The high-current output stage of the UCC28C4x-Q1 has been redesigned to drive the external power switch in approximately half the time of the earlier devices. To drive a power MOSFET directly, the totem-pole OUT driver sinks or source up to 1 A peak of current. The OUT of the UCC28C40-Q1, UCC28C42-Q1, and UCC28C43-Q1 devices switch at the same frequency as the oscillator and can operate near 100% duty cycle. In the UCC28C41-Q1, UCC28C44-Q1, and UCC28C45-Q1, the switching frequency of OUT is one-half that of the oscillator due to an internal T flip-flop. This limits the maximum duty cycle in the UCC28C41-Q1, UCC28C44-Q1, and UCC28C45-Q1 to < 50%.
The UCC28C4x-Q1 family houses unique totem pole drivers exhibiting a 10-Ω impedance to the upper rail and a 5.5‑Ω impedance to ground, typically. This reduced impedance on the low-side switch helps minimize turnoff losses at the power MOSFET, whereas the higher turnon impedance of the high-side is intended to better match the reverse recovery characteristics of many high-speed output rectifiers. Transition times, rising and falling edges, are typically 25 nanoseconds and 20 nanoseconds, respectively, for a 10% to 90% change in voltage.
A low impedance MOS structure in parallel with a bipolar transistor, or BiCMOS construction, comprises the totem-pole output structure. This more efficient utilization of silicon delivers the high peak current required along with sharp transitions and full rail-to-rail voltage swings. Furthermore, the output stage is self-biasing, active low during under-voltage lockout type. With no VDD supply voltage present, the output actively pulls low if an attempt is made to pull the output high. This condition frequently occurs at initial power-up with a power MOSFET as the driver load.