VCC is the power input connection for this device. In normal operation, VCC is powered through a current limiting resistor to a low-impedance source. To prevent noise problems, bypass VCC to GND with a 0.1-µF ceramic capacitor in parallel as close to the VCC pin as possible. An electrolytic capacitor can also be used in addition to the ceramic capacitor.
Although quiescent VCC current is very low, total supply current is higher, depending on the OUT current. Total VCC current is the sum of quiescent VCC current and the average OUT current. Knowing the switching frequency f and the MOSFET gate charge (Qg), average OUT current can be calculated from Equation 3.
The UCCx813-x has a lower VCC (supply voltage) clamp of 13.5 V typical versus 30 V on the UC3842. For applications that require a higher VCC voltage, a resistor must be placed in series with VCC to increase the source impedance. The maximum value of this resistor is calculated with Equation 4.
Additionally, the UCCx813-x has an on-chip Zener diode to limit VCC to 13.5 V, which also limits the maximum OUT voltage. If the bias-supply source is always lower than 12 V, it may be connected directly to VCC. With UVLO thresholds at 4.1 V and 3.6 V for the UCCx813-3 and UCCx813-5, respectively, 5-V PWM operation is now possible.