Datasheet LT3574 (Analog Devices) - 8

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