Datasheet LT3573 (Analog Devices) - 5

LT3573
PIN FUNCTIONS GND:
Ground.
SS
: Soft-Start Pin. Place a soft-start capacitor here to
TEST:
This pin is used for testing purposes only and must limit start-up inrush current and output voltage ramp be connected to ground for the part to operate properly. rate. Switching starts when the voltage at this pin reaches ~0.7V.
SW
: Collector Node of the Output Switch. This pin has large currents flowing through it. Keep the traces to the
VC
: Compensation Pin for Internal Error Amplifier. Connect switching components as short as possible to minimize a series RC from this pin to ground to compensate the electromagnetic radiation and voltage spikes. switching regulator. A 100pF capacitor in parallel helps eliminate noise.
VIN
: Input Voltage. This pin supplies current to the internal start-up circuitry and as a reference voltage for the DCM
RFB
: Input Pin for External Feedback Resistor. This pin is comparator and feedback circuitry. This pin must be locally connected to the transformer primary (VSW). The ratio bypassed with a capacitor. of this resistor to the RREF resistor, times the internal bandgap reference, determines the output voltage (plus
BIAS
: Bias Voltage. This pin supplies current to the switch the effect of any non-unity transformer turns ratio). The driver and internal circuitry of the LT3573. This pin must average current through this resistor during the flyback be locally bypassed with a capacitor. This pin may also be period should be approximately 200µA. For nonisolated connected to VIN if a third winding is not used and if VIN applications, this pin should be connected to VIN. ≤ 15V. If a third winding is used, the BIAS voltage should be lower than the input voltage for proper operation.
RREF
: Input Pin for External Ground-Referred Reference Resistor. This resistor should be in the range of 6k, but
SHDN/UVLO
: Shutdown/Undervoltage Lockout. A resistor for convenience, need not be precisely this value. For divider connected to VIN is tied to this pin to program the nonisolated applications, a traditional resistor voltage minimum input voltage at which the LT3573 will operate. divider may be connected to this pin. At a voltage below ~0.7V, the part draws no quiescent current. When below 1.22V and above ~0.7V, the part will
TC
: Output Voltage Temperature Compensation. Connect draw 10µA of current, but internal circuitry will remain off. a resistor to ground to produce a current proportional to Above 1.25V, the internal circuitry will start and a 10µA absolute temperature to be sourced into the RREF node. current will be fed into the SS pin. When this pin falls ITC = 0.55V/RTC. below 1.25V, 2.5µA will be pulled from the pin to provide
Exposed Pad:
Ground. The Exposed Pad of the package programmable hysteresis for UVLO. provides both electrical contact to ground and good thermal
R
contact to the printed circuit board. The Exposed Pad must
ILIM
: Maximum Current Limit Adjust Pin. A resistor should be tied to this pin to ground to set the current be soldered to the circuit board for proper operation and limit. Use a 10k resistor for the full current capabilities should be well connected with many vias to an internal of the switch. ground plane. 3573fd For more information www.linear.com/LT3573 5 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