The traditional 5-V amplitude bandgap reference voltage of the UC3842 family can be also found on the UCC2800, UCC2801, UCC2802, and UCC2804 devices. However, the reference voltage of the UCC2803 and UCC2805 device 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 UVLO status indication.

Figure 9-4 Required Reference Bypass

Note that the 4-V reference voltage on the UCC2803 and UCC2805 is derived from the supply voltage (VCC) and requires about 0.5 V of headroom to maintain regulation. Whenever Vcc is below approximately 4.5 V, the reference voltage also drops outside of its specified range for normal operation. The relationship between VCC and V_REF during this excursion is shown in Figure 9-5.

Figure 9-5 UCC2803 REF Output vs V_VCC

The noninverting input to the error amplifier is tied to half of the PWM's reference voltage, V_REF. Note that this input is 2 V on the UCC2803 and UCC2805 and 2.5 V on the higher reference voltage parts: the UCC2800, UCC2801, UCC2802, and UCC2804.