The device has an internal wide input VCC regulator that is sourced from the BIAS pin. The wide input VCC regulator allows the BIAS pin to be connected directly to supply voltages from 2.9 V to 45 V (transient protection up to 50 V).

The VCC regulator turns on when the device is in Standby or Run mode. When the BIAS pin voltage is below the VCC regulation target, the VCC output tracks BIAS with a small dropout voltage. When the BIAS pin voltage is greater than the VCC regulation target, the VCC regulator provides a 5-V supply (typical) for the device and the internal N-channel MOSFET driver.

The VCC regulator sources current into the capacitor connected to the VCC pin. Connect a 5-Ω resistor in series with a 1-µF ceramic bypass capacitor from this pin to PGND.

The minimum supply voltage after start-up can be further decreased by supplying the BIAS pin from the boost converter output or from an external power supply as shown in Figure 9-5. Also, this configuration allows the device to handle more power when V_SUPPLY is less than 5 V. Practical minimum supply voltage after start-up is decided by the maximum duty cycle limit (D_MAX).

Figure 9-5 Decrease the Minimum Operating Voltage After Start-Up

In flyback topology, the internal power dissipation of the device can be decreased by supplying the BIAS using an additional transformer winding, especially in PSR flyback. In this configuration, the external BIAS supply voltage (V_AUX) must be greater than the regulation target of the external LDO, and the BIAS pin voltage must always be greater than 2.9 V.

Figure 9-6 External BIAS Supply (PSR Flyback)