# Supply-Scalable High-Speed I/O Interfaces
## 3 Design Considerations of Supply-Scalable I/O
### TX Driver Topology  
TX output driver topology is another important factor that impacts the efficiency of scalable IO, since the TX output driver is mainly responsible for the signaling power we discussed above, although a separate supply voltage is usually dedicated for the TX driver to decouple the TX swing from the supply scaling [12; 25]. The requirements for the TX driver are summarized as a proper output impedance for signal integrity, enough output swing to guarantee sufficient SNR, and optional FFE equalization for a high-loss channel. Figure 12 shows three popular topologies of TX driver implementation: a current-mode driver, P-over-N voltage-mode driver, and N-over-N voltage-mode driver [25; 59; 60; 61; 62]. In the current-mode driver, the transistors operate in the saturation region, the output impedance of the driver is generally dominated by the passive load resistance R. As a result, it provides good impedance matching across a wide range of output swing. Compared to the CML circuit that has an output swing of IBIASR, the current-mode driver has a halved swing of IBIASR/2 (single-ended), because the IBIAS splits into the load resistance and RX termination. For example, with 50-\(\Omega\) termination, 20-mA current consumption is needed for 500-mV swing (=Swing/25\(\Omega\)). On the other hand, the voltage-mode drivers consume less current than the current-mode driver, because there are not spilt current paths. Such a difference originates from where the termination is placed relative to the signal path, such that the termination is placed in parallel with the signal path in the current-mode driver but in series with the signal path in the voltage-mode driver. For that reason, the voltage-mode driver is also referred to the source-series terminated (SST) driver. In a P-over-N voltage-mode driver, an inverter-like PMOS-over-NMOS structure drives the channel and the RX termination. In an N-over-N voltage-mode driver, NMOS transistors are used for both pull-up and pull-down, but they are driven by an opposite polarity of input voltage. As a result, pull-up and pull-down paths are not turned on simultaneously, the current solely flows from \(V_{DD,PU}\) (or \(V_{\mathit{swing}}\)) to \(V_{CM}\) (or \(V_{CM}\) to ground). As a result, assuming matched impedance for both paths, both the output swing and the common-mode voltage (\(V_{CM}\)) are \(V_{DD,PU}\)/2. For example, it leads to the current consumption of 5 mA for a 500-mV output swing, which is only \(\frac{1}{4}\) that of the current-mode driver. The same is also applied to the N-over-N voltage-mode driver. On the other hand, the impedance matching of voltage-mode drivers is more complex than the current-mode driver, because they rely on the active devices rather than a passive resistor. Basically, the transistors should operate as a resistor to assure low output impedance so that they should be in a linear region, which constraints the gate-overdrive voltage (\(V_{OV}\)) to be higher than the drain-source voltage (\(V_{DS}\)). As a result, the P-over-N is more appropriate for high output swing, whereas the  
Figure 11: Comparison of ring- and LC-based clock generators in terms of figure-of-merit and silicon area, based on the works presented in ISSCC 2010–2019 [19]. N-over-N fits better for low output swing [12]. A passive resistor is usually placed in series with the driver to reduce the \(V_{DS}\) for better linearity [60; 63]. As shown in Figure 12b,c, the voltage-mode drivers are driven by CMOS inverters, whereas the current-mode driver needs to be driven by a CML pre-driver, so the voltage-mode drivers are more appropriate for supply-scalable design, as we discussed in Section 3.1. However, there are constraints on the supply voltage of such inverters; hence, it limits the scalability. Since the output impedance is set by the transistors, and it is dominated by the \(V_{GS}\) of the transistors, the supply voltage can only be adjusted in a range where the output impedance is matched within the configurable range of the driver segmentation.