LM2585S-3.3

LM2585 SIMPLE SWITCHER 3A Flyback Regulator February 2007 LM2585 SIMPLE SWITCHER® 3A Flyback Regulator General Description 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. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ 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 ■ ■ ■ ■ Flyback regulator Multiple-output regulator Simple boost regulator Forward converter Connection Diagrams Bent, Staggered Leads 5-Lead TO-220 (T) Top View Bent, Staggered Leads 5-Lead TO-220 (T) Side View 1251515 1251514 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 1251517 1251516 Order Number LM2585S-3.3, LM2585S-5.0, LM2585S-12 or LM2585S-ADJ See NS Package Number TS5B SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation © 2007 National Semiconductor Corporation 12515 www.national.com LM2585 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 LM2585T-3.3, LM2585T-5.0, LM2585T-12, LM2585T-ADJ LM2585S-3.3, LM2585S-5.0, LM2585S-12, LM2585S-ADJ LM2585SX-3.3, LM2585SX-5.0, LM2585SX-12, LM2585SX-ADJ www.national.com 2 LM2585 Absolute Maximum Ratings (Note 1) 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 ≤ VSW ≤ 65V Internally Limited −0.4V ≤ VCOMP ≤ 2.4V −0.4V ≤ VFB ≤ 2V −65°C to +150°C 260°C −0.4V ≤ VIN ≤ 45V Maximum Junction Temperature  (Note 3) Power Dissipation (Note 3) Minimum ESD Rating (C = 100 pF, R = 1.5 kΩ) Operating Ratings Supply Voltage Output Switch Voltage Output Switch Current Junction Temperature Range 0V ≤ VSW ≤ 60V −40°C ≤ TJ ≤ +125°C 4V ≤ VIN ≤ 40V ISW ≤ 3.0A 150°C Internally Limited 2 kV 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 VOUT ΔVOUT/ ΔVIN ΔVOUT/ ΔILOAD η VREF ΔVREF GM AVOL Efficiency Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain ICOMP = −30 μA to +30 μA VCOMP = 1.0V VCOMP = 0.5V to 1.6V RCOMP = 1.0 MΩ (Note 6) 260 151/75 V/V 1.193 0.678 2.259 mmho Load Regulation Parameters Output Voltage Line Regulation Conditions VIN = 4V to 12V 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 Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 2.0 mV 76 3.3 3.242/3.234 3.358/3.366 % V 20 50/100 mV 20 50/100 mV Typical 3.3 Min 3.17/3.14 Max 3.43/3.46 Units V SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) UNIQUE DEVICE PARAMETERS (Note 5) LM2585-5.0 Symbol VOUT ΔVOUT/ ΔVIN ΔVOUT/ ΔILOAD η VREF Efficiency Output Reference Voltage Load Regulation Parameters Output Voltage Line Regulation Conditions VIN = 4V to 12V 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 Measured at Feedback Pin VCOMP = 1.0V 80 5.0 4.913/4.900 5.088/5.100 % V 20 50/100 mV 20 50/100 mV Typical 5.0 Min 4.80/4.75 Max 5.20/5.25 Units V SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4) UNIQUE DEVICE PARAMETERS (Note 5) 3 www.national.com LM2585 Symbol ΔVREF GM AVOL Parameters Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain Conditions VIN = 4V to 40V ICOMP = −30 μA to +30 μA VCOMP = 1.0V VCOMP = 0.5V to 1.6V RCOMP = 1.0 MΩ (Note 6) Typical 3.3 0.750 165 Min Max Units mV 0.447 99/49 1.491 mmho V/V LM2585-12 Symbol VOUT ΔVOUT/ ΔVIN ΔVOUT/ ΔILOAD η VREF ΔVREF GM AVOL Efficiency Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain ICOMP = −30 μA to +30 μA VCOMP = 1.0V VCOMP = 0.5V to 1.6V RCOMP = 1.0 MΩ (Note 6) 70 41/21 V/V 0.328 0.186 0.621 mmho Load Regulation Parameters Output Voltage Line Regulation Conditions VIN = 4V to 10V 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 Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 7.8 mV 93 12.0 11.79/11.76 12.21/12.24 % V 20 100/200 mV 20 100/200 mV Typical 12.0 Min 11.52/11.40 Max 12.48/12.60 Units V SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) UNIQUE DEVICE PARAMETERS (Note 5) LM2585-ADJ Symbol VOUT ΔVOUT/ ΔVIN ΔVOUT/ ΔILOAD η VREF ΔVREF GM AVOL Efficiency Output Reference Voltage Reference Voltage Line Regulation Error Amp Transconductance Error Amp Voltage Gain ICOMP = −30 μA to +30 μA VCOMP = 1.0V VCOMP = 0.5V to 1.6V RCOMP = 1.0 MΩ (Note 6) 670 400/200 V/V 3.200 1.800 6.000 mmho Load Regulation Parameters Output Voltage Line Regulation Conditions VIN = 4V to 10V 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 Measured at Feedback Pin VCOMP = 1.0V VIN = 4V to 40V 1.5 mV 93 1.230 1.208/1.205 1.252/1.255 % V 20 100/200 mV 20 100/200 mV Typical 12.0 Min 11.52/11.40 Max 12.48/12.60 Units V SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4) UNIQUE DEVICE PARAMETERS (Note 5) www.national.com 4 LM2585 Symbol IB Parameters Error Amp Input Bias Current Conditions VCOMP = 1.0V Typical 125 Min Max 425/600 Units nA Electrical Characteristics (All Versions) Symbol IS Parameters Input Supply Current VUV fO Input Supply Undervoltage Lockout Oscillator Frequency Measured at Switch Pin RLOAD = 100Ω VCOMP = 1.0V fSC Short-Circuit Frequency VEAO Error Amplifier Output Swing Measured at Switch Pin RLOAD = 100Ω VFEEDBACK = 1.15V Upper Limit (Note 7) Lower Limit (Note 8) IEAO Error Amp Output Current (Source or Sink) ISS D IL VSUS VSAT ICL θJA θJA θJC θJA θJA θJA θJC Soft Start Current Maximum Duty Cycle Switch Leakage Current Switch Sustaining Voltage Switch Saturation Voltage NPN Switch Current Limit Thermal Resistance T Package, Junction to Ambient (Note 10) T Package, Junction to Ambient (Note 11) T Package, Junction to Case S Package, Junction to Ambient (Note 12) S Package, Junction to Ambient (Note 13) S Package, Junction to Ambient (Note 14) S Package, Junction to Case 65 45 2 56 35 26 2 °C/W 4.0 3.0 7.0 A ISWITCH = 3.0A 0.45 0.65/0.9 V VFEEDBACK = 0.92V VCOMP = 1.0V RLOAD = 100Ω (Note 7) Switch Off VSWITCH = 60V dV/dT = 1.5V/ns 65 V 15 300/600 μA 98 93/90 % 11.0 8.0/7.0 17.0/19.0 μA (Note 9) 165 110/70 260/320 μA 0.25 0.40/0.55 V 2.8 2.6/2.4 V 25 kHz 100 85/75 115/125 kHz Conditions (Switch Off) (Note 8) ISWITCH = 1.8A RLOAD = 100Ω 50 3.30 3.05 100/115 3.75 mA V Typical 11 Min Max 15.5/16.5 Units mA COMMON DEVICE PARAMETERS for all versions (Note 5) 5 www.national.com LM2585 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). 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 × 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 at its high value (specified in (Note 8) . 7) and Note 10: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with ½ 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 ½ 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. www.national.com 6 LM2585 Typical Performance Characteristics Supply Current vs Temperature Reference Voltage vs Temperature 1251502 1251503 ΔReference Voltage vs Supply Voltage Supply Current vs Switch Current 1251504 1251505 Current Limit vs Temperature Feedback Pin Bias Current vs Temperature 1251506 1251507 7 www.national.com LM2585 Switch Saturation Voltage vs Temperature Switch Transconductance vs Temperature 1251508 1251509 Oscillator Frequency vs Temperature Error Amp Transconductance vs Temperature 1251510 1251511 Error Amp Voltage Gain vs Temperature Short Circuit Frequency vs Temperature 1251512 1251513 www.national.com 8 LM2585 Flyback Regulator 1251501 Block Diagram 1251518 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. 9 www.national.com LM2585 Test Circuits 1251519 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 1251520 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 10 LM2585 Flyback Regulator Operation 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 prima- ry. 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. 1251521 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 11 www.national.com LM2585 1251522 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 1251523 FIGURE 6. VOUT Load Current Step Response www.national.com 12 LM2585 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. 1251524 FIGURE 7. Single-Output Flyback Regulator 1251525 FIGURE 8. Single-Output Flyback Regulator 13 www.national.com LM2585 1251526 FIGURE 9. Single-Output Flyback Regulator 1251527 FIGURE 10. Dual-Output Flyback Regulator www.national.com 14 LM2585 1251528 FIGURE 11. Dual-Output Flyback Regulator 1251529 FIGURE 12. Triple-Output Flyback Regulator 15 www.national.com LM2585 TRANSFORMER SELECTION (T) Figure 13 lists the standard transformers available for flyback regulator applications. Included in the table are the turns ratio Applications Transformers VIN VOUT1 IOUT1 (Max) N1 VOUT2 IOUT2 (Max) N2 VOUT3 IOUT3 (Max) N3 Figure 7 T7 4V–6V 3.3V 1.4A 1 Figure 8 T7 4V–6V 5V 1A 1 (s) for each transformer, as well as the output voltages, input voltage ranges, and the maximum load currents for each circuit. Figure 10 T6 4V–6V 12V 0.15A 1.2 −12V 0.15A 1.2 Figure 11 T6 18V–36V 12V 0.6A 1.2 −12V 0.6A 1.2 Figure 12 T5 18V–36V 5V 1.8A 0.5 12V 0.25A 1.15 −12V 0.25A 1.15 Figure 9 T7 8V–16V 12V 0.8A 1 FIGURE 13. Transformer Selection Table Transform er Type T5 T6 T7 Note 15: Coilcraft Inc. Manufacturers' Part Numbers Coilcraft (Note 15) Q4338-B Q4339-B S6000-A Phone: (800) 322-2645 Fax: (708) 639-1469 Fax: (619) 674-8262 Coilcraft (Note 15) Surface Mount Q4437-B Q4438-B S6057-A Pulse (Note 16) Surface Mount PE-68413 PE-68414 — Pulse (Note 16) — — PE-68482 Renco (Note 17) RL-5532 RL-5533 RL-5751 Schott (Note 18) 67140890 67140900 26606 1102 Silver Lake Road, Cary, IL 60013: Note 16: Pulse Engineering Inc. Phone: (619) 674-8100 12220 World Trade Drive, San Diego, CA 92128: Note 17: Renco Electronics Inc. Note 18: Schott Corp. 60 Jeffryn Blvd. East, Deer Park, NY 11729: Phone: (800) 645-5828 Fax: (516) 586-5562 Fax: (612) 475-1786 Phone: (612) 475-1173 1000 Parkers Lane Road, Wayzata, MN 55391: FIGURE 14. Transformer Manufacturer Guide www.national.com 16 LM2585 TRANSFORMER FOOTPRINTS Figure 15 through Figure 29 show the footprints of each transformer, listed in Figure 14. T7 T7 1251534 Top View 1251530 Top View FIGURE 15. Coilcraft S6000-A FIGURE 19. Coilcraft S6057-A (Surface Mount) T6 T6 1251535 1251531 Top View FIGURE 16. Coilcraft Q4339-B T5 Top View FIGURE 20. Coilcraft Q4438-B (Surface Mount) T7 1251536 Top View FIGURE 21. Pulse PE-68482 1251532 T6 Top View FIGURE 17. Coilcraft Q4437-B (Surface Mount) T5 1251537 Top View FIGURE 22. Pulse PE-68414 (Surface Mount) 1251533 Top View FIGURE 18. Coilcraft Q4338-B 17 www.national.com LM2585 T5 T7 1251544 FIGURE 27. Top View Schott 26606 1251539 Top View FIGURE 23. Pulse PE-68413 (Surface Mount) T7 T6 1251546 Top View 1251540 FIGURE 28. Schott 67140900 T5 Top View FIGURE 24. Renco RL-5751 T6 1251542 Top View FIGURE 25. Renco RL-5533 T5 1251547 Top View FIGURE 29. Schott 67140890 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. 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 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. 1251543 Top View FIGURE 26. Renco RL-5532 www.national.com 18 LM2585 1251548 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 1251549 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 1251550 FIGURE 32. VOUT Response to Load Current Step 19 www.national.com LM2585 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 num- ber(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. 1251551 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 (Note 19) D03316-153 Note 19: Coilcraft Inc. Pulse (Note 20) PE-53898 Phone: (800) 322-2645 Fax: (708) 639-1469 Renco (Note 21) RL-5471-7 Schott (Note 22) 67146510 Schott (Note 22) (Surface Mount) 67146540 1102 Silver Lake Road, Cary, IL 60013 Note 20: Pulse Engineering Inc. Phone: (619) 674-8100 12220 World Trade Drive, San Diego, CA 92128 Note 21: Renco Electronics Inc. Note 22: Schott Corp. 60 Jeffryn Blvd. East, Deer Park, NY 11729 Fax: (619) 674-8262 Phone (800) 645-5828 Fax: (516) 586-5562 Fax: (612) 475-1786 Phone: (612) 475-1173 1000 Parkers Lane Road, Wayzata, MN 55391 FIGURE 34. Inductor Selection Table 1251552 FIGURE 35. +12V to +24V Boost Regulator www.national.com 20 LM2585 1251553 FIGURE 36. +24V to +36V Boost Regulator 1251554 *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 21 www.national.com LM2585 Application Hints 1251555 FIGURE 38. Boost Regulator 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: R1 = R2 (VOUT/VREF − 1) where VREF = 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 38), 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 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. 1251556 FIGURE 39. Flyback Regulator www.national.com 22 LM2585 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. 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), 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. 23 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 × VIN × 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). 1251557 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. www.national.com LM2585 1251558 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. 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). Total power dissipated (PD) by the LM2585 can be estimated as follows: 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: ΔTJ = PD × θJA. Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction temperature: TJ = ΔTJ + 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: ΔTJ = PD × (θJC + θInterface + θHeat Sink) Again, the operating junction temperature will be: TJ = ΔTJ + TA 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 3½″ 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). 24 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 LM2585 European Magnetic Vendor Contacts Please contact the following addresses for details of local distributors or representatives: 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 Coilcraft 21 Napier Place Wardpark North Cumbernauld, Scotland G68 0LL 25 www.national.com LM2585 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 26 LM2585 Order Number LM2585S-3.3, LM2585S-5.0, LM2585S-12 or LM2585S-ADJ NS Package Number TS5B 27 www.national.com LM2585 SIMPLE SWITCHER 3A Flyback Regulator Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. 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All other brand or product names may be trademarks or registered trademarks of their respective holders. Copyright© 2007 National Semiconductor Corporation For the most current product information visit us at www.national.com National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530-85-86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +49 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 www.national.com