The traditional 5-V band-gap-derived reference voltage of the UC3842 family can be also found on the UCCx813-[0,1,2,4] devices. However, the reference voltage of the UCCx813-[3,5] devices is 4 V. This change was necessary to facilitate operation with input supply voltages below 5 V. Many of the reference voltage specifications are similar to the UC3842 devices although the test conditions have been changed, indicative of lower-current PWM applications. Similar to their bipolar counterparts, the BiCMOS devices internally pull the reference voltage low during UVLO, which can be used as a logic status indication.
The 4-V reference voltage on the UCCx813-[3,5] is derived from the supply voltage (VVCC) and requires about 0.5 V of headroom to maintain regulation. Whenever VVCC is below approximately 4.5 V, the reference voltage also drops outside of its specified range for normal operation. The relationship between VVCC and VREF during this excursion is shown in Figure 14.
The noninverting input to the error amplifier is tied to one-half of the controller's reference voltage (VREF). This input is 2 V on the UCCx813-[3,5] and 2.5 V on the higher reference voltage parts: the UCCx813-[0,1,2,4].