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LT3574 Datasheet(PDF) 8 Page - Linear Technology |
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LT3574 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3574 3574f ERROR AMPLIFIER—PSEUDO DC THEORY In the Block Diagram, the RREF (R4) and RFB (R3) resistors can be found. They are external resistors used to program theoutputvoltage.TheLT3574operatesmuchthesameway astraditionalcurrentmodeswitchers,themajordifference being a different type of error amplifier which derives its feedback information from the flyback pulse. Operation is as follows: when the output switch, Q1, turns off, its collector voltage rises above the VIN rail. The am- plitude of this flyback pulse, i.e., the difference between it and VIN, is given as: VFLBK = (VOUT + VF + ISEC • ESR) • NPS VF = D1 forward voltage ISEC = Transformer secondary current ESR = Total impedance of secondary circuit NPS = Transformer effective primary-to-secondary turns ratio The flyback voltage is then converted to a current by the action of RFB and Q2. Nearly all of this current flows through resistor RREF to form a ground-referred voltage. This voltage is fed into the flyback error amplifier. The flyback error amplifier samples this output voltage infor- mation when the secondary side winding current is zero. The error amplifier uses a bandgap voltage, 1.23V, as the reference voltage. The relatively high gain in the overall loop will then cause the voltage at the RREF resistor to be nearly equal to the bandgap reference voltage VBG. The relationship between VFLBK and VBG may then be expressed as: a V R V R or V V R R FLBK FB BG REF FLBK BG FB REF = = , 1 a a = Ratio of Q1 IC to IE, typically ≈0.986 VBG = Internal bandgap reference applications inForMation In combination with the previous VFLBK expression yields an expression for VOUT, in terms of the internal reference, programming resistors, transformer turns ratio and diode forward voltage drop: V V R R N V I ES OUT BG FB REF PS F SEC = − − 1 a ( RR) Additionally, it includes the effect of nonzero secondary output impedance (ESR). This term can be assumed to be zero in boundary control mode. More details will be discussed in the next section. Temperature Compensation The first term in the VOUT equation does not have a tem- perature dependence, but the diode forward drop has a significant negative temperature coefficient. To compen- sate for this, a positive temperature coefficient current source is connected to the RREF pin. The current is set by a resistor to ground connected to the TC pin. To cancel the temperature coefficient, the following equation is used: d d d d d V T R R N V T or R R N V F FB TC PS TC TC FB PS = − = − • • , • 1 1 FF TC FB PS T V T R N / • d d d ≈ ( dVF/dT) = Diode’s forward voltage temperature coefficient ( dVTC/dT) = 2mV VTC = 0.55V The resistor value given by this equation should also be verifiedexperimentally,andadjustedifnecessarytoachieve optimal regulation over temperature. The revised output voltage is as follows: V V R R N V V R OUT BG FB REF PS F TC TC = − − 1 a • – ( ) R N I ESR FB PS SEC a |
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