Datasheet LT3748 (Analog Devices) - 9

LT3748
applications inForMation Pseudo-DC Theory of Operation
Combining with the previous VFLBK expression yields an The R expression for VOUT, in terms of the internal reference, REF and RFB resistors as depicted in the Block Diagram are external resistors used to program the output voltage. programming resistors, transformer turns ratio and diode The LT3748 operates much the same way as traditional forward voltage drop: current mode switchers with the exception of the unique    error amplifier which derives its feedback information V RFB   1   OUT = VBG from the flyback pulse. R N − VF −ISEC (ESR)  REF  PS  Operation is as follows: when the NMOS output switch Additionally, it includes the effect of nonzero secondary turns off, its drain voltage rises above VIN. The amplitude output impedance (ESR). This term can be assumed to of this flyback pulse (i.e., the difference between it and be zero in boundary control mode. VIN) is given as: V
Temperature Compensation
FLBK = (VOUT + VF + ISEC • ESR) • NPS V The first term in the V F = DOUT forward voltage OUT equation does not have a tem- perature dependence, but the diode forward drop, V I F , has a SEC = Transformer secondary current significant negative temperature coefficient. To compensate ESR = Total impedance of secondary circuit for this, a positive temperature coefficient current source N is internally connected to the RREF pin. The current is set PS = Transformer effective primary-to-secondary turns ratio by resistor RTC to ground connected between the TC pin and ground. To cancel the temperature coefficient, the The flyback voltage is converted to a current by RFB and following equation is used: Q2. Nearly all of this current flows through resistor RREF to form a ground-referred voltage. This voltage is fed into the dVF R = − FB dV • 1 • TC or, flyback error amplifier. The flyback error amplifier samples dT RTC NPS dT this output voltage information when the secondary-side R winding current reaches zero. The error amplifier uses a dV R TC FB TC = −RFB • 1 • ≈ bandgap voltage, 1.223V, as the reference voltage. N dV dT N PS F / dT PS The relatively high gain in the overall loop will then cause (dV the voltage at the R F/dT) = Diode’s forward voltage temperature coef- REF resistor to be nearly equal to the ficient bandgap reference voltage, VBG. The relationship between VFLBK and VBG may then be expressed as: (dVTC/dT) = 1.85mV/°C  V  VTC = 0.55V FLBK V  BG   or R  = The resistor value given by this equation should also  FB  RREF be verified experimentally and adjusted, if necessary, to   achieve optimal regulation over temperature. V RFB   FLBK = VBG R  The revised output voltage is as follows:  REF   R  1  V V FB    BG = Internal bandgap reference OUT = VBG R   − VF  REF  N  PS   V  − TC   • RFB –ISEC (ESR) RTC  NPS 3748fb For more information www.linear.com/LT3748 9 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 Package Description Typical Application Related Parts