Defining high performance for analog ICs

Electronic News, Feb 19, 1996 by Larry Sample

High-performance analog integrated circuits (ICs) have become a rallying cry for semiconductor companies in Silicon Valley, but what they are is not often defined, probably because high-performance analog is not easy to define.

For all the difficulty and cost of manufacturing them, high-performance digital ICs are pretty easy to define. They are digital, usually CMOS ICs with the fastest, smallest and most tightly packed transistors, operating at the highest possible speed, and performing the greatest number of logic operations in a given time.

Just as PC and cellular phone manufacturers are the engines that have driven semiconductor sales for the past several months, digital IC technology is driving semiconductor equipment manufacturers and CAD vendors to push the envelope to accommodate smaller transistor dimensions on wafers of constantly increasing diameter. Digital circuits demand higher speed and more components per unit area for high performance.

High-performance analog is quite comfortable using wafer sizes and manufacturing equipment tolerances that our digital colleagues abandoned years ago. Circuit cleverness is more the aim than component size or circuit density. The value-added in high-performance analog comes from solving a subsystem problem that enables a system designer to forget the analog block of the diagram and get back to the software or feature implementation that forms the system's heart.

Analog problem-solving often includes power management, whether in the form of lower dropout in a regulator for battery-powered systems or providing a PCMCIA power matrix subsystem that embraces full specification compliance with nominal tolerance regulated input voltages.

The system designer has enough problems to confront and relies on high-performance analog components to contain the headaches on the power management piece of the board. Digital circuits respond to a minimum set of conditions that turn transistors on or off under software or firmware control. Analog circuits have to respond to a host of different electrical occurrences with a spectrum of voltage, current and frequency conditions that may turn them part-way on and part-way off under their own control. Analog circuit functions have to condition outputs in controllable proportion to their inputs. That requirement involves a much more arcane science in the integrated circuit of design. High-performance analog integrated circuit manufacturers produce a range of seemingly unrelated functions. Among them are super low dropout in linear voltage regulators, high efficiency and simple-to-use switching voltage regulators, high-speed switching matrixes and MOSFET drivers, high-side drive MOSFET drivers, high-performance op amps, and specialized packaging such as Micrel's IttyBitty package for microminiature PC board layout.

High-performance analog today means analog with custom-like specialized performance capability that system designers require. Because of that specialization, high performance commands a higher price. But because the products are standard, they serve a broader market than custom circuits. The new analog ICs tend to sell over a long period of time in small quantities compared to digital circuits, but sell to a large number of users often with completely divergent design goals for use in everything from toys to supercomputers.

For example, take low dropout voltage regulators. "Dropout" is that point at which the circuit drops out of regulation and the output voltage is no longer in control. "High performance" here means the lower the dropout voltage, the better. The importance of having a lower dropout is that the regulator can operate from a lower supply voltage, such as a nearly depleted battery, while maintaining output regulation. Such an IC is particularly valuable as more and more electronic equipment uses battery power. Regulators began with Darlington NPN transistors and a 2.5V dropout differential between input and output. That is, if the output voltage had to be 5V for standard logic, the input voltage had to be 7.5V. That is an unhandy voltage for battery-powered equipment. Consequently, a new generation evolved using a PNP transistor to drive an NPN. That is the approach of Linear Technology and it yields a 1.5V differential between input and output.

The Micrel approach uses a PNP power transistor, which allows a typical dropout voltage of 300mV at rated output current and correspondingly lower dropout voltages at lower output currents. High performance also means lower ground current while achieving dropout voltages less than 300mV. Other semiconductor manufacturers have PNP-based regulator designs, but the ground current is as high as 15-20 percent of the output current. This ground current represents wasted power and limits the practical output current to about 1 ampere maximum. Until Micrel developed the SuperBeta PNP transistor, there was no way around the limitation. The Micrel technology allows a ground current of 1-2 percent of output current which pushes a linear regulator's upper output current limit to 7.5 amps or more and provides high efficiency.