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. n n n n n 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. n n n n 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 n n n n 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 www.national.com 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 www.national.com 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 www.national.com 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 www.national.com 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 5 www.national.com 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 www.national.com 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 7 www.national.com 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 www.national.com 8 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 9 www.national.com 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 www.national.com 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 www.national.com LM2587 Typical Flyback Regulator Applications (Continued) 01231613 FIGURE 7. Single-Output Flyback Regulator 01231614 FIGURE 8. Single-Output Flyback Regulator www.national.com 12 LM2587 Typical Flyback Regulator Applications (Continued) 01231615 FIGURE 9. Single-Output Flyback Regulator 01231616 FIGURE 10. Dual-Output Flyback Regulator 13 www.national.com 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. www.national.com 14 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 www.national.com 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) www.national.com FIGURE 22. Pulse PE-68412 (Surface Mount) 16 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 www.national.com 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 www.national.com 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 www.national.com 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 www.national.com 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 www.national.com 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 www.national.com 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 www.national.com 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. www.national.com 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 www.national.com 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 www.national.com 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 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.