Fairchild FSDH321 Manual

Also see for FPS FSDH0265RN: Design guide

Contents

Fairchild FSDH321 Manual Manual pdf 3 page image

FSDH321, FSDL3213Pin DefinitionsPin ConfigurationFigure 3. Pin Configuration (Top View)Pin Number Pin Name Pin Function Description1 GND Sense FET source terminal on primary side and internal control ground.2 VccPositive supply voltage input. Although connected to an auxiliary transform-er winding, current is supplied from pin 5 (Vstr) via an internal switch duringstartup (see Internal Block Diagram section). It is not until Vcc reaches theUVLO upper threshold (12V) that the internal start-up switch opens and de-vice power is supplied via the auxiliary transformer winding.3 VfbThe feedback voltage pin is the non-inverting input to the PWM comparator.It has a 0.9mA current source connected internally while a capacitor and op-tocoupler are typically connected externally. A feedback voltage of 6V trig-gers over load protection (OLP). There is a time delay while chargingexternal capacitor Cfb from 3V to 6V using an internal 5uA current source.This time delay prevents false triggering under transient conditions, but stillallows the protection mechanism to operate under true overload conditions.4 IpkThis pin adjusts the peak current limit of the Sense FET. The feedback0.9mA current source is diverted to the parallel combination of an internal2.8kΩ resistor and any external resistor to GND on this pin to determine thepeak current limit. If this pin is tied to Vcc or left floating, the typical peak cur-rent limit will be 0.7A.5 VstrThis pin connects directly to the rectified AC line voltage source. At start upthe internal switch supplies internal bias and charges an external storagecapacitor placed between the Vcc pin and ground. Once the Vcc reaches12V, the internal switch is opened.6, 7, 8 DrainThe drain pins are designed to connect directly to the primary lead of thetransformer and are capable of switching a maximum of 650V. Minimizingthe length of the trace connecting these pins to the transformer will decreaseleakage inductance.1234 5678GNDVccVfbIpk VstrDrainDrainDrain8DIP8LSOP