Datasheet LT3573 (Analog Devices) - 8

LT3573
APPLICATIONS INFORMATION ERROR AMPLIFIER—PSEUDO DC THEORY
In combination with the previous VFLBK expression yields In the Block Diagram, the R an expression for VOUT, in terms of the internal reference, REF (R4) and RFB (R3) resistors can be found. They are external resistors used to program programming resistors, transformer turns ratio and diode the output voltage. The LT3573 operates much the same forward voltage drop: way as traditional current mode switchers, the major  R   1  difference being a different type of error amplifier which V FB OUT = VBG derives its feedback information from the flyback pulse.  R  a N REF  PS  − VF −ISEC (ESR) 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 dV R 1 dV through resistor R F FB TC REF to form a ground-referred volt- = − • • or, age. This voltage is fed into the flyback error amplifier. dT RTC NPS dT The flyback error amplifier samples this output voltage 1 dV R information when the secondary side winding current is R TC FB TC = −RFB • • ≈ zero. The error amplifier uses a bandgap voltage, 1.23V, NPS dVF /dT dT NPS as the reference voltage. (dVF/dT) = Diode’s forward voltage temperature The relatively high gain in the overall loop will then cause coefficient the voltage at the RREF resistor to be nearly equal to the bandgap reference voltage V (dV BG. The relationship between TC/dT) = 2mV VFLBK and VBG may then be expressed as: VTC = 0.55V  V  V The resistor value given by this equation should also be a FLBK BG or,  R verified experimentally, and adjusted if necessary to achieve FB  = RREF optimal regulation over temperature.  R   1 The revised output voltage is as follows: V FB  FLBK = VBG    RREF  a  R   1  V FB OUT = VBG a = Ratio of Q1 I  R   N  − VF C to IE, typically ≈ 0.986 REF PS a VBG = Internal bandgap reference  V  R − TC FB –I  R N SEC (ESR) TC  • PS a 3573fd 8 For more information www.linear.com/LT3573 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