Datasheet LT3512 (Analog Devices) - 8

LT3512
applicaTions inForMaTion PSEUDO DC THEORY
the effect of nonzero secondary output impedance (ESR). In the Block Diagram, R Boundary control mode minimizes the effect of this im- REF (R4) and RFB (R3) are external resistors used to program the output voltage. The LT3512 pedance term. operates similar to traditional current mode switchers,
Temperature Compensation
except in the use of a unique error amplifier, which derives its feedback information from the flyback pulse. The first term in the VOUT equation does not have tem- perature dependence, but the diode forward drop has a Operation is as follows: when the output switch, Q1, turns significant negative temperature coefficient. A positive off, its collector voltage rises above the VIN rail. The am- temperature coefficient current source connects to the plitude of this flyback pulse, i.e., the difference between R it and V REF pin to compensate. A resistor to ground from the IN, is given as: TC pin sets the compensation current. VFLBK = (VOUT + VF + ISEC • ESR) • NPS The following equation explains the cancellation of the VF = D1 forward voltage temperature coefficient: ISEC = Transformer secondary current dVF R 1 = − FB dV • • TC or, ESR = Total impedance of secondary circuit dT RTC NPS dT N 1 dV R PS = Transformer effective primary-to-secondary turns R • • TC ≈ FB ratio TC = −RFB NPS dVF / dT dT NPS RFB and Q2 convert the flyback voltage into a current. Nearly (dV all of this current flows through R F/dT) = Diode’s forward voltage temperature coefficient REF to form a ground- referred voltage. The resulting voltage forms the input (dVTC/dT) = 2mV to the flyback error amplifier. The flyback error amplifier VTC = 0.55V samples the voltage information when the secondary side winding current is zero. The bandgap voltage, 1.20V, acts Experimentally verify the resulting value of RTC and adjust as as the reference for the flyback error amplifier. necessary to achieve optimal regulation over temperature. The relatively high gain in the overall loop will then cause The addition of a temperature coefficient current modifies the voltage at R the expression of output voltage as follows: REF to be nearly equal to the bandgap reference voltage VBG. The resulting relationship between  RFB   1  V V FLBK and VBG approximately equals: OUT = VBG  RREF  NPS  − VF  VFLBK  VBG  R  or V FB  V  R FLBK = VBG − TC FB –ISEC (ESR)  RFB  = RREF  RREF   RTC  • NPS VBG = Internal bandgap reference
Output Power
Combination of the preceding expression with earlier A flyback converter has a complicated relationship be- derivation of VFLBK results in the following equation: tween the input and output current compared to a buck  R   1  or a boost. A boost has a relatively constant maximum V FB OUT = VBG input current regardless of input voltage and a buck has a  RREF   NPS  − VF −ISEC (ESR) relatively constant maximum output current regardless of The expression defines V input voltage. This is due to the continuous nonswitching OUT in terms of the internal ref- erence, programming resistors, transformer turns ratio behavior of the two currents. A flyback converter has both and diode forward voltage drop. Additionally, it includes discontinuous input and output currents which makes it 3512fb 8 Document Outline Features Applications Description 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 Typical Application Related Parts