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LT3825EFE-TR Datasheet(PDF) 10 Page - Linear Technology |
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LT3825EFE-TR Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  LT3825 10 3525fa OPERATION The LT3825 is a current mode switcher controller IC de- signed specifically for use in an isolated flyback topology employing synchronous rectification. The LT3825 operation is similar to traditional current mode switchers. The major difference is that output voltage feedback is derived via sensing the output voltage through the transformer. This precludes the need of an optoisolator in isolated designs greatly improving dynamic response and reliability. The LT3825 has a unique feedback amplifier that samples a transformer winding voltage during the flyback period and uses that voltage to control output voltage. The internal blocks are similar to many current mode controllers. The differences lie in the flyback feedback amplifier and load compensation circuitry. The logic block also contains circuitry to control the special dynamic requirements of flyback control. For more information on the basics of current mode switcher/controllers and isolated flyback converters see Application Note 19. Feedback Amplifier—Pseudo DC Theory For the following discussion refer to the simplified Fly- back Feedback Amplifier diagram. When the primary side MOSFET switch MP turns off, its drain voltage rises above the VIN rail. Flyback occurs when the primary MOSFET is off and the synchronous secondary MOSFET is on. Dur- ing flyback the voltage on nondriven transformer pins is determined by the secondary voltage. The amplitude of this flyback pulse as seen on the third winding is given as: V VI ESR R N FLBK OUT SEC DS ON SF = ++ () • () RDS(ON) = on resistance of the synchronous MOSFET MS ISEC = transformer secondary current ESR = impedance of secondary circuit capacitor, winding and traces NSF=transformereffectivesecondary-to-feedbackwinding turns ratio (i.e., NS/NFLBK) The flyback voltage is scaled by an external resistive divider R1/R2 and presented at the FB pin. The feedback amplifier compares the voltage to the internal bandgap reference. The feedback amp is actually a transconductance ampli- fier whose output is connected to VC only during a period in the flyback time. An external capacitor on the VC pin integrates the net feedback amp current to provide the control voltage to set the current mode trip point. The regulation voltage at the FB pin is nearly equal to the bandgap reference VFB because of the high gain in the overall loop. The relationship between VFLBK and VFB is expressed as: V RR R V FLBK FB = + 12 2 • Combining this with the previous VFLBK expression yields an expression for VOUT in terms of the internal reference, programming resistors and secondary resistances: VOUT = R1+ R2 R2 •VFB •NSF –ISEC •ESR +RDS(ON) () The effect of nonzero secondary output impedance is dis- cussed in further detail; see Load Compensation Theory. The practical aspects of applying this equation for VOUT are found in the Applications Information. Feedback Amplifier Dynamic Theory So far, this has been a pseudo-DC treatment of flyback feedback amplifier operation. But the flyback signal is a pulse, not a DC level. Provision must be made to enable the flyback amplifier only when the flyback pulse is present. This is accomplished by the “Enable” line in the diagram. Timing signals are then required to enable and disable the flyback amplifier. There are several timing signals which are required for proper LT3825 operation. Please refer to the Timing Diagram. Minimum Output Switch On Time (tON(MIN)) The LT3825 affects output voltage regulation via flyback pulse action. If the output switch is not turned on, there is no flyback pulse and output voltage information is not available. This causes irregular loop response and start-up/latch-up problems. The solution is to require the primary switch to be on for an absolute minimum time per each oscillator cycle. If the output load is less than |
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