My cousin introduced me to the Texas two-step a couple years ago. I enjoyed swing dancing in college but had never tried two-step before. My first few attempts were quite abysmal. Fortunately, I was with friends and we could laugh about it. After several more tries, I was able to get the hang of it.

It can also be a little daunting to learn a new converter topology.** **You might be familiar with the conventional buck converter. The simplicity and beauty of this converter has made it popular for decades. TI recently introduced the TPS54A20 based on the series capacitor buck converter. It is a new topology that enables efficient, high-frequency operation of small point-of-load voltage regulators.

**Figure 1: The** **two-phase series capacitor buck converter**

Today we are going to learn the “steps” of the series capacitor buck converter shown in Figure 1. Like any new dance, it may be challenging at first. After walking through the steps of steady-state operation a few times, I think you will find that it is not that difficult. You might even like it! This will be a brief beginner’s class; if you want more details, check out this application note. So let’s begin by considering a converter with a 12V input switching at 5MHz per phase.

The first step, or time interval, occurs when the high side switch of phase A (Q_{1a}) is on as shown in Fig. 2. The series capacitor (C_{t}) connects to the input by switch Q_{1a}. Because the nominal voltage across the series capacitor is half the input voltage (approximately 6V in this case), the phase A switch-node voltage (V_{SWa}) is roughly half the input voltage, shown in blue in Figure 2. The phase A inductor current (I_{La}) rises in a triangular fashion just like a normal buck converter (no resonant behavior) and simultaneously charges C_{t}. In fact, the series capacitor current (I_{Ct}) is equal to I_{La} during this step. The differential series capacitor voltage (V_{Ct}) increases by a few hundred millivolts due to the added charge. During this step, the phase B low-side switch (Q_{2b}) is on, connecting the phase B switch node (V_{SWb}) to ground. The phase B inductor current (I_{Lb}) decreases linearly as a result.

**Figure 2: High side switch of phase A (Q _{1a}) on (step 1) **

Both low-side switches (Q_{2a} and Q_{2b}) are on during step two, as shown in Fig 3. This connects both V_{SWa} and V_{SWb} to ground just like a conventional two-phase buck converter. Both I_{La} and I_{Lb} have negative slopes. Because the series capacitor has no current flowing through it (because I_{Ct} is zero), V_{Ct} remains constant.** **

**Figure 3: Both low side switch on (step 2)**

Step three is where things get interesting, so pay attention to Fig. 4. Switch Q_{2a} is still on, connecting V_{SWa} to ground. Switch Q_{2a} is also connecting the negative side of C_{t} to ground. When the phase B high-side switch (Q_{1b}) turns on, the positive side of the series capacitor connects to V_{SWb}. Now the series capacitor is acting like an input capacitor for phase B! I_{Lb} ramps up and simultaneously discharges the series capacitor. This is evident from the negative I_{Ct} and the small decrease in V_{Ct}. I_{La} continues to ramp down.

**Figure 4: High side switch of phase B (step 3)**

Step four is identical to step two as shown in Fig. 5. Q_{2a} and Q_{2b} are on and V_{SWa} and V_{SWb} are grounded. Both I_{La} and I_{Lb} ramp down. V_{Ct} remains fixed because I_{Ct} is zero. After step four, the whole cycle repeats.

**Figure 5: Both low side switches on (step 4) **

How was that? Not too hard, right? Check out the additional resources to learn more about this exciting new topology. Now it’s time to hit the design floor and take the series capacitor buck converter for a spin.

**Additional resources**

- Explore the “Tiny, Low Profile 10 A Point-of-load Voltage Regulator Reference Design.”
- Read the white paper “Breakthrough power delivery for space constrained applications.”
- Start a design now with WEBENCH® power designer.
- Order the TPS54A20 evaluation module.