SLUS336A June 1998 – December 2016 UC1854 , UC2854 , UC3854
PRODUCTION DATA.
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Bypass the VCC pin directly to the GND pin, using a ceramic capacitor of at least 0.1 µF. This bypass capacitor absorbs supply current spikes required to charge external MOSFET gate capacitances.
VCC must be connected to a stable source that can deliver at least 20 mA. The VCC supply must exceed the VCC turnon threshold to start switching operation and must remain above the VCC turnoff threshold for normal operation.
A secondary winding on the PFC boost inductor can be used to deliver a regulated auxiliary bias supply with few external components as shown in Figure 12. Unlike more conventional and unregulated single diode or bridge rectifier techniques, this approach uses two diodes in a full wave configuration. This arrangement develops two separate voltages across capacitors C1 and C2 each with 120-Hz components. However, when these two are summed at capacitor C3, the line variations are cancelled, and a regulated auxiliary bias is obtained. The number of turns on the secondary winding adjusts the bias supply voltage.
A bootstrap resistor and storage capacitor must be added, as shown in Figure 12 when VCC is obtained from a PFC boost inductor auxiliary winding. These parts must be added to ensure the UC3854 controller has sufficient VCC voltage to start up and operate through the soft-start process until sufficient voltage is available from the auxiliary winding.