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LT3512EMSPBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3512EMSPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3512 8 3512f PSUEDO DC THEORY In the Block Diagram, RREF (R4) and RFB (R3) are external resistors used to program the output voltage. The LT3512 operates similar to traditional current mode switchers, except in the use of a unique 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 RFBandQ2converttheflybackvoltageintoacurrent.Nearly all of this current flows through RREF to form a ground- referred voltage. The resulting voltage forms the input to the flyback error amplifier. The flyback error amplifier samples the voltage information when the secondary side winding current is zero. The bandgap voltage, 1.20V, acts as the reference for the flyback error amplifier. The relatively high gain in the overall loop will then cause the voltage at RREF to be nearly equal to the bandgap reference voltage VBG. The resulting relationship between VFLBK and VBG approximately equals: VFLBK RFB ⎛ ⎝⎜ ⎞ ⎠⎟ = VBG RREF or VFLBK = VBG RFB RREF ⎛ ⎝⎜ ⎞ ⎠⎟ VBG = Internal bandgap reference Combination of the preceding expression with earlier derivation of VFLBK results in the following equation: VOUT = VBG RFB RREF ⎛ ⎝⎜ ⎞ ⎠⎟ 1 NPS ⎛ ⎝⎜ ⎞ ⎠⎟ − VF −ISEC (ESR) The expression defines VOUT in terms of the internal ref- erence, programming resistors, transformer turns ratio and diode forward voltage drop. Additionally, it includes APPLICATIONS INFORMATION the effect of nonzero secondary output impedance (ESR). Boundary control mode minimizes the effect of this im- pedance term. Temperature Compensation The first term in the VOUT equation does not have tem- perature dependence, but the diode forward drop has a significant negative temperature coefficient. A positive temperature coefficient current source connects to the RREF pin to compensate. A resistor to ground from the TC pin sets the compensation current. The following equation explains the cancellation of the temperature coefficient: δVF δT =− RFB RTC • 1 NPS • δVTC δT or, RTC = −RFB NPS • 1 δVF / δT • δVTC δT ≈ RFB NPS ( δVF/δT) = Diode’s forward voltage temperature coefficient ( δVTC/δT) = 2mV VTC = 0.55V Experimentally verify the resulting value of RTCandadjustas necessary to achieve optimal regulation over temperature. The addition of a temperature coefficient current modifies the expression of output voltage as follows: VOUT = VBG RFB RREF ⎛ ⎝⎜ ⎞ ⎠⎟ 1 NPS ⎛ ⎝⎜ ⎞ ⎠⎟ − VF − VTC RTC ⎛ ⎝⎜ ⎞ ⎠⎟ • RFB NPS –ISEC (ESR) Output Power A flyback converter has a complicated relationship be- tween the input and output current compared to a buck or a boost. A boost has a relatively constant maximum input current regardless of input voltage and a buck has a relatively constant maximum output current regardless of input voltage. This is due to the continuous nonswitching behavior of the two currents. A flyback converter has both discontinuous input and output currents which makes it |
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