LM2594
SIMPLE SWITCHER ® Power Converter 150 kHz 0.5A
Step-Down Voltage Regulator
General Description
The LM2594/LM2594HV series of regulators are monolithic
integrated circuits that provide all the active functions for a
step-down (buck) switching regulator, capable of driving a
0.5A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, 12V,
and an adjustable output version, and are packaged in a
8-lead DIP and a 8-lead surface mount package.
The LM2594HV is for applications requiring an input voltage
up to 60V.
Features
n 3.3V, 5V, 12V, and adjustable output versions
n Adjustable version output voltage range, 1.2V to 37V
(57V for the HV version) ± 4% max over line and load
conditions
n Available in 8-pin surface mount and DIP-8 package
n Guaranteed 0.5A output current
n Input voltage range up to 60V
n Requires only 4 external components
n 150 kHz fixed frequency internal oscillator
n TTL Shutdown capability
n Low power standby mode, IQ typically 85 µA
n High Efficiency
n Uses readily available standard inductors
n Thermal shutdown and current limit protection
Requiring a minimum number of external components, these
regulators are simple to use and feature internal frequency
compensation†, a fixed-frequency oscillator, and improved
line and load regulation specifications.
The LM2594/LM2594HV 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. Because of its high efficiency, the copper traces on the printed circuit board are normally the only
heat sinking needed.
A standard series of inductors (both through hole and surface mount types) are available from several different manufacturers optimized for use with the LM2594/LM2594HV series. This feature greatly simplifies the design of
switch-mode power supplies.
Other features include a guaranteed ± 4% tolerance on output voltage under all conditions of input voltage and output
load conditions, and ± 15% on the oscillator frequency. External shutdown is included, featuring typically 85 µA
standby current. Self protection features include a two stage
frequency reducing current limit for the output switch and an
over temperature shutdown for complete protection under
fault conditions.
Typical Application
Applications
n
n
n
n
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to Negative convertor
(Fixed Output Voltage Versions)
DS012439-1
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LM2594
Connection Diagrams and Order Information
8-Lead DIP (N)
8-Lead Surface Mount (M)
DS012439-2
Top View
Order Number
LM2594N-3.3, LM2594N-5.0,
LM2594N-12 or LM2594N-ADJ
LM2594HVN-3.3, LM2594HVN-5.0,
LM2594HVN-12 or LM2594HVN-ADJ
See NS Package Number N08E
DS012439-3
Top View
Order Number LM2594M-3.3,
LM2594M-5.0, LM2594M-12 or
LM2594M-ADJ
LM2594HVM-3.3, LM2594HVM-5.0,
LM2594HVM-12 or LM2594HVM-ADJ
See NS Package Number M08A
*No internal connection, but should be soldered to pc board for best heat transfer.
‡ Patent Number 5,382,918.
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LM2594
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
LM2594
LM2594HV
ON /OFF Pin Input Voltage
Feedback Pin Voltage
Output Voltage to Ground
(Steady State)
Power Dissipation
Storage Temperature Range
ESD Susceptibility
45V
60V
−0.3 ≤ V ≤ +25V
−0.3 ≤ V ≤+25V
−1V
Internally limited
−65˚C to +150˚C
Human Body Model (Note 2)
Lead Temperature
M8 Package
Vapor Phase (60 sec.)
Infrared (15 sec.)
N Package (Soldering, 10 sec.)
Maximum Junction Temperature
2 kV
+215˚C
+220˚C
+260˚C
+150˚C
Operating Conditions
−40˚C ≤ TJ +125˚C
Temperature Range
Supply Voltage
LM2594
LM2594HV
4.5V to 40V
4.5V to 60V
LM2594/LM2594HV-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.VINmax = 40V for the LM2594 and 60V for the LM2594HV.
Symbol
Parameter
Conditions
LM2594/LM2594HV-3.3
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
4.75V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
VIN = 12V, ILOAD = 0.5A
3.3
V
3.168/3.135
V(min)
3.432/3.465
V(max)
%
80
LM2594/LM2594HV-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
Conditions
LM2594/LM2594HV-5.0
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
7V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
VIN = 12V, ILOAD = 0.5A
5.0
V
4.800/4.750
V(min)
5.200/5.250
V(max)
%
82
LM2594/LM2594HV-12
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
Conditions
LM2594/LM2594HV-12
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
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15V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
VIN = 25V, ILOAD = 0.5A
12.0
88
3
V
11.52/11.40
V(min)
12.48/12.60
V(max)
%
2014 NOV
LM2594
LM2594/LM2594HV-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
Conditions
LM2594/LM2594HV-ADJ
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VFB
Feedback Voltage
4.5V ≤ VIN ≤ VINmax, 0.1A ≤ ILOAD ≤ 0.5A
1.230
VOUT programmed for 3V. Circuit of Figure 1
η
Efficiency
VIN = 12V, ILOAD = 0.5A
V
1.193/1.180
V(min)
1.267/1.280
V(max)
%
80
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range . Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V version. ILOAD = 100 mA
Symbol
Parameter
Conditions
LM2594/LM2594HV-XX
Typ
Limit
(Note 3)
(Note 4)
50/100
Units
(Limits)
DEVICE PARAMETERS
Ib
Feedback Bias Current
Adjustable Version Only, VFB = 1.3V
10
fO
Oscillator Frequency
(Note 6)
150
VSAT
DC
ICL
Saturation Voltage
IOUT = 0.5A (Note 7) (Note 8)
127/110
kHz(min)
173/173
kHz(max)
1.1/1.2
V(max)
0.9
Max Duty Cycle (ON)
(Note 8)
100
Min Duty Cycle (OFF)
(Note 9)
0
Current Limit
Peak Current, (Note 7) (Note 8)
V
%
0.8
A
0.65/0.58
IL
Output Leakage Current
Output = 0V
(Note 7) (Note 9) (Note 10)
Output = −1V
IQ
ISTBY
θJA
Quiescent Current
(Note 9)
A(max)
50
µA(max)
15
mA(max)
10
mA(max)
200/250
µA(max)
250/300
µA(max)
mA
5
ON/OFF pin = 5V (OFF)
Current
LM2594
LM2594HV
140
Thermal Resistance
N Package, Junction to Ambient (Note 11)
95
M Package, Junction to Ambient (Note 11)
150
(Note 10)
A(min)
1.3/1.4
2
Standby Quiescent
nA
kHz
mA
85
µA
˚C/W
ON/OFF CONTROL Test Circuit Figure 1
ON /OFF Pin Logic Input
VIH
Low (Regulator ON)
ON /OFF Pin
High (Regulator OFF)
VLOGIC = 2.5V (Regulator OFF)
VIL
IH
1.3
Threshold Voltage
VLOGIC = 0.5V (Regulator ON)
V(max)
2.0
V(min)
15
µA(max)
5
µA(max)
5
Input Current
IL
V
0.6
µA
0.02
µA
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.
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LM2594
All Output Voltage Versions
Electrical Characteristics (Continued)
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 3: Typical numbers are at 25˚C and represent the most likely norm.
Note 4: 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 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator system performance. When the LM2594/LM2594HV is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section
of Electrical Characteristics.
Note 6: 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 7: No diode, inductor or capacitor connected to output pin.
Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 9: 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.
Note 10: VIN = 40V for the LM2594 and 60V for the LM2594HV.
Note 11: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will
lower thermal resistance further. See application hints in this data sheet and the thermal model in Switchers Made Simple ® software.
Typical Performance Characteristics
Normalized
Output Voltage
Line Regulation
Efficiency
DS012439-6
DS012439-5
DS012439-4
Switch Saturation
Voltage
Dropout Voltage
Switch Current Limit
DS012439-8
DS012439-9
DS012439-7
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LM2594
Typical Performance Characteristics
(Continued)
Standby
Quiescent Current
Quiescent Current
Minimum Operating
Supply Voltage
DS012439-10
DS012439-11
ON /OFF Threshold
Voltage
ON /OFF Pin
Current (Sinking)
DS012439-12
Switching Frequency
DS012439-15
DS012439-13
DS012439-14
Feedback Pin
Bias Current
DS012439-16
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LM2594
Typical Performance Characteristics
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 400 mA
L = 100 µH, COUT = 120 µF, COUT ESR = 140 mΩ
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 200 mA
L = 33 µH, COUT = 220 µF, COUT ESR = 60 mΩ
DS012439-17
DS012439-18
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
Horizontal Time Base: 2 µs/div.
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 200 mA to 500 mA
L = 100 µH, COUT = 120 µF, COUT ESR = 140 mΩ
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 100 mA to 200 mA
L = 33 µH, COUT = 220 µF, COUT ESR = 60 mΩ
DS012439-20
DS012439-19
A: Output Voltage, 50 mV/div. (AC)
B: 200 mA to 500 mA Load Pulse
A: Output Voltage, 50 mV/div. (AC)
B: 100 mA to 200 mA Load Pulse
Horizontal Time Base: 200 µs/div.
Horizontal Time Base: 50 µs/div.
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LM2594
Typical Circuit and Layout Guidelines
Fixed Output Voltage Versions
DS012439-22
CIN — 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 1A, 40V Schottky Rectifier, 1N5819
L1 — 100 µH, L20
Select components with higher voltage ratings for designs using the LM2594HV with an input voltage between 40V and 60V.
Adjustable Output Voltage Versions
DS012439-23
CIN — 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 1A, 40V Schottky Rectifier, 1N5819
L1 — 100 µH, L20
R1 — 1 kΩ, 1%
CFF — See Application Information Section
FIGURE 1. Typical Circuits and Layout Guides
If open core inductors are used, special care must be
taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and COUT 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.)
As in any switching regulator, layout is very important. Rapidly 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.
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LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed
Output)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
Given:
VOUT = Regulated Output Voltage (3.3V, 5V or 12V)
VIN(max) = Maximum DC Input Voltage
ILOAD(max) = Maximum Load Current
Given:
VOUT = 5V
VIN(max) = 12V
ILOAD(max) = 0.4A
1. Inductor Selection (L1)
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 0.4A vertical line is 100 µH, and the inductor code is
L20.
C. The inductance value required is 100 µH. From the table
in Figure 8, go to the L20 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.)
A. Select the correct inductor value selection guide from Figures 4, 5 or Figure 6. (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.
2. Output Capacitor Selection (COUT)
2. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 82 µF and
220 µF and low ESR solid tantalum capacitors between 15
µF and 100 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
220 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 4.1 or later.
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 0.5A 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.
120 µF 25V Panasonic HFQ Series
120 µF 25V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V or
more is needed. But, in this example, even a low ESR,
switching grade, 120 µF 10V aluminum electrolytic capacitor
would exhibit approximately 400 mΩ of ESR (see the curve
in Figure 14 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 voltage rating (lower ESR) should be selected. A 16V or 25V capacitor will reduce the ripple voltage
by approximately half.
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LM2594
LM2594/LM2594HV 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 LM2594. 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 LM2594 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
1N4001 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
1A, 20V, 1N5817 Schottky diode will provide the best performance, and will not be overstressed even for a shorted output.
4. Input Capacitor (CIN)
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 13 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 recommended 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 VIN pin.
4. Input Capacitor (CIN)
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 VIN) 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 400 mA load, a capacitor with a RMS
current rating of at least 200 mA is needed. The curves
shown in Figure 13 can be used to select an appropriate input capacitor. From the curves, locate the 25V line and note
which capacitor values have RMS current ratings greater
than 200 mA. Either a 47 µF or 68 µF, 25V capacitor could be
used.
For a through hole design, a 68 µF/25V 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.
For additional information, see section on input capacitors in Application Information section.
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LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed
Output) (Continued)
Conditions
Inductor
Output Capacitor
Through Hole
Surface Mount
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
0.5
5
33
L14
220/16
220/16
100/16
100/6.3
0.2
5
0.5
0.2
12
0.5
0.2
7
47
L13
120/25
120/25
100/16
100/6.3
10
68
L21
120/25
120/25
100/16
100/6.3
40
100
L20
120/35
120/35
100/16
100/6.3
6
68
L4
120/25
120/25
100/16
100/6.3
10
150
L10
120/16
120/16
100/16
100/6.3
40
220
L9
120/16
120/16
100/16
100/6.3
8
47
L13
180/16
180/16
100/16
33/25
10
68
L21
180/16
180/16
100/16
33/25
15
100
L20
120/25
120/25
100/16
33/25
40
150
L19
120/25
120/25
100/16
33/25
9
150
L10
82/16
82/16
100/16
33/25
20
220
L9
120/16
120/16
100/16
33/25
40
330
L8
120/16
120/16
100/16
33/25
15
68
L21
82/25
82/25
100/16
15/25
18
150
L19
82/25
82/25
100/16
15/25
30
220
L27
82/25
82/25
100/16
15/25
40
330
L26
82/25
82/25
100/16
15/25
15
100
L11
82/25
82/25
100/16
15/25
20
220
L9
82/25
82/25
100/16
15/25
40
330
L17
82/25
82/25
100/16
15/25
FIGURE 2. LM2594/LM2594HV Fixed Voltage Quick Design Component Selection Table
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable
Output)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = Regulated Output Voltage
VIN(max) = Maximum Input Voltage
ILOAD(max) = Maximum Load Current
Given:
VOUT = 20V
VIN(max) = 28V
ILOAD(max) = 0.5A
F = Switching Frequency (Fixed at a nominal 150 kHz).
F = Switching Frequency (Fixed at a nominal 150 kHz).
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LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version)
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
Select R1 to be 1 kΩ, 1%. Solve for R2.
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1.
Use the following formula to select the appropriate resistor
values.
R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R2 = 15.4 kΩ.
Select a value for R1 between 240Ω and 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant (E • T)
,
2. Inductor Selection (L1)
A. Calculate the inductor Volt microsecond constant
E • T (V • µs) , from the following formula:
where VSAT = internal switch saturation voltage = 0.9V
and VD = 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
identified 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.
B. E • T = 35.2 (V • µs)
C. ILOAD(max) = 0.5A
D. From the inductor value selection guide shown in Figure 7,
the inductance region intersected by the 35 (V • µs) horizontal line and the 0.5A vertical line is 150 µH, and the inductor
code is L19.
E. From the table in Figure 8, locate line L19, and select an
inductor part number from the list of manufacturers part numbers.
3. Output Capacitor SeIection (COUT)
A. See section on COUT 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.
82 µF 50V Panasonic HFQ Series
120 µF 50V Nichicon PL Series
3. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 220 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 220 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.
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EXAMPLE (Adjustable Output Voltage Version)
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.
12
2014 NOV
LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
4. Feedforward Capacitor (CFF) (See Figure 1 )
4. Feedforward Capacitor (CFF)
The table shown in Figure 3 contains feed forward capacitor
values for various output voltages. In this example, a 1 nF
capacitor is needed.
For output voltages greater than approximately 10V, an additional capacitor is required. The compensation capacitor is
typically between 50 pF and 10 nF, and is wired in parallel
with the output voltage setting resistor, R2. 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.)
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 1A, 40V,
1N5819 Schottky diode would be a good choice. The 1A diode rating is more than adequate and will not be overstressed even for a shorted output.
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 LM2594. 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 LM2594 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.
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13
2014 NOV
LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (CIN)
6. Input Capacitor (CIN)
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 13 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.
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
VIN) 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 VIN) 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 400 mA load, a capacitor with a RMS
current rating of at least 200 mA is needed.
The curves shown in Figure 13 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 200 mA. A 47 µF/50V low ESR electrolytic
capacitor capacitor is needed.
For a through hole design, a 47 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 are
recommended. 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 bypassing, because it may cause severe ringing at the VIN pin.
For additional information, see section on input capacitors in application information section.
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 4.1 or later)
is available from National’s web site, www.national.com.
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)
1.2
220/25
220/25
0
220/10
220/10
0
4
180/25
180/25
4.7 nF
100/10
120/10
4.7 nF
6
82/25
82/25
4.7 nF
100/10
120/10
4.7 nF
9
82/25
82/25
3.3 nF
100/16
100/16
3.3 nF
12
82/25
82/25
2.2 nF
100/16
100/16
2.2 nF
15
82/25
82/25
1.5 nF
68/20
100/20
1.5 nF
24
82/50
120/50
1 nF
10/35
15/35
220 pF
28
82/50
120/50
820 pF
10/35
15/35
220 pF
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
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14
2014 NOV
LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS012439-24
DS012439-25
FIGURE 4. LM2594/LM2594HV-3.3
FIGURE 5. LM2594/LM2594HV-5.0
DS012439-26
DS012439-27
FIGURE 6. LM2594/LM2594HV-12
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FIGURE 7. LM2594/LM2594HV-ADJ
15
2014 NOV
LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure
(Continued)
Inductance
(µH)
Current
(A)
Through
Surface
Through
Surface
Through
Surface
Hole
Mount
Hole
Mount
Hole
Mount
Mount
L1
220
0.18
67143910
67144280 RL-5470-3
RL1500-220 PE-53801
PE-53801-S
DO1608-224
L2
150
0.21
67143920
67144290 RL-5470-4
RL1500-150 PE-53802
PE-53802-S
DO1608-154
L3
100
0.26
67143930
67144300 RL-5470-5
RL1500-100 PE-53803
PE-53803-S
DO1608-104
L4
68
0.32
67143940
67144310 RL-1284-68
RL1500-68
PE-53804
PE-53804-S
DO1608-68
L5
47
0.37
67148310
67148420 RL-1284-47
RL1500-47
PE-53805
PE-53805-S
DO1608-473
L6
33
0.44
67148320
67148430 RL-1284-33
RL1500-33
PE-53806
PE-53806-S
DO1608-333
L7
22
0.60
67148330
67148440 RL-1284-22
RL1500-22
PE-53807
PE-53807-S
DO1608-223
L8
330
0.26
67143950
67144320 RL-5470-2
RL1500-330 PE-53808
PE-53808-S
DO3308-334
L9
220
0.32
67143960
67144330 RL-5470-3
RL1500-220 PE-53809
PE-53809-S
DO3308-224
L10
150
0.39
67143970
67144340 RL-5470-4
RL1500-150 PE-53810
PE-53810-S
DO3308-154
L11
100
0.48
67143980
67144350 RL-5470-5
RL1500-100 PE-53811
PE-53811-S
DO3308-104
L12
68
0.58
67143990
67144360 RL-5470-6
RL1500-68
PE-53812
PE-53812-S
DO1608-683
L13
47
0.70
67144000
67144380 RL-5470-7
RL1500-47
PE-53813
PE-53813-S
DO3308-473
L14
33
0.83
67148340
67148450 RL-1284-33
RL1500-33
PE-53814
PE-53814-S
DO1608-333
L15
22
0.99
67148350
67148460 RL-1284-22
RL1500-22
PE-53815
PE-53815-S
DO1608-223
L16
15
1.24
67148360
67148470 RL-1284-15
RL1500-15
PE-53816
PE-53816-S
DO1608-153
L17
330
0.42
67144030
67144410 RL-5471-1
RL1500-330 PE-53817
PE-53817-S
DO3316-334
L18
220
0.55
67144040
67144420 RL-5471-2
RL1500-220 PE-53818
PE-53818-S
DO3316-224
L19
150
0.66
67144050
67144430 RL-5471-3
RL1500-150 PE-53819
PE-53819-S
DO3316-154
L20
100
0.82
67144060
67144440 RL-5471-4
RL1500-100 PE-53820
PE-53820-S
DO3316-104
L21
68
0.99
67144070
67144450 RL-5471-5
RL1500-68
PE-53821
PE-53821-S
DDO3316-683
L26
330
0.80
67144100
67144480 RL-5471-1
—
PE-53826
PE-53826-S
—
L27
220
1.00
67144110
67144490 RL-5471-2
—
PE-53827
PE-53827-S
—
Schott
Renco
Pulse Engineering
Coilcraft
Surface
FIGURE 8. Inductor Manufacturers Part Numbers
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16
2014 NOV
LM2594
LM2594/LM2594HV Series Buck Regulator Design Procedure
VR
(Continued)
1A Diodes
Surface Mount
Schottky
Through Hole
Ultra Fast
Schottky
Ultra Fast
1N5817
All of these
diodes are
SR102
diodes are
rated to at
1N5818
rated to at
least 60V.
SR103
least 60V.
Recovery
20V
Recovery
All of
these
MBRS130
30V
11DQ03
40V
50V
or
more
MBRS140
MURS120
1N5819
MUR120
10BQ040
10BF10
SR104
HER101
10MQ040
11DQ04
11DF1
MBRS160
SR105
10BQ050
MBR150
10MQ060
11DQ05
MBRS1100
MBR160
10MQ090
SB160
SGL41-60
11DQ10
SS16
FIGURE 11. Diode Selection Table
Block Diagram
DS012439-21
FIGURE 12.
Application Information
PIN FUNCTIONS
+VIN — This is the positive input supply for the IC switching
regulator. A suitable input bypass capacitor must be present
at this pin to minimize voltage transients and to supply the
switching currents needed by the regulator.
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Ground — Circuit ground.
Output — Internal switch. The voltage at this pin switches
between (+VIN − VSAT) and approximately −0.5V, with a duty
cycle of VOUT/VIN. To minimize coupling to sensitive circuitry,
the PC board copper area connected to this pin should be
kept to a minimum.
17
2014 NOV
LM2594
Application Information
“Standard” electrolytic capacitors typically have much higher
ESR numbers, lower RMS current ratings and typically have
a shorter operating lifetime.
(Continued)
Feedback — Senses the regulated output voltage to complete the feedback loop.
ON /OFF — Allows the switching regulator circuit to be shut
down using logic level signals thus dropping the total input
supply current to approximately 80 µA. Pulling this pin below
a threshold voltage of approximately 1.3V turns the regulator
on, and pulling this pin above 1.3V (up to a maximum of 25V)
shuts the regulator down. If this shutdown feature is not
needed, the ON /OFF pin can be wired to the ground pin or
it can be left open, in either case the regulator will be in the
ON condition.
Because of their small size and excellent performance, surface mount solid tantalum capacitors are often used for input
bypassing, but several precautions must be observed. A
small percentage of solid tantalum capacitors can short if the
inrush current rating is exceeded. This can happen at turn on
when the input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents. Several capacitor manufacturers do a 100% surge current testing on their products to minimize this potential problem. If
high turn on currents are expected, it may be necessary to
limit this current by adding either some resistance or inductance before the tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the
RMS ripple current rating must be sized to the load current.
EXTERNAL COMPONENTS
CIN — A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be located near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the input, and provides the instantaneous current needed each
time the switch turns on.
The important parameters for the Input capacitor are the
voltage rating and the RMS current rating. Because of the
relatively high RMS currents flowing in a buck regulator’s input capacitor, this capacitor should be chosen for its RMS
current rating rather than its capacitance or voltage ratings,
although the capacitance value and voltage rating are directly related to the RMS current rating.
The RMS current rating of a capacitor could be viewed as a
capacitor’s power rating. The RMS current flowing through
the capacitors internal ESR produces power which causes
the internal temperature of the capacitor to rise. The RMS
current rating of a capacitor is determined by the amount of
current required to raise the internal temperature approximately 10˚C above an ambient temperature of 105˚C. The
ability of the capacitor to dissipate this heat to the surrounding air will determine the amount of current the capacitor can
safely sustain. Capacitors that are physically large and have
a large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS current
rating.
The consequences of operating an electrolytic capacitor
above the RMS current rating is a shortened operating life.
The higher temperature speeds up the evaporation of the capacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple current. For a maximum ambient temperature of 40˚C, a general guideline would be to select a capacitor with a ripple
current rating of approximately 50% of the DC load current.
For ambient temperatures up to 70˚C, a current rating of
75% of the DC load current would be a good choice for a
conservative design. The capacitor voltage rating must be at
least 1.25 times greater than the maximum input voltage,
and often a much higher voltage capacitor is needed to satisfy the RMS current requirements.
A graph shown in Figure 13 shows the relationship between
an electrolytic capacitor value, its voltage rating, and the
RMS current it is rated for. These curves were obtained from
the Nichicon “PL” series of low ESR, high reliability electrolytic capacitors designed for switching regulator applications.
Other capacitor manufacturers offer similar types of capacitors, but always check the capacitor data sheet.
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DS012439-28
FIGURE 13. RMS Current Ratings for Low ESR
Electrolytic Capacitors (Typical)
OUTPUT CAPACITOR
COUT — An output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter.
The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a low ESR
value is needed. This value is determined by the maximum
allowable output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely
low, there is a possibility of an unstable feedback loop, resulting in an oscillation at the output. Using the capacitors
listed in the tables, or similar types, will provide design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is required, refer to the section on Output Voltage Ripple and
Transients for a post ripple filter.
An aluminum electrolytic capacitor’s ESR value is related to
the capacitance value and its voltage rating. In most cases,
Higher voltage electrolytic capacitors have lower ESR values
18
2014 NOV
LM2594
Application Information
(Continued)
(see Figure 14 ). Often, capacitors with much higher voltage
ratings may be needed to provide the low ESR values required for low output ripple voltage.
The output capacitor for many different switcher designs often can be satisfied with only three or four different capacitor
values and several different voltage ratings. See the quick
design component selection tables in Figure 2 and Figure 3
for typical capacitor values, voltage ratings, and manufacturers capacitor types.
Electrolytic capacitors are not recommended for temperatures below −25˚C. The ESR rises dramatically at cold temperatures and typically rises 3X @ −25˚C and as much as
10X at −40˚C. See curve shown in Figure 15 .
Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below −25˚C.
DS012439-30
FIGURE 15. Capacitor ESR Change vs Temperature
INDUCTOR SELECTION
All switching regulators have two basic modes of operation;
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulators
performance and requirements. Most switcher designs will
operate in the discontinuous mode when the load current is
low.
The LM2594 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of operation.
In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower peak
switch, inductor and diode currents, and can have lower output ripple voltage. But it does require larger inductor values
to keep the inductor current flowing continuously, especially
at low output load currents and/or high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 4 through
Figure 7 ). This guide assumes that the regulator is operating
in the continuous mode, and selects an inductor that will allow a peak-to-peak inductor ripple current to be a certain
percentage of the maximum design load current. This
peak-to-peak inductor ripple current percentage is not fixed,
but is allowed to change as different design load currents are
selected. (See Figure 16.)
DS012439-29
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2594 using
short leads and short printed circuit traces.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI 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.
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19
2014 NOV
LM2594
Application Information
may be a better choice. It would use an inductor that would
be physically smaller, and would need only one half to one
third the inductance value needed for a continuous mode design. The peak switch and inductor currents will be higher in
a discontinuous design, but at these low load currents
(200 mA and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The output pin (switch) waveform can have some damped sinusoidal ringing present. (See photo titled; Discontinuous Mode
Switching Waveforms) This ringing is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous operation, there is a period of time
where neither the switch or the diode are conducting, and
the inductor current has dropped to zero. During this time, a
small amount of energy can circulate between the inductor
and the switch/diode parasitic capacitance causing this characteristic ringing. Normally this ringing is not a problem, unless the amplitude becomes great enough to exceed the input voltage, and even then, there is very little energy present
to cause damage.
(Continued)
DS012439-31
FIGURE 16. (∆IIND) Peak-to-Peak
Inductor Ripple Current
(as a Percentage of the Load Current) vs Load Current
Different inductor types and/or core materials produce different amounts of this characteristic ringing. Ferrite core inductors have very little core loss and therefore produce the most
ringing. The higher core loss of powdered iron inductors produce less ringing. If desired, a series RC could be placed in
parallel with the inductor to dampen the ringing. The computer aided design software Switchers Made Simple (version 4.1) will provide all component values for continuous
and discontinuous modes of operation.
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size can
be kept relatively low.
When operating in the continuous mode, the inductor current
waveform ranges from a triangular to a sawtooth type of
waveform (depending on the input voltage), with the average
value of this current waveform equal to the DC output load
current.
Inductors are available in different styles such as pot core,
toroid, E-core, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. The least expensive, the bobbin, rod or stick core, consists of wire wrapped
on a ferrite bobbin. This type of construction makes for a inexpensive inductor, but since the magnetic flux is not completely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus causing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope readings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.
The inductors listed in the selection chart include ferrite
E-core construction for Schott, ferrite bobbin core for Renco
and Coilcraft, and powdered iron toroid for Pulse Engineering.
DS012439-32
FIGURE 17. Post Ripple Filter Waveform
Exceeding an inductor’s maximum current rating may cause
the inductor to overheat because of the copper wire losses,
or the core may saturate. If the inductor begins to saturate,
the inductance decreases rapidly and the inductor begins to
look mainly resistive (the DC resistance of the winding). This
can cause the switch current to rise very rapidly and force
the switch into a cycle-by-cycle current limit, thus reducing
the DC output load current. This can also result in overheating of the inductor and/or the LM2594. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
The inductor manufacturers data sheets include current and
energy limits to avoid inductor saturation.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply operating in
the continuous mode will contain a sawtooth ripple voltage at
the switcher frequency, and may also contain short voltage
spikes at the peaks of the sawtooth waveform.
The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output capacitor. A
typical output ripple voltage can range from approximately
0.5% to 3% of the output voltage. To obtain low ripple voltage, the ESR of the output capacitor must be low, however,
caution must be exercised when using extremely low ESR
capacitors because they can affect the loop stability, resulting in oscillation problems. If very low output ripple voltage is
needed (less than 15 mV), a post ripple filter is recommended. (See Figure 1.) The inductance required is typically
between 1 µH and 5 µH, with low DC resistance, to maintain
good load regulation. A low ESR output filter capacitor is also
required to assure good dynamic load response and ripple
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
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20
2014 NOV
LM2594
Application Information
nomographs shown in Figure 4 through Figure 7 are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 18 shows the range of (∆IIND) that can be expected
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current (∆IIND) changes as you
go from the lower border to the upper border (for a given load
current) within an inductance region. The upper border represents a higher input voltage, while the lower border represents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode operation, and only if the inductor selection guides are used to select the inductor value
Consider the following example:
VOUT = 5V, maximum load current of 300 mA
VIN = 15V, nominal, varying between 11V and 20V.
(Continued)
reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop. The
photo shown in Figure 17 shows a typical output ripple voltage, with and without a post ripple filter.
When observing output ripple with a scope, it is essential
that a short, low inductance scope probe ground connection
be used. Most scope probe manufacturers provide a special
probe terminator which is soldered onto the regulator board,
preferable at the output capacitor. This provides a very short
scope ground thus eliminating the problems associated with
the 3 inch ground lead normally provided with the probe, and
provides a much cleaner and more accurate picture of the
ripple voltage waveform.
The voltage spikes are caused by the fast switching action of
the output switch and the diode, and the parasitic inductance
of the output filter capacitor, and its associated wiring. To
minimize these voltage spikes, the output capacitor should
be designed for switching regulator applications, and the
lead lengths must be kept very short. Wiring inductance,
stray capacitance, as well as the scope probe used to evaluate these transients, all contribute to the amplitude of these
spikes.
When a switching regulator is operating in the continuous
mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input
voltage). For a given input and output voltage, the
peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or decreases,
the entire sawtooth current waveform also rises and falls.
The average value (or the center) of this current waveform is
equal to the DC load current.
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher designs (irregardless how large the inductor value is) will be forced to run discontinuous if the output is lightly loaded. This is a perfectly
acceptable mode of operation.
The selection guide in Figure 5 shows that the vertical line
for a 0.3A load current, and the horizontal line for the 15V input voltage intersect approximately midway between the upper and lower borders of the 150 µH inductance region. A
150 µH inductor will allow a peak-to-peak inductor current
(∆IIND) to flow that will be a percentage of the maximum load
current. Referring to Figure 18, follow the 0.3A line approximately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆IIND) on the left hand
axis (approximately 150 mA p-p).
As the input voltage increases to 20V, it approaches the upper border of the inductance region, and the inductor ripple
current increases. Referring to the curve in Figure 18, it can
be seen that for a load current of 0.3A, the peak-to-peak inductor ripple current (∆IIND) is 150 mA with 15V in, and can
range from 175 mA at the upper border (20V in) to 120 mA at
the lower border (11V in).
Once the ∆IIND value is known, the following formulas can be
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2.
Minimum load current before the circuit becomes discontinuous
3.
Output Ripple Voltage
= (∆IIND)x(ESR of COUT)
= 0.150Ax0.240Ω = 36 mV p-p
or
4.
ESR of COUT
DS012439-33
FIGURE 18. Peak-to-Peak Inductor
Ripple Current vs Load Current
In a switching regulator design, knowing the value of the
peak-to-peak inductor ripple current (∆IIND) can be useful for
determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch current, minimum load current before the circuit becomes discontinuous,
output ripple voltage and output capacitor ESR can all be
calculated from the peak-to-peak ∆IIND. When the inductor
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OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
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LM2594
Application Information
heat also depends on the size, quantity and spacing of other
components on the board. Furthermore, some of these components such as the catch diode will add heat to the PC
board and the heat can vary as the input voltage changes.
For the inductor, depending on the physical size, type of core
material and the DC resistance, it could either act as a heat
sink taking heat away from the board, or it could add heat to
the board.
(Continued)
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the inductor’s magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the magnetic field, and the distance between the copper trace and
the inductor, determine the amount of voltage generated in
the copper trace. Another way of looking at this inductive
coupling is to consider the PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is
used, it’s possible that the location of the inductor with respect to other PC traces may be the problem. To determine
if this is the problem, temporarily raise the inductor away
from the board by several inches and then check circuit operation. If the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem.
Substituting a closed core inductor such as a torroid or
E-core will correct the problem, or re-arranging the PC layout
may be necessary. Magnetic flux cutting the IC device
ground trace, feedback trace, or the positive or negative
traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin inductor will provide good results, provided it is exactly in the
center of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. If flux problems are
present, even the direction of the inductor winding can make
a difference in some circuits.
This discussion on open core inductors is not to frighten the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or “stick” inductors are an inexpensive, simple way of making a compact efficient inductor, and they are used by the millions in many different applications.
DS012439-35
Circuit Data for Temperature Rise Curve (DIP-8)
Through hole electrolytic
Inductor
Through hole, Schott, 100 µH
Diode
Through hole, 1A 40V, Schottky
PC board
4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 19. Junction Temperature Rise, DIP-8
THERMAL CONSIDERATIONS
The LM2594/LM2594HV is available in two packages, an
8-pin through hole DIP (N) and an 8-pin surface mount SO-8
(M). Both packages are 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 LM2594 and
the other heat producing components.
For best thermal performance, wide copper traces should be
used and all ground and unused pins should be soldered to
generous amounts of printed circuit board copper, such as a
ground plane (one exception to this is the output (switch) pin,
which should not have large areas of copper). Large areas of
copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and even double-sided or multilayer boards provide a better heat path to the surrounding
air. Unless power levels are small, sockets are not recommended because of the added thermal resistance it adds
and the resultant higher junction temperatures.
DS012439-34
Circuit Data for Temperature Rise Curve
(Surface Mount)
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect the junction temperature. Some of these factors include board size, shape, thickness, position, location,
and even board temperature. Other factors are, trace width,
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board, and the amount of solder on
the board. The effectiveness of the PC board to dissipate
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Capacitors
Capacitors
Surface mount tantalum, molded “D” size
Inductor
Surface mount, Coilcraft DO33, 100 µH
Diode
Surface mount, 1A 40V, Schottky
PC board
4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 20. Junction Temperature Rise, SO-8
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Application Information
DELAYED STARTUP
The circuit in Figure 21 uses the the ON /OFF pin to provide
a time delay between the time the input voltage is applied
and the time the output voltage comes up (only the circuitry
pertaining to the delayed start up is shown). As the input voltage rises, the charging of capacitor C1 pulls the ON /OFF pin
high, keeping the regulator off. Once the input voltage
reaches its final value and the capacitor stops charging, and
resistor R2 pulls the ON /OFF pin low, thus allowing the circuit to start switching. Resistor R1 is included to limit the
maximum voltage applied to the ON /OFF pin (maximum of
25V), reduces power supply noise sensitivity, and also limits
the capacitor, C1, discharge current. When high input ripple
voltage exists, avoid long delay time, because this ripple can
be coupled into the ON /OFF pin and cause problems.
(Continued)
The curves shown in Figure 19 and Figure 20 show the
LM2594 junction temperature rise above ambient temperature with a 500 mA load for various input and output voltages. This data was taken with the circuit operating as a
buck switcher with all components mounted on a PC board
to simulate the junction temperature under actual operating
conditions. This curve is typical, and can be used for a quick
check on the maximum junction temperature for various conditions, but keep in mind that there are many factors that can
affect the junction temperature.
This delayed startup feature is useful in situations where the
input power source is limited in the amount of current it can
deliver. It allows the input voltage to rise to a higher voltage
before the regulator starts operating. Buck regulators require
less input current at higher input voltages.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An undervoltage lockout feature applied to a buck regulator is
shown in Figure 22, while Figure 23 and Figure 24 applies
the same feature to an inverting circuit. The circuit in Figure
23 features a constant threshold voltage for turn on and turn
off (zener voltage plus approximately one volt). If hysteresis
is needed, the circuit in Figure 24 has a turn ON voltage
which is different than the turn OFF voltage. The amount of
hysteresis is approximately equal to the value of the output
voltage. If zener voltages greater than 25V are used, an additional 47 kΩ resistor is needed from the ON /OFF pin to the
ground pin to stay within the 25V maximum limit of the ON
/OFF pin.
DS012439-36
FIGURE 21. Delayed Startup
DS012439-37
FIGURE 22. Undervoltage Lockout
for Buck Regulator
INVERTING REGULATOR
The circuit in Figure 25 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulators ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regulates it.
DS012439-38
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 23. Undervoltage Lockout for Inverting Regulator
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Application Information
(Continued)
DS012439-39
This circuit has hysteresis
Regulator starts switching at VIN = 13V
Regulator stops switching at VIN = 8V
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
DS012439-40
CIN — 68 µF/25V Tant. Sprague 595D
120 µF/35V Elec. Panasonic HFQ
COUT — 22 µF/20V Tant. Sprague 595D
39 µF/16V Elec. Panasonic HFQ
FIGURE 25. Inverting −5V Regulator with Delayed Startup
This example uses the LM2594-5 to generate a −5V output,
but other output voltages are possible by selecting other output voltage versions, including the adjustable version. Since
this regulator topology can produce an output voltage that is
either greater than or less than the input voltage, the maximum output current greatly depends on both the input and
output voltage. The curve shown in Figure 26 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
The maximum voltage appearing across the regulator is the
absolute sum of the input and output voltage, and this must
be limited to a maximum of 40V. For example, when converting +20V to −12V, the regulator would see 32V between the
input pin and ground pin. The LM2594 has a maximum input
voltage spec of 40V (60V for the LM2594HV).
Additional diodes are required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the CIN capacitor to the output, under light
or no load conditions. Also, this diode isolation changes the
topology to closley resemble a buck configuration thus providing good closed loop stability. A Schottky diode is recommended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a fast recovery diode
could be used.
Without diode D3, when the input voltage is first applied, the
charging current of CIN can pull the output positive by several volts for a short period of time. Adding D3 prevents the
output from going positive by more than a diode voltage.
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DS012439-41
FIGURE 26. Inverting Regulator Typical Load Current
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to select the
inductor value. In the majority of designs, a 100 µH, 1A inductor is the best choice. Capacitor selection can also be
narrowed down to just a few values. Using the values shown
in Figure 25 will provide good results in the majority of inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2594 current limit (approx 0.8A) are needed for at least 2 ms or more, until the out-
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LM2594
Application Information
INVERTING REGULATOR SHUTDOWN METHODS
To use the ON /OFF pin in a standard buck configuration is
simple, pull it below 1.3V ( @25˚C, referenced to ground) to
turn regulator ON, pull it above 1.3V to shut the regulator
OFF. With the inverting configuration, some level shifting is
required, because the ground pin of the regulator is no
longer at ground, but is now setting at the negative output
voltage level. Two different shutdown methods for inverting
regulators are shown in Figure 27 and Figure 28.
(Continued)
put reaches its nominal output voltage. The actual time depends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the relatively high startup currents required by the inverting topology,
the delayed startup feature (C1, R1 and R2) shown in Figure
25 is recommended. By delaying the regulator startup, the
input capacitor is allowed to charge up to a higher voltage
before the switcher begins operating. A portion of the high input current needed for startup is now supplied by the input
capacitor (CIN). For severe start up conditions, the input capacitor can be made much larger than normal.
DS012439-42
FIGURE 27. Inverting Regulator Ground Referenced Shutdown
DS012439-43
FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device
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Application Information
(Continued)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X SIZE)
DS012439-44
CIN —
COUT —
D1 —
L1 —
10 µF, 35V, Solid Tantalum AVX, “TPS series”
100 µF, 10V Solid Tantalum AVX, “TPS series”
1A, 40V Schottky Rectifier, surface mount
100 µH, L20, Coilcraft DO33
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X SIZE)
DS012439-45
CIN —
COUT —
D1 —
L1 —
R1 —
R2 —
CFF —
10 µF, 35V, Solid Tantalum AVX, “TPS series”
100 µF, 10V Solid Tantalum AVX, “TPS series”
1A, 40V Schottky Rectifier, surface mount
100 µH, L20, Coilcraft DO33
1 kΩ, 1%
Use formula in Design Procedure
See Figure 3.
FIGURE 29. PC Board Layout
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