The UCC21220 and UCC21220A output stages feature a pull-up structure which delivers the highest peak-source current when it is most needed, during the Miller plateau region of the power-switch turn on transition (when the power switch drain or collector voltage experiences dV/dt). The output stage pull-up structure features a P-channel MOSFET and an additional Pull-Up N-channel MOSFET in parallel. The function of the N-channel MOSFET is to provide a boost in the peak-sourcing current, enabling fast turn on. This is accomplished by briefly turning on the N-channel MOSFET during a narrow instant when the output is changing states from low to high. The on-resistance of this N-channel MOSFET (R_NMOS) is approximately 1.47 Ω when activated.

The R_OH parameter is a DC measurement and it is representative of the on-resistance of the P-channel device only. This is because the Pull-Up N-channel device is held in the off state in DC condition and is turned on only for a brief instant when the output is changing states from low to high. Therefore the effective resistance of the UCC21220 and UCC21220A pull-up stage during this brief turn-on phase is much lower than what is represented by the R_OH parameter.

The pull-down structure of the UCC21220 and UCC21220A are composed of an N-channel MOSFET. The R_OL parameter, which is also a DC measurement, is representative of the impedance of the pull-down state in the device. Both outputs of the UCC21220 and UCC21220A are capable of delivering 4-A peak source and 6-A peak sink current pulses. The output voltage swings between VDD and VSS provides rail-to-rail operation, thanks to the MOS-out stage which delivers very low drop-out.

Figure 8-2 Output Stage