Design Ideas July 20, 1995

Buck regulator terminates fast data buses

Bruce Moore,
Maxim Integrated Products, Sunnyvale, CA

The limitations of 5 and 3.3V CMOS buses are spurring the development of high-speed, low-voltage buses for the next generation of computers. These buses, such as Futurebus and Rambus, require low-voltage supply rails to reduce signal-voltage swings. Other buses, such as CTT (center-terminated transceiver) and HSTL (high-speed transceiver logic), have center termination and, therefore, require a power source that can sink current as well as source it.

The termination supply for a CTT or HSTL bus must generate an output of about 0.75V. This output must be capable of sourcing and sinking current into many 50 Ohm terminating resistors. Designing such supplies can cause headaches for two reasons: First, the headroom that the emitter-follower pass element in a linear regulator needs makes sinking current at such a low voltage difficult. Second, 0.75V is below the "magic" 1.25V level bandgap that circuits produce as a feedback reference level in most linear and switch-mode power-supply ICs.

The efficient, synchronous buck regulator in Fig 1 overcomes both problems. Sink capability at low voltage arises from the use of a synchronous switch (Q₂) and from allowing the inductor current to reverse. IC₁, like most buck-regulator ICs, includes current-limiting circuitry that prevents inductor-current reversals. The IC also includes a logic input (SKIP) that lets you disable that circuitry.

In noise-sensitive wireless applications, pulling SKIP high forces the inductor current to be continuous, thereby avoiding the ringing associated with an otherwise discontinuous inductor current. In this circuit, pulling SKIP high allows current to flow from the circuit's output back into the inductor and through the synchronous switch to ground.

The circuit overcomes the problem of regulating an output level below the 1.25V bandgap threshold by dividing down the reference voltage and feeding it to an external integrator amplifier (IC₂). Summing this reduced reference voltage with a directly coupled feedback signal ensures an excellent transient response. The summation is an integrated feedback signal that feeds directly into the regulator IC's main high-speed PFM (pulse-frequency-modulation) comparator. Current that the output sinks does not 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, the numerous other 5V loads absorb this excess current. (DI #1731)

PICTURE 1

Modifications to a conventional buck-regulator circuit produce a 0.75V, 3A supply with sink and source capabilities.