FSDH321, FSDL321

3

Pin Definitions

Pin Configuration

Figure 3. Pin Configuration (Top View)

Pin Number

Pin Name

Pin Function Description

1

GND

Sense FET source terminal on primary side and internal control ground.

2Vcc

Positive supply voltage input. Although connected to an auxiliary transform-

er winding, current is supplied from pin 5 (Vstr) via an internal switch during

startup (see Internal Block Diagram section). It is not until Vcc reaches the

UVLO upper threshold (12V) that the internal start-up switch opens and de-

vice power is supplied via the auxiliary transformer winding.

3Vfb

The 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 charging

external capacitor Cfb from 3V to 6V using an internal 5uA current source.

This time delay prevents false triggering under transient conditions, but still

allows the protection mechanism to operate under true overload conditions.

4Ipk

This pin adjusts the peak current limit of the Sense FET. The feedback

0.9mA current source is diverted to the parallel combination of an internal

2.8k

Ω resistor and any external resistor to GND on this pin to determine the

peak current limit. If this pin is tied to Vcc or left floating, the typical peak cur-

rent limit will be 0.7A.

5Vstr

This pin connects directly to the rectified AC line voltage source. At start up

the internal switch supplies internal bias and charges an external storage

capacitor placed between the Vcc pin and ground. Once the Vcc reaches

12V, the internal switch is opened.

6, 7, 8

Drain

The drain pins are designed to connect directly to the primary lead of the

transformer and are capable of switching a maximum of 650V. Minimizing

the length of the trace connecting these pins to the transformer will decrease

leakage inductance.

1

2

3

45

6

7

8

GND

Vcc

Vfb

Ipk

Vstr

Drain

Drain

Drain

8DIP

8LSOP