Datasheet LT3825 (Analog Devices) - 10

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