
3
C4
33
2
C2
2
C3
2
C4
47
2
C2
2
C3
2
C4
68
1
C2
1
C3
1
C4
22
3
C2
1
C4
3
C4
33
2
C2
1
C3
2
C4
47
1
C3
1
C4
1
C6
68
1
C2
1
C3
1
C4
33
2
C5
1
C6
2
C8
47
1
C5
1
C6
2
C8
68
1
C5
1
C6
1
C8
100
1
C4
1
C5
1
C8
33
1
C5
1
C6
2
C8
47
1
C5
1
C6
2
C8
68
1
C5
1
C6
1
C8
100
1
C5
1
C6
1
C8
33
1
C6
1
C8
1
C8
47
1
C6
1
C8
1
C8
68
1
C6
1
C8
1
C8
100
1
C6
1
C8
1
C8
33
1
C8
1
C10
2
C10
47
1
C8
1
C9
2
C10
68
1
C8
1
C9
2
C10
100
1
C8
1
C9
1
C10
33
2
C9
2
C11
2
C11
47
1
C10
1
C12
1
C11
68
1
C9
1
C12
1
C11
100
1
C9
1
C12
1
C11
10
4
C13
8
C12
15
3
C13
5
C12
2
C13
4
C12
33
1
C13
3
C12
47
1
C13
2
C12
68
1
C13
2
C12
22
No Values Available
23
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LM2678
Capacitor Selection Guides
(Continued)
Output Capacitors for Adjustable Output Voltage Applications (continued)
Through Hole
Output Voltage
(V)
1.21 to 2.50
2.5 to 3.75
3.75 to 5
5 to 6.25
6.25 to 7.5
7.5 to 10
10 to 12.5
12.5 to 15
15 to 20
Inductance
(µH)
Sanyo OS-CON SA
Series
30 to 37
Nichicon PL Series
No.
C Code
No.
C Code
No.
C Code
No.
2
C3
5
C1
5
C3
3
C
47*
2
C2
4
C1
3
C3
2
C5
33*
1
C3
3
C1
3
C1
2
C5
47*
1
C2
2
C1
2
C3
1
C5
22
1
C3
3
C1
3
C1
2
C5
33
1
C2
2
C1
2
C1
1
C5
47
1
C2
2
C1
1
C3
1
C5
22
1
C5
2
C6
2
C3
2
C5
33
1
C4
1
C6
2
C1
1
C5
47
1
C4
1
C6
1
C3
1
C5
68
1
C4
1
C6
1
C1
1
C5
22
1
C5
1
C6
2
C1
1
C5
33
1
C4
1
C6
1
C3
1
C5
47
1
C4
1
C6
1
C1
1
C5
68
1
C4
1
C2
1
C1
1
C5
33
1
C7
1
C6
1
C14
1
C5
47
1
C7
1
C6
1
C14
1
C5
68
1
C7
1
C2
1
C14
1
C2
100
1
C7
1
C2
1
C14
1
C2
33
1
C7
1
C6
1
C14
1
C5
47
1
C7
1
C2
1
C14
1
C5
C Code
68
1
C7
1
C2
1
C9
1
C2
100
1
C7
1
C2
1
C9
1
C2
33
1
C9
1
C10
1
C15
1
C2
47
1
C9
1
C10
1
C15
1
C2
68
1
C9
1
C10
1
C15
1
C2
100
1
C9
1
C10
1
C15
1
C2
33
1
C10
1
C7
1
C15
1
C2
47
1
C10
1
C7
1
C15
1
C2
68
1
C10
1
C7
1
C15
1
C2
100
1
C10
1
C7
1
C15
1
C2
47
No Values
68
Available
1
C7
1
C16
1
C2
1
C7
1
C16
1
C2
1
C7
1
C16
1
C2
100
1
C7
1
C16
1
C2
10
1
C12
1
C20
1
C10
15
1
C11
1
C20
1
C11
1
C11
1
C20
1
C10
22
No Values
33
Available
1
C11
1
C20
1
C10
47
1
C11
1
C20
1
C10
68
1
C11
1
C20
1
C10
* Set to a higher value for a practical design solution. See Applications Hints section
No. represents the number of identical capacitor types to be connected in parallel
C Code indicates the Capacitor Reference number in Table 2 for identifying the specific component from the manufacturer.
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Panasonic HFQ
Series
33*
33
20 to 30
Sanyo MV-GX Series
24
LM2678
Physical Dimensions
inches (millimeters)
unless otherwise noted
TO-263 Surface Mount Power Package
Order Number LM2678S-3.3, LM2678S-5.0,
LM2678S-12 or LM2678S-ADJ
NS Package Number TS7B
25
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LM2678
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
TO-220 Power Package
Order Number LM2678T-3.3, LM2678T-5.0,
LM2678T-12 or LM2678T-ADJ
NS Package Number TA07B
14-Lead LLP Package
NS Package Number SRC14A
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26
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LM2678 SIMPLE SWITCHER High Efficiency 5A Step-Down Voltage Regulator
Notes