As the form factor of wireless handsets, PDAs, and other portable electronics continues to shrink while their complexity level continues to grow, design engineers face an increasing number of problems with battery life, board space and heat or power dissipation.
Efficiency is often the main objective when using DC/DC converters. Many design requirements involve converting the battery voltage to a low supply voltage. Although a linear regulator can be used for that, it cannot achieve the efficiency of switching regulator-based designs. This article will cover some of the most common issues designers have to face when trading off solution size, performance and cost.
Large-signal vs. small-signal response
Switching converters are based on fairly complex regulation schemes to keep high efficiency at heavy and light load. Modern CPU core supplies require a fast and friendly, large-signal response of the regulator. For instance, when a processor is switching from idle to full speed operation mode, the current drawn by the core can rise very fast from a few tens of micro-amps to a couple of hundreds of milli-amps.
As the load conditions change, the loop rapidly responds to the new requirements in order to keep the voltage within the regulation limits. The amount and rate of load change determines whether the loop response is called a large-signal response or a small-signal response. We define the small-signal parameters based on a steady-state operating point. Consequently we can typically consider variations below 10% of a steady-state operating point as being a small-signal variation.
In practice, the error amplifier is in slew limit and it does not control the loop because the load transient is occurring faster than the error amplifier can respond, so the output capacitors satisfy the transient current until the inductor current can "catch up."
Large-signal response temporarily takes the loop out of operation. However, the loop must respond gracefully going into and out of large-signal response. The wider the loop bandwidth, the faster the load transient the loop can respond to.
Even though the regulation loop, from a small signal prospective, may show enough gain and phase margin the switching converter can still exhibit instability and ringing during line or load transients. When selecting external components power supply designers need to be aware of these limitations otherwise their designs could cross the edge of disaster.