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LM385 feedback provides regulator isolation

Posted: 11 Apr 2001     Print Version  Bookmark and Share



TL H 11049 LM385FeedbackProvidesRegulatorIsolationAN-715 National Semiconductor Application Note 715 Robert Pease Fran Hoffart November 1990 LM385 Feedback Provides Regulator Isolation You can use a conventional 4N27 optocoupler in a feed- back arrangement (Figure 1) to design a switching regulator with a floating output The LM3524 switch-mode-regulator IC is configured as a simple flyback power supply with trans- former-isolated output The LM385 acts as a reference and comparison amplifier that satisifes the 4N27's current de- mands for balancing the dc feedback to pin 2 of the LM3524 thereby closing the loop The LM385 automatically compensates for any LED degradation or optical-coupling lossT he 4N27 specs a 0 1 dc to 1 6 dc gain range the ac gain varies from 0 05 to 1 0 Fortunately the ``Miller'' damper from pins 5 to 6 nullifies the effect of the wide gain varianceM oreover the damper provides excellent loop stability for a wide range of 4N27 optocouplers from several manufactur- ers In the example shown the LM385 provides 0 5 mA to 5 mA to the optocouplerI f the 5V output available from this circuit does not meet your needs choose R2 e R1 (VOUT b 1 25) 1 25 If VOUT is greater than 6V insert a zener diode between points X and Y with VZ e VOUT b 5V this addition prevents the voltage across the LM385 from exceeding its 5 3V max limitT he circuit is suitable for regulated output voltages from 3 2V to 25V T L H 11049-1 L e 15 turns 22 wire wrapped on W resistor bodyD 1 e optional (see text)F IGURE 1 Optocoupler-based feedback circuit produces galvanically-isolated regulated voltages The heavy negative feedback compensates for wide variations in optocoupler gains The values portrayed in this schematic yield 5V output by varying R2 you can obtain voltages from 3 2V to 25VC 1995 National Semiconductor Corporation RRD-B30M75 Printed in U S A AN-715LM385FeedbackProvidesRegulatorIsolation LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein1 Life support devices or systems are devices or 2 A critical component is any component of a life systems which (a) are intended for surgical implant support device or system whose failure to perform can into the body or (b) support or sustain life and whose be reasonably expected to cause the failure of the life failure to perform when properly used in accordance support device or system or to affect its safety or with instructions for use provided in the labeling can effectivenessb e reasonably expected to result in a significant injury to the userN ational Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductores National Semiconductor Corporation GmbH Japan Ltd Hong Kong Ltd Do Brazil Ltda (Australia) Pty Ltd 2900 Semiconductor Drive Livry-Gargan-Str 10 Sumitomo Chemical 13th Floor Straight Block Rue Deputado Lacorda Franco Building 16 P O Box 58090 D-82256 F4urstenfeldbruck Engineering Center Ocean Centre 5 Canton Rd 120-3A Business Park Drive Santa Clara CA 95052-8090 Germany Bldg 7F Tsimshatsui Kowloon Sao Paulo-SP Monash Business Park Tel 1(800) 272-9959 Tel (81-41) 35-0 1-7-1 Nakase Mihama-Ku Hong Kong Brazil 05418-000 Nottinghill Melbourne TWX (910) 339-9240 Telex 527649 Chiba-City Tel (852) 2737-1600 Tel (55-11) 212-5066 Victoria 3168 Australia Fax (81-41) 35-1 Ciba Prefecture 261 Fax (852) 2736-9960 Telex 391-1131931 NSBR BR Tel (3) 558-9999 Tel (043) 299-2300 Fax (55-11) 212-1181 Fax (3) 558-9998 Fax (043) 299-2500 National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications

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