The termination supply for an HSTL or CTT bus must generate an output of about 0.75V, capable of sourcing and sinking current into a bunch of 50Ω terminating resistors. Designing such supplies can be a headache for two reasons. First, the headroom needed by an emitter-follower pass element in a linear regulator makes it difficult to sink current at such a low voltage. Second, 0.75V is below the magic 1.25V level produced by bandgap circuits as a feedback reference in most linear and switch-mode power-supply ICs.
An efficient, synchronous buck regulator (Figure 1) avoids both of these problems. Sink capability at low voltage is accomplished by the use of a synchronous switch (Q2) and by allowing the inductor current to reverse. IC1 includes current-limiting circuitry that prevents inductor-current reversals (as do most buck-regulator ICs), but it also includes a logic input (active-low SKIP) that lets you disable that circuitry.
Figure 1. Modifications to a conventional buck-regulator circuit produce a 0.75V,3A output with sink/source capabilities, useful as a termination supply for high-speed data buses.
In noise-sensitive wireless applications, pulling active-low SKIP high forces the inductor current to be continuous, thereby avoiding the ringing associated with an otherwise discontinuous inductor current. In this circuit, pulling active-low SKIP high allows current to flow from the circuit output back into the inductor and through the synchronous switch to ground.
The other problem-that of regulating an output level below the 1.25V bandgap threshold-is overcome by dividing down the reference voltage and feeding it to an external integrator amplifier (IC2). Summing this reduced reference with a directly coupled feedback signal ensures an excellent transient response, and produces an integrated feedback signal that feeds directly into the IC's main high-speed PFM comparator.
Current sunk by the output doesn't flow directly to ground as it would in a linear-regulator termination supply. Instead, the buck topology works in reverse and becomes a boost topology, producing a net positive current flow into the 5V supply. In most systems, this excess current is absorbed by the numerous other 5V loads.