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LT3574 Datasheet(PDF) 7 Page - Linear Technology |
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LT3574 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 24 page  LT3574 3574f operation The LT3574 is a current mode switching regulator IC designed specifically for the isolated flyback topology. The special problem normally encountered in such cir- cuits 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,thishasbeendonewithopto-isola- tors or extra transformer windings. Opto-isolator circuits waste output power and the extra components increase the cost and physical size of the power supply. Opto- isolators can also exhibit trouble due to limited dynamic response, nonlinearity, unit-to-unit variation and aging over life. Circuits employing extra transformer windings also exhibit deficiencies. Using an extra winding adds to the transformer’s physical size and cost, and dynamic response is often mediocre. The LT3574 derives its information about the isolated output voltage by examining the primary side flyback pulse waveform. In this manner, no opto-isolator nor extra transformer winding is required for regulation. The output voltageiseasilyprogrammedwithtworesistors.Sincethis IC operates in boundary control mode, the output voltage is calculated from the switch pin when the secondary cur- rent is almost zero. This method improves load regulation without external resistors and capacitors. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in traditional switching regulators including: internal bias regulator, oscillator, logic, current amplifier and comparator, driver, and output switch. The novel sections include a special flyback error amplifier and a temperature compensation circuit. In addition, the logic system contains additional logic for boundary mode operation, and the sampling error amplifier. The LT3574 features a boundary mode control method, where the part operates at the boundary between continu- ousconductionmodeanddiscontinuousconductionmode. TheVCpincontrolsthecurrentleveljustasitdoesinnormal current mode operation, but instead of turning the switch on at the start of the oscillator period, the part detects when the secondary-side winding current is zero. Boundary Mode Operation Boundary mode is a variable frequency, current mode switching scheme. The switch turns on and the inductor currentincreasesuntilaVCpincontrolledcurrentlimit.The voltage on the SW pin rises to the output voltage divided by the secondary-to-primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls below VIN. A discontinuous conduction mode (DCM) comparator detects this event and turns the switch back on. Boundary mode returns the secondary current to zero every cycle, so the parasitic resistive voltage drops do not cause load regulation errors. Boundary mode also allows the use of a smaller transformer compared to continuous conduction mode and no subharmonic oscillation. At low output currents the LT3574 delays turning on the switch, and thus operates in discontinuous mode. Unlike a traditional flyback converter, the switch has to turn on to update the output voltage information. Below 0.6V on the VC pin, the current comparator level decreases to its minimum value, and the internal oscillator frequency decreases in frequency. With the decrease of the internal oscillator, the part starts to operate in DCM. The output current is able to decrease while still allowing a minimum switch off-time for the error amp sampling circuitry. The typical minimum internal oscillator frequency with VC equal to 0V is 40kHz. |
Similar Part No. - LT3574_15 |
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Similar Description - LT3574_15 |
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