Innovations in design are helping automotive manufacturers to simplify systems and reduce component count, by implementing more functions while reducing board space requirements and power consumption. By Thomas Mueller, Product Manager, Automotive ASICs with ams AG.
SBCs (System Basis Chips) have been in use in automotive applications for many years. They work as a link between the high voltage and low voltage world in a vehicle’s electric system. The high voltage world comprises the major power source – the battery – and connections to a bus via the cable harness. High voltage pulses, the load dump and EMI (Electromagnetic Interference) are the most notable characteristics of this world.
On the low voltage side, sensitive electronic devices such as microcontrollers and sensor interfaces implement the complex safety and comfort functions needed in a modern vehicle.
In acting as the bridge between the high and low voltage worlds, today’s SBCs combine power supply, protection and actuation functions together with physical layer interfaces to bus systems. Compared to circuits made of discrete components, they offer a significant reduction in component count and board space. SBCs can today be found in systems such as body controllers, gateways, air conditioning units and door modules, as well as parts of the vehicle which require higher power, most notably the powertrain. A classic example is the TLE7263E from Infineon, which features CAN/LIN physical layers, an LDO (low drop-out regulator) and reset generation and switches.
In a typical ECU, however, a multitude of supply voltages is needed for microcontrollers, external sensors and other components. This has led to a situation today in which circuits might combine multiple standard SBCs as well as many discrete components. This increase in component count is only set to worsen as work starts to implement demanding new functional safety standards, such as ISO 26262, which require the use of extensive diagnostic features. At the same time, automotive manufacturers are subject — as ever — to tight space and cost limitations, while meeting very high standards of reliability and reducing energy consumption. Today’s approach, using multiple components, is far from ideal.
A new generation of SBC ASICs (custom ICs) is stretching the limits of SBC integration, not only providing multiple output voltages but also adding many other functions such as switch read-in, monitoring of external signals and safety features.
Reducing Power Dissipation
A key innovation in SBC ASICs is the use of Switch-Mode Power Supply (SMPS) DC-DC converters in place of the traditional LDO. DC-DC converters are far more efficient than LDOs, but at one time the EMC problems associated with the switching activity prevented their adoption in automotive applications. These problems have now been solved by using controlled slew-rates and small inductances. Buck, boost and fly-back converter types are in use in vehicles today. Both boost and fly-back converters enable the system to operate on very low battery voltages.
The integration of a DC-DC converter core in an SBC ASIC with internal or external switching transistors produces a marked reduction in power dissipation compared to an equivalent SBC using only an LDO for power regulation (see Figure 1). The power saved in conversion effectively increases the available power budget for other blocks in the chip.
Just as important, under this architectural approach just one component can manage every power supply that the ECU needs. SBC ASICs can therefore be used in a wide range of applications, integrating a much wider range of functions than has previously been specified in SBC designs. Electric Power Assisted Steering (EPAS or EPS) is one example (see Figure 2). Here, a single SBC ASIC can supply the specified voltages to the main microcontroller, the torque sensor and the support motor-position sensor, as well as providing independent diagnostics. Another example is access control in the car interior: an SBC ASIC can read the start-stop switch and the door switches, power and communicate with the electric steering lock, switch the ignition relays and power the fuel injection ECU. Important functional safety features related to the steering lock can also be implemented on-chip.
Figure 3 shows how an SBC ASIC is partitioned, to provide power management via an SMPS, physical layer interfaces, drivers, and switch-reading and safety functions.
A low-power pre-regulator powers the DC-DC core and the wake-up circuits directly from the battery. This pre-regulator also powers the circuit that controls the various operating modes. In Sleep mode, with active wake-up detection operating, the current draw can be designed to be lower than 100µA over the whole temperature range, in line with typical automotive requirements. An SBC ASIC usually provides over-temperature detection and shut-down to protect the device from thermal damage. In the SBC shown in Figure 3, the over-temperature detection, a voltage level controller and a window watchdog are the reset sources for the microcontroller.
The DC-DC converter core can be configured to handle a range of input and output voltages. For instance, a buck converter can reduce the input voltage to the LDO from the battery voltage to a value of 1-2V higher than the LDO’s output voltage — this reduces by a large amount the losses in the LDO compared to a design in which the LDO’s input is the battery voltage.
Equally, the DC-DC converter core can be used as a boost converter, only operating when the input (battery) voltage drops below a given threshold. Or, a fly-back transformer configuration can be used across the range of input voltages, from a normal level down to very low battery voltages of less than 5V. With more than one secondary coil, a flyback converter can provide several outputs. One coil output is used for the regulation loop; the others run freely (proportional to their winding count).
Both boost and flyback converters guarantee a viable power supply to the microcontroller — the first, LDO-based design will not operate with low battery voltages. This is becoming more and more important as an increasing number of safety-critical systems, such as EPAS, must work even when the battery is almost completely discharged.
The Limits Of Integration
During the architectural design phase of SBC ASIC projects, ECU designers commonly ask whether it is worthwhile to integrate the microcontroller in the SBC. This would serve to reduce component count and board space, as well as providing other potential benefits. In practice, it is difficult to realise the expected cost benefit since such a device would entail a complex fabrication process that could combine high voltage capability with, for instance, Flash memory blocks. Finally, the wide variety of off-the-shelf microcontrollers offers the ECU designer great flexibility and choice that would be lost if fixed microcontroller IP were embedded within an ASIC.
In any case, the strong economic and technical case for implementing an advanced SBC ASIC does not depend on absorbing the discrete microcontroller. The limits of integration are defined most of all by the functions that are compatible with the high-voltage process technology that SBCs must use. The potential to integrate drivers such as low-side, high-side or push-pull is constrained by the total power dissipation the IC can tolerate. Switching 0.5A alone, with an on-resistance in the order of 1Ω, results in 0.25W of power dissipation in the chip. For larger currents it might be necessary to use an external power transistor driven by internal gate drivers. For n-channel high-side switches, a charge pump or bootstrap architecture can be employed.
In addition, the load on the pins supplied by the battery must be considered carefully. An IC manufactured in an HV (High Voltage) CMOS process can withstand load dump pulses of up to 120V. But this does not account for the requirements of pulses 1 and 2 as specified in the ISO 7637 standard. For that, external protection comprising capacitors, a serial diode or a varistor is needed. The pulses 3a/b of ISO 7637 are short enough to be dealt with by the ESD protection capability of the pins and small external capacitances.
When it comes to communication, CAN and LIN physical layers are common features of an SBC ASIC. But basic compliance with the LIN and CAN standards is, in many cases, not adequate in automotive applications. Conformity to far more demanding requirements, such as the German OEM standards (system level ESD according to IEC 61000 4-2) and immunity to very high DPI (direct power injection) and BCI (bulk current injection) levels can be provided in an SBC ASIC.
With the ISO 26262 standard now approaching final release, functional safety topics are becoming more and more important. True, ICs have been used in safety-critical applications for a long time, but this new standard establishes a firm base for proving the reliability of electronic devices, with standardised documents such as the FMEDA (Failure Mode, Effect and Detection Analysis). An FMEDA analysis for an SBC ASIC reveals which additional functional safety features are needed to meet the safety integrity (SIL or ASIL) requirement in the application.
Examples include supervision of the internal voltages and current with the help of an independent reference, redundant reading of switches, and read-back of driver status. Furthermore, the ISO standard requires not only a viable system architecture but also a ‘development execution’ compliant with the standard.
The packaging of SBC ASICs is also worth careful consideration because of the large power dissipation within the chip, attributable mainly to the LDO, drivers and the CAN physical interface. Leaded packages with an exposed pad have in the past been preferred by automotive suppliers. Leadless packages have a somewhat controversial reputation because of alleged stability issues and problems with soldering inspection. On the other hand, QFN-like packages are becoming popular since they are, at least for higher pin counts, far more cost effective. Furthermore, research on wettable pin flanks is in progress, making the inspection of the device easier.
Finally, the IC technology best suited for the manufacturing of SBC ASICs is a perennial topic of debate. BCD (Bipolar CMOS DMOS) and pure High-Voltage CMOS (HV-CMOS) are the most viable candidates. Both allow for the integration of HV transistors with low voltage circuits. In general, it is safe to say that the lower the complexity and the higher the proportion of low-resistance switches on the chip, the more suitable BCD is. With increasing complexity, a larger die area devoted to digital circuits and more diagnostic features, HV-CMOS seems to be the better alternative.
SBC ASICs offer a marked reduction in the space needed for an ECU’s board. This is because of an ASIC’s ability to integrate multiple power management and special functions, such as switch-readout, high/low-side drivers and supervision circuits in a single IC. Using custom SBCs, an integration level far beyond that feasible with conventional, standard SBCs can be realised.
This in turn reduces overall ECU costs. Additionally, by using one main IC and a limited number of external components, reliability is greatly enhanced. Furthermore, a tailor-made functional safety approach to reaching the required SIL /ASIL level in the application can only be implemented in an ASIC.