LM2585
LM2585 SIMPLE SWITCHER 3A Flyback Regulator
Literature Number: SNVS120E
LM2585
February 2, 2010
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 ad-
justable.
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 regula-
tors. 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 lim-
iting 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, cur-
rent 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 volt-
ages 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
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Bent, Staggered Leads
5-Lead TO-220 (T)
Side View
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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
1251516
5-Lead TO-263 (S)
Side View
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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
© 2010 National Semiconductor Corporation 12515 www.national.com
LM2585 SIMPLE SWITCHER 3A Flyback Regulator
Ordering Information
Package Type NSC Package Order Number
Drawing
5-Lead TO-220 Bent, Staggered Leads T05D LM2585T-3.3, LM2585T-5.0, LM2585T-12, LM2585T-ADJ
5-Lead TO-263 TS5B 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
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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 −0.4V ≤ VIN ≤ 45V
Switch Voltage −0.4V ≤ VSW ≤ 65V
Switch Current (Note 2) Internally Limited
Compensation Pin Voltage −0.4V ≤ VCOMP ≤ 2.4V
Feedback Pin Voltage −0.4V ≤ VFB ≤ 2V
Storage Temperature Range −65°C to +150°C
Lead Temperature
(Soldering, 10 sec.) 260°C
Maximum Junction Temperature
(Note 3) 150°C
Power Dissipation (Note 3) Internally Limited
Minimum ESD Rating
(C = 100 pF, R = 1.5 kΩ) 2 kV
Operating Ratings
Supply Voltage 4V ≤ VIN ≤ 40V
Output Switch Voltage 0V ≤ VSW ≤ 60V
Output Switch Current ISW ≤ 3.0A
Junction Temperature Range −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 Typical Min Max Units
SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4)
VOUT Output Voltage VIN = 4V to 12V 3.3 3.17/3.14 3.43/3.46 V
ILOAD = 0.3A to 1.2A
ΔVOUT/Line Regulation VIN = 4V to 12V 20 50/100 mV
ΔVIN ILOAD = 0.3A
ΔVOUT/Load Regulation VIN = 12V 20 50/100 mV
ΔILOAD ILOAD = 0.3A to 1.2A
ηEfficiency VIN = 5V, ILOAD = 0.3A 76 %
UNIQUE DEVICE PARAMETERS (Note 5)
VREF Output Reference Measured at Feedback Pin 3.3 3.242/3.234 3.358/3.366 V
Voltage VCOMP = 1.0V
ΔVREF Reference Voltage VIN = 4V to 40V 2.0 mV
Line Regulation
GMError Amp ICOMP = −30 μA to +30 μA1.193 0.678 2.259 mmho
Transconductance VCOMP = 1.0V
AVOL Error Amp VCOMP = 0.5V to 1.6V 260 151/75 V/V
Voltage Gain RCOMP = 1.0 MΩ (Note 6)
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LM2585
LM2585-5.0
Symbol Parameters Conditions Typical Min Max Units
SYSTEM PARAMETERS Test Circuit of Figure 2 (Note 4)
VOUT Output Voltage VIN = 4V to 12V 5.0 4.80/4.75 5.20/5.25 V
ILOAD = 0.3A to 1.1A
ΔVOUT/Line Regulation VIN = 4V to 12V 20 50/100 mV
ΔVIN ILOAD = 0.3A
ΔVOUT/Load Regulation VIN = 12V 20 50/100 mV
ΔILOAD ILOAD = 0.3A to 1.1A
ηEfficiency VIN = 12V, ILOAD = 0.6A 80 %
UNIQUE DEVICE PARAMETERS (Note 5)
VREF Output Reference Measured at Feedback Pin 5.0 4.913/4.900 5.088/5.100 V
Voltage VCOMP = 1.0V
ΔVREF Reference Voltage VIN = 4V to 40V 3.3 mV
Line Regulation
GMError Amp ICOMP = −30 μA to +30 μA0.750 0.447 1.491 mmho
Transconductance VCOMP = 1.0V
AVOL Error Amp VCOMP = 0.5V to 1.6V 165 99/49 V/V
Voltage Gain RCOMP = 1.0 MΩ (Note 6)
LM2585-12
Symbol Parameters Conditions Typical Min Max Units
SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4)
VOUT Output Voltage VIN = 4V to 10V 12.0 11.52/11.40 12.48/12.60 V
ILOAD = 0.2A to 0.8A
ΔVOUT/Line Regulation VIN = 4V to 10V 20 100/200 mV
ΔVIN ILOAD = 0.2A
ΔVOUT/Load Regulation VIN = 10V 20 100/200 mV
ΔILOAD ILOAD = 0.2A to 0.8A
ηEfficiency VIN = 10V, ILOAD = 0.6A 93 %
UNIQUE DEVICE PARAMETERS (Note 5)
VREF Output Reference Measured at Feedback Pin 12.0 11.79/11.76 12.21/12.24 V
Voltage VCOMP = 1.0V
ΔVREF Reference Voltage VIN = 4V to 40V 7.8 mV
Line Regulation
GMError Amp ICOMP = −30 μA to +30 μA0.328 0.186 0.621 mmho
Transconductance VCOMP = 1.0V
AVOL Error Amp VCOMP = 0.5V to 1.6V 70 41/21 V/V
Voltage Gain RCOMP = 1.0 MΩ (Note 6)
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LM2585
LM2585-ADJ
Symbol Parameters Conditions Typical Min Max Units
SYSTEM PARAMETERS Test Circuit of Figure 3 (Note 4)
VOUT Output Voltage VIN = 4V to 10V 12.0 11.52/11.40 12.48/12.60 V
ILOAD = 0.2A to 0.8A
ΔVOUT/Line Regulation VIN = 4V to 10V 20 100/200 mV
ΔVIN ILOAD = 0.2A
ΔVOUT/Load Regulation VIN = 10V 20 100/200 mV
ΔILOAD ILOAD = 0.2A to 0.8A
ηEfficiency VIN = 10V, ILOAD = 0.6A 93 %
UNIQUE DEVICE PARAMETERS (Note 5)
VREF Output Reference Measured at Feedback Pin 1.230 1.208/1.205 1.252/1.255 V
Voltage VCOMP = 1.0V
ΔVREF Reference Voltage VIN = 4V to 40V 1.5 mV
Line Regulation
GMError Amp ICOMP = −30 μA to +30 μA3.200 1.800 6.000 mmho
Transconductance VCOMP = 1.0V
AVOL Error Amp VCOMP = 0.5V to 1.6V 670 400/200 V/V
Voltage Gain RCOMP = 1.0 MΩ (Note 6)
IBError Amp VCOMP = 1.0V 125 425/600 nA
Input Bias Current
Electrical Characteristics (All Versions)
Symbol Parameters Conditions Typical Min Max Units
COMMON DEVICE PARAMETERS for all versions (Note 5)
ISInput Supply (Switch Off) 11 15.5/16.5 mA
Current (Note 8)
ISWITCH = 1.8A 50 100/115 mA
VUV Input Supply RLOAD = 100Ω 3.30 3.05 3.75 V
Undervoltage Lockout
fOOscillator Frequency Measured at Switch Pin
RLOAD = 100Ω 100 85/75 115/125 kHz
VCOMP = 1.0V
fSC Short-Circuit Measured at Switch Pin
Frequency RLOAD = 100Ω 25 kHz
VFEEDBACK = 1.15V
VEAO Error Amplifier Upper Limit 2.8 2.6/2.4 V
Output Swing (Note 7)
Lower Limit 0.25 0.40/0.55 V
(Note 8)
IEAO Error Amp (Note 9)
Output Current 165 110/70 260/320 μA
(Source or Sink)
ISS Soft Start Current VFEEDBACK = 0.92V 11.0 8.0/7.0 17.0/19.0 μA
VCOMP = 1.0V
D Maximum Duty RLOAD = 100Ω 98 93/90 %
Cycle (Note 7)
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LM2585
Symbol Parameters Conditions Typical Min Max Units
ILSwitch Leakage Switch Off 15 300/600 μA
Current VSWITCH = 60V
VSUS Switch Sustaining dV/dT = 1.5V/ns 65 V
Voltage
VSAT Switch Saturation ISWITCH = 3.0A 0.45 0.65/0.9 V
Voltage
ICL NPN Switch 4.0 3.0 7.0 A
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 °C/W
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
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 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 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.
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LM2585
Typical Performance Characteristics
Supply Current
vs Temperature
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Reference Voltage
vs Temperature
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ΔReference Voltage
vs Supply Voltage
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Supply Current
vs Switch Current
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Current Limit
vs Temperature
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Feedback Pin Bias
Current vs Temperature
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LM2585
Switch Saturation
Voltage vs Temperature
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Switch Transconductance
vs Temperature
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Oscillator Frequency
vs Temperature
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Error Amp Transconductance
vs Temperature
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Error Amp Voltage
Gain vs Temperature
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Short Circuit Frequency
vs Temperature
1251513
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LM2585
Flyback Regulator
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Block Diagram
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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.
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LM2585
Test Circuits
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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
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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
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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 out-
put voltage that is inside the range of the input voltage. This
feature is unique to flyback regulators and cannot be dupli-
cated 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 pro-
portional to the switch current (i.e., inductor current during the
switch on time). The comparator terminates the switch on time
when the two voltages are equal, thereby controlling the peak
switch current to maintain a constant output voltage.
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As shown in Figure 4, the 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
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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
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FIGURE 6. VOUT Load Current Step Response
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LM2585
Typical Flyback Regulator
Applications
Figure 7 through Figure 12 show six typical flyback applica-
tions, 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.
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FIGURE 7. Single-Output Flyback Regulator
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FIGURE 8. Single-Output Flyback Regulator
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LM2585
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FIGURE 9. Single-Output Flyback Regulator
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FIGURE 10. Dual-Output Flyback Regulator
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LM2585
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FIGURE 11. Dual-Output Flyback Regulator
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FIGURE 12. Triple-Output Flyback Regulator
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LM2585
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 cir-
cuit.
Applications Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12
Transformers T7 T7 T7 T6 T6 T5
VIN 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
N11 1 1 1.2 1.2 0.5
VOUT2 −12V −12V 12V
IOUT2 (Max) 0.15A 0.6A 0.25A
N2 1.2 1.2 1.15
VOUT3 −12V
IOUT3 (Max) 0.25A
N3 1.15
FIGURE 13. Transformer Selection Table
Transform
er
Type
Manufacturers' Part Numbers
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. Phone: (800) 322-2645
1102 Silver Lake Road, Cary, IL 60013: Fax: (708) 639-1469
Note 16: Pulse Engineering Inc. Phone: (619) 674-8100
12220 World Trade Drive, San Diego, CA 92128: Fax: (619) 674-8262
Note 17: Renco Electronics Inc. Phone: (800) 645-5828
60 Jeffryn Blvd. East, Deer Park, NY 11729: Fax: (516) 586-5562
Note 18: Schott Corp. Phone: (612) 475-1173
1000 Parkers Lane Road, Wayzata, MN 55391: Fax: (612) 475-1786
FIGURE 14. Transformer Manufacturer Guide
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LM2585
TRANSFORMER FOOTPRINTS
Figure 15 through Figure 29 show the footprints of each trans-
former, listed in Figure 14.
T7
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Top View
FIGURE 15. Coilcraft S6000-A
T6
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Top View
FIGURE 16. Coilcraft Q4339-B
T5
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Top View
FIGURE 17. Coilcraft Q4437-B
(Surface Mount)
T5
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Top View
FIGURE 18. Coilcraft Q4338-B
T7
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Top View
FIGURE 19. Coilcraft S6057-A
(Surface Mount)
T6
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Top View
FIGURE 20. Coilcraft Q4438-B
(Surface Mount)
T7
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Top View
FIGURE 21. Pulse PE-68482
T6
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Top View
FIGURE 22. Pulse PE-68414
(Surface Mount)
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LM2585
T5
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Top View
FIGURE 23. Pulse PE-68413
(Surface Mount)
T7
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Top View
FIGURE 24. Renco RL-5751
T6
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Top View
FIGURE 25. Renco RL-5533
T5
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Top View
FIGURE 26. Renco RL-5532
T7
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FIGURE 27. Top View
Schott 26606
T6
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Top View
FIGURE 28. Schott 67140900
T5
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Top View
FIGURE 29. Schott 67140890
Step-Up (Boost) Regulator
Operation
Figure 30 shows the LM2585 used as a step-up (boost) reg-
ulator. 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.
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LM2585
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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
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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
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FIGURE 32. VOUT Response to Load Current Step
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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.
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FIGURE 33. +5V to +12V Boost Regulator
Figure 34 contains a table of standard inductors, by part num-
ber 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 Fax: (708) 639-1469
Note 20: Pulse Engineering Inc. Phone: (619) 674-8100
12220 World Trade Drive, San Diego, CA 92128 Fax: (619) 674-8262
Note 21: Renco Electronics Inc. Phone (800) 645-5828
60 Jeffryn Blvd. East, Deer Park, NY 11729 Fax: (516) 586-5562
Note 22: Schott Corp. Phone: (612) 475-1173
1000 Parkers Lane Road, Wayzata, MN 55391 Fax: (612) 475-1786
FIGURE 34. Inductor Selection Table
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FIGURE 35. +12V to +24V Boost Regulator
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LM2585
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FIGURE 36. +24V to +36V Boost Regulator
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*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
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LM2585
Application Hints
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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 fol-
lowing 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 out-
put is shorted (see Figure 38), current flows directly from the
input, through the inductor and the diode, to the output, by-
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 ex-
ternally 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 stan-
dard 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 capaci-
tors 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 stor-
age 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 ap-
pearing at the switch, when it is off, is set by the transformer
turns ratio, N, the output voltage, VOUT, and the maximum in-
put 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 recov-
ery 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 sup-
pressor in series with a diode is inserted across the trans-
former 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 trans-
former 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 rec-
tifier(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.
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the max-
imum switch voltage minus a diode drop. In a flyback regula-
tor, 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 fol-
lowing equation:
Theoretically, the maximum output voltage can be as large as
desired—just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from in-
creasing 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 in-
stability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic os-
cillations, a minimum value of inductance is required to en-
sure 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.
23 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 gener-
ate 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 ap-
proximately 15°C cooler than the maximum junction temper-
ature 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:
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 regu-
lators). The duty cycle is given by:
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 re-
quired. 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 pa-
rameters or different component values. It can also calculate
the heat sink thermal resistance required to maintain the reg-
ulator 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 Semi-
conductor sales office in your area or the National Semicon-
ductor Customer Response Center (1-800-272-9959).
www.national.com 24
LM2585
European Magnetic Vendor
Contacts
Please contact the following addresses for details of local
distributors or representatives:
Coilcraft
21 Napier Place
Wardpark North
Cumbernauld, Scotland G68 0LL
Phone: +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
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
Notes
LM2585 SIMPLE SWITCHER 3A Flyback Regulator
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