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LM2587 SIMPLE SWITCHER 5A Flyback Regulator
March 1996
LM2587 SIMPLE SWITCHER 5A Flyback Regulator
General Description
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 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 g4% within specified input voltages and output load conditions is guaranteed for the power supply system
Features
Y Y Y Y Y
Y Y
Y
Y
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 g4% max over line and load conditions
Typical Applications
Y Y Y Y
Flyback regulator Multiple-output regulator Simple boost regulator Forward converter
Flyback Regulator
TL H 12316 - 1
Ordering Information
Package Type 5-Lead TO-220 Bent Staggered Leads 5-Lead TO-263 5-Lead TO-263 Tape and Reel NSC Package Drawing T05D TS5B TS5B Order Number LM2587T-3 3 LM2587T-5 0 LM2587T-12 LM2587T-ADJ LM2587S-3 3 LM2587S-5 0 LM2587S-12 LM2587S-ADJ LM2587SX-3 3 LM2587SX-5 0 LM2587SX-12 LM2587SX-ADJ
SIMPLE SWITCHER and Switchers Made Simple C1996 National Semiconductor Corporation
are registered trademarks of National Semiconductor Corporation RRD-B30M36 Printed in U S A
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Absolute Maximum Ratings
(Note 1) Storage Temperature Range Lead Temperature (Soldering 10 sec ) Maximum Junction Temperature (Note 3) Minimum ESD Rating (C e 100 pF R e 1 5 kX
b 65 C to a 150 C
If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Input Voltage Switch Voltage Switch Current (Note 2) Compensation Pin Voltage Feedback Pin Voltage Power Dissipation (Note 3)
b 0 4V s VIN s 45V b 0 4V s VSW s 65V
260 C 150 C 2 kV
Internally Limited
b 0 4V s VCOMP s 2 4V b 0 4V s VFB s 2 VOUT
Operating Ratings
Supply Voltage Output Switch Voltage Output Switch Current Junction Temperature Range 4V s VIN s 40V 0V s VSW s 60V ISW s 5 0A
b 40 C s TJ s a 125 C
Internally Limited
Electrical Characteristics
Specifications with standard type face are for TJ e 25 C and those in bold type face apply over full Operating Temperature Range Unless otherwise specified VIN e 5V
LM2587-3 3
Symbol Parameters Conditions Typical Min Max Units SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) VOUT DVOUT DVIN DVOUT DILOAD h Output Voltage Line Regulation Load Regulation Efficiency VIN e 4V to 12V ILOAD e 400 mA to 1 75A VIN e 4V to 12V ILOAD e 400 mA VIN e 12V ILOAD e 400 mA to 1 75A VIN e 12V ILOAD e 1A 33 20 20 75 3 17 3 14 3 43 3 46 50 100 50 100 V mV mV %
UNIQUE DEVICE PARAMETERS (Note 5) VREF DVREF GM AVOL Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain Measured at Feedback Pin VCOMP e 1 0V VIN e 4V to 40V ICOMP e b30 mA to a 30 mA VCOMP e 1 0V VCOMP e 0 5V to 1 6V RCOMP e 1 0 MX (Note 6) 33 20 1 193 260 0 678 151 75 2 259 3 242 3 234 3 358 3 366 V mV mmho VV
LM2587-5 0
Symbol Parameters Conditions Typical Min Max Units SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) VOUT DVOUT DVIN DVOUT DILOAD h Output Voltage Line Regulation Load Regulation Efficiency VIN e 4V to 12V ILOAD e 500 mA to 1 45A VIN e 4V to 12V ILOAD e 500 mA VIN e 12V ILOAD e 500 mA to 1 45A VIN e 12V ILOAD e 750 mA 50 20 20 80 4 80 4 75 5 20 5 25 50 100 50 100 V mV mV %
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Electrical Characteristics
Specifications with standard type face are for TJ e 25 C and those in bold type face apply over full Operating Temperature Range Unless otherwise specified VIN e 5V (Continued)
LM2587-5 0 (Continued)
Symbol Parameters Conditions Typical Min Max Units UNIQUE DEVICE PARAMETERS (Note 5) VREF DVREF GM AVOL Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain Measured at Feedback Pin VCOMP e 1 0V VIN e 4V to 40V ICOMP e b30 mA to a 30 mA VCOMP e 1 0V VCOMP e 0 5V to 1 6V RCOMP e 1 0 MX (Note 6) 50 33 0 750 165 0 447 99 49 1 491 4 913 4 900 5 088 5 100 V mV mmho VV
LM2587-12
Symbol Parameters Conditions Typical Min Max Units SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) VOUT DVOUT DVIN DVOUT DILOAD h Output Voltage Line Regulation Load Regulation Efficiency VIN e 4V to 10V ILOAD e 300 mA to 1 2A VIN e 4V to 10V ILOAD e 300 mA VIN e 10V ILOAD e 300 mA to 1 2A VIN e 10V ILOAD e 1A 12 0 20 20 90 11 52 11 40 12 48 12 60 100 200 100 200 V mV mV %
UNIQUE DEVICE PARAMETERS (Note 5) VREF DVREF GM AVOL Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain Measured at Feedback Pin VCOMP e 1 0V VIN e 4V to 40V ICOMP e b30 mA to a 30 mA VCOMP e 1 0V VCOMP e 0 5V to 1 6V RCOMP e 1 0 MX (Note 6) 12 0 78 0 328 70 0 186 41 21 0 621 11 79 11 76 12 21 12 24 V mV mmho VV
LM2587-ADJ
Symbol Parameters Conditions Typical Min Max Units SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) VOUT DVOUT DVIN DVOUT DILOAD h Output Voltage Line Regulation Load Regulation Efficiency VIN e 4V to 10V ILOAD e 300 mA to 1 2A VIN e 4V to 10V ILOAD e 300 mA VIN e 10V ILOAD e 300 mA to 1 2A VIN e 10V ILOAD e 1A 12 0 20 20 90 11 52 11 40 12 48 12 60 100 200 100 200 V mV mV %
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Electrical Characteristics
Specifications with standard type face are for TJ e 25 C and those in bold type face apply over full Operating Temperature Range Unless otherwise specified VIN e 5V (Continued)
LM2587-ADJ (Continued)
Symbol Parameters Conditions Typical Min Max Units UNIQUE DEVICE PARAMETERS (Note 5) VREF DVREF GM AVOL IB Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain Error Amp Input Bias Current Measured at Feedback Pin VCOMP e 1 0V VIN e 4V to 40V ICOMP e b30 mA to a 30 mA VCOMP e 1 0V VCOMP e 0 5V to 1 6V RCOMP e 1 0 MX (Note 6) VCOMP e 1 0V 1 230 15 3 200 670 125 1 800 400 200 425 600 6 000 1 208 1 205 1 252 1 255 V mV mmho VV nA
COMMON DEVICE PARAMETERS for all versions (Note 5) Symbol IS Parameters Input Supply Current Conditions (Switch Off) (Note 8) ISWITCH e 3 0A VUV fO Input Supply Undervoltage Lockout Oscillator Frequency RLOAD e 100X Measured at Switch Pin RLOAD e 100X VCOMP e 1 0V Measured at Switch Pin RLOAD e 100X VFEEDBACK e 1 15V Upper Limit (Note 7) Lower Limit (Note 8) IEAO Error Amp Output Current (Source or Sink) Soft Start Current Maximum Duty Cycle Switch Leakage Current Switch Sustaining Voltage Switch Saturation Voltage NPN Switch Current Limit (Note 9) 165 VFEEDBACK e 0 92V VCOMP e 1 0V RLOAD e 100X (Note 7) Switch Off VSWITCH e 60V dV dT e 1 5V ns ISWITCH e 5 0A 110 70 260 320 mA Typical 11 85 3 30 140 3 05 Min Max 15 5 16 5 165 3 75 Units mA mA V
100
85 75
115 125
kHz
fSC
Short-Circuit Frequency Error Amplifier Output Swing
25
kHz
VEAO
28 0 25
26 24 0 40 0 55
V V
ISS D IL VSUS VSAT ICL
11 0 98 15
80 70 93 90
17 0 19 0
mA %
300 600 65
mA V
07 65 50
11 14 95
V A
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Electrical Characteristics
Specifications with standard type face are for TJ e 25 C and those in bold type face apply over full Operating Temperature Range Unless otherwise specified VIN e 5V (Continued) COMMON DEVICE PARAMETERS (Note 4) (Continued) Symbol iJA iJA iJC iJA iJA iJA iJC Parameters Thermal Resistance Conditions T Package Junction to Ambient (Note 10) T Package Junction to Ambient (Note 11) T Package Junction to Case S Package S Package S Package S Package Junction to Ambient (Note 12) Junction to Ambient (Note 13) Junction to Ambient (Note 14) Junction to Case Typical 65 45 2 56 35 26 2 CW Min Max Units
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 (iJA) 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 c iJA a TA(MAX) t TJ(MAX) For a safe thermal design check that the maximum power dissipated by the device is less than PD s TJ(MAX) b TA(MAX)) iJA 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 Figures 2 and 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 MX 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 e 1 05V 3 3V VFB e 2 81V 5 0V VFB e 4 25V 12V VFB e 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 e 1 41V 3 3V VFB e 3 80V 5 0V VFB e 5 75V 12V VFB e 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 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 board containing approximately 4 square inches of (1oz ) copper area surrounding the leads inch leads in a socket or on a inch leads soldered to a PC
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
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Typical Performance Characteristics
Supply Current vs Temperature Reference Voltage vs Temperature DReference Voltage vs Supply Voltage
Supply Current vs Switch Current
Current Limit vs Temperature
Feedback Pin Bias Current vs Temperature
Switch Saturation Voltage vs Temperature
Switch Transconductance vs Temperature
Oscillator Frequency vs Temperature
Error Amp Transconductance vs Temperature
Error Amp Voltage Gain vs Temperature
Short Circuit Frequency vs Temperature
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Connection Diagrams
Bent Staggered Leads 5-Lead TO-220 (T) Top View Bent Staggered Leads 5-Lead TO-220 (T) Side View
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Order Number LM2587T-3 3 LM2587T-5 0 LM2587T-12 or LM2587T-ADJ See NS Package Number T05D 5-Lead TO-263 (S) Top View 5-Lead TO-263 (S) Side View
TL H 12316 - 6 TL H 12316 - 5
Order Number LM2587S-3 3 LM2587S-5 0 LM2587S-12 or LM2587S-ADJ See NS Package Number TS5B
Block Diagram
For Fixed Versions 3 3V R1 e 3 4k R2 e 2k 5V R1 e 6 15k R2 e 2k 12V R1 e 8 73k R2 e 1k For Adj Version R1 e Short (0X) R2 e Open
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FIGURE 1
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Test Circuits
CIN1 CIN2 T D CC RC
100 mF 25V Aluminum Electrolytic 0 1 mF Ceramic 67141450
22 mH 1 1 Schott 1N5820
COUT 2k
680 mF 16V Aluminum Electrolytic
0 47 mF Ceramic
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FIGURE 2 LM2587-3 3 and LM2587-5 0
CIN1 CIN2 L D CC RC
100 mF 25V Aluminum Electrolytic 0 1 mF Ceramic RL-5472-5
15 mH Renco 1N5820
COUT 2k
680 mF 16V Aluminum Electrolytic
0 47 mF Ceramic
For 12V Devices R1 e Short (0X) and R2 e Open For ADJ Devices R1 e 48 75k g0 1% and R2 e 5 62k g1%
TL H 12316 - 9
FIGURE 3 LM2587-12 and LM2587-ADJ
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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 collapses 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
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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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Typical Performance Characteristics
A Switch Voltage 10 V div B Switch Current 5 A div C Output Rectifier Current 5 A div D Output Ripple Voltage 100 mV div AC-Coupled Horizontal 2 ms div
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FIGURE 5 Switching Waveforms
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FIGURE 6 VOUT Load Current Step Response
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Typical Flyback Regulator Applications
Figures 7 through 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 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
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FIGURE 7 Single-Output Flyback Regulator
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FIGURE 8 Single-Output Flyback Regulator
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Typical Flyback Regulator Applications (Continued)
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FIGURE 9 Single-Output Flyback Regulator
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FIGURE 10 Dual-Output Flyback Regulator
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Typical Flyback Regulator Applications (Continued)
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FIGURE 11 Dual-Output Flyback Regulator
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FIGURE 12 Triple-Output Flyback Regulator
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Typical Flyback Regulator Applications (Continued)
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
Applications Transformers VIN VOUT1 IOUT1 (Max) N1 VOUT2 IOUT2 (Max) N2 VOUT3 IOUT3 (Max) N3 FIGURE 13 Transformer Selection Table
Figure 7
T1 4V - 6V 3 3V 1 8A 1
Figure 8
T1 4V - 6V 5V 1 4A 1
Figure 9
T1 8V -16V 12V 1 2A 1
Figure 10
T2 4V -6V 12V 0 3A 25
b 12V
Figure 11
T3 18V -36V 12V 1A 08
b 12V
Figure 12
T4 18V -36V 5V 2 5A 0 35 12V 0 5A 08
b 12V
0 3A 25
1A 08
0 5A 08
Transformer Type T1 T2 T3 T4
Manufacturers' Part Numbers Coilcraft1 Q4434-B Q4337-B Q4343-B Q4344-B
Note 1 Coilcraft Inc 1102 Silver Lake Road Cary IL 60013 Note 2 Pulse Engineering Inc 12220 World Trade Drive San Diego CA 92128 Note 3 Renco Electronics Inc 60 Jeffryn Blvd East Deer Park NY 11729 Note 4 Schott Corp 1000 Parkers Lane Road Wayzata MN 55391
Coilcraft1 Surface Mount Q4435-B Q4436-B
Pulse2 Surface Mount PE-68411 PE-68412 PE-68421 PE-68422
Phone (800) 322-2645 Fax (708) 639-1469 Phone (619) 674-8100 Fax (619) 674-8262 Phone (800) 645-5828 Fax (516) 586-5562 Phone (612) 475-1173 Fax (612) 475-1786
Renco3 RL-5530 RL-5531 RL-5534 RL-5535
Schott4 67141450 67140860 67140920 67140930
FIGURE 14 Transformer Manufacturer Guide
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Typical Flyback Regulator Applications (Continued)
Transformer Footprints
Figures 15 through 32 show the footprints of each transformer listed in Figure 14
T1
T2
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Top View
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FIGURE 15 Coilcraft Q4434-B
Top View FIGURE 16 Coilcraft Q4337-B
T3
T4
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Top View FIGURE 17 Coilcraft Q4343-B
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Top View FIGURE 18 Coilcraft Q4344-B
T1
T2
TL H 12316 - 34
TL H 12316 - 35
Top View FIGURE 19 Coilcraft Q4435-B (Surface Mount)
Top View FIGURE 20 Coilcraft Q4436-B (Surface Mount)
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Typical Flyback Regulator Applications (Continued)
T1
T2
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Top View FIGURE 21 Pulse PE-68411 (Surface Mount)
Top View FIGURE 22 Pulse PE-68412 (Surface Mount)
T3
T4
TL H 12316-38
Top View FIGURE 23 Pulse PE-68421 (Surface Mount) FIGURE 24 Pulse PE-68422 (Surface Mount)
TL H 12316 - 39
Top View
T1
T2
TL H 12316-40
TL H 12316 - 41
Top View FIGURE 25 Renco RL-5530
Top View FIGURE 26 Renco RL-5531
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Typical Flyback Regulator Applications (Continued)
T3
T4
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Top View FIGURE 27 Renco RL-5534
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Top View FIGURE 28 Renco RL-5535
T1
T2
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Top View FIGURE 29 Schott 67141450
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Top View FIGURE 30 Schott 67140860
T3
T4
TL H 12316 - 45
Top View FIGURE 31 Schott 67140920 FIGURE 32 Schott 67140930
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Top View
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Step-Up (Boost) Regulator Operation
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 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 b 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
TL H 12316 - 19
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
A Switch Voltage 10 V div B Switch Current 5 A div C Inductor Current 5 A div D Output Ripple Voltage 100 mV div AC-Coupled Horizontal 2 ms div
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FIGURE 34 Switching Waveforms
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FIGURE 35 VOUT Response to Load Current Step
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Typical Boost Regulator Applications
Figures 36 and 38 through 40 show four typical boost applications) one fixed and three using the adjustable version of the LM2587 Each drawing contains the part 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
TL H 12316 - 22
FIGURE 36 a 5V to a 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
Coilcraft1 R4793-A Pulse2 PE-53900 Renco3 RL-5472-5 Schott4 67146520
Note 1 Coilcraft Inc 1102 Silver Lake Road Cary IL 60013 Note 2 Pulse Engineering Inc 12220 World Trade Drive San Diego CA 92128 Note 3 Renco Electronics Inc 60 Jeffryn Blvd East Deer Park NY 11729 Note 4 Schott Corp 1000 Parkers Lane Road Wayzata MN 55391
Phone (800) 322-2645 Fax (708) 639-1469 Phone (619) 674-8100 Fax (619) 674-8262 Phone (800) 645-5828 Fax (516) 586-5562 Phone (612) 475-1173 Fax (612) 475-1786
FIGURE 37 Inductor Selection Table
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Typical Boost Regulator Applications (Continued)
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FIGURE 38 a 12V to a 24V Boost Regulator
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FIGURE 39 a 24V to a 36V Boost Regulator
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FIGURE 40 a 24V to a 48V Boost Regulator
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
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Application Hints
TL H 12316 - 26
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 VOUT e VREF (1 a R1 R2) where VREF e 1 23V 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 R1 e R2 (VOUT VREF b 1) where VREF e 1 23V For best temperature coefficient and stability with time use 1% metal film resistors 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 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 stor-
TL H 12316 - 27
FIGURE 42 Flyback Regulator
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Application Hints (Continued)
age capacitor (t100 mF) 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 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 mF ceramic capacitor between VIN and ground as close as possible to the device 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) e VIN (Max) a (VOUT a 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 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 b0 4V The resistor may range in value between 10X and 1 kX and the capacitor will vary from 0 001 mF to 0 1 mF 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 b0 4V The reverse voltage rating of the diode must be greater than the switch off voltage FIGURE 43 Input Line Filter 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 c VIN c D (1 b D) The duty cycle of a flyback regulator is determined by the following equation VOUT a VF VOUT N(VIN b VSAT) a VOUT a VF N(VIN) a VOUT 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) De 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 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 mH and rated at 100 mA) 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 2 92 (VIN(Min) b VSAT) c (2D(Max) b1) (mH) 1 b D(Max) where VSAT is the switch saturation voltage and can be found in the Characteristic Curves L(Min) e
TL H 12316 - 28
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Application Hints (Continued)
TL H 12316 - 29
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 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) 4) LM2587 package thermal resistances iJA and iJC (given in the Electrical Characteristics) Total power dissipated (PD) by the LM2587 can be estimated as follows Boost PD e 0 15X c Flyback 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 RILOAD is the sum of the maximum load currents for multiple-output flyback regulators) The duty cycle is given by Boost De Flyback De VOUT a VF N(VIN b VSAT) a VOUT a VF VOUT N(VIN) a VOUT VOUT a VF b VIN VOUT a VF b VSAT VOUT b VIN VOUT
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 DTJ e PD c iJA Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction temperature TJ e DTJ a TA 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 DTJ e PD c (iJC a iInterface a iHeat Sink) Again the operating junction temperature will be TJ e DTJ a TA
ILOAD 1bD
J
2 cDa
ILOAD c D c VIN 50 c (1bD)
PD e 0 15X c
a
N c RILOAD c D c VIN 50 c (1bD)
N c RILOAD 1bD
J
2 cD
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Application Hints (Continued)
As before if the maximum junction temperature is exceeded a larger heat sink is required (one that has a lower thermal resistance) 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 inputoutput 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
European Magnetic Vendor Contacts
Please contact the following addresses for details of local distributors or representatives
Coilcraft
21 Napier Place Wardpark North Cumbernauld Scotland G68 0LL Phone a 44 1236 730 595 Fax a 44 1236 730 627
To further simplify the flyback regulator design procedure National Semiconductor is making available computer design software Switchers Made Simple software is available on a (3 ) diskette for IBM compatable computers from a National Semiconductor sales office in your area or the National Semiconductor Customer Response Center (1-800-272-9959)
Pulse Engineering
Dunmore Road Tuam Co Galway Ireland Phone a 353 93 24 107 Fax a 353 93 24 459
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Physical Dimensions inches (millimeters)
Order Number LM2587T-3 3 LM2587T-5 0 LM2587T-12 or LM2587T-ADJ NS Package Number T05D
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LM2587 SIMPLE SWITCHER 5A Flyback Regulator
Physical Dimensions inches (millimeters) (Continued)
Order Number LM2587S-3 3 LM2587S-5 0 LM2587S-12 or LM2587S-ADJ NS Package Number TS5B
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