Flexibility—A Mixed-Signal Blessing

Bob Klosterboer explains why a wide variety of applications can benefit from the latest mixed-signal semiconductor technologies.

Conventional mixed-signal semiconductor technology allows analogue control and signal processing functions, such as amplifiers, ADCs and filters, to be combined with digital functionality, such as microcontrollers, memory, timers and logic control functions on a single, chip.

Some of the more recent mixed-signal semiconductor developments, however, have significantly simplified the implementation of mixed-signal solutions by allowing many more functions to be integrated into a single device. The latest mixed-signal processes allow high voltage functionality to be integrated into an IC alongside the relatively low voltages required for the more conventional mixed-signal functions.

Low data rate wireless communications can also be deployed in mixed-signal ICs, opening up a range of options for the addition of wireless and telemetry capabilities to new or existing designs. Simultaneously, technologies that reduce power consumption have come to the fore, in turn creating opportunities for mixed-signal ICs in applications where previously battery life would have limited their use.

SENSOR INTERFACE ASICS
Many applications need high-performance sensor solutions that deliver accuracy and functionality in the smallest possible form factors with the lowest power consumptions. As a result, low-power mixed-signal ASIC technologies are increasingly being chosen to implement 'Smart sensor' SoCs. Figure 1 shows an example of some of the elements that can be integrated into a 'smart' sensor interface ASIC. As the diagram indicates, available IP includes solutions for signal conditioning, conversion and processing of the signal received from the sensor element including 8051, 6502 and ARM7 TDMI processor options. Functionality can also cover capabilities such as temperature sensing, calibration, diagnostics and memory components. A variety of methods for output of the data to the user (including the low data rate wireless communications option described below) can also be integrated into a single 'smart sensor' IC.

LOW DATA RATE WIRELESS
The latest mixed-signal technologies significantly simplify the addition of low data rate wireless communications to new and existing applications. Take, for example, the AMIS ASTRIC (Application Specific Transmit and Receive IC) mixed-signal RF ASIC technology platform. This platform can be used to develop a standalone transceiver IC or can be incorporated alongside other functional blocks into a highly integrated single chip solution. Based around a proven and highly integrated 0.35µm CMOS process that features up to five metal layers, ASTRIC designs allow developers to make use of many analogue and digital IP building blocks to reduce both design complexity and development time. These blocks include solutions for clock management and power management analogue front ends, as well as microcontroller options and embedded memory solutions.

Figure 2 shows a functional block diagram of the AMIS-52000 transceiver IC, a device that AMIS has developed using the ASTRIC technology. This device provides a 433.92MHz narrowband RF link using ASK modulation and operates in half duplex mode with user selectable data rates of between 1.0 and 19.2kbps. A low-power RC oscillator contributes to a very low current consumption of just 7.5mA while receiving data, and 25mA during transmit. A power conservation mode (SniffMode) makes the technology suitable for implantable medical devices and other low power applications, as it enables the device to 'wakeup' from a low-power state, poll for the presence of an RF signal, and then shut down again if no RF source is detected, all in less than 100µsec. The fast sniff cycle in combination with appropriate duty cycling makes it possible to achieve low average power consumption when monitoring the RF channel.

HIGH-VOLTAGE MIXED-SIGNAL TECHNOLOGIES
Using a single IC that combines analogue and digital capabilities with higher voltage output stages for actuation and protection is a goal of many industrial, medical and automotive applications. Solutions such as AMI Semiconductor's I3T 'Smart Power' technology are helping engineers to achieve this goal by delivering high-density mixed-signal ASICs capable of handling voltages up to 80V.

The latest I3T50 technology, for example, can handle voltages up to 50V and can reduce the size of sensor interface ICs by up to 60%. This is because of a proprietary deep trench isolation (DTI) technique that allows isolation distances between an ASIC's high-voltage devices to be reduced. This means the chip area of an I3T50 ASIC or ASSP is between 10% and 60% lower than SoC solutions that use standard junction isolation schemes.

The 50V technology is built around a low-voltage (LV) 0.35µm CMOS semiconductor process featuring metal-metal capacitors and high-ohmic resistors. It is available with a library of high-voltage DMOS and bipolar devices including floating vertical nDMOS transistors. These transistors have a drain-to-drain on resistance (RDS (ON)) of below 48mΩ.mm2 at a breakdown voltage in excess of 50V. Electrostatic discharge (ESD) withstand capability for the I3T technology is rated at 4.5kV HBM (human body model) and 750V CDM (charge device model).

MIXED-SIGNAL APPLICATIONS
One emerging trend in the medical electronics sector is the move towards portable equipment and implantable devices. These include blood glucose monitoring systems, insulin pumps, body temperature sensors, defibrillators, nerve stimulators, spinal stimulators, neurological stimulators and pacemakers and hearing aids.

These applications have a number of fundamental design requirements. In the case of implantable devices, a key design element is a long and relatively maintenance-free life. This means battery life must be as long as possible (typically over ten years). At the same time, size will be critical, as will the method with which a medical team can program, communicate with, or control the device. There is also growing demand for devices to offer high levels of built-in intelligence and memory storage, as well as delivering improved sensing functionality and enhanced configurability.

In cardiac rhythm management (CRM) applications, control output stages are likely to involve high voltages ranging from 20V (pacemakers) to almost 1000V (defibrillator control), while operating frequencies can range from 0.1 to 1000Hz. By using the most recent mixed-signal technologies, designers are able to address all of these criteria while minimising overall time-to-market.

In the automotive arena engineers seek increasingly integrated and cost-effective solutions for the control of and interface to in-vehicle systems. This in turn is driving demand for in-vehicle networking (IVN) semiconductor solutions that combine digital, analogue and high-voltage capabilities in a single IC.

Take for example semiconductors for LIN bus systems used for controlling distributed electrical systems in non-time critical automotive applications. Such applications include the control of DC and stepper motors for windows, mirrors and headlamps, or the management of information from sensors for climate control or seat position feedback. Traditionally, LIN slave nodes have comprised transceivers, microcontrollers, and sensor interfaces or actuator drivers made from discrete components. More recently, dedicated microcontrollers with a built-in LIN UART have become available. These microcontrollers are typically used with a companion chip that integrates the remaining slave node blocks such as the LIN transceiver, voltage regulator, watchdog timer, actuator drivers and sensor interfaces. New high-voltage mixed-signal technologies, however, allow full integration of the key slave node blocks into a single, cost-effective chip built around standard IP modules as shown in Figure 3.

Another example of how single-chip solutions based on high-voltage mixed-signal technology can be used to reduce the component count and simplify the implementation of automotive applications is shown in Figure 4. The diagram shows a LIN-based stepper motor driver that has been developed by AMIS as an application specific standard part (ASSP) to be used in applications such as headlamp levelling and swivelling. Acting as a LIN slave node, this single-chip solution integrates bus connection, positioning electronics and a micro stepping motor driver in a single SOP20 IC. An advanced state machine performs the positioning tasks, while high-power drivers activate the stepper motor coils. Detailed diagnostics feedback helps the LIN master to obtain all necessary information relating to motor operation. A stall detection feature is also built into the device. Flexibility in configuring motion parameters such as maximum speed, acceleration and driving current mean that this standard device platform can be used with a variety of different motor types, load conditions and positioning ranges.

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