Datasheet LT3511 (Analog Devices) - 8

LT3511
APPLICATIONS INFORMATION PSEUDO DC THEORY
the effect of nonzero secondary output impedance (ESR). Boundary control mode minimizes the effect of this im- In the Block Diagram, RREF (R4) and RFB (R3) are external pedance term. resistors used to program the output voltage. The LT3511 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 δV δ F = R 1 V – FB • • TC or, ESR = Total impedance of secondary circuit δT R δ TC NPS T 1 δV NPS = Transformer effective primary-to-secondary turns R = –RFB TC ≈ RFB TC • • ratio N δV δ F / δT T N PS PS RFB and Q2 convert the flyback voltage into a current. Nearly (δV all of this current flows through R F/δT) = Diode’s forward voltage temperature coefficient REF to form a ground- referred voltage. The resulting voltage forms the input (δVTC/δT) = 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 ⎛ R ⎞⎛ ⎞ V = FB FLBK and VBG approximately equals: VOUT VBG ⎜ ⎟ 1 ⎜ ⎟ – V ⎝ F R ⎠⎝N ⎠ ⎛ REF PS V ⎞ ⎛ ⎞ FLBK ⎜ ⎟ = VBG R or V = FB ⎛ ⎞ FLBK VBG ⎜ ⎟ V R – TC ⎜ ⎟ • FB – I ( ) ⎝ RFB ⎠ RREF ⎝RREF ⎠ SEC ESR ⎝R N TC ⎠ PS VBG = Internal bandgap reference
Output Power
Combination of the preceding expression with earlier derivation of V A flyback converter has a complicated relationship be- FLBK results in the following equation: tween the input and output current compared to a buck ⎛ R ⎞⎛ ⎞ V = FB ( ) or a boost. A boost has a relatively constant maximum OUT VBG ⎜ ⎟ 1 ⎜ ⎟ – V ⎝ F – ISEC ESR R ⎠⎝N ⎠ input current regardless of input voltage and a buck has a REF PS relatively constant maximum output current regardless of The expression defines VOUT in terms of the internal ref- input voltage. This is due to the continuous nonswitching 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 3511fc 8