LM2599
LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage
Regulator, with Features
Literature Number: SNVS123B
LM2599
SIMPLE SWITCHER®Power Converter 150 kHz 3A
Step-Down Voltage Regulator, with Features
General Description
The LM2599 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 3A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-
able output version.
This series of switching regulators is similar to the LM2596
series, with additional supervisory and performance features
added.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation, improved line and load specifications, fixed-
frequency oscillator, Shutdown/Soft-start, error flag delay
and error flag output.
The LM2599 series operates at a switching frequency of 150
kHz thus allowing smaller sized filter components than what
would be needed with lower frequency switching regulators.
Available in a standard 7-lead TO-220 package with several
different lead bend options, and a 7-lead TO-263 Surface
mount package.
A standard series of inductors (both through hole and sur-
face mount types) are available from several different manu-
facturers optimized for use with the LM2599 series. This
feature greatly simplifies the design of switch-mode power
supplies.
Other features include a guaranteed ±4% tolerance on out-
put voltage under all conditions of input voltage and output
load conditions, and ±15% on the oscillator frequency. Ex-
ternal shutdown is included, featuring typically 80 µA
standby current. Self protection features include a two stage
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions.
Features
n3.3V, 5V, 12V, and adjustable output versions
nAdjustable version output voltage range, 1.2V to 37V
±4% max over line and load conditions
nGuaranteed 3A output current
nAvailable in 7-pin TO-220 and TO-263 (surface mount)
Package
nInput voltage range up to 40V
n150 kHz fixed frequency internal oscillator
nShutdown/Soft-start
nOut of regulation error flag
nError output delay
nLow power standby mode, I
Q
typically 80 µA
nHigh Efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
Applications
nSimple high-efficiency step-down (buck) regulator
nEfficient pre-regulator for linear regulators
nOn-card switching regulators
nPositive to Negative converter
Note: Patent Number 5,382,918.
Typical Application (Fixed Output Voltage
Versions)
01258201
SIMPLE SWITCHER®and Switchers Made Simple®are registered trademarks of National Semiconductor Corporation.
December 2000
LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator, with
Features
© 2004 National Semiconductor Corporation DS012582 www.national.com
Connection Diagrams and Order Information
Bent and Staggered Leads, Through Hole Package
7-Lead TO-220 (T)
Surface Mount Package
7-Lead TO-263 (S)
01258250
Order Number LM2599T-3.3, LM2599T-5.0,
LM2599T-12 or LM2599T-ADJ
See NS Package Number TA07B
01258223
Order Number LM2599S-3.3, LM2599S-5.0,
LM2599S-12 or LM2599S-ADJ
See NS Package Number TS7B
LM2599
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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 Supply Voltage (V
IN
) 45V
SD /SS Pin Input Voltage (Note 2) 6V
Delay Pin Voltage (Note 2) 1.5V
Flag Pin Voltage −0.3 V45V
Feedback Pin Voltage −0.3 V+25V
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally limited
Storage Temperature Range −65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 3) 2 kV
Lead Temperature
S Package
Vapor Phase (60 sec.) +215˚C
Infrared (10 sec.) +245˚C
T Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C T
J
+125˚C
Supply Voltage 4.5V to 40V
LM2599-3.3
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-3.3 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 4.75V V
IN
40V, 0.2A I
LOAD
3A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
=3A 73 %
LM2599-5.0
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-5.0 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 7V V
IN
40V, 0.2A I
LOAD
3A 5 V
4.800/4.750 V(min)
5.200/5.250 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
=3A 80 %
LM2599
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LM2599-12
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-12 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
OUT
Output Voltage 15V V
IN
40V, 0.2A I
LOAD
3A 12 V
11.52/11.40 V(min)
12.48/12.60 V(max)
ηEfficiency V
IN
= 25V, I
LOAD
=3A 90 %
LM2599-ADJ
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range.
Symbol Parameter Conditions LM2599-ADJ Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit Figure 1
V
FB
Feedback Voltage 4.5V V
IN
40V, 0.2A I
LOAD
3A 1.230 V
V
OUT
programmed for 3V. Circuit of Figure 1. 1.193/1.180 V(min)
1.267/1.280 V(max)
ηEfficiency V
IN
= 12V, V
OUT
= 3V, I
LOAD
=3A 73 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version and V
IN
= 24V for the 12V version.
I
LOAD
= 500 mA
Symbol Parameter Conditions LM2599-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, V
FB
= 1.3V 10 nA
50/100 nA (max)
f
O
Oscillator Frequency (Note 7) 150 kHz
127/110 kHz(min)
173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
= 3A (Note 8) (Note 9) 1.16 V
1.4/1.5 V(max)
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
CL
Current Limit Peak Current, (Note 8) (Note 9) 4.5 A
3.6/3.4 A(min)
6.9/7.5 A(max)
I
L
Output Leakage Current (Note 8) (Note 10) (Note 11) Output = 0V 50 µA(max)
Output = −1V 2 mA
30 mA(max)
I
Q
Operating Quiescent SD /SS Pin Open (Note 10) 5mA
Current 10 mA(max)
LM2599
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
= 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version and V
IN
= 24V for the 12V version.
I
LOAD
= 500 mA
Symbol Parameter Conditions LM2599-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
STBY
Standby Quiescent SD /SS pin = 0V (Note 11) 80 µA
Current 200/250 µA(max)
θ
JC
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
θ
JA
TO220 Package, Juncton to Ambient (Note 12) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 13) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 14) 30 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 15) 20 ˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of Figure 1
V
SD
Shutdown Threshold 1.3 V
Voltage Low, (Shutdown Mode) 0.6 V(max)
High, (Soft-start Mode) 2V(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 A
10 µA(max)
I
SS
Soft-start Current V
Soft-start
= 2.5V 1.6 µA
5 µA(max)
FLAG/DELAY CONTROL Test Circuit of Figure 1
Regulator Dropout Detector Low (Flag ON) 96 %
Threshold Voltage 92 %(min)
98 %(max)
VF
SAT
Flag Output Saturation I
SINK
= 3 mA 0.3 V
Voltage V
DELAY
= 0.5V 0.7/1.0 V(max)
IF
L
Flag Output Leakage Current V
FLAG
= 40V 0.3 µA
Delay Pin Threshold 1.25 V
Voltage Low (Flag ON) 1.21 V(min)
High (Flag OFF) and V
OUT
Regulated 1.29 V(max)
Delay Pin Source Current V
DELAY
= 0.5V 3 µA
6 µA(max)
Delay Pin Saturation Low (Flag ON) 55 mV
350/400 mV(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
intended 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 1 mA.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: Typical numbers are at 25˚C and represent the most likely norm.
Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2599
is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 7: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current
overload.
Note 8: No diode, inductor or capacitor connected to output pin.
Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 10: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor
switch OFF.
LM2599
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All Output Voltage Versions
Electrical Characteristics (Continued)
Note 11: VIN = 40V.
Note 12: Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the leads soldered to
a printed circuit board with (1 oz.) copper area of approximately 1 in2.
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2of (1 oz.) copper area.
Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2of (1 oz.) copper area.
Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2of (1 oz.) copper area on
the LM2599S side of the board, and approximately 16 in2of copper on the other side of the p-c board. See application hints in this data sheet and the thermal model
in Switchers Made Simple version 4.2.1 (or later) software.
Typical Performance Characteristics
(Circuit of Figure 1)
Normalized
Output Voltage Line Regulation
01258202 01258203
Efficiency
Switch Saturation
Voltage
01258204 01258205
LM2599
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Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Switch Current Limit Dropout Voltage
01258206 01258207
Operating
Quiescent Current
Shutdown
Quiescent Current
01258208 01258209
Minimum Operating
Supply Voltage
Feedback Pin
Bias Current
01258210 01258211
LM2599
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Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Flag Saturation
Voltage Switching Frequency
01258212 01258213
Soft-start
Shutdown /Soft-start
Current
01258214 01258215
Daisy Pin Current Soft-start Response
01258216 01258218
LM2599
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Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Shutdown/Soft-start
Threshold Voltage
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
=2A
L=3H,C
OUT
= 220 µF, C
OUT
ESR=50m
01258253
01258220
A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div.
C: Output Ripple Voltage, 50 mV/div.
Horizontal Time Base: 2 µs/div.
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA
L=1H,C
OUT
= 330 µF, C
OUT
ESR=45m
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L=3H,C
OUT
= 220 µF, C
OUT
ESR=50m
01258219
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 100 mV/div.
Horizontal Time Base: 2 µs/div.
01258221
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 50 µs/div.
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L=1H,C
OUT
= 330 µF, C
OUT
ESR=45m
01258222
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 200 µs/div.
LM2599
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
01258224
Component Values shown are for VIN = 15V,
VOUT = 5V, ILOAD = 3A.
CIN 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 5A, 40V Schottky Rectifier, 1N5825
L1 68 µH, L38
Typical Values
CSS 0.1 µF
CDELAY 0.1 µF
RPull Up 4.7k
LM2599
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Test Circuit and Layout Guidelines (Continued)
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the wires indicated by
heavy lines should be wide printed circuit traces and
should be kept as short as possible. For best results,
external components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and C
OUT
wiring can cause problems.
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the
associated wiring. Physically locate both resistors near the
IC, and route the wiring away from the inductor, especially an
open core type of inductor. (See application section for more
information.)
Adjustable Output Voltage Versions
01258225
where VREF = 1.23V
Select R1to be approximately 1 k, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 3A.
CIN: 470 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT: 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”
D1 5A, 30V Schottky Rectifier, 1N5824
L1 68 µH, L38
R1—1k,1%
R2 7.15k, 1%
CFF 3.3 nF, See Application Information Section
RFF —3k, See Application Information Section
Typical Values
CSS 0.1 µF
CDELAY 0.1 µF
RPULL UP 4.7k
FIGURE 1. Standard Test Circuits and Layout Guides
LM2599
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LM2599 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V or 12V)
V
IN
(max) = Maximum DC Input Voltage
I
LOAD
(max) = Maximum Load Current
Given:
V
OUT
=5V
V
IN
(max) = 12V
I
LOAD
(max) = 3A
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from
Figure 4,Figure 5,or6. (Output voltages of 3.3V, 5V, or
12V respectively.) For all other voltages, see the design
procedure for the adjustable version.
B. From the inductor value selection guide, identify the
inductance region intersected by the Maximum Input
Voltage line and the Maximum Load Current line. Each
region is identified by an inductance value and an
inductor code (LXX).
C. Select an appropriate inductor from the four
manufacturer’s part numbers listed in Figure 8.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version
shown in Figure 5.
B. From the inductor value selection guide shown in
Figure 5, the inductance region intersected by the 12V
horizontal line and the 3A vertical line is 33 µH, and the
inductor code is L40. C. The inductance value required is
33 µH. From the table in Figure 8, go to the L40 line and
choose an inductor part number from any of the four
manufacturers shown. (In most instance, both through
hole and surface mount inductors are available.)
2. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR (Equivalent
Series Resistance) electrolytic capacitors between 82 µF
and 820 µF and low ESR solid tantalum capacitors
between 10 µF and 470 µF provide the best results. This
capacitor should be located close to the IC using short
capacitor leads and short copper traces. Do not use
capacitors larger than 820 µF.
For additional information, see section on output
capacitors in application information section.
B. To simplify the capacitor selection procedure, refer to
the quick design component selection table shown in
Figure 2. This table contains different input voltages,
output voltages, and load currents, and lists various
inductors and output capacitors that will provide the best
design solutions.
C. The capacitor voltage rating for electrolytic capacitors
should be at least 1.5 times greater than the output
voltage, and often much higher voltage ratings are
needed to satisfy the low ESR requirements for low
output ripple voltage.
D. For computer aided design software, see Switchers
Made Simple (version 4.2.1 or later).
2. Output Capacitor Selection (C
OUT
)
A. See section on output capacitors in application
information section.
B. From the quick design component selection table
shown in Figure 2, locate the 5V output voltage section.
In the load current column, choose the load current line
that is closest to the current needed in your application,
for this example, use the 3A line. In the maximum input
voltage column, select the line that covers the input
voltage needed in your application, in this example, use
the 15V line. Continuing on this line are recommended
inductors and capacitors that will provide the best
overall performance.
The capacitor list contains both through hole electrolytic
and surface mount tantalum capacitors from four
different capacitor manufacturers. It is recommended
that both the manufacturers and the manufacturer’s
series that are listed in the table be used.
In this example aluminum electrolytic capacitors from
several different manufacturers are available with the
range of ESR numbers needed.
330 µF 35V Panasonic HFQ Series
330 µF 35V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V
or more is needed. But even a low ESR, switching grade,
220 µF 10V aluminum electrolytic capacitor would exhibit
approximately 225 mof ESR (see the curve in Figure 17
for the ESR vs voltage rating). This amount of ESR would
result in relatively high output ripple voltage. To reduce
the ripple to 1% of the output voltage, or less, a capacitor
with a higher value or with a higher voltage rating (lower
ESR) should be selected. A 16V or 25V capacitor will
reduce the ripple voltage by approximately half.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous
output short, the diode should have a current rating
equal to the maximum current limit of the LM2599. The
most stressful condition for this diode is an overload or
shorted output condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time)
and must be located close to the LM2599 using short
leads and short printed circuit traces. Because of their
fast switching speed and low forward voltage drop,
Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in
low output voltage applications. Ultra-fast recovery, or
High-Efficiency rectifiers also provide good results.
Ultra-fast recovery diodes typically have reverse
recovery times of 50 ns or less. Rectifiers such as the
IN5400 series are much too slow and should not be used.
3. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 11. In this example,
a 5A, 20V, 1N5823 Schottky diode will provide the best
performance, and will not be overstressed even for a
shorted output.
4. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least
1
2
the DC load current.
The capacitor manufacturers data sheet must be checked
to assure that this current rating is not exceeded. The
curve shown in Figure 16 shows typical RMS current
ratings for several different aluminum electrolytic
capacitor values.
This capacitor should be located close to the IC using
short leads and the voltage rating should be
approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is
recomended that they be surge current tested by the
manufacturer.
Use caution when using ceramic capacitors for input
bypassing, because it may cause severe ringing at the
V
IN
pin.
For additional information, see section on input
capacitors in Application Information section.
4. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the
input voltage rating and the RMS current rating. With a
nominal input voltage of 12V, an aluminum electrolytic
capacitor with a voltage rating greater than 18V (1.5 x V
IN
)
would be needed. The next higher capacitor voltage
rating is 25V.
The RMS current rating requirement for the input
capacitor in a buck regulator is approximately
1
2
the DC
load current. In this example, with a 3A load, a capacitor
with a RMS current rating of at least 1.5A is needed. The
curves shown in Figure 16 can be used to select an
appropriate input capacitor. From the curves, locate the
35V line and note which capacitor values have RMS
current ratings greater than 1.5A. A 680 µF, 35V capacitor
could be used.
For a through hole design, a 680 µF/35V electrolytic
capacitor (Panasonic HFQ series or Nichicon PL series
or equivalent) would be adequate. other types or other
manufacturers capacitors can be used provided the RMS
ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors are
recommended. The TPS series available from AVX, and
the 593D series from Sprague are both surge current
tested.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given:
V
OUT
= Regulated Output Voltage
V
IN
(max) = Maximum Input Voltage
I
LOAD
(max) = Maximum Load Current
F = Switching Frequency (Fixed at a nominal 150 kHz).
Given:
V
OUT
= 20V
V
IN
(max) = 28V
I
LOAD
(max) = 3A
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in Figure 1)
Use the following formula to select the appropriate resistor
values.
Select a value for R
1
between 240and 1.5 k. The lower
resistor values minimize noise pickup in the sensitive feed-
back pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in Figure 1)
Select R
1
to be 1 k, 1%. Solve for R
2
.
R
2
= 1k (16.26 1) = 15.26k, closest 1% value is 15.4 k.
R
2
= 15.4 k.
Conditions Inductor Output Capacitor
Through Hole Electrolytic Surface Mount Tantalum
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (µH) (#) HFQ Series PL Series Series 595D Series
(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)
3.3
3
5 22 L41 470/25 560/16 330/6.3 390/6.3
7 22 L41 560/35 560/35 330/6.3 390/6.3
10 22 L41 680/35 680/35 330/6.3 390/6.3
40 33 L40 560/35 470/35 330/6.3 390/6.3
6 22 L33 470/25 470/35 330/6.3 390/6.3
2 10 33 L32 330/35 330/35 330/6.3 390/6.3
40 47 L39 330/35 270/50 220/10 330/10
5
3
8 22 L41 470/25 560/16 220/10 330/10
10 22 L41 560/25 560/25 220/10 330/10
15 33 L40 330/35 330/35 220/10 330/10
40 47 L39 330/35 270/35 220/10 330/10
9 22 L33 470/25 560/16 220/10 330/10
2 20 68 L38 180/35 180/35 100/10 270/10
40 68 L38 180/35 180/35 100/10 270/10
12
3
15 22 L41 470/25 470/25 100/16 180/16
18 33 L40 330/25 330/25 100/16 180/16
30 68 L44 180/25 180/25 100/16 120/20
40 68 L44 180/35 180/35 100/16 120/20
15 33 L32 330/25 330/25 100/16 180/16
2 20 68 L38 180/25 180/25 100/16 120/20
40 150 L42 82/25 82/25 68/20 68/25
FIGURE 2. LM2599 Fixed Voltage Quick Design Component Selection Table
LM2599
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LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
2. Inductor Selection (L1)
A. Calculate the inductor Volt microsecond constant
ET(Vµs), from the following formula:
where V
SAT
= internal switch saturation voltage = 1.16V and
V
D
= diode forward voltage drop = 0.5V
B. Use the E T value from the previous formula and match it
with the E T number on the vertical axis of the Inductor Value
Selection Guide shown in Figure 7.
C. on the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E T value
and the Maximum Load Current value. Each region is identi-
fied by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8.
2. Inductor Selection (L1)
A. Calculate the inductor Volt microsecond constant
(E T),
B. ET = 34.2 (V µs)
C. I
LOAD
(max) = 3A
D. From the inductor value selection guide shown in Figure 7,
the inductance region intersected by the 34 (V µs) horizontal
line and the 3A vertical line is 47 µH, and the inductor code is
L39.
E. From the table in Figure 8, locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
3. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 820 µF provide the
best results. This capacitor should be located close to the IC
using short capacitor leads and short copper traces. Do not
use capacitors larger than 820 µF. For additional
information, see section on output capacitors in
application information section.
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in Figure 3. This table contains
different output voltages, and lists various output capacitors
that will provide the best design solutions.
C. The capacitor voltage rating should be at least 1.5 times
greater than the output voltage, and often much higher
voltage ratings are needed to satisfy the low ESR
requirements needed for low output ripple voltage.
3. Output Capacitor SeIection (C
OUT
)
A. See section on C
OUT
in Application Information
section.
B. From the quick design table shown in Figure 3,
locate the output voltage column. From that column,
locate the output voltage closest to the output voltage
in your application. In this example, select the 24V line.
Under the output capacitor section, select a capacitor
from the list of through hole electrolytic or surface
mount tantalum types from four different capacitor
manufacturers. It is recommended that both the
manufacturers and the manufacturers series that are
listed in the table be used.
In this example, through hole aluminum electrolytic
capacitors from several different manufacturers are
available.
220/35 Panasonic HFQ Series
150/35 Nichicon PL Series
C. For a 20V output, a capacitor rating of at least 30V or
more is needed. In this example, either a 35V or 50V
capacitor would work. A 50V rating was chosen because
it has a lower ESR which provides a lower output ripple
voltage.
Other manufacturers or other types of capacitors may
also be used, provided the capacitor specifications
(especially the 100 kHz ESR) closely match the types
listed in the table. Refer to the capacitor manufacturers
data sheet for this information.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
4. Feedforward Capacitor (C
FF
)(See Figure 1)
For output voltages greater than approximately 10V, an
additional capacitor is required. The compensation capacitor
is typically between 100 pF and 33 nF, and is wired in parallel
with the output voltage setting resistor, R
2
. It provides
additional stability for high output voltages, low input-output
voltages, and/or very low ESR output capacitors, such as
solid tantalum capacitors.
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capacitors
made with Z5U material, they are not recommended.)
4. Feedforward Capacitor (C
FF
)
The table shown in Figure 3 contains feed forward
capacitor values for various output voltages. In this
example, a 560 pF capacitor is needed.
5. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3
times greater than the maximum load current. Also, if the
power supply design must withstand a continuous
output short, the diode should have a current rating
equal to the maximum current limit of the LM2599. The
most stressful condition for this diode is an overload or
shorted output condition.
B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time)
and must be located close to the LM2599 using short
leads and short printed circuit traces. Because of their
fast switching speed and low forward voltage drop,
Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in
low output voltage applications. Ultra-fast recovery, or
High-Efficiency rectifiers are also a good choice, but
some types with an abrupt turn-off characteristic may
cause instability or EMl problems. Ultra-fast recovery
diodes typically have reverse recovery times of 50 ns or
less. Rectifiers such as the 1N4001 series are much too
slow and should not be used.
5. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 11. Schottky
diodes provide the best performance, and in this
example a 3A, 40V, 1N5825 Schottky diode would be a
good choice. The 3A diode rating is more than adequate
and will not be overstressed even for a shorted output.
LM2599
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LM2599 Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground to prevent
large voltage transients from appearing at the input. In
addition, the RMS current rating of the input capacitor
should be selected to be at least
1
2
the DC load current.
The capacitor manufacturers data sheet must be
checked to assure that this current rating is not
exceeded. The curve shown in Figure 16 shows typical
RMS current ratings for several different aluminum
electrolytic capacitor values.
This capacitor should be located close to the IC using
short leads and the voltage rating should be
approximately 1.5 times the maximum input voltage.
If solid tantalum input capacitors are used, it is
recomended that they be surge current tested by the
manufacturer.
Use caution when using a high dielectric constant
ceramic capacitor for input bypassing, because it may
cause severe ringing at the V
IN
pin.
For additional information, see section on input
capacitor in application information section.
6. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the
input voltage rating and the RMS current rating. With a
nominal input voltage of 28V, an aluminum electrolytic
aluminum electrolytic capacitor with a voltage rating
greater than 42V (1.5 x V
IN
) would be needed. Since the
the next higher capacitor voltage rating is 50V, a 50V
capacitor should be used. The capacitor voltage rating
of (1.5 x V
IN
) is a conservative guideline, and can be
modified somewhat if desired.
The RMS current rating requirement for the input
capacitor of a buck regulator is approximately
1
2
the DC
load current. In this example, with a 3A load, a capacitor
with a RMS current rating of at least 1.5A is needed.
The curves shown in Figure 16 can be used to select an
appropriate input capacitor. From the curves, locate the
50V line and note which capacitor values have RMS
current ratings greater than 1.5A. Either a 470 µF or
680 µF, 50V capacitor could be used.
For a through hole design, a 680 µF/50V electrolytic
capacitor (Panasonic HFQ series or Nichicon PL series
or equivalent) would be adequate. Other types or other
manufacturers capacitors can be used provided the
RMS ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors
can be used, but caution must be exercised with regard
to the capacitor sure current rating (see Application
Information or input capacitors in this data sheet). The
TPS series available from AVX, and the 593D series
from Sprague are both surge current tested.
To further simplify the buck regulator design procedure,
National Semiconductor is making available computer
design software to be used with the Simple Switcher
line ot switching regulators. Switchers Made Simple
(version 4.2.1 or later) is available on a 3
1
2
" diskette for
IBM compatible computers.
Output
Voltage
(V)
Through Hole Output Capacitor Surface Mount Output Capacitor
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
HFQ Series Series Capacitor Series 595D Series Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
2820/35 820/35 33 nF 330/6.3 470/4 33 nF
4560/35 470/35 10 nF 330/6.3 390/6.3 10 nF
6470/25 470/25 3.3 nF 220/10 330/10 3.3 nF
9330/25 330/25 1.5 nF 100/16 180/16 1.5 nF
12 330/25 330/25 1 nF 100/16 180/16 1 nF
15 220/35 220/35 680 pF 68/20 120/20 680 pF
24 220/35 150/35 560 pF 33/25 33/25 220 pF
28 100/50 100/50 390 pF 10/35 15/50 220 pF
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
LM2599
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LM2599 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
01258226
FIGURE 4. LM2599-3.3
01258227
FIGURE 5. LM2599-5.0
01258228
FIGURE 6. LM2599-12
01258229
FIGURE 7. LM2599-ADJ
LM2599
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