LM2585 SIMPLE SWITCHER ® 3A Flyback Regulator General Description Features The LM2585 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 3.0A 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 n n n n n Requires few external components Family of standard inductors and transformers NPN output switches 3.0A, 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 DS012515-1 Ordering Information Package Type NSC Package Order Number Drawing 5-Lead TO-220 Bent, Staggered Leads T05D 5-Lead TO-263 TS5B LM2585T-3.3, LM2585T-5.0, LM2585T-12, LM2585T-ADJ LM2585S-3.3, LM2585S-5.0, LM2585S-12, LM2585S-ADJ 5-Lead TO-263 Tape and Reel TS5B LM2585SX-3.3, LM2585SX-5.0, LM2585SX-12, LM2585SX-ADJ SIMPLE SWITCHER and Switchers Made Simple™ are registered trademarks of National Semiconductor Corporation. © 1999 National Semiconductor Corporation DS012515 www.national.com LM2585 SIMPLE SWITCHER 3A Flyback Regulator March 1998 Absolute Maximum Ratings (Note 1) Maximum Junction Temperature (Note 3) Power Dissipation (Note 3) Minimum ESD Rating (C = 100 pF, R = 1.5 kΩ) 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 Storage Temperature Range Lead Temperature (Soldering, 10 sec.) −0.4V ≤ VIN ≤ 45V −0.4V ≤ VSW ≤ 65V Internally Limited −0.4V ≤ VCOMP ≤ 2.4V −0.4V ≤ VFB ≤ 2 VOUT −65˚C to +150˚C 150˚C Internally Limited 2 kV Operating Ratings Supply Voltage Output Switch Voltage Output Switch Current Junction Temperature Range 260˚C 4V ≤ VIN ≤ 40V 0V ≤ VSW ≤ 60V ISW ≤ 3.0A −40˚C ≤ TJ ≤ +125˚C Electrical Characteristics LM2585-3.3 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 SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) VOUT Output Voltage VIN = 4V to 12V ∆VOUT/ Line Regulation ∆VIN ∆VOUT/ Load Regulation ∆ILOAD η Efficiency ILOAD = 0.3A to 1.2A VIN = 4V to 12V ILOAD = 0.3A VIN = 12V ILOAD = 0.3A to 1.2A VIN = 5V, ILOAD = 0.3A Typical Min Max Units 3.3 3.17/3.14 3.43/3.46 V 20 50/100 mV 20 50/100 mV % 76 UNIQUE DEVICE PARAMETERS (Note 5) VREF Output Reference Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 3.3 1.193 0.678 Error Amp ICOMP = −30 µA to +30 µA VCOMP = 1.0V VCOMP = 0.5V to 1.6V 260 151/75 Voltage Gain RCOMP = 1.0 MΩ (Note 6) Typical Min Max Units 5.0 4.80/4.75 5.20/5.25 V 20 50/100 mV 20 50/100 mV Voltage ∆VREF Reference Voltage 3.242/3.234 3.358/3.366 2.0 V mV Line Regulation GM Error Amp Transconductance AVOL 2.259 mmho V/V LM2585-5.0 Symbol Parameters Conditions SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) VOUT Output Voltage VIN = 4V to 12V ∆VOUT/ Line Regulation ∆VIN ∆VOUT/ Load Regulation ∆ILOAD η Efficiency ILOAD = 0.3A to 1.1A VIN = 4V to 12V ILOAD = 0.3A VIN = 12V ILOAD = 0.3A to 1.1A VIN = 12V, ILOAD = 0.6A % 80 UNIQUE DEVICE PARAMETERS (Note 5) VREF ∆VREF Output Reference Measured at Feedback Pin VCOMP = 1.0V 5.0 Voltage Reference Voltage VIN = 4V to 40V 3.3 Line Regulation www.national.com 2 4.913/4.900 5.088/5.100 V mV LM2585-5.0 Symbol (Continued) Parameters Conditions Typical Min Max Units 0.750 0.447 1.491 mmho 165 99/49 Typical Min Max Units 12.0 11.52/11.40 12.48/12.60 V 20 100/200 mV 20 100/200 mV UNIQUE DEVICE PARAMETERS (Note 5) GM Error Amp ICOMP = −30 µA to +30 µA VCOMP = 1.0V VCOMP = 0.5V to 1.6V Voltage Gain RCOMP = 1.0 MΩ (Note 6) Error Amp Transconductance AVOL V/V LM2585-12 Symbol Parameters Conditions SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) VOUT Output Voltage VIN = 4V to 10V ∆VOUT/ Line Regulation ∆VIN ∆VOUT/ Load Regulation ∆ILOAD η Efficiency ILOAD = 0.2A to 0.8A VIN = 4V to 10V ILOAD = 0.2A VIN = 10V ILOAD = 0.2A to 0.8A VIN = 10V, ILOAD = 0.6A % 93 UNIQUE DEVICE PARAMETERS (Note 5) VREF Output Reference Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 12.0 0.328 0.186 Error Amp ICOMP = −30 µA to +30 µA VCOMP = 1.0V VCOMP = 0.5V to 1.6V 70 41/21 Voltage Gain RCOMP = 1.0 MΩ (Note 6) Voltage ∆VREF Reference Voltage 11.79/11.76 12.21/12.24 7.8 V mV Line Regulation GM Error Amp Transconductance AVOL 0.621 mmho V/V LM2585-ADJ Symbol Parameters Conditions SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) VOUT Output Voltage VIN = 4V to 10V ∆VOUT/ Line Regulation ∆VIN ∆VOUT/ Load Regulation ∆ILOAD η Efficiency ILOAD = 0.2A to 0.8A VIN = 4V to 10V ILOAD = 0.2A VIN = 10V ILOAD = 0.2A to 0.8A VIN = 10V, ILOAD = 0.6A Typical Min Max Units 12.0 11.52/11.40 12.48/12.60 V 20 100/200 mV 20 100/200 mV % 93 UNIQUE DEVICE PARAMETERS (Note 5) VREF Output Reference Voltage ∆VREF Reference Voltage Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 1.230 1.208/1.205 ICOMP = −30 µA to +30 µA VCOMP = 1.0V VCOMP = 0.5V to 1.6V 3.200 1.800 670 400/200 1.252/1.255 1.5 V mV Line Regulation GM Error Amp Transconductance AVOL Error Amp Voltage Gain IB Error Amp RCOMP = 1.0 MΩ (Note 6) VCOMP = 1.0V 125 6.000 mmho V/V 425/600 nA Input Bias Current 3 www.national.com Electrical Characteristics (All Versions) Symbol Parameters Conditions Typical Min Max Units 15.5/16.5 mA COMMON DEVICE PARAMETERS for all versions (Note 5) IS VUV Input Supply (Switch Off) Current (Note 8) 11 ISWITCH = 1.8A RLOAD = 100Ω 100/115 mA Input Supply 3.30 3.05 3.75 V Measured at Switch Pin RLOAD = 100Ω 100 85/75 115/125 kHz 50 Undervoltage Lockout fO Oscillator Frequency VCOMP = 1.0V fSC Short-Circuit Measured at Switch Pin RLOAD = 100Ω VFEEDBACK = 1.15V 25 Error Amplifier Upper Limit 2.8 Output Swing (Note 7) Frequency VEAO Lower Limit kHz 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 D Maximum Duty VFEEDBACK = 0.92V VCOMP = 1.0V RLOAD = 100Ω Cycle (Note 7) Switch Leakage Switch Off VSWITCH = 60V dV/dT = 1.5V/ns IL Current VSUS Switch Sustaining 15 % 300/600 65 µA V Voltage VSAT Switch Saturation ISWITCH = 3.0A 0.45 0.65/0.9 V 7.0 A Voltage ICL NPN Switch 4.0 3.0 Current Limit 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 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 ˚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 LM2585 is used as a step-up regulator. To prevent damage to the switch, the output current must be externally limited to 3A. However, output current is internally limited when the LM2585 is used as a flyback regulator (see the Application Hints section for more information). www.national.com 4 Electrical Characteristics (All Versions) (Continued) 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 LM2585 is used as shown in Figures 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 LM2585 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 Supply Voltage Reference Voltage vs Temperature DS012515-2 Supply Current vs Switch Current DS012515-4 DS012515-3 Current Limit vs Temperature Feedback Pin Bias Current vs Temperature DS012515-5 DS012515-6 5 DS012515-7 www.national.com Typical Performance Characteristics Switch Saturation Voltage vs Temperature (Continued) Switch Transconductance vs Temperature DS012515-8 Oscillator Frequency vs Temperature DS012515-9 Error Amp Transconductance vs Temperature DS012515-10 Error Amp Voltage Gain vs Temperature DS012515-11 Short Circuit Frequency vs Temperature DS012515-12 DS012515-13 Connection Diagrams Bent, Staggered Leads 5-Lead TO-220 (T) Top View Bent, Staggered Leads 5-Lead TO-220 (T) Side View DS012515-15 DS012515-14 Order Number LM2585T-3.3, LM2585T-5.0, LM2585T-12 or LM2585T-ADJ See NS Package Number T05D 5-Lead TO-263 (S) Top View 5-Lead TO-263 (S) Side View DS012515-17 DS012515-16 Order Number LM2585S-3.3, LM2585S-5.0, LM2585S-12 or LM2585S-ADJ See NS Package Number TS5B www.national.com 6 Block Diagram DS012515-18 For Fixed Versions 3.3V, R1 = 3.4k, R2 = 2k 5V, R1 = 6.15k, R2 = 2k 12V, R1 = 8.73k, R2 = 1k For Adj. Version R1 = Short (0Ω), R2 = Open FIGURE 1. 7 www.national.com Test Circuits DS012515-19 CIN1 — 100 µF, 25V Aluminum Electrolytic CIN2 — 0.1 µF Ceramic T — 22 µH, 1:1 Schott #67141450 D — 1N5820 COUT — 680 µF, 16V Aluminum Electrolytic CC — 0.47 µF Ceramic RC — 2k FIGURE 2. LM2585-3.3 and LM2585-5.0 DS012515-20 CIN1 — 100 µF, 25V Aluminum Electrolytic CIN2 — 0.1 µF Ceramic L — 15 µH, Renco #RL-5472-5 D — 1N5820 COUT — 680 µF, 16V Aluminum Electrolytic CC — 0.47 µF Ceramic RC — 2k For 12V Devices: R1 = Short (0Ω) and R2 = Open For ADJ Devices: R1 = 48.75k, ± 0.1% and R2 = 5.62k, ± 1% FIGURE 3. LM2585-12 and LM2585-ADJ www.national.com 8 Flyback Regulator Operation 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. The LM2585 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- DS012515-21 As shown in Figure 4, the LM2585 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 DS012515-22 A: Switch Voltage, 20 V/div B: Switch Current, 2 A/div C: Output Rectifier Current, 2 A/div D: Output Ripple Voltage, 50 mV/div AC-Coupled Horizontal: 2 µs/div FIGURE 5. Switching Waveforms 9 www.national.com Flyback Regulator Operation (Continued) DS012515-23 FIGURE 6. VOUT Load Current Step Response Typical Flyback Regulator Applications Figure 7 through Figure 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 LM2585-ADJ — or different output configurations that do not match the standard configurations, refer to the Switchers Made Simple ® software. DS012515-24 FIGURE 7. Single-Output Flyback Regulator www.national.com 10 Typical Flyback Regulator Applications (Continued) DS012515-25 FIGURE 8. Single-Output Flyback Regulator DS012515-26 FIGURE 9. Single-Output Flyback Regulator 11 www.national.com Typical Flyback Regulator Applications (Continued) DS012515-27 FIGURE 10. Dual-Output Flyback Regulator DS012515-28 FIGURE 11. Dual-Output Flyback Regulator www.national.com 12 Typical Flyback Regulator Applications (Continued) DS012515-29 FIGURE 12. Triple-Output Flyback Regulator 13 www.national.com 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 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Transformers T7 T7 T7 T6 T6 T5 4V–6V 4V–6V 8V–16V 4V–6V 18V–36V 18V–36V VOUT1 3.3V 5V 12V 12V 12V 5V IOUT1 (Max) 1.4A 1A 0.8A 0.15A 0.6A 1.8A 1 1 1 VIN N1 Figure 12 1.2 1.2 0.5 VOUT2 −12V −12V 12V IOUT2 (Max) 0.15A 0.6A 0.25A 1.2 1.2 N2 1.15 VOUT3 −12V IOUT3 (Max) 0.25A N3 1.15 FIGURE 13. Transformer Selection Table Transformer Type Coilcraft Coilcraft Pulse Pulse Renco Schott (Note 15) (Note 15) (Note 16) (Note 16) (Note 17) (Note 18) Surface Mount Surface Mount T5 Q4338-B Q4437-B PE-68413 — RL-5532 67140890 T6 Q4339-B Q4438-B PE-68414 — RL-5533 67140900 T7 S6000-A S6057-A — PE-68482 RL-5751 26606 Note 15: Coilcraft Inc. Manufacturers’ Part Numbers 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 www.national.com 14 Typical Flyback Regulator Applications (Continued) Transformer Footprints Figure 15 through Figure 29 show the footprints of each transformer, listed in Figure 14. T7 T7 DS012515-30 Top View DS012515-34 FIGURE 15. Coilcraft S6000-A Top View FIGURE 19. Coilcraft S6057-A (Surface Mount) T6 T6 DS012515-31 Top View FIGURE 16. Coilcraft Q4339-B DS012515-35 Top View T5 FIGURE 20. Coilcraft Q4438-B (Surface Mount) T7 DS012515-36 DS012515-32 Top View Top View FIGURE 17. Coilcraft Q4437-B (Surface Mount) FIGURE 21. Pulse PE-68482 T6 T5 DS012515-37 Top View DS012515-33 FIGURE 22. Pulse PE-68414 (Surface Mount) Top View FIGURE 18. Coilcraft Q4338-B 15 www.national.com Typical Flyback Regulator Applications (Continued) T7 T5 DS012515-44 FIGURE 27. Top View Schott 26606 T6 DS012515-39 Top View FIGURE 23. Pulse PE-68413 (Surface Mount) DS012515-46 T7 Top View FIGURE 28. Schott 67140900 T5 DS012515-40 Top View FIGURE 24. Renco RL-5751 T6 DS012515-47 Top View FIGURE 29. Schott 67140890 DS012515-42 Top View FIGURE 25. Renco RL-5533 T5 DS012515-43 Top View FIGURE 26. Renco RL-5532 www.national.com 16 Step-Up (Boost) Regulator Operation Figure 30 shows the LM2585 used as a step-up (boost) regulator. This is a switching regulator that produces an output voltage greater than the input supply voltage. 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. A brief explanation of how the LM2585 Boost Regulator works is as follows (refer to Figure 30). 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 DS012515-48 By adding a small number of external components (as shown in Figure 30), the LM2585 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 31. Typical performance of this regulator is shown in Figure 32. FIGURE 30. 12V Boost Regulator DS012515-49 A: Switch Voltage, 10 V/div B: Switch Current, 2 A/div C: Inductor Current, 2 A/div D: Output Ripple Voltage, 100 mV/div, AC-Coupled Horizontal: 2 µs/div FIGURE 31. Switching Waveforms DS012515-50 FIGURE 32. VOUT Response to Load Current Step 17 www.national.com Typical Boost Regulator Applications Figure 33 and Figure 35 through Figure 37 show four typical boost applications) — one fixed and three using the adjustable version of the LM2585. 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 34. For applications with different output voltages, refer to the Switchers Made Simple software. DS012515-51 FIGURE 33. +5V to +12V Boost Regulator Figure 34 contains a table of standard inductors, by part number and corresponding manufacturer, for the fixed output regulator of Figure 33. Coilcraft Pulse Renco Schott Schott (Note 22) (Note 19) (Note 20) (Note 21) (Note 22) (Surface Mount) D03316-153 PE-53898 RL-5471-7 67146510 67146540 Note 19: Coilcraft Inc. 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 34. Inductor Selection Table DS012515-52 FIGURE 35. +12V to +24V Boost Regulator www.national.com 18 Typical Boost Regulator Applications (Continued) DS012515-53 FIGURE 36. +24V to +36V Boost Regulator DS012515-54 *The LM2585 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 37. +24V to +48V Boost Regulator 19 www.national.com Application Hints DS012515-55 FIGURE 38. Boost Regulator passing 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 3A. In a flyback regulator application (Figure 39), using the standard transformers, the LM2585 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. PROGRAMMING OUTPUT VOLTAGE (SELECTING R1 AND R2) Referring to the adjustable regulator in Figure 38, the output voltage is programmed by the resistors R1 and R2 by the following formula: VOUT = VREF (1 + R1/R2) where VREF = 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: where VREF = 1.23V R1 = R2 (VOUT/VREF − 1) 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 38), current flows directly from the input, through the inductor and the diode, to the output, by- DS012515-56 FIGURE 39. Flyback Regulator www.national.com 20 Application Hints also shown in Figure 39. 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. (Continued) 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 39). Both are required due to the inherent operation of a flyback regulator. To keep a stable or constant voltage supply to the LM2585, a storage capacitor (≥100 µF) is required. If the input source is a rectified 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 µF ceramic capacitor between VIN and ground as close as possible to the device. 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: 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 LM2585 switch, the output diode(s), and the transformer — such as reverse recovery time of the output diode (mentioned above). 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 39 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 LM2585 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 39. 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), DS012515-57 FIGURE 40. Input Line Filter NOISY INPUT LINE CONDITION A small, low-pass RC filter should be used at the input pin of the LM2585 if the input voltage has an unusual large amount of transient noise, such as with an input switch that bounces. The circuit in Figure 40 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). 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: where VSAT is the switch saturation voltage and can be found in the Characteristic Curves. 21 www.national.com Application Hints (Continued) DS012515-58 FIGURE 41. 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 41). When using the Adjustable version, physically locate the programming resistors as near the regulator IC as possible, to keep the sensitive feedback wiring short. 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. HEAT SINK/THERMAL CONSIDERATIONS In many cases, no heat sink is required to keep the LM2585 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 LM2585). For a safe, conservative design, a temperature approximately 15˚C cooler than the maximum junction temperature should be selected (110˚C). 4) LM2585 package thermal resistances θJA and θJC (given in the Electrical Characteristics). Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction temperature: TJ = ∆TJ + 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: ∆TJ = PD x (θJC + θInterface + θHeat Sink) Again, the operating junction temperature will be: TJ = ∆TJ + TA Total power dissipated (PD) by the LM2585 can be estimated as follows: 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 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. To further simplify the flyback regulator design procedure, National Semiconductor is making available computer design software to be used with the Simple Switcher line of switching regulators. Switchers Made Simple is available on a 31⁄2" diskette for IBM compatible computers from a National Semiconductor sales office in your area or the National Semiconductor Customer Response Center (1-800-272-9959). 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: www.national.com 22 European Magnetic Vendor Contacts Phone: +44 1236 730 595 Fax: +44 1236 730 627 Please contact the following addresses for details of local distributors or representatives: Pulse Engineering Dunmore Road Tuam Coilcraft Co. Galway, Ireland Phone: +353 93 24 107 Fax: +353 93 24 459 21 Napier Place Wardpark North Cumbernauld, Scotland G68 0LL 23 www.national.com Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM2585T-3.3, LM2585T-5.0, LM2585T-12 or LM2585T-ADJ NS Package Number T05D www.national.com 24 LM2585 SIMPLE SWITCHER 3A Flyback Regulator Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Order Number LM2585S-3.3, LM2585S-5.0, LM2585S-12 or LM2585S-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. 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