Using analogue as a building block

10 June 2008

Typical system level functional block hierarchy
Typical system level functional block hierarchy

A new breed of analogue/mixed-signal/power ICs deliver a quick time-to-market with a building block approach to design

Whether the project is to design a standalone desktop appliance, or a sub-system module to update part of a larger system or appliance, breaking the system down into major functional blocks has always been part of the initial design process. Rather than arbitrarily defining these blocks, the engineer has the option of aligning the functional breakdown against new ‘off-the-shelf’ analogue/mixedsignal ICs that combine commonly needed functions.

Silicon technology, such as Freescale's SmartMOS, has brought to the market many ‘analogue’ ICs that combine analogue/mixedsignal functions and power functions traditionally only seen in discrete MOSFETs and rectifiers. It combines BiCMOS-type process monolithically integrating precision analogue circuitry, power FETs, and dense CMOS logic.

The engineer will break the design into major functional blocks by grouping like functions together and then indicating the I/O relationship between blocks. At this stage, the organisation of the breakdown can be arbitrary and conceptual, since hardware has not yet been designed.

The engineer may, for convenience, arbitrarily group all of the power driver functions together, (such as motor, solenoid, and transducer drivers), as all these functions require similar sized/rated discrete power transistors to control higher currents. In the past, such an architecture decision would influence the physical breakdown of the design, resulting in an array of power transistors comprising a plug-in ‘driver card’. This approach had advantages when the hardware consisted of modules built up from discrete components.

However, such alignment of working blocks in accordance with functional similarity and I/O relationship may not yield the quickest time-to-market. This is especially true if blocks of proven hardware design are available for re-use.

By using standard, readily available, analogue/mixed-signal ICs to fulfill major functional blocks of system architecture, the engineer can focus on the novel aspects of the design. Standard analogue ASICs from nearly all the major semiconductor manufacturers range from single function devices, like the MC33887 H-bridge driver, to poly-functioned devices, like the MC33287 contact monitoring and dual low-side protected switch.

SMARTMOS technologies span operation from 1V to 90V and are capable of driving loads up to 10A. Its CMOS logic typically incorporates several hundred thousand gates on the same chip with precision analogue and power driver circuitry. Such advanced ICs can embody several functions, disparate or associated, such as linear voltage regulation, DC/DC power conversion, and more.

The next step is to categorise and subgroup analogue/mixed-signal devices into application areas or functional categories. Exploring the product areas of semiconductor manufacturers’ websites would help the engineer understand how applications engineers view system breakdown via analogue/mixed-signal building blocks, before beginning the conceptual hierarchical breakdown of the project, along pre-defined boundaries of building blocks.

The best candidates to use as building blocks in the design need to be determined. Understanding how manufacturers group and categoriese products will help when navigating websites for datasheets.

As these are gathered, the engineer can start checking the maximum voltage and current ratings of the device specifications against the requirements of the system design. After discarding devices that are too slow, the remaining candidates represent building blocks that can be used to complete the system design.

BAHER AHMAD is systems and architecture engineer, Freescale

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