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## X-11181-002 Datasheet(PDF) 11 Page - Linear Technology

 Part No. X-11181-002 Description Isolated No Opto-Coupler Flyback Controller Download 20 Pages Scroll/Zoom 100% Maker LINER [Linear Technology] Homepage http://www.linear.com Logo

 11 page LT3798113798fProgramming Output CurrentThe maximum output current depends on the supply volt-age and the output voltage in a flyback topology. With theVIN_SENSE pin connected to 100μA current source and a DCsupply voltage, the maximum output current is determinedat the minimum supply voltage, and the maximum outputvoltage using the following equation:IOUT(MAX) = 2•(1– D)•NPS42 • RSENSEwhereD=VOUT •NPSVOUT •NPS + VINThe maximum control voltage to achieve this maximumoutput current is 2V • (1-D).It is suggested to operate at 95% of these values to givemargin for the part’s tolerances.When designing for power factor correction, the outputcurrent waveform is going to have a half sine wave squaredshape and will no longer be able to provide the abovecurrents. By taking the integral of a sine wave squaredover half a cycle, the average output current is found tobe half the value of the peak output current. In this case,the recommended maximum average output current isas follows:IOUT(MAX) = 2•(1−D) •NPS42 • RSENSE• 47.5%whereD=VOUT •NPSVOUT •NPS + VINThe maximum control voltage to achieve this maximumoutput current is (1-D) • 47.5%.For control voltages below the maximum, the output cur-rent is equal to the following equation:IOUT = CTRL •NPS42 • RSENSEThe VREF pin supplies a 2V reference voltage to be usedwith the control pins. To set an output current, a resistordivider is used from the 2V reference to one of the controlpins. The following equation sets the output current witha resistor divider:R1= R22NPS42 •IOUT •RSENSE–1⎛⎝⎜⎞⎠⎟where R1 is the resistor connected to the VREF pin and theCTRL pin and R2 is the resistor connected to the CTRLpin and ground.Setting VIN_SENSE ResistorThe VIN_SENSE resistor sets the current feeding the internalmultiplier that modulates the current limit for power factorcorrection. At the maximum line voltage, VMAX, the currentis set to 360μA. Under this condition, the resistor value isequal to (VMAX/360μA).For DC input or non-PFC AC input applications, connecta 25k resistor from VIN_SENSE to INTVCC instead of theAC line voltage.Critical Conduction Mode OperationCritical conduction mode is a variable frequency switchingscheme that always returns the secondary current to zerowith every cycle. The LT3798 relies on boundary mode anddiscontinuous mode to calculate the critical current becausethe sensing scheme assumes the secondary current returnsto zero with every cycle. The DCM pin uses a fast currentinput comparator in combination with a small capacitor todetect dv/dt on the third winding. To eliminate false trip-ping due to leakage inductance ringing, a blanking time ofbetween 600ns and 2μs is applied after the switch turns off,depending on the current limit shown in the Leakage In-ductance Blanking Time vs SENSE Current Limit Thresholdcurve in the Typical Performance Characteristics section.The detector looks for 80μA of current through the DCMpin due to falling voltage on the third winding when thesecondary diode turns off. This detection is important sincethe output current is calculated using this comparator’soutput. This is not the optimal time to turn the switch onbecause the switch voltage is still close to VIN + VOUT • NPSand would waste all the energy stored in the parasitic ca-pacitance on the switch node. Discontinuous ringing beginswhen the secondary current reaches zero and the energyin the parasitic capacitance on the switch node transfersOPERATION