NSC LM2587S-ADJ Simple switcherâ® 5a flyback regulator Datasheet

LM2587
SIMPLE SWITCHER ® 5A Flyback Regulator
General Description
Features
The LM2587 series of regulators are monolithic integrated
circuits specifically designed for flyback, step-up (boost), and
forward converter applications. The device is available in 4
different output voltage versions: 3.3V, 5.0V, 12V, and adjustable.
Requiring a minimum number of external components, these
regulators are cost effective, and simple to use. Included in
the datasheet are typical circuits of boost and flyback regulators. Also listed are selector guides for diodes and capacitors and a family of standard inductors and flyback transformers designed to work with these switching regulators.
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The power switch is a 5.0A NPN device that can stand-off
65V. Protecting the power switch are current and thermal
limiting circuits, and an undervoltage lockout circuit. This IC
contains a 100 kHz fixed-frequency internal oscillator that
permits the use of small magnetics. Other features include
soft start mode to reduce in-rush current during start up,
current mode control for improved rejection of input voltage
and output load transients and cycle-by-cycle current limiting. An output voltage tolerance of ± 4%, within specified
input voltages and output load conditions, is guaranteed for
the power supply system.
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Requires few external components
Family of standard inductors and transformers
NPN output switches 5.0A, can stand off 65V
Wide input voltage range: 4V to 40V
Current-mode operation for improved transient
response, line regulation, and current limit
100 kHz switching frequency
Internal soft-start function reduces in-rush current during
start-up
Output transistor protected by current limit, under
voltage lockout, and thermal shutdown
System Output Voltage Tolerance of ± 4% max over line
and load conditions
Typical Applications
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Flyback regulator
Multiple-output regulator
Simple boost regulator
Forward converter
Flyback Regulator
01231601
Ordering Information
Package Type
NSC Package
Order Number
Drawing
5-Lead TO-220 Bent, Staggered Leads
T05D
LM2587T-3.3, LM2587T-5.0, LM2587T-12, LM2587T-ADJ
5-Lead TO-263
TS5B
LM2587S-3.3, LM2587S-5.0, LM2587S-12, LM2587S-ADJ
5-Lead TO-263 Tape and Reel
TS5B
LM2587SX-3.3, LM2587SX-5.0, LM2587SX-12,
LM2587SX-ADJ
SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation
DS012316
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LM2587 SIMPLE SWITCHER 5A Flyback Regulator
February 2004
LM2587
Absolute Maximum Ratings (Note 1)
Maximum Junction
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Power Dissipation (Note 3)
Temperature (Note 3)
Minimum ESD Rating
−0.4V ≤ VIN ≤ 45V
Input Voltage
150˚C
Internally Limited
(C = 100 pF, R = 1.5 kΩ
2 kV
−0.4V ≤ VSW ≤ 65V
Switch Voltage
Switch Current (Note 2)
Internally Limited
Compensation Pin Voltage
−0.4V ≤ VCOMP ≤ 2.4V
Feedback Pin Voltage
−0.4V ≤ VFB ≤ 2 VOUT
Storage Temperature Range
Operating Ratings
4V ≤ VIN ≤ 40V
Supply Voltage
Output Switch Voltage
−65˚C to +150˚C
0V ≤ VSW ≤ 60V
ISW ≤ 5.0A
Output Switch Current
Lead Temperature
(Soldering, 10 sec.)
Junction Temperature Range
260˚C
−40˚C ≤ TJ ≤ +125˚C
LM2587-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 5V.
Symbol
Parameters
Conditions
Typical
Min
Max
Units
3.3
3.17/3.14
3.43/3.46
V
20
50/100
mV
20
50/100
mV
SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4)
VOUT
Output Voltage
VIN = 4V to 12V
ILOAD = 400 mA to 1.75A
∆VOUT/
Line Regulation
VIN = 4V to 12V
Load Regulation
VIN = 12V
∆VIN
∆VOUT/
ILOAD = 400 mA
∆ILOAD
η
ILOAD = 400 mA to 1.75A
Efficiency
VIN = 12V, ILOAD = 1A
75
%
UNIQUE DEVICE PARAMETERS (Note 5)
VREF
Output Reference
Voltage
VCOMP = 1.0V
∆VREF
Reference Voltage
VIN = 4V to 40V
Measured at Feedback Pin
3.3
3.242/3.234
3.358/3.366
2.0
V
mV
Line Regulation
GM
AVOL
Error Amp
ICOMP = −30 µA to +30 µA
Transconductance
VCOMP = 1.0V
Error Amp
VCOMP = 0.5V to 1.6V
Voltage Gain
RCOMP = 1.0 MΩ (Note 6)
1.193
0.678
260
151/75
2.259
mmho
V/V
LM2587-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 5V.
Symbol
Parameters
Conditions
Typical
Min
Max
Units
5.0
4.80/4.75
5.20/5.25
V
20
50/100
mV
20
50/100
mV
SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4)
VOUT
Output Voltage
VIN = 4V to 12V
∆VOUT/
Line Regulation
VIN = 4V to 12V
ILOAD = 500 mA to 1.45A
∆VIN
∆VOUT/
ILOAD = 500 mA
Load Regulation
∆ILOAD
η
VIN = 12V
ILOAD = 500 mA to 1.45A
Efficiency
VIN = 12V, ILOAD = 750 mA
80
%
UNIQUE DEVICE PARAMETERS (Note 5)
VREF
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Output Reference
Measured at Feedback Pin
5.0
2
4.913/4.900
5.088/5.100
V
Symbol
∆VREF
LM2587
LM2587-5.0
Electrical Characteristics
(Continued)
Parameters
Conditions
Voltage
VCOMP = 1.0V
Reference Voltage
VIN = 4V to 40V
Typical
Min
Max
3.3
Units
mV
Line Regulation
GM
Error Amp
ICOMP = −30 µA to +30 µA
Transconductance
VCOMP = 1.0V
AVOL
Error Amp
VCOMP = 0.5V to 1.6V
Voltage Gain
RCOMP = 1.0 MΩ (Note 6)
0.750
0.447
165
99/49
1.491
mmho
V/V
LM2587-12
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 5V.
Symbol
Parameters
Conditions
Typical
Min
Max
Units
12.0
11.52/11.40
12.48/12.60
V
20
100/200
mV
20
100/200
mV
SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4)
VOUT
Output Voltage
VIN = 4V to 10V
∆VOUT/
Line Regulation
VIN = 4V to 10V
ILOAD = 300 mA to 1.2A
∆VIN
∆VOUT/
ILOAD = 300 mA
Load Regulation
∆ILOAD
η
VIN = 10V
ILOAD = 300 mA to 1.2A
Efficiency
VIN = 10V, ILOAD = 1A
90
%
UNIQUE DEVICE PARAMETERS (Note 5)
VREF
∆VREF
Output Reference
Measured at Feedback Pin
Voltage
VCOMP = 1.0V
Reference Voltage
VIN = 4V to 40V
12.0
11.79/11.76
12.21/12.24
7.8
V
mV
Line Regulation
GM
AVOL
Error Amp
ICOMP = −30 µA to +30 µA
Transconductance
VCOMP = 1.0V
Error Amp
VCOMP = 0.5V to 1.6V
Voltage Gain
RCOMP = 1.0 MΩ (Note 6)
0.328
0.186
70
41/21
0.621
mmho
V/V
LM2587-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 5V.
Symbol
Parameters
Conditions
Typical
Min
Max
Units
12.0
11.52/11.40
12.48/12.60
V
20
100/200
mV
20
100/200
mV
SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4)
VOUT
Output Voltage
VIN = 4V to 10V
ILOAD = 300 mA to 1.2A
∆VOUT/
Line Regulation
∆VIN
∆VOUT/
ILOAD = 300 mA
Load Regulation
∆ILOAD
η
VIN = 4V to 10V
VIN = 10V
ILOAD = 300 mA to 1.2A
Efficiency
VIN = 10V, ILOAD = 1A
90
%
UNIQUE DEVICE PARAMETERS (Note 5)
VREF
Output Reference
Measured at Feedback Pin
Voltage
VCOMP = 1.0V
1.230
3
1.208/1.205
1.252/1.255
V
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LM2587
LM2587-ADJ
Electrical Characteristics
(Continued)
Symbol
Conditions
Parameters
∆VREF
Reference Voltage
Typical
VIN = 4V to 40V
Min
Max
Units
1.5
mV
Line Regulation
GM
AVOL
IB
Error Amp
ICOMP = −30 µA to +30 µA
Transconductance
VCOMP = 1.0V
Error Amp
VCOMP = 0.5V to 1.6V
Voltage Gain
RCOMP = 1.0 MΩ (Note 6)
Error Amp
VCOMP = 1.0V
3.200
1.800
670
400/200
6.000
mmho
V/V
125
425/600
nA
Input Bias Current
All Output Voltage Versions
Electrical Characteristics (Note 5)
Specifications with standard type face are for TJ = 25˚C, and those in bold type face apply over full Operating Temperature
Range. Unless otherwise specified, VIN = 5V.
Symbol
IS
Parameters
Input Supply Current
Conditions
(Switch Off)
Typical
Min
11
Max
Units
15.5/16.5
mA
(Note 8)
VUV
Input Supply
ISWITCH = 3.0A
85
140/165
mA
RLOAD = 100Ω
3.30
3.05
3.75
V
100
85/75
115/125
kHz
Undervoltage Lockout
fO
Oscillator Frequency
Measured at Switch Pin
RLOAD = 100Ω
VCOMP = 1.0V
fSC
Short-Circuit
Measured at Switch Pin
Frequency
RLOAD = 100Ω
25
kHz
VFEEDBACK = 1.15V
VEAO
Error Amplifier
Upper Limit
Output Swing
(Note 7)
2.8
Lower Limit
2.6/2.4
0.25
V
0.40/0.55
V
(Note 8)
IEAO
Error Amp
(Note 9)
Output Current
165
110/70
260/320
µA
11.0
8.0/7.0
17.0/19.0
µA
98
93/90
(Source or Sink)
ISS
Soft Start Current
VFEEDBACK = 0.92V
VCOMP = 1.0V
D
Maximum Duty Cycle
RLOAD = 100Ω
IL
Switch Leakage
Switch Off
Current
VSWITCH = 60V
Switch Sustaining
dV/dT = 1.5V/ns
%
(Note 7)
VSUS
15
300/600
65
µA
V
Voltage
VSAT
Switch Saturation
ISWITCH = 5.0A
0.7
1.1/1.4
V
9.5
A
Voltage
ICL
NPN Switch
6.5
Current Limit
COMMON DEVICE PARAMETERS (Note 4)
θJA
Thermal Resistance
T Package, Junction to Ambient (Note 10)
65
θJA
T Package, Junction to Ambient (Note 11)
45
θJC
T Package, Junction to Case
2
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4
5.0
Symbol
Parameters
5)
LM2587
All Output Voltage Versions
Electrical Characteristics (Note
(Continued)
Conditions
Typical
θJA
S Package, Junction to Ambient (Note 12)
56
θJA
S Package, Junction to Ambient (Note 13)
35
θJA
S Package, Junction to Ambient (Note 14)
26
θJC
S Package, Junction to Case
2
Min
Max
Units
˚C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions the device is intended to
be functional, but device parameter specifications may not be guaranteed under these conditions. For guaranteed specifications and test conditions, see the
Electrical Characteristics.
Note 2: Note that switch current and output current are not identical in a step-up regulator. Output current cannot be internally limited when the LM2587 is used as
a step-up regulator. To prevent damage to the switch, the output current must be externally limited to 5A. However, output current is internally limited when the
LM2587 is used as a flyback regulator (see the Application Hints section for more information).
Note 3: The junction temperature of the device (TJ) is a function of the ambient temperature (TA), the junction-to-ambient thermal resistance (θJA), and the power
dissipation of the device (PD). A thermal shutdown will occur if the temperature exceeds the maximum junction temperature of the device: PD x θJA + TA(MAX) ≥
TJ(MAX). For a safe thermal design, check that the maximum power dissipated by the device is less than: PD ≤ [TJ(MAX) − TA(MAX))]/θJA. When calculating the
maximum allowable power dissipation, derate the maximum junction temperature — this ensures a margin of safety in the thermal design.
Note 4: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. When the LM2587 is used as
shown in Figure 2 and Figure 3, system performance will be as specified by the system parameters.
Note 5: All room temperature limits are 100% production tested, and all limits at temperature extremes are guaranteed via correlation using standard Statistical
Quality Control (SQC) methods.
Note 6: A 1.0 MΩ resistor is connected to the compensation pin (which is the error amplifier output) to ensure accuracy in measuring AVOL.
Note 7: To measure this parameter, the feedback voltage is set to a low value, depending on the output version of the device, to force the error amplifier output high.
Adj: VFB = 1.05V; 3.3V: VFB = 2.81V; 5.0V: VFB = 4.25V; 12V: VFB = 10.20V.
Note 8: To measure this parameter, the feedback voltage is set to a high value, depending on the output version of the device, to force the error amplifier output
low. Adj: VFB = 1.41V; 3.3V: VFB = 3.80V; 5.0V: VFB = 5.75V; 12V: VFB = 13.80V.
Note 9: To measure the worst-case error amplifier output current, the LM2587 is tested with the feedback voltage set to its low value (specified in Note 7) and at
its high value (specified in Note 8).
Note 10: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 1⁄2 inch leads in a socket, or on a
PC board with minimum copper area.
Note 11: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 1⁄2 inch leads soldered to a PC board
containing approximately 4 square inches of (1oz.) copper area surrounding the leads.
Note 12: Junction to ambient thermal resistance for the 5 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the
TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 13: Junction to ambient thermal resistance for the 5 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area
of the TO-263 package) of 1 oz. (0.0014 in. thick) copper.
Note 14: Junction to ambient thermal resistance for the 5 lead TO-263 mounted horizontally against a PC board copper area of 1.0064 square inches (7.4 times
the area of the TO-263 package) of 1 oz. (0.0014 in. thick) copper. Additional copper area will reduce thermal resistance further. See the thermal model in Switchers
Made Simple ® software.
Typical Performance Characteristics
Supply Current
vs Temperature
Reference Voltage
vs Temperature
01231648
01231649
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LM2587
Typical Performance Characteristics
(Continued)
∆Reference Voltage
vs Supply Voltage
Supply Current
vs Switch Current
01231650
01231651
Current Limit
vs Temperature
Feedback Pin Bias
Current vs Temperature
01231653
01231652
Switch Saturation
Voltage vs Temperature
Switch Transconductance
vs Temperature
01231654
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01231655
6
LM2587
Typical Performance Characteristics
(Continued)
Oscillator Frequency
vs Temperature
Error Amp Transconductance
vs Temperature
01231657
01231656
Error Amp Voltage
Gain vs Temperature
Short Circuit Frequency
vs Temperature
01231659
01231658
Connection Diagrams
Bent, Staggered Leads
5-Lead TO-220 (T)
Side View
Bent, Staggered Leads
5-Lead TO-220 (T)
Top View
01231604
01231603
Order Number LM2587T-3.3, LM2587T-5.0,
LM2587T-12 or LM2587T-ADJ
See NS Package Number T05D
5-Lead TO-263 (S)
Side View
5-Lead TO-263 (S)
Top View
01231606
01231605
Order Number LM2587S-3.3, LM2587S-5.0,
LM2587S-12 or LM2587S-ADJ
See NS Package Number TS5B
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LM2587
Block Diagram
01231607
For Fixed Versions3.3V, R1 = 3.4k, R2 = 2k5V, R1 = 6.15k, R2 = 2k12V, R1 = 8.73k, R2 = 1kFor Adj. VersionR1 = Short (0Ω), R2 = Open
FIGURE 1.
Test Circuits
01231608
CIN1 — 100 µF, 25V Aluminum ElectrolyticCIN2 — 0.1 µF CeramicT — 22 µH, 1:1 Schott #67141450D — 1N5820COUT — 680 µF, 16V Aluminum
ElectrolyticCC — 0.47 µF CeramicRC — 2k
FIGURE 2. LM2587-3.3 and LM2587-5.0
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LM2587
Test Circuits
(Continued)
01231609
CIN1 — 100 µF, 25V Aluminum ElectrolyticCIN2 — 0.1 µF CeramicL — 15 µH, Renco #RL-5472-5D — 1N5820COUT — 680 µF, 16V Aluminum
ElectrolyticCC — 0.47 µF CeramicRC — 2kFor 12V Devices: R1 = Short (0Ω) and R2 = OpenFor ADJ Devices: R1 = 48.75k, ± 0.1% and R2 = 5.62k, ± 1%
FIGURE 3. LM2587-12 and LM2587-ADJ
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LM2587
lapses, reversing the voltage polarity of the primary and
secondary windings. Now rectifier D1 is forward biased and
current flows through it, releasing the energy stored in the
transformer. This produces voltage at the output.
The output voltage is controlled by modulating the peak
switch current. This is done by feeding back a portion of the
output voltage to the error amp, which amplifies the difference between the feedback voltage and a 1.230V reference.
The error amp output voltage is compared to a ramp voltage
proportional to the switch current (i.e., inductor current during the switch on time). The comparator terminates the
switch on time when the two voltages are equal, thereby
controlling the peak switch current to maintain a constant
output voltage.
Flyback Regulator Operation
The LM2587 is ideally suited for use in the flyback regulator
topology. The flyback regulator can produce a single output
voltage, such as the one shown in Figure 4, or multiple
output voltages. In Figure 4, the flyback regulator generates
an output voltage that is inside the range of the input voltage.
This feature is unique to flyback regulators and cannot be
duplicated with buck or boost regulators.
The operation of a flyback regulator is as follows (refer to
Figure 4): when the switch is on, current flows through the
primary winding of the transformer, T1, storing energy in the
magnetic field of the transformer. Note that the primary and
secondary windings are out of phase, so no current flows
through the secondary when current flows through the primary. When the switch turns off, the magnetic field col-
01231610
As shown in Figure 4, the LM2587 can be used as a flyback regulator by using a minimum number of external components. The switching waveforms of this
regulator are shown in Figure 5. Typical Performance Characteristics observed during the operation of this circuit are shown in Figure 6.
FIGURE 4. 12V Flyback Regulator Design Example
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10
LM2587
Typical Performance Characteristics
01231611
A: Switch Voltage, 10 V/divB: Switch Current, 5 A/divC: Output Rectifier Current, 5 A/divD: Output Ripple Voltage, 100 mV/div
AC-Coupled
Horizontal: 2 µs/div
FIGURE 5. Switching Waveforms
01231612
FIGURE 6. VOUT Load Current Step Response
component except the transformer. For the transformer part
numbers and manufacturers names, see the table in Figure
13.
For
applications
with
different
output
voltages — requiring the LM2587-ADJ — or different output
configurations that do not match the standard configurations,
refer to the Switchers Made Simple software.
Typical Flyback Regulator
Applications
Figures 7, 8, 9, 11, 12 show six typical flyback applications,
varying from single output to triple output. Each drawing
contains the part number(s) and manufacturer(s) for every
11
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LM2587
Typical Flyback Regulator Applications
(Continued)
01231613
FIGURE 7. Single-Output Flyback Regulator
01231614
FIGURE 8. Single-Output Flyback Regulator
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12
LM2587
Typical Flyback Regulator Applications
(Continued)
01231615
FIGURE 9. Single-Output Flyback Regulator
01231616
FIGURE 10. Dual-Output Flyback Regulator
13
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LM2587
Typical Flyback Regulator Applications
(Continued)
01231617
FIGURE 11. Dual-Output Flyback Regulator
01231618
FIGURE 12. Triple-Output Flyback Regulator
TRANSFORMER SELECTION (T)
Figure 13 lists the standard transformers available for flyback regulator applications. Included in the table are the
turns ratio(s) for each transformer, as well as the output
voltages, input voltage ranges, and the maximum load currents for each circuit.
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LM2587
Typical Flyback Regulator Applications
(Continued)
Applications
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Transformers
T1
T1
T1
T2
T3
T4
18V–36V
VIN
4V–6V
4V–6V
8V–16V
4V–6V
18V–36V
VOUT1
3.3V
5V
12V
12V
12V
5V
IOUT1 (Max)
1.8A
1.4A
1.2A
0.3A
1A
2.5A
1
1
1
2.5
0.8
0.35
VOUT2
−12V
−12V
12V
IOUT2 (Max)
0.3A
1A
0.5A
2.5
0.8
0.8
N1
N2
VOUT3
−12V
IOUT3 (Max)
0.5A
N3
0.8
FIGURE 13. Transformer Selection Table
Transformer
Type
Manufacturers’ Part Numbers
Coilcraft
Coilcraft (Note 15)
Pulse (Note 16)
Renco
Schott
(Note 15)
Surface Mount
Surface Mount
(Note 17)
(Note 18)
T1
Q4434-B
Q4435-B
PE-68411
RL-5530
67141450
T2
Q4337-B
Q4436-B
PE-68412
RL-5531
67140860
T3
Q4343-B
—
PE-68421
RL-5534
67140920
T4
Q4344-B
—
PE-68422
RL-5535
67140930
Note 15: Coilcraft Inc.,:
Phone: (800) 322-2645
1102 Silver Lake Road, Cary, IL 60013:
Note 16: Pulse Engineering Inc.,:
Fax: (708) 639-1469
Phone: (619) 674-8100
12220 World Trade Drive, San Diego, CA 92128:
Note 17: Renco Electronics Inc.,:
60 Jeffryn Blvd. East, Deer Park, NY 11729:
Note 18: Schott Corp.,:
Fax: (619) 674-8262
Phone: (800) 645-5828
Fax: (516) 586-5562
Phone: (612) 475-1173
1000 Parkers Lane Road, Wayzata, MN 55391:
Fax: (612) 475-1786
FIGURE 14. Transformer Manufacturer Guide
TRANSFORMER FOOTPRINTS
Figures 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
29, 30, 31 and Figure 32 show the footprints of each transformer, listed in Figure 14.
T2
T1
01231631
Top View
01231630
Top View
FIGURE 16. Coilcraft Q4337-B
FIGURE 15. Coilcraft Q4434-B
15
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LM2587
Typical Flyback Regulator
Applications (Continued)
T2
T3
01231635
Top View
01231632
FIGURE 20. Coilcraft Q4436-B
(Surface Mount)
Top View
FIGURE 17. Coilcraft Q4343-B
T1
T4
01231636
Top View
01231633
FIGURE 21. Pulse PE-68411
(Surface Mount)
Top View
FIGURE 18. Coilcraft Q4344-B
T2
T1
01231634
01231637
Top View
Top View
FIGURE 19. Coilcraft Q4435-B
(Surface Mount)
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FIGURE 22. Pulse PE-68412
(Surface Mount)
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LM2587
Typical Flyback Regulator
Applications (Continued)
T2
T3
01231641
Top View
FIGURE 26. Renco RL-5531
01231638
Top View
T3
FIGURE 23. Pulse PE-68421
(Surface Mount)
T4
01231646
Top View
FIGURE 27. Renco RL-5534
T4
01231639
Top View
FIGURE 24. Pulse PE-68422
(Surface Mount)
T1
01231642
Top View
FIGURE 28. Renco RL-5535
01231640
Top View
T1
FIGURE 25. Renco RL-5530
01231643
Top View
FIGURE 29. Schott 67141450
17
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LM2587
Typical Flyback Regulator
Applications (Continued)
T4
T2
01231647
Top View
01231644
Top View
FIGURE 32. Schott 67140930
FIGURE 30. Schott 67140860
T3
01231645
Top View
FIGURE 31. Schott 67140920
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18
Figure 33 shows the LM2587 used as a step-up (boost)
regulator. This is a switching regulator that produces an
output voltage greater than the input supply voltage.
A brief explanation of how the LM2587 Boost Regulator
works is as follows (refer to Figure 33). When the NPN
switch turns on, the inductor current ramps up at the rate of
01231619
By adding a small number of external components (as shown in Figure 33), the LM2587 can be used to produce a regulated output voltage that is greater than
the applied input voltage. The switching waveforms observed during the operation of this circuit are shown in Figure 34. Typical performance of this regulator is
shown in Figure 35.
FIGURE 33. 12V Boost Regulator
Typical Performance Characteristics
01231620
A: Switch Voltage, 10 V/divB: Switch Current, 5 A/divC: Inductor Current, 5 A/divD: Output Ripple Voltage,
100 mV/div, AC-Coupled
Horizontal: 2 µs/div
FIGURE 34. Switching Waveforms
19
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LM2587
VIN/L, storing energy in the inductor. When the switch turns
off, the lower end of the inductor flies above VIN, discharging
its current through diode (D) into the output capacitor (COUT)
at a rate of (VOUT − VIN)/L. Thus, energy stored in the
inductor during the switch on time is transferred to the output
during the switch off time. The output voltage is controlled by
adjusting the peak switch current, as described in the flyback
regulator section.
Step-Up (Boost) Regulator
Operation
LM2587
Typical Performance Characteristics
(Continued)
01231621
FIGURE 35. VOUT Response to Load Current Step
number(s) and manufacturer(s) for every component. For
the fixed 12V output application, the part numbers and
manufacturers’ names for the inductor are listed in a table in
Figure 40. For applications with different output voltages,
refer to the Switchers Made Simple software.
Typical Boost Regulator
Applications
Figure 36 and Figures 38, 39 and Figure 40 show four typical
boost applications) — one fixed and three using the adjustable version of the LM2587. Each drawing contains the part
01231622
FIGURE 36. +5V to +12V Boost Regulator
Figure 37 contains a table of standard inductors, by part
number and corresponding manufacturer, for the fixed output regulator of Figure 36.
Note 19: Coilcraft Inc.,:
Coilcraft
(Note 19)
Pulse
(Note 20)
Renco
(Note 21)
Schott
(Note 22)
R4793-A
PE-53900
RL-5472-5
67146520
Phone: (800) 322-2645
1102 Silver Lake Road, Cary, IL 60013:
Note 20: Pulse Engineering Inc.,:
Fax: (708) 639-1469
Phone: (619) 674-8100
12220 World Trade Drive, San Diego, CA 92128:
Note 21: Renco Electronics Inc.,:
60 Jeffryn Blvd. East, Deer Park, NY 11729:
Note 22: Schott Corp.,:
Fax: (619) 674-8262
Phone: (800) 645-5828
Fax: (516) 586-5562
Phone: (612) 475-1173
1000 Parkers Lane Road, Wayzata, MN 55391:
Fax: (612) 475-1786
FIGURE 37. Inductor Selection Table
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20
LM2587
Typical Boost Regulator Applications
(Continued)
01231623
FIGURE 38. +12V to +24V Boost Regulator
01231624
FIGURE 39. +24V to +36V Boost Regulator
01231625
*The LM2587 will require a heat sink in these applications. The size of the heat sink will depend on the maximum ambient temperature. To calculate the thermal
resistance of the IC and the size of the heat sink needed, see the “Heat Sink/Thermal Considerations” section in the Application Hints.
FIGURE 40. +24V to +48V Boost Regulator
21
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LM2587
Application Hints
01231626
FIGURE 41. Boost Regulator
PROGRAMMING OUTPUT VOLTAGE
(SELECTING R1 AND R2)
Referring to the adjustable regulator in Figure 41, the output
voltage is programmed by the resistors R1 and R2 by the
following formula:
where VREF = 1.23V
VOUT = VREF (1 + R1/R2)
Resistors R1 and R2 divide the output voltage down so that
it can be compared with the 1.23V internal reference. With
R2 between 1k and 5k, R1 is:
where VREF = 1.23V
R1 = R2 (VOUT/VREF − 1)
For best temperature coefficient and stability with time, use
1% metal film resistors.
the main output. When the output voltage drops to 80% of its
nominal value, the frequency will drop to 25 kHz. With a
lower frequency, off times are larger. With the longer off
times, the transformer can release all of its stored energy
before the switch turns back on. Hence, the switch turns on
initially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the
device.
FLYBACK REGULATOR INPUT CAPACITORS
A flyback regulator draws discontinuous pulses of current
from the input supply. Therefore, there are two input capacitors needed in a flyback regulator; one for energy storage
and one for filtering (see Figure 42). Both are required due to
the inherent operation of a flyback regulator. To keep a
stable or constant voltage supply to the LM2587, a storage
capacitor (≥100 µF) is required. If the input source is a
recitified DC supply and/or the application has a wide temperature range, the required rms current rating of the capacitor might be very large. This means a larger value of capacitance or a higher voltage rating will be needed of the input
capacitor. The storage capacitor will also attenuate noise
which may interfere with other circuits connected to the
same input supply voltage.
SHORT CIRCUIT CONDITION
Due to the inherent nature of boost regulators, when the
output is shorted (see Figure 41), current flows directly from
the input, through the inductor and the diode, to the output,
bypassing the switch. The current limit of the switch does not
limit the output current for the entire circuit. To protect the
load and prevent damage to the switch, the current must be
externally limited, either by the input supply or at the output
with an external current limit circuit. The external limit should
be set to the maximum switch current of the device, which is
5A.
In a flyback regulator application (Figure 42), using the standard transformers, the LM2587 will survive a short circuit to
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22
LM2587
Application Hints
(Continued)
01231627
FIGURE 42. Flyback Regulator
In addition, a small bypass capacitor is required due to the
noise generated by the input current pulses. To eliminate the
noise, insert a 1.0 µF ceramic capacitor between VIN and
ground as close as possible to the device.
switch pin is caused by the output diode capacitance and the
transformer leakage inductance forming a resonant circuit at
the secondary(ies). The resonant circuit generates the “ringing” voltage, which gets reflected back through the transformer to the switch pin. There are two common methods to
avoid this problem. One is to add an RC snubber around the
output rectifier(s), as in Figure 42. The values of the resistor
and the capacitor must be chosen so that the voltage at the
Switch pin does not drop below −0.4V. The resistor may
range in value between 10Ω and 1 kΩ, and the capacitor will
vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)
reduce the efficiency of the overall circuit.
The other method to reduce or eliminate the “ringing” is to
insert a Schottky diode clamp between pins 4 and 3
(ground), also shown in Figure 42. This prevents the voltage
at pin 4 from dropping below −0.4V. The reverse voltage
rating of the diode must be greater than the switch off
voltage.
SWITCH VOLTAGE LIMITS
In a flyback regulator, the maximum steady-state voltage
appearing at the switch, when it is off, is set by the transformer turns ratio, N, the output voltage, VOUT, and the
maximum input voltage, VIN (Max):
VSW(OFF) = VIN (Max) + (VOUT +VF)/N
where VF is the forward biased voltage of the output diode,
and is 0.5V for Schottky diodes and 0.8V for ultra-fast recovery diodes (typically). In certain circuits, there exists a voltage spike, VLL, superimposed on top of the steady-state
voltage (see Figure 5, waveform A). Usually, this voltage
spike is caused by the transformer leakage inductance
and/or the output rectifier recovery time. To “clamp” the
voltage at the switch from exceeding its maximum value, a
transient suppressor in series with a diode is inserted across
the transformer primary (as shown in the circuit on the front
page and other flyback regulator circuits throughout the
datasheet). The schematic in Figure 42 shows another
method of clamping the switch voltage. A single voltage
transient suppressor (the SA51A) is inserted at the switch
pin. This method clamps the total voltage across the switch,
not just the voltage across the primary.
If poor circuit layout techniques are used (see the “Circuit
Layout Guideline” section), negative voltage transients may
appear on the Switch pin (pin 4). Applying a negative voltage
(with respect to the IC’s ground) to any monolithic IC pin
causes erratic and unpredictable operation of that IC. This
holds true for the LM2587 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 4) can go negative when the switch turns on. The “ringing” voltage at the
01231628
FIGURE 43. Input Line Filter
23
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LM2587
Application Hints
The circuit in Figure 43 demonstrates the layout of the filter,
with the capacitor placed from the input pin to ground and
the resistor placed between the input supply and the input
pin. Note that the values of RIN and CIN shown in the schematic are good enough for most applications, but some
readjusting might be required for a particular application. If
efficiency is a major concern, replace the resistor with a
small inductor (say 10 µH and rated at 100 mA).
(Continued)
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the
maximum switch voltage minus a diode drop. In a flyback
regulator, the maximum output voltage is determined by the
turns ratio, N, and the duty cycle, D, by the equation:
VOUT ≈ N x VIN x D/(1 − D)
The duty cycle of a flyback regulator is determined by the
following equation:
STABILITY
All current-mode controlled regulators can suffer from an
instability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic
oscillations, a minimum value of inductance is required to
ensure stability for all boost and flyback regulators. The
minimum inductance is given by:
Theoretically, the maximum output voltage can be as large
as desired — just keep increasing the turns ratio of the transformer. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from
increasing to infinity. The physical limitations are capacitances and inductances in the LM2587 switch, the output
diode(s), and the transformer — such as reverse recovery
time of the output diode (mentioned above).
where VSAT is the switch saturation voltage and can be
found in the Characteristic Curves.
NOISY INPUT LINE CONDITION)
A small, low-pass RC filter should be used at the input pin of
the LM2587 if the input voltage has an unusual large amount
of transient noise, such as with an input switch that bounces.
01231629
FIGURE 44. Circuit Board Layout
CIRCUIT LAYOUT GUIDELINES
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, keep the length of the
leads and traces as short as possible. Use single point
grounding or ground plane construction for best results.
Separate the signal grounds from the power grounds (as
indicated in Figure 44). When using the Adjustable version,
physically locate the programming resistors as near the
regulator IC as possible, to keep the sensitive feedback
wiring short.
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HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2587
junction temperature within the allowed operating range. For
each application, to determine whether or not a heat sink will
be required, the following must be identified:
1) Maximum ambient temperature (in the application).
2) Maximum regulator power dissipation (in the application).
3) Maximum allowed junction temperature (125˚C for the
LM2587). For a safe, conservative design, a temperature
approximately 15˚C cooler than the maximum junction temperature should be selected (110˚C).
24
If the operating junction temperature exceeds the maximum
junction temperatue in item 3 above, then a heat sink is
required. When using a heat sink, the junction temperature
rise can be determined by the following:
∆TJ = PD x (θJC + θInterface + θHeat Sink)
(Continued)
4) LM2587 package thermal resistances θJA and θJC (given
in the Electrical Characteristics).
Total power dissipated (PD) by the LM2587 can be estimated
as follows:
Again, the operating junction temperature will be:
TJ = ∆TJ + TA
As before, if the maximum junction temperature is exceeded,
a larger heat sink is required (one that has a lower thermal
resistance).
Boost:
Included in the Switchers Made Simple design software is
a more precise (non-linear) thermal model that can be used
to determine junction temperature with different input-output
parameters or different component values. It can also calculate the heat sink thermal resistance required to maintain the
regulator junction temperature below the maximum operating temperature.
VIN is the minimum input voltage, VOUT is the output voltage,
N is the transformer turns ratio, D is the duty cycle, and ILOAD
is the maximum load current (and ∑ILOAD is the sum of the
maximum load currents for multiple-output flyback regulators). The duty cycle is given by:
To further simplify the flyback regulator design procedure,
National Semiconductor is making available computer design software. Switchers Made Simple software is available
on a (31⁄2") diskette for IBM compatable computers from a
National Semiconductor sales office in your area or the
National Semiconductor Customer Response Center (1-800272-9959).
Boost:
European Magnetic Vendor
Contacts
Please contact the following addresses for details of local
distributors or representatives:
Coilcraft
where VF is the forward biased voltage of the diode and is
typically 0.5V for Schottky diodes and 0.8V for fast recovery
diodes. VSAT is the switch saturation voltage and can be
found in the Characteristic Curves.
When no heat sink is used, the junction temperature rise is:
∆TJ = PD x θJA.
Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction temperature:
TJ = ∆TJ + TA.
21 Napier Place
Wardpark North Cumbernauld, Scotland G68 0LL Phone:
+44 1236 730 595 Fax: +44 1236 730 627
Pulse Engineering
Dunmore Road
Tuam Co. Galway, Ireland Phone: +353 93 24 107 Fax: +353
93 24 459
25
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LM2587
Application Hints
LM2587
Physical Dimensions
inches (millimeters)
unless otherwise noted
Order Number LM2587T-3.3, LM2587T-5.0,
LM2587T-12 or LM2587T-ADJ
NS Package Number T05D
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26
LM2587 SIMPLE SWITCHER 5A Flyback Regulator
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
Order Number LM2587S-3.3, LM2587S-5.0,
LM2587S-12 or LM2587S-ADJ
NS Package Number TS5B
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