Defining high performance for analog ICs

Electronic News, Feb 19, 1996 by Larry Sample

Super low-dropout regulators have an even more important function working with the new high-speed Pentium and PowerPC microprocessors. Newer processors are particularly sensitive to power transients under changing power load conditions. The microprocessors require regulators with extremely fast transient response. Transient response time deteriorates when a regulator goes into dropout. Many so-called low-dropout regulators actually have up to 1.5-Volt dropout points. Micrel's SuperBeta process for PNP transistors produces regulators with only 300 millivolt dropout at full load. The extra 1.2V of linear operation from a Micrel regulator provides an additional margin for fast transient response. The super low-dropout regulators respond much better to input surges and load transients than any other LDO regulators on the market when the input-to-output differential is less than 1 volt.

Super low-dropout not only enables fast transient response but also yields high efficiency, another measure of high performance in analog IC products. So-called "green" computers and super-efficient linear power supplies require low-dropout regulators. Features of high-performance linear regulators include such quasi-smart-power additions as thermal protection, overvoltage lockout and reverse battery protection. A regulator based on an NPN output power transistor may require an external diode to prevent damage in some applications using large output capacitors. Micrel's SuperBeta PNP process does not require this additional external component.

Switching as well as linear regulators are included among the new high-performance analog types. And they come in different shapes and topologies, such as boost, buck, current-fed, and in either voltage or current control modes. Switching regulators are not new. The new thing about recent ones is the simplicity of designing with them and the extraordinary efficiencies possible with the designs. Analog IC makers such as Micrel perform the hard part of the design inside a monolithic IC, to simplify the task of the user who is increasingly likely not to be interested in becoming an analog expert. Leading edge switching regulator ICs take as few as four external components to build a power supply. And a lot of application advice is available through application notes, field application engineers and data sheets.

The switching frequencies of monolithic switching regulators cover a wide spectrum. With switching frequencies as high as 200kHz, the monolithic MIC4574 series of step-down switching regulators, for example, reduces component sizes while offering extraordinary power supply efficiencies for on-card switching, positive-to-negative converters, and split plus or minus 5 volt or 12 volt power supplies. The frequencies of high-performance pulse width modulators or PWM switching regulator controllers exceed 500kHz and dramatically minimize component sizes while boosting efficiencies in off-line power supplies.

Analog "high performance" sometimes means simply "fully spec compliant" because meeting the specification poses special challenges that it takes smart analog designers to overcome. One case is that of the PCMCIA card socket switching matrices. The ICs have to be fully integrated and meet tough requirements for low RDS(ON) current. Failure to meet the specification means that a card will not play in a PCMCIA system. Micrel's MIC2562 single channel and MIC2563 dual channel switching matrices are single package devices that meet the critical PCMCIA 1A output current specification for Vcc and up to 200mA supply current for Vpp.