
X11181002 Datasheet(PDF) 11 Page  Linear Technology 

11 page LT3798 11 3798f Programming Output Current The maximum output current depends on the supply volt age and the output voltage in a flyback topology. With the VIN_SENSE pin connected to 100μA current source and a DC supply voltage, the maximum output current is determined at the minimum supply voltage, and the maximum output voltage using the following equation: IOUT(MAX) = 2•(1– D)• NPS 42 • RSENSE where D = VOUT •NPS VOUT •NPS + VIN The maximum control voltage to achieve this maximum output current is 2V • (1D). It is suggested to operate at 95% of these values to give margin for the part’s tolerances. When designing for power factor correction, the output current waveform is going to have a half sine wave squared shape and will no longer be able to provide the above currents. By taking the integral of a sine wave squared over half a cycle, the average output current is found to be half the value of the peak output current. In this case, the recommended maximum average output current is as follows: IOUT(MAX) = 2•(1−D) • NPS 42 • RSENSE • 47.5% where D = VOUT •NPS VOUT •NPS + VIN The maximum control voltage to achieve this maximum output current is (1D) • 47.5%. For control voltages below the maximum, the output cur rent is equal to the following equation: IOUT = CTRL • NPS 42 • RSENSE The VREF pin supplies a 2V reference voltage to be used with the control pins. To set an output current, a resistor divider is used from the 2V reference to one of the control pins. The following equation sets the output current with a resistor divider: R1 = R2 2NPS 42 •IOUT •RSENSE –1 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ where R1 is the resistor connected to the VREF pin and the CTRL pin and R2 is the resistor connected to the CTRL pin and ground. Setting VIN_SENSE Resistor The VIN_SENSE resistor sets the current feeding the internal multiplier that modulates the current limit for power factor correction. At the maximum line voltage, VMAX, the current is set to 360μA. Under this condition, the resistor value is equal to (VMAX/360μA). For DC input or nonPFC AC input applications, connect a 25k resistor from VIN_SENSE to INTVCC instead of the AC line voltage. Critical Conduction Mode Operation Critical conduction mode is a variable frequency switching scheme that always returns the secondary current to zero with every cycle. The LT3798 relies on boundary mode and discontinuous mode to calculate the critical current because the sensing scheme assumes the secondary current returns to zero with every cycle. The DCM pin uses a fast current input comparator in combination with a small capacitor to detect dv/dt on the third winding. To eliminate false trip ping due to leakage inductance ringing, a blanking time of between 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 Threshold curve in the Typical Performance Characteristics section. The detector looks for 80μA of current through the DCM pin due to falling voltage on the third winding when the secondary diode turns off. This detection is important since the output current is calculated using this comparator’s output. This is not the optimal time to turn the switch on because the switch voltage is still close to VIN + VOUT • NPS and would waste all the energy stored in the parasitic ca pacitance on the switch node. Discontinuous ringing begins when the secondary current reaches zero and the energy in the parasitic capacitance on the switch node transfers OPERATION 
