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LT1424-5 Datasheet(PDF) 8 Page - Linear Technology |
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LT1424-5 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1424-5 sn14245 14245fs OPERATION The LT1424-5 is a current mode switching regulator IC that has been designed specifically for the isolated fly- back topology. The special problem normally encoun- tered in such circuits is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation. Historically, this has been done with optoisolators or extra transformer windings. Optoisolator circuits waste output power and the extra components they require increase the cost and physical volume of the power supply. Optoisolators can also exhibit trouble due to limited dynamic response (tempo- ral), nonlinearity, unit-to-unit variation and aging over life. Circuits employing extra transformer windings also exhibit deficiencies. The extra winding adds to the transformer’s physical size and cost. Dynamic response is often mediocre. There is usually no method for main- taining load regulation versus load. The LT1424-5 derives its information about the isolated output voltage by examining the primary side flyback pulse waveform. In this manner no optoisolator nor extra transformer winding is required. This IC is a quantum improvement over previous approaches because: target output voltage is programmed by resistor ratio, regula- tion is maintained well into discontinuous mode and optional load compensation is available. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in tradi- tional designs including: internal bias regulator, oscilla- tor, logic, current amplifier and comparator, driver and output switch. The novel sections include a special flyback error amplifier and a load compensation mecha- nism. Also, due to the special dynamic requirements of flyback control, the logic system contains additional functionality not found in conventional designs. The RREF, RFB and ROCOMP resistors in the Block Diagram are application-specific thin-film resistors internal to the LT1424-5. The capacitor connected to the RCCOMP pin is external. The LT1424-5 operates much the same as traditional current mode switchers, the major difference being a different type of error amplifier which derives its feedback information from the flyback pulse. Due to space con- straints, this discussion will not reiterate the basics of current mode switcher/controllers and isolated flyback converters. A good source of information on these topics is LTC’s Application Note 19. ERROR AMPLIFIER—PSEUDO DC THEORY Please refer to the simplified diagram of the Flyback Error Amplifier. Operation is as follows: when output switch Q4 turns off, its collector voltage rises above the VIN rail. The amplitude of this flyback pulse, i.e., the difference between it and VIN, is given as: VFLBK = VF = D1 forward voltage ISEC = Transformer secondary current ESR = Total impedance of secondary circuit NSP = Transformer effective secondary-to-primary turns ratio VOUT + VF + (ISEC)(ESR) NSP The flyback voltage is then converted to a current by the action of RFB and Q1. Nearly all of this current flows through resistor RREF to form a ground-referred voltage. This is then compared to the internal bandgap reference by the differential transistor pair Q2/Q3. The collector current from Q2 is mirrored around and subtracted from fixed current source IFXD at the VC pin. An external capacitor integrates this net current to provide the control voltage to set the current mode trip point. The relatively high gain in the overall loop will then cause the voltage at the RREF resistor to be nearly equal to the bandgap reference VBG. The relationship between VFLBK and VBG may then be expressed as: VFLBK RFB VFLBK = VBG α = Ratio of Q1 IC to IE VBG = Internal bandgap reference α = or, RFB RREF VBG RREF ) ) 1 α )) |
Similar Part No. - LT1424-5_15 |
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Similar Description - LT1424-5_15 |
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