Issue link: https://iconnect007.uberflip.com/i/1359517
APRIL 2021 I DESIGN007 MAGAZINE 47 resistance is set to 20 mOhms and the L and C in the PDN is selected such that sqrt(L/C) equals the source resistance, just as we would do with a single-lump transmission-line model in signal integrity. It is this matching of these three numbers that guarantees the flat imped- ance profile and clean transient response. Why did we choose 1 nH for this illustra- tion? Simply because we may get 1 nH induc- tance from a single via, though when we assume 10A DC current, it is not a good idea to let it go through a single via. In a real system the 1 nH series inductance may represent the inductance of the entire PCB structure. Fig- ure 4 shows the simulated impedance looking back from the load and the transient response to a load current step. We see from the clean response that it is 2.5 mF capacitance, all what it takes to balance a 1 nH inductance at 20 mOhm impedance. We can take the case in Figures 3 and 4 as the baseline and see what happens if for any reason the series inductance gets higher. For instance, we can increase the inductance to 10nH and leave everything else (including the parasitics) unchanged. e result is shown in Figure 5. In the frequency response we get a peak at 1 MHz going up to 100 mOhm and correspondingly we get a big 1 MHz ringing in the transient response. In a real system the 10 nH inductance may come from a connector or short wire, or may represent the equivalent output induc- tance of a very wide-band voltage regulator. To compensate for the increased inductance, our only choice is to increase capacitance propor- tionally. If we simulate the circuit of Figure 3 with 10 nH inductance and 25 mF capacitance (and leave everything else unchanged), we get Figure 4: Impedance profile (on the left) and transient response (on the right) of the circuit in Figure 3. Figure 5: Impedance profile (on the left) and transient response (on the right) of the circuit in Figure 2 when we change the series inductance from 1 nH to 10 nH.