Datasheet

UCC28050, UCC28051
UCC38050, UCC38051
SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009
16
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Bias Current
The bias voltage is supplied by a bias winding on the inductor. Select the turns ratio so that sufficient bias voltage
can be achieved at low ac line voltage. The bias capacitor must be large enough to maintain sufficient voltage
with ac line variations. Be sure to connect a 0.1-μF bypass capacitor between the VCC pin and the GND pin
as close to the integrated circuit as possible. For wide line variations, a resistor, R
B
, is necessary in order to
permit clamping action. The bias voltage should also be clamped with an external zener diode to a maximum
of 18 V.
Zero Current Detection
The zero current detection activates when the ZCD voltage falls below 1.4 V. The bias winding can provide the
necessary voltage. This pin has a clamp at approximately 5 V. Add a current limiting resistor, R
ZC
, to keep the
maximum current below 1 mA.