Datasheet LT1425 (Analog Devices) - 8

LT1425
U OPERATION
Within the dashed lines in the Block Diagram can be found VFLBK α VBG the R = or, REF, RFB and ROCOMP resistors. They are external RFB RREF resistors on the user-programmable LT1425. The capaci- tor connected to the R R 1 CCOMP pin is also external. V FB ) FLBK = VBG R α The LT1425 operates much the same as traditional current REF mode switchers, the major difference being a different α = Ratio of Q1 IC to IE type of error amplifier which derives its feedback informa- VBG = Internal bandgap reference tion from the flyback pulse. Due to space constraints, this discussion will not reiterate the basics of current mode Combination with the previous V switcher/controllers and isolated flyback converters. A FLBK expression yields an expression for V good source of information on these topics is LTC’s OUT, in terms of the internal reference, programming resistors, transformer turns ratio and diode Application Note 19. forward voltage drop:
ERROR AMPLIFIER—PSEUDO DC THEORY
R N V FB ) SP ) OUT = VBG – VF – ISEC (ESR) R α Please refer to the simplified diagram of the Flyback Error REF Amplifier. Operation is as follows: when output switch Q4 Additionally, it includes the effect of nonzero secondary turns off, its collector voltage rises above the VIN rail. The output impedance. See Load Compensation for details. amplitude of this flyback pulse, i.e., the difference between The practical aspects of applying this equation for V it and V OUT are IN, is given as: found in the Applications Information section. V So far, this has been a pseudo-DC treatment of flyback V OUT + VF + (ISEC)(ESR) FLBK = N error amplifier operation. But the flyback signal is a pulse, SP not a DC level. Provision must be made to enable the VF = D1 forward voltage flyback amplifier only when the flyback pulse is present. I SEC = Transformer secondary current This is accomplished by the dashed line connections to the ESR = Total impedance of secondary circuit block labeled “ENABLE.” Timing signals are then required NSP = Transformer effective secondary-to-primary to enable and disable the flyback amplifier. turns ratio The flyback voltage is then converted to a current by the
ERROR AMPLIFIER—DYNAMIC THEORY
action of RFB and Q1. Nearly all of this current flows There are several timing signals that are required for through resistor RREF to form a ground-referred voltage. proper LT1425 operation. Please refer to the Timing This is then compared to the internal bandgap reference by Diagram. the differential transistor pair Q2/Q3. The collector current from Q2 is mirrored around and subtracted from fixed
Minimum Output Switch ON Time
current source IFXD at the VC pin. An external capacitor integrates this net current to provide the control voltage to The LT1425 effects output voltage regulation via flyback set the current mode trip point. pulse action. If the output switch is not turned on at all, there will be no flyback pulse, and output voltage informa- The relatively high gain in the overall loop will then cause tion is no longer available. This would cause irregular loop the voltage at the RREF resistor to be nearly equal to the response and start-up/latchup problems. The solution bandgap reference VBG. (VBG is not present in final output chosen is to require the output switch to be on for an voltage setting equation. See Applications Information absolute minimum time per each oscillator cycle. This in section.) The relationship between VFLBK and VBG may turn establishes a minimum load requirement to maintain then be expressed as: 8