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LM2594HV

Datasheet LM2594, LM2594HV Datasheet (National Semiconductor)

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LM2594/LM2594HV 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
Requiring a minimum number of external components, these regulators are simple to use and feature internal frequency compensation line and load regulation specifications.
Other features include a guaranteed put voltage under all conditions of input voltage and output load conditions, and ternal 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.
†
, a fixed-frequency oscillator, and improved
±
4%tolerance on out-
±
15%on the oscillator frequency. Ex-
December 1999
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) 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, I n High Efficiency n Uses readily available standard inductors n Thermal shutdown and current limit protection
±
4%max over line and load
typically 85 µA
Q
Applications
n Simple high-efficiency step-down (buck) regulator n Efficient pre-regulator for linear regulators n On-card switching regulators n Positive to Negative convertor
LM2594/LM2594HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Regulator
Typical Application (Fixed Output Voltage Versions)
DS012439-1
SIMPLE SWITCHER and
© 1999 National Semiconductor Corporation DS012439 www.national.com
Switchers Made Simple
™
are registered trademarks of National Semiconductor Corporation.
Connection Diagrams and Order Information
8-Lead DIP (N)
LM2594/LM2594HV
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
*No internal connection, but should be soldered to pc board for best heat transfer.
‡
Patent Number 5,382,918.
8-Lead Surface Mount (M)
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
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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 LM2594 45V LM2594HV 60V ON /OFF Pin Input Voltage 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
−0.3 ≤ V ≤ +25V
Human Body Model (Note 2) 2 kV Lead Temperature M8 Package
Vapor Phase (60 sec.) +215˚C
Infrared (15 sec.) +220˚C N Package (Soldering, 10 sec.) +260˚C Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C ≤ TJ+125˚C Supply Voltage
LM2594 4.5V to 40V
LM2594HV 4.5V to 60V
LM2594/LM2594HV-3.3 Electrical Characteristics
12V, I
=
25˚C, and those with boldface type apply over full Operating Tempera-
J
Typ Limit
(Note 3) (Note 4)
Figure 1
INmax
, 0.1A ≤ I
≤ 0.5A 3.3 V
LOAD
3.168/3.135 V(min)
3.432/3.465 V(max)
=
0.5A 80
LOAD
(Limits)
%
Specifications with standard type face are for T ture Range.V
=
40V for the LM2594 and 60V for the LM2594HV.
INmax
Symbol Parameter Conditions LM2594/LM2594HV-3.3 Units
SYSTEM PARAMETERS (Note 5) Test Circuit
V
OUT
η Efficiency V
Output Voltage 4.75V ≤ VIN≤ V
=
IN
LM2594/LM2594HV
LM2594/LM2594HV-5.0 Electrical Characteristics
12V, I
=
25˚C, and those with boldface type apply over full Operating Tempera-
J
Typ Limit
(Note 3) (Note 4)
Figure 1
INmax
, 0.1A ≤ I
≤ 0.5A 5.0 V
LOAD
4.800/4.750 V(min)
5.200/5.250 V(max)
=
0.5A 82
LOAD
(Limits)
%
Specifications with standard type face are for T
ture Range
Symbol Parameter Conditions LM2594/LM2594HV-5.0 Units
SYSTEM PARAMETERS (Note 5) Test Circuit
V
OUT
η Efficiency V
Output Voltage 7V ≤ VIN≤ V
=
IN
LM2594/LM2594HV-12 Electrical Characteristics
25V, I
=
25˚C, and those with boldface type apply over full Operating Tempera-
J
Typ Limit
(Note 3) (Note 4)
Figure 1
INmax
, 0.1A ≤ I
≤ 0.5A 12.0 V
LOAD
11.52/11.40 V(min)
12.48/12.60 V(max)
=
0.5A 88
LOAD
(Limits)
%
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Specifications with standard type face are for T
ture Range
Symbol Parameter Conditions LM2594/LM2594HV-12 Units
SYSTEM PARAMETERS (Note 5) Test Circuit
V
OUT
η Efficiency V
Output Voltage 15V ≤ VIN≤ V
=
IN
LM2594/LM2594HV-ADJ Electrical Characteristics
Specifications with standard type face are for T
ture Range
Symbol Parameter Conditions LM2594/LM2594HV-ADJ Units
LM2594/LM2594HV
SYSTEM PARAMETERS (Note 5) Test Circuit
V
FB
η Efficiency V
Feedback Voltage 4.5V ≤ VIN≤ V
V
OUT
=
IN
=
25˚C, and those with boldface type apply over full Operating Tempera-
J
Figure 1
, 0.1A ≤ I
INmax
LOAD
programmed for 3V. Circuit of
12V, I
=
0.5A 80
LOAD
Typ Limit
(Note 3) (Note 4)
≤ 0.5A 1.230 V
Figure 1
1.193/1.180 V(min)
1.267/1.280 V(max)
All Output Voltage Versions Electrical Characteristics
Specifications with standard type face are for T ture Range . Unless otherwise specified, V sion. I
LOAD
=
100 mA
Symbol Parameter Conditions LM2594/LM2594HV-XX Units
DEVICE PARAMETERS
I
b
f
O
V
Feedback Bias Current Adjustable Version Only, VFB=1.3V 10 50/100 nA Oscillator Frequency (Note 6) 150 kHz
Saturation Voltage I
SAT
DC Max Duty Cycle (ON) (Note 8) 100
Min Duty Cycle (OFF) (Note 9) 0
I
I
I
I
CL
L
Q
STBY
Current Limit Peak Current, (Note 7) (Note 8) 0.8 A
Output Leakage Current (Note 7) (Note 9) (Note 10) Output=0V 50 µA(max)
Quiescent Current (Note 9) 5 mA
Standby Quiescent ON/OFF pin=5V (OFF) (Note 10) 85 µA Current LM2594 200/250 µA(max)
LM2594HV 140 250/300 µA(max)
θ
JA
Thermal Resistance N Package, Junction to Ambient (Note 11) 95 ˚C/W
M Package, Junction to Ambient (Note 11) 150
ON/OFF CONTROL Test Circuit
Figure 1
ON /OFF Pin Logic Input 1.3 V V V I
H
Threshold Voltage Low (Regulator ON) 0.6 V(max)
IH IL
High (Regulator OFF) 2.0 V(min) ON /OFF Pin V Input Current 15 µA(max)
I
L
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.
V
=
25˚C, and those with boldface type apply over full Operating Tempera-
J
=
12V for the 3.3V, 5V, and Adjustable version and V
IN
=
24V for the 12V ver-
IN
Typ Limit
(Note 3) (Note 4)
127/110 kHz(min) 173/173 kHz(max)
=
0.5A (Note 7) (Note 8) 0.9 V
OUT
1.1/1.2 V(max)
0.65/0.58 A(min)
1.3/1.4 A(max)
Output=−1V 2 mA
15 mA(max)
10 mA(max)
=
2.5V (Regulator OFF) 5 µA
LOGIC
=
0.5V (Regulator ON) 0.02 µA
LOGIC
5 µA(max)
(Limits)
%
(Limits)
%
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All Output Voltage Versions Electrical Characteristics
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%produc-
tion 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 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: V 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
=
40V for the LM2594 and 60V for the LM2594HV.
IN
(Continued)
Figure 1
test circuit, system performance will be as shown in system parameters section
®
software.
Typical Performance Characteristics
LM2594/LM2594HV
Normalized Output Voltage
Switch Saturation Voltage
DS012439-4
DS012439-7
Line Regulation
Switch Current Limit
DS012439-5
DS012439-8
Efficiency
DS012439-6
Dropout Voltage
DS012439-9
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Typical Performance Characteristics (Continued)
Quiescent Current
LM2594/LM2594HV
ON /OFF Threshold Voltage
Feedback Pin Bias Current
DS012439-10
DS012439-13
Standby Quiescent Current
ON /OFF Pin Current (Sinking)
DS012439-11
DS012439-14
Minimum Operating Supply Voltage
DS012439-12
Switching Frequency
DS012439-15
DS012439-16
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Typical Performance Characteristics
LM2594/LM2594HV
Continuous Mode Switching Waveforms
=
V
20V, V
IN
L=100 µH, C
A: Output Pin Voltage, 10V/div. B: Inductor Current 0.2A/div. C: Output Ripple Voltage, 20 mV/div.
OUT
OUT
=
5V, I
=
LOAD
120 µF, C
=
400 mA
ESR=140 mΩ
OUT
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode
=
V
20V, V
IN
L=100 µH, C
OUT
OUT
=
5V, I
=
LOAD
120 µF, C
=
200 mA to 500 mA
ESR=140 mΩ
OUT
DS012439-17
Discontinuous Mode Switching Waveforms
=
V
20V, V
IN
L=33 µH, C
A: Output Pin Voltage, 10V/div. B: Inductor Current 0.2A/div. C: Output Ripple Voltage, 20 mV/div.
OUT
OUT
=
5V, I
=
220 µF, C
LOAD
=
200 mA
ESR=60 mΩ
OUT
DS012439-18
Horizontal Time Base: 2 µs/div.
Load Transient Response for Discontinuous Mode
=
V
20V, V
IN
L=33 µH, C
OUT
OUT
=
5V, I
=
220 µF, C
=
100 mA to 200 mA
LOAD
OUT
ESR=60 mΩ
A: Output Voltage, 50 mV/div. (AC) B: 200 mA to 500 mA Load Pulse
Horizontal Time Base: 50 µs/div.
DS012439-19
A: Output Voltage, 50 mV/div. (AC) B: 100 mA to 200 mA Load Pulse
Horizontal Time Base: 200 µs/div.
DS012439-20
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Typical Circuit and Layout Guidelines
Fixed Output Voltage Versions
LM2594/LM2594HV
CIN— 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
— 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
C
OUT
D1 — 1A, 40V Schottky Rectifier, 1N5819 L1 — 100 µH, L20
DS012439-22
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”
— 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
C
OUT
D1 — 1A, 40V Schottky Rectifier, 1N5819 L1 — 100 µH, L20 R
1
C
FF
%
—1kΩ,1
— SeeApplication Information Section
FIGURE 1. Typical Circuits and Layout Guides
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
ternal 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 induc­tor.Allowing the inductor flux to intersect sensitive feedback, lC groundpath and C
wiring can cause problems.
OUT
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LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
=
V
Regulated Output Voltage (3.3V, 5V or 12V)
OUT
(max)=Maximum DC Input Voltage
V
IN
(max)=Maximum Load Current
I
LOAD
1. Inductor Selection (L1) A. Select the correct inductor value selection guide from
ures 4, 5
or
Figure 6
. (Output voltages of 3.3V, 5V, or 12V re-
Fig-
spectively.) 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
2. Output Capacitor Selection (C
Figure 8
.
OUT
)
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
Simple
version 4.1 or later.
Switchers Made
Given:
=
V
5V
OUT
(max)=12V
V
IN
(max)=0.4A
I
LOAD
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 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.)
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 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.
Figure 5
LM2594/LM2594HV
,
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LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed Output)
3. Catch Diode Selection (D1) A. The catch diode current rating must be at least 1.3 times
LM2594/LM2594HV
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.
4. Input Capacitor (C
Alow 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 data sheet must be checked to assure that this current rating is not exceeded. The curve shown in 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 V
For additional information, see section on input capaci­tors in Application Information section.
(Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1) A. Refer to the table shown in
Figure 11
1A, 20V, 1N5817 Schottky diode will provide the best perfor­mance, and will not be overstressed even for a shorted out­put.
)
IN
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
1
⁄2the DC load current. The capacitor manufacturers
Figure 13
shows typical
a voltage rating greater than 18V (1.5 x V needed. The next higher capacitor voltage rating is 25V.
The RMS current rating requirement for the input capacitor in a buck regulator is approximately 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)
pin.
IN
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.
. In this example, a
) would be
IN
1
⁄2the DC load current. In
www.national.com 10
LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed Output)
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (µH) (
(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
(Continued)
Conditions Inductor Output Capacitor
Through Hole Surface Mount
#
) HFQ Series PL Series Series 595D Series
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
0.2 10 150 L10 120/16 120/16 100/16 100/6.3 40 220 L9 120/16 120/16 100/16 100/6.3
5 0.5 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
0.2 20 220 L9 120/16 120/16 100/16 33/25 40 330 L8 120/16 120/16 100/16 33/25
12 0.5 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
0.2 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
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given:
=
V
Regulated Output Voltage
OUT
(max)=Maximum Input Voltage
V
IN
(max)=Maximum Load Current
I
LOAD
F=Switching Frequency
(Fixed at a nominal 150 kHz).
Given:
=
V
20V
OUT
(max)=28V
V
IN
(max)=0.5A
I
LOAD
F=Switching Frequency
(Fixed at a nominal 150 kHz).
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LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
1. Programming Output Voltage (Selecting R
shown in
LM2594/LM2594HV
Use the following formula to select the appropriate resistor values.
Select a value for R1between 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.)
(Continued)
Figure 1
and R2,as
.
1
1. Programming Output Voltage (Selecting R1and R2,as shown in
Select R
R R
Figure 1
)
to be 1 kΩ,1%. Solve for R2.
1
=
1k (16.26 − 1)=15.26k, closest 1%value is 15.4 kΩ.
2
=
15.4 kΩ.
2
2. Inductor Selection (L1) A. Calculate the inductor Volt microsecond constant
T(V•µs) , from the following formula:
E
•
where V and V B. Use the E
it with the E Value Selection Guide shown in
=
internal switch saturation voltage=0.9V
SAT
=
diode forward voltage drop=0.5V
D
T value from the previous formula and match
•
T number on the vertical axis of the Inductor
•
Figure 7
.
C. on the horizontal axis, select the maximum load current. D. Identify the inductance region intersected by the E
•
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
3. Output Capacitor Selection (C
Figure 8
.
OUT)
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 informa-
tion, 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.
2. Inductor Selection (L1) A. Calculate the inductor Volt
microsecond constant (E•T)
•
,
B. E•T=35.2 (V•µs)
(max)=0.5A
C. I
LOAD
D. From the inductor value selection guide shown in the inductance region intersected by the 35 (V
T
tal 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 (C A. See section on C
in Application Information section.
OUT
B. From the quick design table shown in
OUT
)
Figure 3
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
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.
Figure 7
µs) horizon-
•
, locate the
,
www.national.com 12
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
4. Feedforward Capacitor (C
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, R tional 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. 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.
(Continued)
FF
) (See
Figure 1
)
. It provides addi-
2
4. Feedforward Capacitor (C
The table shown in values for various output voltages. In this example,a1nF capacitor is needed.
5. Catch Diode Selection (D1) A. Refer to the table shown in
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.
Figure 3
)
FF
contains feed forward capacitor
Figure 11
. Schottky diodes
LM2594/LM2594HV
www.national.com13
LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
Alow ESR aluminum or tantalum bypass capacitor is needed
LM2594/LM2594HV
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 data sheet must be checked to assure that this current rating is not exceeded. The curve shown in 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
For additional information, see section on input capaci­tors in application information section.
(Continued)
)
IN
1
⁄2the DC load current. The capacitor manufacturers
Figure 13
shows typical
pin.
IN
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
) would be needed. Since the the next higher capacitor
IN
voltage rating is 50V, a 50V capacitor should be used. The capacitor voltage rating of (1.5 x V line, and can be modified somewhat if desired.
The RMS current rating requirement for the input capacitor of a buck regulator is approximately
) is a conservative guide-
IN
1
⁄2the 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.
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
Volt-
age
(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
www.national.com 14
LM2594/LM2594HV Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
LM2594/LM2594HV
DS012439-24
FIGURE 4. LM2594/LM2594HV-3.3
DS012439-26
FIGURE 6. LM2594/LM2594HV-12
DS012439-25
FIGURE 5. LM2594/LM2594HV-5.0
DS012439-27
FIGURE 7. LM2594/LM2594HV-ADJ
www.national.com15
LM2594/LM2594HV Series Buck Regulator Design Procedure (Continued)
Induc-
tance
(µH)
LM2594/LM2594HV
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 —
Cur-
rent
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
FIGURE 8. Inductor Manufacturers Part Numbers
Coilcraft Inc. Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft Inc., Europe Phone +44 1236 730 595
FAX +44 1236 730 627
Pulse Engineering Inc. Phone (619) 674-8100
FAX (619) 674-8262
Pulse Engineering Inc., Phone +353 93 24 107 Europe FAX +353 93 24 459 Renco Electronics Inc. Phone (800) 645-5828
FAX (516) 586-5562
Schott Corp. Phone (612) 475-1173
FAX (612) 475-1786
FIGURE 9. Inductor Manufacturers Phone Numbers
www.national.com 16
Nichicon Corp. Phone (708) 843-7500
FAX (708) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Corp. Phone (803) 448-9411
FAX (803) 448-1943
Sprague/Vishay Phone (207) 324-7223
FAX (207) 324-4140
FIGURE 10. Capacitor Manufacturers Phone Numbers
LM2594/LM2594HV Series Buck Regulator Design Procedure (Continued)
VR 1A Diodes
Surface Mount Through Hole
Schottky
20V All of
MBRS130 rated to at 1N5818 rated to at
30V least 60V. SR103 least 60V.
MBRS140 MURS120 1N5819 MUR120
40V 10BQ040 10BF10 SR104 HER101
10MQ040 11DQ04 11DF1
50V
more
MBRS160 SR105
or
10BQ050 MBR150 10MQ060 11DQ05
MBRS1100 MBR160
10MQ090 SB160 SGL41-60 11DQ10
SS16
Ultra Fast Schot-
tky
Recovery Recovery
1N5817 All of these
these
diodes are SR102 diodes are
11DQ03
FIGURE 11. Diode Selection Table
Ultra Fast
Block Diagram
LM2594/LM2594HV
Application Information
PIN FUNCTIONS
—This is the positive input supply for the IC switching
+V
IN
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.
FIGURE 12.
DS012439-21
Ground —Circuit ground. Output — Internal switch. The voltage at this pin switches
between (+V cycle of V
OUT/VIN
the PC board copper area connected to this pin should be kept to a minimum.
) and approximately −0.5V, with a duty
IN−VSAT
. To minimize coupling to sensitive circuitry,
www.national.com17
Application Information (Continued)
Feedback — Senses the regulated output voltage to com-
plete 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
LM2594/LM2594HV
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.
EXTERNAL COMPONENTS C
—A low ESR aluminum or tantalum bypass capacitor is
IN
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 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.
Figure 13
shows the relationship between
DS012439-28
FIGURE 13. RMS Current Ratings for Low ESR
Electrolytic Capacitors (Typical)
OUTPUT CAPACITOR C
—An output capacitor is required to filter the output
OUT
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
www.national.com 18
Application Information (Continued)
(see
Figure 14
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 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 10X at −40˚C. See curve shown in
Solid tantalum capacitors have a much better ESR spec for cold temperatures and are recommended for temperatures below −25˚C.
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
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.
). Often, capacitors with much higher voltage
Figure 2
and
@
−25˚C and as much as
Figure 15
.
DS012439-29
(Typical Low ESR Electrolytic Capacitor)
Figure 3
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 7
). This guide assumes that the regulator is operating
Figure 4
through
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
.)
LM2594/LM2594HV
www.national.com19
Application Information (Continued)
LM2594/LM2594HV
FIGURE 16. (∆I
Inductor Ripple Current
(as a Percentage of the Load Current) vs Load Current
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.
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.
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
) Peak-to-Peak
IND
DS012439-31
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.
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.
DS012439-32
FIGURE 17. Post Ripple Filter Waveform
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.
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. Alow ESR output filter capacitor is also required to assure good dynamic load response and ripple
www.national.com 20
Application Information (Continued)
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.
Figure 17
shows a typical output ripple volt-
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
for different load currents. The curve also shows how the peak-to-peak inductor ripple current (∆I go from the lower border to the upper border (for a given load
shows the range of (∆I
) that can be expected
IND
) changes as you
IND
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:
=
V
5V, maximum load current of 300 mA
OUT
=
15V, nominal, varying between 11V and 20V.
V
IN
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 (∆I
) to flow that will be a percentage of the maximum load
IND
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 (∆I axis (approximately 150 mA p-p).
) on the left hand
IND
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
,itcan be seen that for a load current of 0.3A, the peak-to-peak in­ductor ripple current (∆I range from 175 mAat the upper border (20V in) to 120 mA at
) is 150 mA with 15V in, and can
IND
the lower border (11V in). Once the ∆I
used to calculate additional information about the switching
value is known, the following formulas can be
IND
regulator circuit.
1. Peak Inductor or peak switch current
LM2594/LM2594HV
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 (∆I determining a number of other circuit parameters. Param-
) can be useful for
IND
eters 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 ∆I
. When the inductor
IND
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage
=
)x(ESR of C
(∆I
IND
=
0.150Ax0.240Ω=36 mV p-p
OUT
)
or
4. ESR of C
OUT
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.
www.national.com21
Application Information (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
LM2594/LM2594HV
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.
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.
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
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.
DS012439-35
Circuit Data for Temperature Rise Curve (DIP-8) Capacitors 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
DS012439-34
Circuit Data for Temperature Rise Curve
(Surface Mount) 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
www.national.com 22
Application Information (Continued)
The curves shown in 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.
FIGURE 22. Undervoltage Lockout
Figure 19
and
Figure 20
DS012439-36
FIGURE 21. Delayed Startup
for Buck Regulator
show the
DS012439-37
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 R cuit to start switching. Resistor R1is included to limit the
pulls the ON /OFF pin low, thus allowing the cir-
2
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.
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 the same feature to an inverting circuit. The circuit in
23
Figure 22
, while
Figure 23
and
Figure 24
applies
Figure
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.
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.
LM2594/LM2594HV
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 23. Undervoltage Lockout for Inverting Regulator
DS012439-38
www.national.com23
Application Information (Continued)
LM2594/LM2594HV
This circuit has hysteresis
Regulator starts switching at V Regulator stops switching at V
=
13V
IN
=
8V
IN
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
CIN— 68 µF/25V Tant. Sprague 595D
120 µF/35V Elec. Panasonic HFQ — 22 µF/20V Tant. Sprague 595D
C
OUT
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.
Additional diodes are required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or noise from coupling through the C or no load conditions. Also, this diode isolation changes the
capacitor to the output, under light
IN
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 C eral volts for a short period of time. Adding D3 prevents the
can pull the output positive by sev-
IN
output from going positive by more than a diode voltage.
DS012439-39
DS012439-40
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 invert-
ing 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-
www.national.com 24
Application Information (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, R
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 (C pacitor can be made much larger than normal.
). For severe start up conditions, the input ca-
IN
and R2) shown in
1
Figure
FIGURE 27. Inverting Regulator Ground Referenced Shutdown
INVERTING REGULATOR SHUTDOWN METHODS
Figure 27
DS012439-42
and
Figure 28
.
LM2594/LM2594HV
DS012439-43
FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device
www.national.com25
Application Information (Continued)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X SIZE)
LM2594/LM2594HV
CIN— 10 µF, 35V, Solid TantalumAVX, “TPS series”
— 100 µF, 10V Solid TantalumAVX, “TPS series”
C
OUT
D1 — 1A, 40V Schottky Rectifier, surface mount L1 — 100 µH, L20, Coilcraft DO33
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X SIZE)
DS012439-44
CIN— 10 µF, 35V, Solid TantalumAVX, “TPS series”
— 100 µF, 10V Solid Tantalum AVX, “TPS series”
C
OUT
D1 — 1A, 40V Schottky Rectifier, surface mount L1 — 100 µH, L20, Coilcraft DO33 R1 — 1 kΩ,1
— Use formula in Design Procedure
R
2
— See
C
FF
%
Figure 3
.
FIGURE 29. PC Board Layout
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DS012439-45
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package,
Order Number LM2594M-3.3, LM2594M-5.0,
LM2594M-12 or LM2594M-ADJ JEDEC
NS Package Number M08A
LM2594/LM2594HV
www.national.com27
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Regulator
LM2594/LM2594HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Order Number LM2594N-3.3, LM2594N-5.0, LM2594N-12 or LM2594N-ADJ
8-Lead (0.300" Wide) Molded Dual-In-Line Package,
NS Package Number N08E
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