Datasheet LM2585S-3.3, LM2585S-12, LM2585T-ADJ, LM2585T-3.3, LM2585T-12 Datasheet (NSC)

LM2585
SIMPLE SWITCHER

®

3A Flyback Regulator

LM2585 SIMPLE SWITCHER 3A Flyback Regulator

March 1998

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,andadjust­able.

Requiring a minimum number of external components,these
regulators are cost effective, and simple to use. Included in
the datasheet are typical circuits of boostandflybackregula­tors.Alsolistedare 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 reducein-rush current during start up, cur­rent mode control forimproved rejection of input voltage and
output load transients and cycle-by-cycle current limiting.An
output voltage tolerance of
ages and output load conditions, is guaranteedforthepower
supply system.

±

4%, within specified input volt-

Flyback Regulator

Features

n Requires few external components
n Family of standard inductors and transformers
n NPN output switches 3.0A, can stand off 65V
n Wide input voltage range: 4V to 40V
n Current-mode operation for improved transient

response, line regulation, and current limit

n 100 kHz switching frequency
n Internal soft-start function reduces in-rush current during

start-up

n Output transistor protected by current limit, under

voltage lockout, and thermal shutdown

n System Output Voltage Tolerance of

and load conditions

±

4%max over line

Typical Applications

n Flyback regulator
n Multiple-output regulator
n Simple boost regulator
n Forward converter

DS012515-1

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,

SIMPLE SWITCHER and

© 1999 National Semiconductor Corporation DS012515 www.national.com

Switchers Made Simple

™

are registered trademarks of National SemiconductorCorporation.

LM2585SX-ADJ

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 ≤ V
Switch Voltage −0.4V ≤ V
Switch Current (Note 2) Internally Limited
Compensation Pin Voltage −0.4V ≤ V
Feedback Pin Voltage −0.4V ≤ V
Storage Temperature Range −65˚C to +150˚C
Lead Temperature

(Soldering, 10 sec.) 260˚C

COMP

FB

IN

SW

≤ 2.4V

≤ 2V

≤ 45V
≤ 65V

OUT

Maximum Junction Temperature

(Note 3) 150˚C
Power Dissipation (Note 3) Internally Limited
Minimum ESD Rating

(C=100 pF, R=1.5 kΩ)2kV

Operating Ratings

Supply Voltage 4V ≤ VIN≤ 40V
Output Switch Voltage 0V ≤ V
Output Switch Current I
Junction Temperature Range −40˚C ≤ T

≤ 60V

SW

≤ 3.0A

SW

≤ +125˚C

J

Electrical Characteristics
LM2585-3.3

Specifications with standard type face are for T
Range. Unless otherwise specified, V

IN

Symbol Parameters Conditions Typical Min Max Units

SYSTEM PARAMETERS Test Circuit of

V

OUT

∆V

OUT

∆V

IN

∆V

OUT

∆I

LOAD

η Efficiency V

Output Voltage V

/ Line Regulation V

/ Load Regulation V

I

LOAD

I

LOAD

I

LOAD

IN

IN

IN

IN

UNIQUE DEVICE PARAMETERS (Note 5)
V

∆V

REF

REF

Output Reference Measured at Feedback Pin 3.3 3.242/3.234 3.358/3.366 V
Voltage V
Reference Voltage V

COMP
IN

Line Regulation

G

M

A

VOL

Error Amp I
Transconductance V
Error Amp V
Voltage Gain R

COMP

COMP
COMP
COMP

=

25˚C, and those in bold type face apply over full Operating Temperature

J

=

5V.

Figure 2

(Note 4)

=

4V to 12V 3.3 3.17/3.14 3.43/3.46 V
=

0.3A to 1.2A

=

4V to 12V 20 50/100 mV
=

0.3A

=

12V 20 50/100 mV
=

0.3A to 1.2A

=

5V, I

=

4V to 40V 2.0 mV

=

=

0.3A 76

LOAD

=

1.0V

−30 µA to +30 µA 1.193 0.678 2.259 mmho

=

1.0V

=

0.5V to 1.6V 260 151/75 V/V

=

1.0 MΩ (Note 6)

%

LM2585-5.0

Symbol Parameters Conditions Typical Min Max Units

SYSTEM PARAMETERS Test Circuit of

V

∆V
∆V
∆V
∆I

OUT

OUT
IN
OUT

LOAD

Output Voltage V

/ Line Regulation V

/ Load Regulation V

η Efficiency V
UNIQUE DEVICE PARAMETERS (Note 5)
V

REF

Output Reference Measured at Feedback Pin 5.0 4.913/4.900 5.088/5.100 V
Voltage V

∆V

REF

Reference Voltage V
Line Regulation

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Figure 2

(Note 4)

=

4V to 12V 5.0 4.80/4.75 5.20/5.25 V

IN

=

I

0.3A to 1.1A

LOAD

=

4V to 12V 20 50/100 mV

IN

=

I

0.3A

LOAD

=

12V 20 50/100 mV

IN

=

I

0.3A to 1.1A

LOAD

=

12V, I

IN

=

COMP

=

4V to 40V 3.3 mV

IN

1.0V

=

0.6A 80

LOAD

%

LM2585-5.0 (Continued)

Symbol Parameters Conditions Typical Min Max Units

UNIQUE DEVICE PARAMETERS (Note 5)

G

M

Error Amp I
Transconductance V

A

VOL

Error Amp V
Voltage Gain R

=

−30 µA to +30 µA 0.750 0.447 1.491 mmho

COMP

=

1.0V

COMP

=

0.5V to 1.6V 165 99/49 V/V

COMP

=

1.0 MΩ (Note 6)

COMP

LM2585-12

Symbol Parameters Conditions Typical Min Max Units

SYSTEM PARAMETERS Test Circuit of

V

∆V
∆V
∆V
∆I

OUT

OUT
IN
OUT

LOAD

Output Voltage V

/ Line Regulation V

/ Load Regulation V

η Efficiency V
UNIQUE DEVICE PARAMETERS (Note 5)
V

REF

Output Reference Measured at Feedback Pin 12.0 11.79/11.76 12.21/12.24 V
Voltage V

∆V

REF

Reference Voltage V
Line Regulation

G

M

Error Amp I
Transconductance V

A

VOL

Error Amp V
Voltage Gain R

Figure 3

(Note 4)

=

4V to 10V 12.0 11.52/11.40 12.48/12.60 V

IN

=

I

0.2A to 0.8A

LOAD

=

4V to 10V 20 100/200 mV

IN

=

I

0.2A

LOAD

=

10V 20 100/200 mV

IN

=

I

0.2A to 0.8A

LOAD

=

10V, I

IN

=

COMP

=

4V to 40V 7.8 mV

IN

=

COMP

=

COMP

=

COMP

=

COMP

=

0.6A 93

LOAD

1.0V

−30 µA to +30 µA 0.328 0.186 0.621 mmho

1.0V

0.5V to 1.6V 70 41/21 V/V

1.0 MΩ (Note 6)

%

LM2585-ADJ

Symbol Parameters Conditions Typical Min Max Units

SYSTEM PARAMETERS Test Circuit of

V

∆V
∆V
∆V
∆I

OUT

OUT
IN
OUT

LOAD

Output Voltage V

/ Line Regulation V

/ Load Regulation V

η Efficiency V
UNIQUE DEVICE PARAMETERS (Note 5)
V

REF

Output Reference Measured at Feedback Pin 1.230 1.208/1.205 1.252/1.255 V
Voltage V

∆V

REF

Reference Voltage V
Line Regulation

G

M

Error Amp I
Transconductance V

A

VOL

Error Amp V
Voltage Gain R

I

B

Error Amp V
Input Bias Current

Figure 3

(Note 4)

=

4V to 10V 12.0 11.52/11.40 12.48/12.60 V

IN

=

I

0.2A to 0.8A

LOAD

=

4V to 10V 20 100/200 mV

IN

=

I

0.2A

LOAD

=

10V 20 100/200 mV

IN

=

I

0.2A to 0.8A

LOAD

=

10V, I

IN

=

COMP

=

4V to 40V 1.5 mV

IN

=

COMP

=

COMP

=

COMP

=

COMP

=

COMP

=

0.6A 93

LOAD

1.0V

−30 µA to +30 µA 3.200 1.800 6.000 mmho

1.0V

0.5V to 1.6V 670 400/200 V/V

1.0 MΩ (Note 6)

1.0V 125 425/600 nA

%

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Electrical Characteristics (All Versions)

Symbol Parameters Conditions Typical Min Max Units

COMMON DEVICE PARAMETERS for all versions (Note 5)

I

S

V

UV

f

O

f

SC

V

EAO

I

EAO

I

SS

D Maximum Duty R

I

L

V

SUS

V

SAT

I

CL

θ

JA

θ

JA

θ

JC

θ

JA

θ

JA

θ

JA

θ

JC

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).

Input Supply (Switch Off) 11 15.5/16.5 mA
Current (Note 8)

=

Input Supply R

I

SWITCH

1.8A 50 100/115 mA

=

100Ω 3.30 3.05 3.75 V

LOAD

Undervoltage
Lockout

Oscillator Frequency Measured at Switch Pin

=

R

100Ω 100 85/75 115/125 kHz

LOAD

=

V

1.0V

COMP

Short-Circuit Measured at Switch Pin
Frequency R

=

100Ω 25 kHz

LOAD

V

FEEDBACK

=

1.15V
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)
Error Amp (Note 9)
Output Current 165 110/70 260/320 µA
(Source or Sink)
Soft Start Current V

FEEDBACK

V

COMP

LOAD

=

0.92V 11.0 8.0/7.0 17.0/19.0 µA

=

1.0V

=

100Ω 98 93/90

%
Cycle (Note 7)
Switch Leakage Switch Off 15 300/600 µA
Current V

SWITCH

=

60V
Switch Sustaining dV/dT=1.5V/ns 65 V
Voltage
Switch Saturation I

SWITCH

=

3.0A 0.45 0.65/0.9 V
Voltage
NPN Switch 4.0 3.0 7.0 A
Current Limit
Thermal Resistance T Package, Junction to

65

Ambient (Note 10)
T Package, Junction to

45

Ambient (Note 11)
T Package, Junction to

2

Case
S Package, Junction to

Ambient (Note 12)
S Package, Junction to

56

˚C/W

35

Ambient (Note 13)
S Package, Junction to

26

Ambient (Note 14)
S Package, Junction to

2

Case

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Electrical Characteristics (All Versions) (Continued)

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

dissipation of the device (P

. For a safe thermal design, check that the maximum power dissipated by the device is less than: PD≤ [T

(MAX)

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

ity 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 measuringA
Note 7: Tomeasure 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: V

1.05V; 3.3V: V

FB

Note 8: Tomeasure 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: V

1.41V; 3.3V: V

FB

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
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
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 leadTO-263 mounted horizontally against a PC board areaof 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

software.

Simple

). A thermal shutdown will occur if the temperature exceedsthe maximum junction temperature of the device: PDx θJA+T

Figure 2

D

and

Figure 3

=

2.81V; 5.0V: V

FB

=

3.80V; 5.0V: V

FB

, system performance will be as specified by the system parameters.

=

FB

=

FB

4.25V; 12V: V

5.75V; 12V: V

=

10.20V.

FB

=

13.80V.

FB

J(MAX)−TA(MAX)

)]/θJA. When calculating the maximum

1

1

⁄2inch leads soldered to a PC board

≥ T

A(MAX)

.

VOL

⁄2inch leads in a socket, or on a PC

Switchers Made

Typical Performance Characteristics

-

J

Supply Current
vs Temperature

Supply Current
vs Switch Current

DS012515-2

DS012515-5

Reference Voltage
vs Temperature

Current Limit
vs Temperature

DS012515-3

DS012515-6

∆Reference Voltage
vs Supply Voltage

DS012515-4

Feedback Pin Bias
Current vs Temperature

DS012515-7

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Typical Performance Characteristics (Continued)

Switch Saturation
Voltage vs Temperature

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Error Amp Transconductance
vs Temperature

DS012515-11

Connection Diagrams

Switch Transconductance
vs Temperature

Error Amp Voltage
Gain vs Temperature

DS012515-9

DS012515-12

Oscillator Frequency
vs Temperature

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Short Circuit Frequency
vs Temperature

DS012515-13

Bent, Staggered Leads

5-Lead TO-220 (T)

Top View

DS012515-14

Order Number LM2585T-3.3, LM2585T-5.0,

LM2585T-12 or LM2585T-ADJ

See NS Package Number T05D

5-Lead TO-263 (S)

Top View

DS012515-16

Order Number LM2585S-3.3, LM2585S-5.0,

LM2585S-12 or LM2585S-ADJ

See NS Package Number TS5B

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Bent, Staggered Leads

5-Lead TO-220 (T)

Side View

DS012515-15

5-Lead TO-263 (S)

Side View

DS012515-17

Block Diagram

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

DS012515-18

FIGURE 1.

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Test Circuits

C

— 100 µF, 25V Aluminum Electrolytic

IN1

— 0.1 µF Ceramic

C

IN2

T— 22 µH, 1:1 Schott
D— 1N5820

— 680 µF, 16V Aluminum Electrolytic

C

OUT

— 0.47 µF Ceramic

C

C

—2k

R

C

C

— 100 µF, 25V Aluminum Electrolytic

IN1

— 0.1 µF Ceramic

C

IN2

L— 15 µH, Renco
D— 1N5820

— 680 µF, 16V Aluminum Electrolytic

C

OUT

— 0.47 µF Ceramic

C

C

—2k

R

C

For 12V Devices: R
For ADJ Devices: R

#

67141450

#

RL-5472-5

=

Short (0Ω) and R

1

=

48.75k,

1

=

Open

2

±

0.1%and R2=5.62k,±1

DS012515-19

FIGURE 2. LM2585-3.3 and LM2585-5.0

DS012515-20

%

FIGURE 3. LM2585-12 and LM2585-ADJ

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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
put voltages. In

Figure 4

, the flyback regulator generates an
output voltage that is inside the range of the input voltage.
This feature is unique to flyback regulators and cannot be
duplicated with buck or boost regulators.

The operation of a flyback regulator is as follows (refer to

Figure 4

): when the switch is on, current flows through the
primary winding of the transformer, T1, storing energy in the
magnetic field of the transformer. Note that the primary and
secondary windings are out of phase, so no current flows
through the secondary when current flows through the pri­mary. When the switch turns off, the magnetic field col-

Figure 4

, or multiple out-

lapses, reversing the voltage polarityof theprimary 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 voltageand a 1.230Vreference.
The error amp output voltage is compared to a rampvoltage
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.

As shown in
regulator are shown in

A: Switch Voltage, 20 V/div
B: Switch Current, 2 A/div
C: Output Rectifier Current, 2A/div
D: Output Ripple Voltage, 50 mV/div
AC-Coupled

Horizontal: 2 µs/div

Figure 4

, the LM2585 can be used as a flyback regulator by using a minimum number of external components. The switching waveforms of this

Figure 5

. Typical Performance Characteristics observed during the operation of this circuit are shown in

FIGURE 4. 12V Flyback Regulator Design Example

DS012515-22

FIGURE 5. Switching Waveforms

DS012515-21

Figure 6

.

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Flyback Regulator Operation (Continued)

FIGURE 6. V

Load Current Step Response

OUT

Typical Flyback Regulator Applications

Figure 7

through

tions, varying from single output to triple output. Each draw-

Figure 12

ing 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

show six typical flyback applica-

DS012515-23

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.

FIGURE 7. Single-Output Flyback Regulator

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DS012515-24

Typical Flyback Regulator Applications (Continued)

FIGURE 8. Single-Output Flyback Regulator

DS012515-25

FIGURE 9. Single-Output Flyback Regulator

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Typical Flyback Regulator Applications (Continued)

FIGURE 10. Dual-Output Flyback Regulator

DS012515-27

FIGURE 11. Dual-Output Flyback Regulator

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DS012515-28

Typical Flyback Regulator Applications (Continued)

FIGURE 12. Triple-Output Flyback Regulator

DS012515-29

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Typical Flyback Regulator Applications (Continued)

Transformer Selection (T)

Figure 13

each transformer, as well as the output voltages, input voltage ranges, and the maximum load currents for each circuit.

lists the standard transformers available forflyback regulator applications.Included in thetable are theturns ratio(s) for

Applications

Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12

Transformers T7 T7 T7 T6 T6 T5

V

IN

V

OUT1

I

(Max) 1.4A 1A 0.8A 0.15A 0.6A 1.8A

OUT1

N

1

V

OUT2

I

(Max) 0.15A 0.6A 0.25A

OUT2

N

2

V

OUT3

I

(Max) 0.25A

OUT3

N

3

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

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

1 1 1 1.2 1.2 0.5

−12V −12V 12V

1.2 1.2 1.15

FIGURE 13. Transformer Selection Table

Trans-

former

Type

Coilcraft Coilcraft Pulse Pulse Renco Schott

(Note 15) (Note 15) (Note 16) (Note 16) (Note 17) (Note 18)

Manufacturers’ Part Numbers

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: [phone]
1102 Silver Lake Road, Cary, IL 60013: Fax: [phone]
Note 16: Pulse Engineering Inc. Phone: [phone]
12220 World Trade Drive, San Diego, CA92128: Fax: [phone]
Note 17: Renco Electronics Inc. Phone: [phone]
60 Jeffryn Blvd. East, Deer Park, NY 11729: Fax: [phone]
Note 18: Schott Corp. Phone: [phone]
1000 Parkers Lane Road, Wayzata, MN 55391: Fax: [phone]

FIGURE 14. Transformer Manufacturer Guide

−12V

1.15

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Typical Flyback Regulator Applications (Continued)

Transformer Footprints

Figure 15

through

Figure 29

show the footprints of each transformer, listed in

Figure 14

.

T7

Top View

FIGURE 15. Coilcraft S6000-A

T6

Top View

FIGURE 16. Coilcraft Q4339-B

T5

T7

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DS012515-34

Top View

FIGURE 19. Coilcraft S6057-A

(Surface Mount)

T6

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Top View

FIGURE 20. Coilcraft Q4438-B

(Surface Mount)

T7

Top View

FIGURE 17. Coilcraft Q4437-B

(Surface Mount)

T5

Top View

FIGURE 18. Coilcraft Q4338-B

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DS012515-36

Top View

FIGURE 21. Pulse PE-68482

T6

DS012515-37

Top View

FIGURE 22. Pulse PE-68414

(Surface Mount)

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Typical Flyback Regulator
Applications

(Continued)

T5

Top View

FIGURE 23. Pulse PE-68413

(Surface Mount)

T7

Top View

FIGURE 24. Renco RL-5751

T6

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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DS012515-42

Top View

FIGURE 25. Renco RL-5533

T5

DS012515-43

Top View

FIGURE 26. Renco RL-5532

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DS012515-47

Top View

FIGURE 29. Schott 67140890

Step-Up (Boost) Regulator Operation

Figure 30

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
switch turns on, the inductor current ramps up at the rate of
V

IN

shows the LM2585 used as a step-up (boost)

Figure 30

). When the NPN

/L, storing energy in the inductor. When the switch turns

off, the lower end ofthe inductorflies above V
its current through diode (D)into theoutput capacitor (C
at a rate of (V
tor during theswitch on time is transferred to the output dur-

)/L. Thus, energy storedin theinduc-

OUT−VIN

, discharging

IN

OUT

ing the switch off time. The output voltage is controlled by
adjusting the peak switch current, as described in the flyback
regulator section.

)

By adding a small number of external components (as shown in
the applied input voltage. The switching waveforms observed during the operation of this circuit are shown in
shown in

Figure 32

.

Figure 30

), the LM2585 can be used to produce a regulated output voltage that is greater than

FIGURE 30. 12V Boost Regulator

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

DS012515-49

FIGURE 31. Switching Waveforms

DS012515-48

Figure 31

. Typical performance of this regulator is

FIGURE 32. V

Response to Load Current Step

OUT

DS012515-50

www.national.com17

Typical Boost Regulator Applications

Figure 33

and

Figure 35

through

Figure 37

show four typical
boost applications)—one fixed and three using the adjust­able version of the LM2585. Each drawing contains the part
number(s) and manufacturer(s) for every component. For

FIGURE 33. +5V to +12V Boost Regulator

Figure 34

of

contains a table of standard inductors, by part number and corresponding manufacturer, for the fixed output regulator

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: [phone]
1102 Silver Lake Road, Cary, IL 60013 Fax: [phone]
Note 20: Pulse Engineering Inc. Phone: [phone]
12220 World Trade Drive, San Diego, CA92128 Fax: [phone]
Note 21: Renco Electronics Inc. Phone [phone]
60 Jeffryn Blvd. East, Deer Park, NY 11729 Fax: [phone]
Note 22: Schott Corp. Phone: [phone]
1000 Parkers Lane Road, Wayzata, MN 55391 Fax: [phone]

FIGURE 34. Inductor Selection Table

the fixed 12V output application, the part numbers and
manufacturers’ names for the inductor are listedin a table in

Figure 34

. For applications with different output voltages, re-

fer to the

Switchers Made Simple

software.

DS012515-51

FIGURE 35. +12V to +24V Boost Regulator

www.national.com 18

DS012515-52

Typical Boost Regulator Applications (Continued)

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

DS012515-53

*

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.

DS012515-54

FIGURE 37. +24V to +48V Boost Regulator

www.national.com19

Application Hints

FIGURE 38. Boost Regulator

PROGRAMMING OUTPUT VOLTAGE
(SELECTING R

Referring to theadjustable regulator in
voltage is programmed bythe resistorsR
lowing formula:

V

OUT

Resistors R
it can be compared with the 1.23V internal reference. With
R

between 1k and 5k, R1is:

2

=

R

1

For best temperature coefficient and stability with time, use
1%metal film resistors.

SHORT CIRCUIT CONDITION

Due to the inherent natureof boost regulators,when the out­put is shorted (see
input, through the inductor and the diode, to the output, by-

AND R2)

1

=

(1+R1/R2) where V

V

REF

and R2divide the output voltage down so that

1

R

2(VOUT/VREF

− 1) where V

Figure 38

), current flows directlyfrom the

Figure 38

, the output

and R2by the fol-

1

=

REF

=

REF

1.23V

1.23V

DS012515-55

passing the switch. The current limit of the switch

does not

limit the output current for the entire circuit. To protect the
load and prevent damage to the switch, the current must be
externally limited, either by the input supply or at the output
with an external current limitcircuit. Theexternal 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 outputvoltage drops to80%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 turnsback on.Hence, theswitch turnson ini­tially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the
device.

FIGURE 39. Flyback Regulator

www.national.com 20

DS012515-56

Application Hints (Continued)

FLYBACK REGULATOR INPUT CAPACITORS

A flyback regulator draws discontinuous pulses of current
from the input supply.Therefore, there are twoinput capaci­tors needed in a flyback regulator; one for energy storage
and one for filtering (see
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. Ifthe 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 inputcurrent pulses.Toeliminate the
noise, insert a 1.0 µF ceramic capacitor between V
ground as close as possible to the device.

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-statevoltage ap­pearing at theswitch, when it is off, isset by the transformer
turns ratio, N, the output voltage, V
put voltage, V

where V
and is 0.5V for Schottkydiodes and0.8V forultra-fast recov-

IN

V

SW(OFF)

is the forward biased voltage of the output diode,

F

ery diodes (typically). In certain circuits, there exists a volt­age spike, V
age (see

, superimposed on top of the steady-state volt-

LL

Figure 5

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 inthe circuiton thefront pageand
other flyback regulator circuits throughout the datasheet).
The schematic in
ing 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 aswell. When usedin 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 capacitanceand the
transformer leakage inductance forminga 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. Oneis toadd anRC snubberaround the
output rectifier(s), as in
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 between10Ω and1kΩ, andthe capacitorwill
vary from 0.001µF to 0.1 µF.Adding a snubber will (slightly)
reduce the efficiency of the overall circuit.

The other method to reduce or eliminatethe “ringing” is to in­sert a Schottky diode clamp between pins 4 and 3 (ground),

Figure 39

). Both are required due to

IN

, and the maximum in-

(Max):

=

V

IN

OUT

(Max) + (V

OUT+VF

)/N

, waveformA). Usually, this voltage spike is

Figure 39

shows another method of clamp-

Figure 39

. The values of the resistor

and

also shown in

Figure 39

. This prevents the voltage at pin 4
from dropping below −0.4V.The reversevoltage 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
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:

V

≈ NxVINxD/(1−D)

OUT

The duty cycle of a flyback regulator is determined bythe fol­lowing equation:

Theoretically, the maximum output voltage can be as large
as desired— just keep increasing the turnsratio of thetrans­former. 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).

DS012515-57

FIGURE 40. Input Line Filter

NOISY INPUT LINE CONDITION

A small, low-pass RC filtershould be used at the inputpin of
the LM2585 if the input voltage hasan unusual largeamount
of transient noise, such as with aninput switch thatbounces.
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 R
matic are good enough for most applications, but some read-

and CINshown in the sche-

IN

justing 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 V
found in the Characteristic Curves.

is the switch saturation voltage and can be

SAT

www.national.com21

Application Hints (Continued)

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 thelength 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
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 sinkis required to keep the LM2585
junction temperature within theallowed operating range. For
each application, to determine whetheror not aheat 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 θ
in the Electrical Characteristics).

Total power dissipated (P
as follows:

VINis the minimum input voltage,V
N is the transformer turns ratio, D is the duty cycle, and I
is the maximum load current (and∑I
maximum load currents for multiple-output flyback regula­tors). The duty cycle is given by:

Figure 41

). When using the Adjustable version,

and θJC(given

JA

) by the LM2585 can be estimated

D

is the output voltage,

OUT

LOAD

is the sum of the

LOAD

DS012515-58

where VFis the forward biased voltage of the diode and is
typically 0.5V for Schottky diodes and 0.8V for fast recovery
diodes. V
found in the Characteristic Curves.

is the switch saturation voltage and can be

SAT

When no heat sink is used, the junction temperature rise is:

=

∆T

x θJA.

P

J

D

Adding the junction temperature rise to the maximum ambi­ent temperature gives the actualoperating junction tempera­ture:

=

T

J

∆T

J+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:

=

∆T

x(θJC+ θ

P

J

D

Interface

+ θ

Heat Sink

)

Again, the operating junction temperature will be:

=

T

∆T

J

J+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 resistancerequired to maintainthe
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

1

ona3

⁄2" diskette for IBM compatible computers from a Na­tional Semiconductor sales office in your area or the National
Semiconductor Customer Response Center
(1-800-272-9959).

www.national.com 22

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: [phone]
Fax: [phone]

Pulse Engineering

Dunmore Road
Tuam
Co. Galway, Ireland
Phone: [phone]
Fax: [phone]

www.national.com23

Physical Dimensions inches (millimeters) unless otherwise noted

Order Number LM2585T-3.3, LM2585T-5.0,

LM2585T-12 or LM2585T-ADJ

NS Package Number T05D

www.national.com 24

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

LM2585 SIMPLE SWITCHER 3A Flyback Regulator

Order Number LM2585S-3.3, LM2585S-5.0,

LM2585S-12 or LM2585S-ADJ

NS Package Number TS5B

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1. Life support devices or systems are devices or
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whose failure to perform when properly used in
accordance with instructions for use provided in the

2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

labeling, can be reasonably expected to result in a
significant injury to the user.

National Semiconductor
Corporation

Americas
Tel: [phone]
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right atany time without notice to change said circuitry and specifications.

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