LT3574 pin Functions BIAS : Bias Voltage. This pin supplies current to the switch SS : Soft-Start Pin. Place a soft-start capacitor here to driver and internal circuitry of the LT3574. This pin must limit start-up inrush current and output voltage ramp be locally bypassed with a capacitor. This pin may also be rate. Switching starts when the voltage at this pin reaches connected to VIN if a third winding is not used and if VIN ~0.7V. ≤ 15V. If a third winding is used, the BIAS voltage should SW : Collector Node of the Output Switch. This pin has be lower than the input voltage for proper operation. large currents flowing through it. Keep the traces to the GND: Ground. switching components as short as possible to minimize R electromagnetic radiation and voltage spikes. FB : Input Pin for External Feedback Resistor. This pin is connected to the transformer primary (VSW). The ratio TC : Output Voltage Temperature Compensation. Connect of this resistor to the RREF resistor, times the internal a resistor to ground to produce a current proportional to bandgap reference, determines the output voltage (plus absolute temperature to be sourced into the RREF node. the effect of any non-unity transformer turns ratio). The ITC = 0.55V/RTC. average current through this resistor during the flyback TEST: This pin is used for testing purposes only and must period should be approximately 200µA. For nonisolated be connected to ground for the part to operate properly. applications, this pin should be connected to VIN. VC : Compensation Pin for Internal Error Amplifier. Connect RILIM : Maximum Current Limit Adjust Pin. A resistor a series RC from this pin to ground to compensate the should be tied to this pin to ground to set the current switching regulator. A 100pF capacitor in parallel helps limit. Use a 10k resistor for the full current capabilities eliminate noise. of the switch. VRIN : Input Voltage. This pin supplies current to the internal REF : Input Pin for External Ground-Referred Reference start-up circuitry and as a reference voltage for the DCM Resistor. This resistor should be in the range of 6k, but comparator and feedback circuitry. This pin must be local y for convenience, need not be precisely this value. For bypassed with a capacitor. nonisolated applications, a traditional resistor voltage divider may be connected to this pin. SHDN/UVLO : Shutdown/Undervoltage Lockout. A resistor divider connected to VIN is tied to this pin to program the minimum input voltage at which the LT3574 will operate. At a voltage below ~0.7V, the part draws no quiescent current. When below 1.22V and above ~0.7V, the part will draw 10µA of current, but internal circuitry will remain off. Above 1.22V, the internal circuitry will start and a 7µA current will be fed into the SS pin. When this pin falls below 1.22V, 2.5µA will be pulled from the pin to provide programmable hysteresis for UVLO. 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
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