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LM2825
Integrated Power Supply 1A DC-DC Converter
General Description
The LM2825 is a complete 1A DC-DC Buck converter pack-
aged in a 24-lead molded Dual-In-Line integrated circuit
package.
Contained within the package are all the active and passive
components for a high efficiency step-down (buck) switching
regulator. Available in fixed output voltages of 3.3V, 5V and
12V, as well as two adjustable versions, these devices can
provide up to 1A of load current with fully guaranteed electri-
cal specifications.
Self-contained, this converter is also fully protected from out-
put fault conditions, such as excessive load current, short
circuits, or excessive temperatures.
Highlights
n
No external components required (fixed output voltage
versions)
n
Integrated circuit reliability
n
MTBF over 20 million hours
n
Radiated EMI meets Class B stipulated by CISPR 22
n
High power density, 35 W/in
3
n
24-pin DIP package profile (1.25 x 0.54 x 0.26 inches)
Features
n
Minimum design time required
n
3.3V, 5V and 12V fixed output versions
n
Two adjustable versions allow 1.23V to 15V outputs
n
Wide input voltage range, up to 40V
n
Low-power standby mode, I
Q
typically 65 A
n
High efficiency, typically 80%
n
4% output voltage tolerance
n
Excellent line and load regulation
n
TTL shutdown capability/programmable Soft-start
n
Thermal shutdown and current limit protection
n
-40C to +85C ambient temperature range
Applications
n
Simple high-efficiency step-down (buck) regulator
n
On-card switching regulators
n
Efficient pre-regulator for linear regulators
n
Distributed power systems
n
DC/DC module replacement
Standard Application
(Fixed output voltage versions)
Radiated EMI
Radiated emission of electromagnetic fields is measured at
10m distance. The emission levels are within the Class B
limits stipulated by CISPR 22.
30. . . .230 MHz
30 dB V/m
230. . . .1000 MHz
37 dB V/m
1. . . .10 GHz
46 dB V/m
Connection Diagram
Ordering Information
Order Number LM2825N-3.3, LM2825N-5.0, LM2825N-12,
LM2825N-ADJ or LM2825HN-ADJ
See NS Package Number NA24F
DS012661-27
DS012661-1
DS012661-2
"NC (Do not use)" pins: See
Figure 11.
Top View
May 1997
LM2825
Integrated
Power
Supply
1A
DC-DC
Converter
1997 National Semiconductor Corporation
DS012661
www.national.com
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Input Supply (V
IN
)
+45V
SD/SS Pin Input Voltage (Note 2)
6V
Output Pin Voltage
(3.3V, 5.0V and ADJ)
-1V
V
9V
(12V and H-ADJ)
-1V
V
16V
ADJ Pin Voltage (ADJ, H-ADJ only)
-0.3V
V
25V
Power Dissipation
Internally Limited
Storage Temperature Range
-40C to +125C
ESD Susceptibility
Human Body Model (Note 3)
2 kV
Lead Temperature (Soldering 10 sec.)
260C
Operating Ratings
Ambient Temperature Range
-40C
T
A
+85C
Junction Temperature Range
-40C
T
J
+125C
Input Supply Voltage (3.3V version)
4.75V to 40V
Input Supply Voltage (5V version)
7V to 40V
Input Supply Voltage (12V version)
15V to 40V
Input Supply Voltage (-ADJ, H-ADJ)
4.5V to 40V
LM2825-3.3 Electrical Characteristics
(Note 4)
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Tempera-
ture Range. Test Circuit
Figure 2.
Symbol
Parameter
Conditions
LM2825-3.3
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
V
OUT
Output Voltage
4.75V
V
IN
40V, 0.1A
I
LOAD
1A
3.3
V
3.168/3.135
V(min)
3.432/3.465
V(max)
Line Regulation
4.75V
V
IN
40V
1.5
mV
I
LOAD
= 100 mA
Load Regulation
0.1A
I
LOAD
1A
8
mV
V
IN
= 12V
Output Ripple Voltage
V
IN
= 12V, I
LOAD
= 1A
40
mV p-p
Efficiency
V
IN
= 12V, I
LOAD
= 0.5A
75
%
LM2825-5.0 Electrical Characteristics
(Note 4)
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Tempera-
ture Range. Test Circuit
Figure 2.
Symbol
Parameter
Conditions
LM2825-5.0
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
V
OUT
Output Voltage
7V
V
IN
40V, 0.1A
I
LOAD
1A
5.0
V
4.800/4.750
V(min)
5.200/5.250
V(max)
Line Regulation
7V
V
IN
40V
2.7
mV
I
LOAD
= 100 mA
Load Regulation
0.1A
I
LOAD
1A
8
mV
V
IN
= 12V
Output Ripple Voltage
V
IN
= 12V, I
LOAD
= 1A
40
mV p-p
Efficiency
V
IN
= 12V, I
LOAD
= 0.5A
80
%
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2
LM2825-12 Electrical Characteristics
(Note 4)
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Tempera-
ture Range. Test Circuit
Figure 2.
Symbol
Parameter
Conditions
LM2825-12
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
V
OUT
Output Voltage
15V
V
IN
40V, 0.1A
I
LOAD
0.75A
12.0
V
11.52/11.40
V(min)
12.48/12.60
V(max)
Line Regulation
15V
V
IN
40V
8.5
mV
I
LOAD
= 100 mA
Load Regulation
0.1A
I
LOAD
0.75A
12
mV
V
IN
= 24V
Output Ripple Voltage
V
IN
= 24V, I
LOAD
= 1A
100
mV p-p
Efficiency
V
IN
= 24V, I
LOAD
= 0.5A
87
%
LM2825-ADJ Electrical Characteristics
(Note 5)
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Tempera-
ture Range. Test Circuit
Figure 3.
Symbol
Parameter
Conditions
LM2825-ADJ
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
V
ADJ
Adjust Pin Voltage
4.5V
V
IN
40V, 0.1A
I
LOAD
1A
1.230
V
1.23V
V
OUT
8V
1.193/1.180
V(min)
1.267/1.280
V(max)
Efficiency
V
IN
= 12V, I
LOAD
= 0.5A
74
%
V
OUT
Programmed for 3V. See Circuit of
Figure 3
LM2825H-ADJ Electrical Characteristics
(Note 5)
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Temperature
Range. Test Circuit
Figure 3.
Symbol
Parameter
Conditions
LM2825H-ADJ
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
V
ADJ
Adjust Pin Voltage
9V
V
IN
40V, 0.1A
I
LOAD
0.55A
1.230
V
7V
V
OUT
15V
1.193/1.180
V(min)
1.267/1.280
V(max)
Efficiency
V
IN
= 24V, I
LOAD
= 0.5A
87
%
V
OUT
Programmed for 12V.
See Circuit of
Figure 3
3
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All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for T
A
= 25C, and those with boldface type apply over full Operating Range. Un-
less otherwise specified, V
IN
= 12V for 3.3V, 5.0V and ADJ versions, V
IN
= 24V for 12V and H-ADJ versions, I
LOAD
= 100 mA.
Symbol
Parameter
Conditions
LM2825-XX
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
I
CL
DC Output Current
Limit
R
L
= 0
1.4
A
1.2
A(min)
2.4
A(max)
I
Q
Operating Quiescent
Current
SD/SS Pin = 3.1V
5
mA
(Note 8)
10
mA(max)
I
STBY
Standby Quiescent
Current
SD/SS Pin = 0V
65
A
(Note 8)
200
A(max)
I
ADJ
Adjust Pin Bias Current
Adjustable Versions Only, V
FB
= 1.3V
6
nA
50/100
nA(max)
f
O
Oscillator Frequency
(Note 9)
150
kHz
JA
Thermal Resistance
Junction to Ambient (Note 10)
30
C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit
Figure 2
V
SD
Shutdown Threshold
Voltage
1.3
V
Low (Shutdown Mode)
0.6
V(max)
High (Soft-start Mode)
2.0
V(min)
V
SS
Soft-start Voltage
V
OUT
= 20% of Nominal Output Voltage
2
V
V
OUT
= 100% of Nominal Output Voltage
3
I
SD
Shutdown Current
V
SHUTDOWN
= 0.5V
5
A
(Note 8)
10
A(max)
I
SS
Soft-start Current
V
SOFT-START
= 2.5V
1.6
A
(Note 8)
5
A(max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is in-
tended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 5 mA.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: When the LM2825 is used as shown in
Figure 2 test circuit, system performance will be as shown in Electrical Characteristics.
Note 5: When the LM2825 is used as shown in
Figure 3 test circuit, system performance will be as shown in Electrical Characteristics.
Note 6: Typical numbers are at 25C and represent the most likely norm.
Note 7: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face) when output current is limited to the value given
in the temperature derating curves. See the application section for curves. All limits at temperature extremes are guaranteed using standard Statistical Quality Control
(SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 8: I
LOAD
= 0A.
Note 9: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current over-
load.
Note 10: Junction to ambient thermal resistance (no external heat sink) for the DIP-24 package with the leads soldered to a printed circuit board with (1 oz.) copper
area of approximately 2 in
2
.
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4
Typical Performance Characteristics
(Circuits of
Figure 2 and Figure 3) Unless otherwise specified,
V
IN
= 12V for 3.3V, 5.0V and ADJ versions, V
IN
= 24V for 12V and H-ADJ versions, I
LOAD
= 100 mA, T
A
= 25C
Normalized
Output Voltage
DS012661-3
Efficiency
DS012661-4
Dropout Voltage
DS012661-5
Line Regulation
DS012661-6
Load Regulation
DS012661-7
Output
Ripple Voltage
DS012661-8
Operating
Quiescent Current
DS012661-9
Shutdown
Quiescent Current
DS012661-10
Switching Frequency
DS012661-11
5
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Typical Performance Characteristics
(Circuits of
Figure 2 and Figure 3) Unless otherwise specified,
V
IN
= 12V for 3.3V, 5.0V and ADJ versions, V
IN
= 24V for 12V and H-ADJ versions, I
LOAD
= 100 mA, T
A
= 25C (Continued)
Soft-start
DS012661-12
Shutdown /Soft-start
Current
DS012661-13
Soft-start Response
DS012661-14
Switch Current
Limit
DS012661-28
Adjust Pin
Bias Current
DS012661-29
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
= 5V, I
L
= 250 mA to 750 mA
DS012661-23
A: Output Voltage 100 mV/div (AC)
B: 250 mA to 750 mA Load Pulse
Horizontal Time Base: 200 s/div
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V, I
L
= 40 mA to 140 mA
DS012661-24
A: Output Voltage 100 mV/div (AC)
B: 40 mA to 140 mA Load Pulse
Horizontal Time Base: 200 s/div
FIGURE 1. Typical Load Transient Response
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6
Test Circuit
Application Information
PROGRAMMING OUTPUT VOLTAGE
(Selecting R1 and R2 as shown in
Figure 3)
The LM2825 is available in two adjustable output versions.
The LM2825-ADJ has been optimized for output voltages
between 1.23V and 8V, while the LM2825H-ADJ covers the
output voltage range of 7V to 15V. Both adjustable versions
are set in the following way.
Select a value for R1 between 240
and 1.5 k
. The lower
resistor values minimize noise pickup at the sensitive adjust
pin. (For lowest temperature coefficient and the best stability
with time, use 1% metal film resistors.)
Select R2 with the following equation.
When programming V
OUT
, keep in mind that V
IN
must be
greater than V
OUT
+ 2V for proper operation.
OPTIONAL EXTERNAL COMPONENTS
SOFT-START CAPACITOR
C
SS
: A capacitor on this pin provides the regulator with a
Soft-start feature (slow start-up). The current drawn from the
source starts out at a low average level with narrow pulses,
and ramps up in a controlled manner as the pulses expand
to their steady-state width. This reduces the startup current
considerably, and delays and slows down the output voltage
rise time.
DS012661-19
*
Optional--Required if package is more than 6" away from main filter or bypass capacitor.
*
*Optional Soft-start Capacitor
V
IN
= 40V (max)
V
OUT
= 3.3V or 5V
@
1A or 12V
@
0.75A
FIGURE 2. Standard Test Circuit
(Fixed Output Voltage Versions)
DS012661-30
*
Optional--Required if package is more than 6" away from main filter or bypass capacitor.
*
*Optional Soft-start Capacitor
***
Optional--See Application Information.
V
IN
= 40V (max)
V
OUT
= 1.23V to 8V (LM2825-ADJ)
7V to 15V (LM2825H-ADJ)
I
LOAD
= I
MAX
(See derating curves)
FIGURE 3. Standard Test Circuit
(Adjustable Output Voltage Versions)
7
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Application Information
(Continued)
It is especially useful in situations where the input power
source is limited in the amount of current it can deliver, since
you avoid loading down this type of power supply.
Under some operating conditions, a Soft-start capacitor is
required for proper operation.
Figure 5 indicates the input
voltage and ambient temperature conditions for which a
Soft-start capacitor may be required.
This curve is typical for full guaranteed output current and
can be used as a guideline. As the output current decreases,
the operating area requiring a Soft-start capacitor de-
creases. Capacitor values between 0.1 F and 1 F are rec-
ommended. Tantalum or ceramic capacitors are appropriate
for this application.
INPUT CAPACITOR
C
IN
: An optional input capacitor is required if the package is
more than 6" away from the main filter or bypass capacitor. A
low ESR aluminum or tantalum bypass capacitor is recom-
mended between the input pin and ground to prevent large
voltage transients from appearing at the input. In addition, to
be conservative, the RMS current rating of the input capaci-
tor should be selected to be at least
1
/
2
the DC load current.
With a 1A load, a capacitor with a RMS current rating of at
least 500 mA is recommended.
The voltage rating should be approximately 1.25 times the
maximum input voltage. With a nominal input voltage of 12V,
an aluminum electrolytic capacitor (Panasonic HFQ series or
Nichicon PL series or equivalent) with a voltage rating
greater than 15V (1.25 x V
IN
) would be needed.
Solid tantalum input capacitors should only be used where
the input source is impedance current limited. High dV/dt ap-
plied at the input can cause excessive charge current
through low ESR tantalum capacitors. This high charge cur-
rent can result in shorting within the capacitor. It is recom-
mended that they be surge current tested by the manufactur-
er.The TPS series available from AVX, and the 593D series
from Sprague are both surge current tested.
Use caution when using ceramic capacitors for input bypass-
ing, because it may cause ringing at the V
IN
pin.
LOWERING OUTPUT RIPPLE
When using the adjustable parts, one can achieve lower out-
put ripple voltage by shorting a resistor internal to the
LM2825. However, if this resistor is shorted, a feed forward
capacitor must be used to keep the regulator stable. For this
reason, this resistor must be left open on all of the fixed out-
put voltage versions or instability will result. See the feed for-
ward capacitor selection below. Shorting the internal resistor
is accomplished by shorting pins 8 and 9 on the LM2825,
and will typically reduce output ripple by 25 to 33%.
FEED FORWARD CAPACITOR SELECTION (C
FF
)
When using an adjustable part and pins 8 and 9 are shorted
to reduce output ripple, a feed forward capacitor is required.
This capacitor is typically between 680 pF and 2700 pF. The
table of
Figure 4 shows the value for C
FF
for a given output
voltage and feedback resistor R
2
(R1 = 1 k
).
SHUTDOWN
The circuit shown in
Figure 10 shows 2 circuits for the
Shutdown/Soft-start feature using different logic signals for
shutdown and using a 0.1 F Soft-start capacitor.
THERMAL CONSIDERATIONS
The LM2825 is available in a 24-pin through hole DIP. The
package is molded plastic with a copper lead frame. When
the package is soldered to the PC board, the copper and the
board are the heat sink for the LM2825.
V
OUT
R2
C
FF
LM2825-ADJ
2
630
N/A
3
1.43k
N/A
4
2.26k
2700 pF
5
3.09k
2700 pF
6
3.92k
2200 pF
7
4.75k
1800 pF
8
5.49k
1500 pF
LM2825H-ADJ
7
4.75k
2700 pF
8
5.49k
2200 pF
9
6.34k
1800 pF
10
7.15k
1500 pF
11
8.06k
1000 pF
12
8.87k
820 pF
13
9.53k
680 pF
14
10.5k
680 pF
15
11.3k
680 pF
FIGURE 4. C
FF
Selection Table
DS012661-22
FIGURE 5. Usage of the Soft-start Capacitor
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8
Application Information
(Continued)
OUTPUT CURRENT DERATING FOR T
J
= -40C to -25C AND T
J
= -25C to 0C
At the lower temperature extremes, the switch current limit drops off sharply. As a result, a lower output current is available in this
temperature range. See
Figure 6 and Figure 7 for the typical available output current at these temperature ranges.
DS012661-32
FIGURE 6. LM2825 Output Current Derating for T
J
= -40C to -25C
DS012661-33
FIGURE 7. LM2825 Output Current Derating for T
J
= -25C to 0C
9
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Application Information
(Continued)
OUTPUT CURRENT DERATING FOR T
A
= 0C to 70C
Due to the limited switch current, the LM2825 cannot supply the full one ampere output current over the entire input and output
voltage range.
Figure 8 shows the typical available output current for any input and output voltage combination. This applies for
all output voltage versions.
OUTPUT CURRENT DERATING FOR T
A
= 70C to 85C
At high these high ambient temperatures, the LM2825 cannot supply the full one ampere over the entire input and output voltage
range. This is due to thermal reasons and
Figure 9 shows the typical available output current for any input and output voltage
combination. This applies for all output voltage versions.
DS012661-34
FIGURE 8. LM2825 Output Current Derating for T
A
= 0C to 70C
DS012661-35
FIGURE 9. LM2825 Output Current Derating for T
A
= 70C to 85C
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10
Application Information
(Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT (2X SIZE), SINGLE SIDED, THROUGH HOLE PLATED
DS012661-25
FIGURE 10. Typical Circuits Using Shutdown /Soft-start Features
DS012661-26
Note: Holes are not shown.
"No Connect Pins" are connected to copper pads for thermal reasons only and must remain electrically isolated.
FIGURE 11. 2X Printed Circuit Board Layout
11
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Physical Dimensions
inches (millimeters) unless otherwise noted
LIFE SUPPORT POLICY
NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE-
VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-
CONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or sys-
tems which, (a) are intended for surgical implant into
the body, or (b) support or sustain life, and whose fail-
ure to perform when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
2. A critical component in any component of a life support
device or system whose failure to perform can be rea-
sonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
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Tel: 81-3-5620-6175
Fax: 81-3-5620-6179
24-Lead (0.600" Wide) Molded Dual-In-Line Package
Order Number LM2825N-3.3, LM2825N-5.0, LM2825N12, LM2825N-ADJ or LM2825HN-ADJ
NS Package Number NA24F
LM2825
Integrated
Power
Supply
1A
DC-DC
Converter
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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