Power Electronics

LDOs See Little Drop-Off in Interest

The quest for higher efficiency has become important in so many power-supply applications that you might begin to wonder whether linear voltage regulators are losing their place in modern equipment designs. Switching regulators can deliver much better efficiency, particularly when higher current levels are needed and when the input- and output-voltage levels differ by a large amount. However, many signs point to the continued importance of linear regulators.

This issue contains two articles discussing aspects of low-dropout linear regulator (LDO) design. In “Shunt Regulator Design Enhances LDO Reliability,” authors Kularatna and Thrimawithana describe a high-current LDO design that enables foldback-current limiting. Then, in “SPICE Model Supports LDO Regulator Designs,” Sandler and Hymowitz describe a device model that improves simulation accuracy for LDO circuits.

Though these articles focus on discrete regulator designs, there's clearly no shortage of interest in monolithic LDOs. Every month, chip vendors continue to introduce new LDO chips with different voltage options, improved dropout performance, smaller packages and other desirable features.

Despite this ongoing LDO activity, it does appear that there are now more new switching regulators and power-supply controllers being introduced than LDOs. But that doesn't mean LDOs are less popular.

Venture Development Corp. (VDC; Natick, Mass.) is one of the firms that tracks demand for power management ICs. In doing so, VDC tracks subcategories such as linear regulators, switching regulators and PWM/PFC controllers. The linear regulator category is comprised of LDOs as well as the older nonLDO parts referred to simply as positive and negative linear regulators. The switching regulators category consists of monolithic dc-dc converter ICs.

According to Peter Walsh, senior analyst at VDC, the total number of LDO ICs sold last year was just less than the total number of switching regulators and PWM/PFC controllers combined. If you expand the LDO category to include all positive and negative linear regulators, the total number of linears shipped last year actually exceeded the switching regulators plus PWM/PFC controllers category by 30% to 40%, said Walsh.

In terms of dollar amounts, linears get beat by the switching regulators and PWM/PFC controllers. Those devices represented a little over $3 billion in sales last year versus a little under $2 billion for linears. These sales numbers are not too surprising, given that the linears represent simpler chip designs and generally cost less than the switching ICs. But, the linear regulators' simplicity and low cost provide much of their appeal and help to explain why these ICs are still so popular more than three decades after the first monolithic linear regulators were introduced.

Other data suggests that the future sales of LDOs, while not growing as quickly as the switching devices, will still be robust. Walsh projects that unit sales for LDOs will increase at a 7.3% compound annual growth rate (CAGR) from 2004 to 2006. Because of falling LDO prices, the corresponding dollar sales during this period are expected to rise at just a 6.4% CAGR. Meanwhile, dollar sales are forecast at 11.9% CAGR for switching regulators and at 9.2% CAGR for PWM/PFC controllers for 2005 through 2008.

LDOs owe much of their popularity to the proliferation of low-value supply voltages and to the space and cost restrictions in portable applications. Small design size, tight regulation and low-noise performance all favor the use of LDOs.

Perhaps the only thing mysterious about the LDO is the term itself. Vendors differ on what input-to-output voltage requirement constitutes “low dropout” — some say <1 V and others say 500 mV or even 300 mV. They only confuse matters further by adding superlatives like “very” low dropout (<300 mV) and “ultra” low dropout (<500 mV or <100 mV). (Load levels also influence the decision of what to call an LDO.)

Though the definition of LDO may be a minor issue, I view the lack of agreement as a healthy sign. It suggests that LDO design is still a dynamic area with vendors trying different design approaches and competing tooth-and-nail for lucrative applications. Fortunately for system designers, competitiveness means more choices when searching for regulator ICs.

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