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LM2587

LM2587 SIMPLE SWITCHER 5A Flyback Regulator

Literature Number: SNVS115C

LM2587

February 2, 2010

SIMPLE SWITCHER® 5A Flyback Regulator

General Description

The LM2587 series of regulators are monolithic integrated

circuits specifically designed for flyback, step-up (boost), and

forward converter applications. The device is available in 4

different output voltage versions: 3.3V, 5.0V, 12V, and 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 5.0A 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 5.0A, can stand off 65V

■Wide input voltage range: 4V to 40V

■Current-mode operation for improved transient response,

line regulation, and current limit

■100 kHz switching frequency

■Internal soft-start function reduces in-rush current during

start-up

■Output transistor protected by current limit, under voltage

lockout, and thermal shutdown

■System Output Voltage Tolerance of ±4% max over line

and load conditions

Typical Applications

■Flyback regulator

■Multiple-output regulator

■Simple boost regulator

■Forward converter

Flyback Regulator

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Ordering Information

Package Type NSC

Package Order Number

Drawing

5-Lead TO-220 Bent, Staggered Leads T05D LM2587T-3.3, LM2587T-5.0, LM2587T-12, LM2587T-ADJ

5-Lead TO-263 TS5B LM2587S-3.3, LM2587S-5.0, LM2587S-12, LM2587S-ADJ

5-Lead TO-263 Tape and Reel TS5B LM2587SX-3.3, LM2587SX-5.0, LM2587SX-12, LM2587SX-ADJ

SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation

LM2587 SIMPLE SWITCHER 5A Flyback Regulator

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 ≤ 2 VOUT

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 kΩ2 kV

Operating Ratings

Supply Voltage 4V ≤ VIN ≤ 40V

Output Switch Voltage 0V ≤ VSW ≤ 60V

Output Switch Current ISW ≤ 5.0A

Junction Temperature Range −40°C ≤ TJ ≤ +125°C

LM2587-3.3

Electrical Characteristics

Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, VIN = 5V.

Symbol Parameters Conditions Typical Min Max Units

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 = 400 mA to 1.75A

ΔVOUT/Line Regulation VIN = 4V to 12V 20 50/100 mV

ΔVIN ILOAD = 400 mA

ΔVOUT/Load Regulation VIN = 12V 20 50/100 mV

ΔILOAD ILOAD = 400 mA to 1.75A

ηEfficiency VIN = 12V, ILOAD = 1A 75 %

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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LM2587

LM2587-5.0

Electrical Characteristics

Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, VIN = 5V.

Symbol Parameters Conditions Typical Min Max Units

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 = 500 mA to 1.45A

ΔVOUT/Line Regulation VIN = 4V to 12V 20 50/100 mV

ΔVIN ILOAD = 500 mA

ΔVOUT/Load Regulation VIN = 12V 20 50/100 mV

ΔILOAD ILOAD = 500 mA to 1.45A

ηEfficiency VIN = 12V, ILOAD = 750 mA 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)

LM2587-12

Electrical Characteristics

Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, VIN = 5V.

Symbol Parameters Conditions Typical Min Max Units

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 = 300 mA to 1.2A

ΔVOUT/Line Regulation VIN = 4V to 10V 20 100/200 mV

ΔVIN ILOAD = 300 mA

ΔVOUT/Load Regulation VIN = 10V 20 100/200 mV

ΔILOAD ILOAD = 300 mA to 1.2A

ηEfficiency VIN = 10V, ILOAD = 1A 90 %

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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LM2587

LM2587-ADJ

Electrical Characteristics

Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, VIN = 5V.

Symbol Parameters Conditions Typical Min Max Units

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 = 300 mA to 1.2A

ΔVOUT/Line Regulation VIN = 4V to 10V 20 100/200 mV

ΔVIN ILOAD = 300 mA

ΔVOUT/Load Regulation VIN = 10V 20 100/200 mV

ΔILOAD ILOAD = 300 mA to 1.2A

ηEfficiency VIN = 10V, ILOAD = 1A 90 %

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

All Output Voltage Versions

Electrical Characteristics (Note 5)

Specifications with standard type face are for TJ = 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, VIN = 5V.

Symbol Parameters Conditions Typical Min Max Units

ISInput Supply Current (Switch Off) 11 15.5/16.5 mA

(Note 8)

ISWITCH = 3.0A 85 140/165 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)

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LM2587

Symbol Parameters Conditions Typical Min Max Units

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 Cycle RLOAD = 100Ω 98 93/90 %

(Note 7)

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 = 5.0A 0.7 1.1/1.4 V

Voltage

ICL NPN Switch 6.5 5.0 9.5 A

Current Limit

COMMON DEVICE PARAMETERS (Note 4)

θJA Thermal Resistance T Package, Junction to Ambient (Note 10) 65

θJA T Package, Junction to Ambient (Note 11) 45

θJC T Package, Junction to Case 2

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 LM2587 is used

as a step-up regulator. To prevent damage to the switch, the output current must be externally limited to 5A. However, output current is internally limited when

the LM2587 is used as a flyback regulator (see the Application Hints section for more information).

Note 3: The junction temperature of the device (TJ) is a function of the ambient temperature (TA), the junction-to-ambient thermal resistance (θJA), and the power

dissipation of the device (PD). A thermal shutdown will occur if the temperature exceeds the maximum junction temperature of the device: PD × θJA + TA(MAX) ≥

TJ(MAX). For a safe thermal design, check that the maximum power dissipated by the device is less than: PD ≤ [TJ(MAX) − TA(MAX))]/θJA. When calculating the

maximum allowable power dissipation, derate the maximum junction temperature—this ensures a margin of safety in the thermal design.

Note 4: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. When the LM2587 is used as

shown in Figure 2 and Figure 3, system performance will be as specified by the system parameters.

Note 5: All room temperature limits are 100% production tested, and all limits at temperature extremes are guaranteed via correlation using standard Statistical

Quality Control (SQC) methods.

Note 6: A 1.0 MΩ resistor is connected to the compensation pin (which is the error amplifier output) to ensure accuracy in measuring AVOL.

Note 7: To measure this parameter, the feedback voltage is set to a low value, depending on the output version of the device, to force the error amplifier output

high. Adj: VFB = 1.05V; 3.3V: VFB = 2.81V; 5.0V: VFB = 4.25V; 12V: VFB = 10.20V.

Note 8: To measure this parameter, the feedback voltage is set to a high value, depending on the output version of the device, to force the error amplifier output

low. Adj: VFB = 1.41V; 3.3V: VFB = 3.80V; 5.0V: VFB = 5.75V; 12V: VFB = 13.80V.

Note 9: To measure the worst-case error amplifier output current, the LM2587 is tested with the feedback voltage set to its low value (specified in Note 7) and at

its high value (specified in Note 8).

Note 10: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with 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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LM2587

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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LM2587

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

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LM2587

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 LM2587T-3.3, LM2587T-5.0,

LM2587T-12 or LM2587T-ADJ

See NS Package Number T05D

5-Lead TO-263 (S)

Top View

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5-Lead TO-263 (S)

Side View

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Order Number LM2587S-3.3, LM2587S-5.0,

LM2587S-12 or LM2587S-ADJ

See NS Package Number TS5B

Block Diagram

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For Fixed Versions3.3V, R1 = 3.4k, R2 = 2k5V, R1 = 6.15k, R2 = 2k12V, R1 = 8.73k, R2 = 1kFor Adj. VersionR1 = Short (0Ω), R2 = Open

FIGURE 1.

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LM2587

Test Circuits

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CIN1—100 μF, 25V Aluminum ElectrolyticCIN2—0.1 μF CeramicT—22 μH, 1:1 Schott #67141450D—1N5820COUT—680 μF, 16V Aluminum ElectrolyticCC—0.47

μF CeramicRC—2k

FIGURE 2. LM2587-3.3 and LM2587-5.0

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CIN1—100 μF, 25V Aluminum ElectrolyticCIN2—0.1 μF CeramicL—15 μH, Renco #RL-5472-5D—1N5820COUT—680 μF, 16V Aluminum ElectrolyticCC—0.47 μF

CeramicRC—2kFor 12V Devices: R1 = Short (0Ω) and R2 = OpenFor ADJ Devices: R1 = 48.75k, ±0.1% and R2 = 5.62k, ±1%

FIGURE 3. LM2587-12 and LM2587-ADJ

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LM2587

Flyback Regulator Operation

The LM2587 is ideally suited for use in the flyback regulator

topology. The flyback regulator can produce a single output

voltage, such as the one shown in Figure 4, or multiple output

voltages. In Figure 4, the flyback regulator generates an 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 LM2587 can be used as a flyback regulator by using a minimum number of external components. The switching waveforms of this

regulator are shown in Figure 5. Typical Performance Characteristics observed during the operation of this circuit are shown in Figure 6.

FIGURE 4. 12V Flyback Regulator Design Example

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LM2587

Typical Performance Characteristics

1231611

A: Switch Voltage, 10 V/divB: Switch Current, 5 A/divC: Output Rectifier Current, 5 A/divD: Output Ripple Voltage, 100 mV/div

AC-Coupled

Horizontal: 2 μs/div

FIGURE 5. Switching Waveforms

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FIGURE 6. VOUT Load Current Step Response

Typical Flyback Regulator

Applications

Figures 7, 8, 9, 11, 12 show six typical flyback applications,

varying from single output to triple output. Each drawing con-

tains the part number(s) and manufacturer(s) for every com-

ponent except the transformer. For the transformer part

numbers and manufacturers names, see the table in Figure

13. For applications with different output voltages—requiring

the LM2587-ADJ—or different output configurations that do

not match the standard configurations, refer to the Switchers

Made Simple software.

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LM2587

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FIGURE 7. Single-Output Flyback Regulator

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FIGURE 8. Single-Output Flyback Regulator

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LM2587

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FIGURE 9. Single-Output Flyback Regulator

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FIGURE 10. Dual-Output Flyback Regulator

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LM2587

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FIGURE 11. Dual-Output Flyback Regulator

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FIGURE 12. Triple-Output Flyback Regulator

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LM2587

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 T1 T1 T1 T2 T3 T4

VIN 4V–6V 4V–6V 8V–16V 4V–6V 18V–36V 18V–36V

VOUT1 3.3V 5V 12V 12V 12V 5V

IOUT1 (Max) 1.8A 1.4A 1.2A 0.3A 1A 2.5A

N11 1 1 2.5 0.8 0.35

VOUT2 −12V −12V 12V

IOUT2 (Max) 0.3A 1A 0.5A

N2 2.5 0.8 0.8

VOUT3 −12V

IOUT3 (Max) 0.5A

N3 0.8

FIGURE 13. Transformer Selection Table

Transformer

Type

Manufacturers' Part Numbers

Coilcraft Coilcraft (Note 15)Pulse (Note 16)Renco Schott

(Note 15)Surface Mount Surface Mount (Note 17) (Note 18)

T1 Q4434-B Q4435-B PE-68411 RL-5530 67141450

T2 Q4337-B Q4436-B PE-68412 RL-5531 67140860

T3 Q4343-B —PE-68421 RL-5534 67140920

T4 Q4344-B —PE-68422 RL-5535 67140930

Note 15: Coilcraft Inc.,: Phone: (800) 322-2645

1102 Silver Lake Road, Cary, IL 60013: 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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LM2587

TRANSFORMER FOOTPRINTS

Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure

20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25 Figure

26 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 and

Figure 32 show the footprints of each transformer, listed in

Figure 14.

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Top View

FIGURE 15. Coilcraft Q4434-B

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Top View

FIGURE 16. Coilcraft Q4337-B

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Top View

FIGURE 17. Coilcraft Q4343-B

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Top View

FIGURE 18. Coilcraft Q4344-B

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Top View

FIGURE 19. Coilcraft Q4435-B

(Surface Mount)

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Top View

FIGURE 20. Coilcraft Q4436-B

(Surface Mount)

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Top View

FIGURE 21. Pulse PE-68411

(Surface Mount)

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Top View

FIGURE 22. Pulse PE-68412

(Surface Mount)

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LM2587

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Top View

FIGURE 23. Pulse PE-68421

(Surface Mount)

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Top View

FIGURE 24. Pulse PE-68422

(Surface Mount)

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Top View

FIGURE 25. Renco RL-5530

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Top View

FIGURE 26. Renco RL-5531

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Top View

FIGURE 27. Renco RL-5534

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Top View

FIGURE 28. Renco RL-5535

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Top View

FIGURE 29. Schott 67141450

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Top View

FIGURE 30. Schott 67140860

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Top View

FIGURE 31. Schott 67140920

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LM2587

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Top View

FIGURE 32. Schott 67140930

Step-Up (Boost) Regulator

Operation

Figure 33 shows the LM2587 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 LM2587 Boost Regulator works

is as follows (refer to Figure 33). When the NPN switch turns

on, the inductor current ramps up at the rate of VIN/L, storing

energy in the inductor. When the switch turns off, the lower

end of the inductor flies above VIN, discharging its current

through diode (D) into the output capacitor (COUT) at a rate of

(VOUT − 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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By adding a small number of external components (as shown in Figure 33), the LM2587 can be used to produce a regulated output voltage that is greater than

the applied input voltage. The switching waveforms observed during the operation of this circuit are shown in Figure 34. Typical performance of this regulator is

shown in Figure 35.

FIGURE 33. 12V Boost Regulator

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LM2587

Typical Performance Characteristics

1231620

A: Switch Voltage, 10 V/divB: Switch Current, 5 A/divC: Inductor Current, 5 A/divD: Output Ripple Voltage,

100 mV/div, AC-Coupled

Horizontal: 2 μs/div

FIGURE 34. Switching Waveforms

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FIGURE 35. VOUT Response to Load Current Step

Typical Boost Regulator

Applications

Figure 36 and Figures 38, 39 and Figure 40 show four typical

boost applications)—one fixed and three using the adjustable

version of the LM2587. Each drawing contains the part 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 40. For

applications with different output voltages, refer to the

Switchers Made Simple software.

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LM2587

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FIGURE 36. +5V to +12V Boost Regulator

Figure 37 contains a table of standard inductors, by part num-

ber and corresponding manufacturer, for the fixed output

regulator of Figure 36.

Coilcraft

(Note 19)

Pulse

(Note 20)

Renco

(Note 21)

Schott

(Note 22)

R4793-A PE-53900 RL-5472-5 67146520

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 37. Inductor Selection Table

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FIGURE 38. +12V to +24V Boost Regulator

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LM2587

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FIGURE 39. +24V to +36V Boost Regulator

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*The LM2587 will require a heat sink in these applications. The size of the heat sink will depend on the maximum ambient temperature. To calculate the thermal

resistance of the IC and the size of the heat sink needed, see the “Heat Sink/Thermal Considerations” section in the Application Hints.

FIGURE 40. +24V to +48V Boost Regulator

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LM2587

Application Hints

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FIGURE 41. Boost Regulator

PROGRAMMING OUTPUT VOLTAGE

(SELECTING R1 AND R2)

Referring to the adjustable regulator in Figure 41, the output

voltage is programmed by the resistors R1 and R2 by the 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 41), 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 5A.

In a flyback regulator application (Figure 42), using the stan-

dard transformers, the LM2587 will survive a short circuit to

the main output. When the output voltage drops to 80% of its

nominal value, the frequency will drop to 25 kHz. With a lower

frequency, off times are larger. With the longer off times, the

transformer can release all of its stored energy before the

switch turns back on. Hence, the switch turns on initially with

zero current at its collector. In this condition, the switch current

limit will limit the peak current, saving the device.

FLYBACK REGULATOR INPUT CAPACITORS

A flyback regulator draws discontinuous pulses of current

from the input supply. Therefore, there are two input capaci-

tors needed in a flyback regulator; one for energy storage and

one for filtering (see Figure 42). Both are required due to the

inherent operation of a flyback regulator. To keep a stable or

constant voltage supply to the LM2587, a storage capacitor

(≥100 μF) is required. If the input source is a recitified DC

supply and/or the application has a wide temperature range,

the required rms current rating of the capacitor might be very

large. This means a larger value of capacitance or a higher

voltage rating will be needed of the input capacitor. The stor-

age capacitor will also attenuate noise which may interfere

with other circuits connected to the same input supply voltage.

www.national.com 22

LM2587

1231627

FIGURE 42. Flyback Regulator

In addition, a small bypass capacitor is required due to the

noise generated by the input current pulses. To eliminate the

noise, insert a 1.0 μF ceramic capacitor between VIN and

ground as close as possible to the device.

SWITCH 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 42 shows another method of clamping

the switch voltage. A single voltage transient suppressor (the

SA51A) is inserted at the switch pin. This method clamps the

total voltage across the switch, not just the voltage across the

primary.

If poor circuit layout techniques are used (see the “Circuit

Layout Guideline” section), negative voltage transients may

appear on the Switch pin (pin 4). Applying a negative voltage

(with respect to the IC's ground) to any monolithic IC pin

causes erratic and unpredictable operation of that IC. This

holds true for the LM2587 IC as well. When used in a flyback

regulator, the voltage at the Switch pin (pin 4) can go negative

when the switch turns on. The “ringing” voltage at the switch

pin is caused by the output diode capacitance and the 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 42. The values of the resistor and the

capacitor must be chosen so that the voltage at the Switch

pin does not drop below −0.4V. The resistor may range in

value between 10Ω and 1 kΩ, and the capacitor will vary from

0.001 μF to 0.1 μF. Adding a snubber will (slightly) reduce the

efficiency of the overall circuit.

The other method to reduce or eliminate the “ringing” is to

insert a Schottky diode clamp between pins 4 and 3 (ground),

also shown in Figure 42. This prevents the voltage at pin 4

from dropping below −0.4V. The reverse voltage rating of the

diode must be greater than the switch off voltage.

1231628

FIGURE 43. Input Line Filter

23 www.national.com

LM2587

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 LM2587 switch, the output diode(s),

and the transformer—such as reverse recovery time of the

output diode (mentioned above).

NOISY INPUT LINE CONDITION)

A small, low-pass RC filter should be used at the input pin of

the LM2587 if the input voltage has an unusual large amount

of transient noise, such as with an input switch that bounces.

The circuit in Figure 43 demonstrates the layout of the filter,

with the capacitor placed from the input pin to ground and the

resistor placed between the input supply and the input pin.

Note that the values of RIN and CIN shown in the schematic

are good enough for most applications, but some readjusting

might be required for a particular application. If efficiency is a

major concern, replace the resistor with a small inductor (say

10 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.

1231629

FIGURE 44. 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

44). When using the Adjustable version, physically locate the

programming resistors as near the regulator IC as possible,

to keep the sensitive feedback wiring short.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, no heat sink is required to keep the LM2587

junction temperature within the allowed operating range. For

each application, to determine whether or not a heat sink will

be required, the following must be identified:

1) Maximum ambient temperature (in the application).

2) Maximum regulator power dissipation (in the application).

3) Maximum allowed junction temperature (125°C for the

LM2587). For a safe, conservative design, a temperature ap-

proximately 15°C cooler than the maximum junction temper-

ature should be selected (110°C).

www.national.com 24

LM2587

4) LM2587 package thermal resistances θJA and θJC (given

in the Electrical Characteristics).

Total power dissipated (PD) by the LM2587 can be estimated

as follows:

Boost:

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:

Boost:

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. Switchers Made Simple software is available on

a (3½″) diskette for IBM compatable computers from a Na-

tional Semiconductor sales office in your area or the National

Semiconductor Customer Response Center

(1-800-272-9959).

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

LM2587

Physical Dimensions inches (millimeters) unless otherwise noted

Order Number LM2587T-3.3, LM2587T-5.0,

LM2587T-12 or LM2587T-ADJ

NS Package Number T05D

www.national.com 26

LM2587

Order Number LM2587S-3.3, LM2587S-5.0,

LM2587S-12 or LM2587S-ADJ

NS Package Number TS5B

27 www.national.com

LM2587

Notes

LM2587 SIMPLE SWITCHER 5A Flyback Regulator

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