Datasheet LT3837 (Analog Devices) - 10

LT3837
OPERATION
The LT3837 is a current mode switcher controller IC amplifier then compares the voltage to the internal bandgap designed specifically for use in an isolated flyback topology reference. The feedback amp is actually a transconductance employing synchronous rectification. The LT3837 operation amplifier whose output is connected to VC only during a is similar to traditional current mode switchers. The major period in the flyback time. An external capacitor on the VC difference is that output voltage feedback is derived via pin integrates the net feedback amp current to provide the sensing the output voltage through the transformer. This control voltage to set the current mode trip point. precludes the need of an optoisolator 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 LT3837 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: See Application Note 19 for more information on the basics of current mode switcher/controllers and isolated  R1+R2 V •V  ( ) flyback converters. OUT = FB •NSF  R2  –ISEC • ESR+RDS(ON)
Feedback Amplifier—Pseudo DC Theory
Rearranging yields the equation for R1. For the following discussion refer to the simplified ⎡ ( ) ⎤ Flyback Feedback Amplifier diagram. When the primary side V R1=R2• OUT +ISEC • ESR+RDS(ON) ⎢ ⎥ MOSFET switch MP turns off, its drain voltage rises above ⎢ N ( SF)(VFB) –1⎥ the V ⎣ ⎦ IN rail. Flyback occurs when the primary MOSFET is off and the synchronous secondary MOSFET is on. Dur- The effect of nonzero secondary output impedance is dis- ing flyback the voltage on nondriven transformer pins is cussed in further detail; see Load Compensation Theory. determined by the secondary voltage. The amplitude of this The practical aspects of applying this equation for VOUT flyback pulse as seen on the third winding is given as: are found in the Applications Information section. V ( )
Feedback Amplifier Dynamic Theory
V OUT + ISEC • ESR + RDS(ON) FLBK = So far, this has been a pseudo-DC treatment of flyback NSF feedback amplifier operation. But the flyback signal is a R pulse, not a DC level. Provision must be made to enable DS(ON) = on resistance of the synchronous MOSFET MS the flyback amplifier only when the flyback pulse is present. ISEC = transformer secondary current This is accomplished by the “Enable” line in the diagram. ESR = impedance of secondary circuit capacitor, winding Timing signals are then required to enable and disable the and traces flyback amplifier. There are several timing signals which N are required for proper LT3837 operation. Please refer to SF = transformer effective secondary-to-feedback winding turns ratio (i.e., N the Timing Diagram. S/NFLBK) The flyback voltage is then scaled by an external resistive divider R1/R2 and presented at the FB pin. The feedback 3837fd 10 Document Outline features applications description Typical Application absolute maximum ratings Pin Configuration order information electrical characteristics typical performance characteristics pin functions block diagram flyback feedback amplifier timing diagram operation applications information typical application Package description Revision History related parts