When it comes to op amps for automotive apps, engineers look for a wide operating temperature and low power consumption. Qualified to AEC-Q100 temperature Grade 0 (−40 to 150°C), STMicroelectronics’ TSU11x series of op amps (single-channel TSU111, dual-channel TSU112, and quad-channel TSU114) specify 900-nA power consumption per channel and 1.2 µA maximum when supplied with 3.3 V. They operate from 1.5 to 5.5 V.
As an example, consider that the TSU111H withstands environments that experience extreme heat, such as in braking systems, combustion-engine exhaust systems, and fuel-cell generators. Its high maximum temperature enables use inside of sensor control units (SCUs) placed close to sensors installed in the hottest areas for optimum measurement accuracy.
As a Grade-0 device, ST’s new op amp can operate continuously at 65°C for more than 25 years if supplied by a CR2032 battery, thereby serving the entire vehicle’s lifetime. Grade-1 devices are specified for only eight years without failure. This suits it for applications such as the battery-management system (BMS) of hybrid and electric vehicles, which is never turned off and must consume the least possible power. In this way, the power op amp detects when the battery is charging/discharging and wakes up the CPU.
The 1.7-µA typical supply current ensures the TSU111H places minimal load on the vehicle electrical supply. In addition, STMicroelectronics said the output voltage is exact to within 250 µV at 25°C, and 600 µV over the full temperature range, certifying high-accuracy signal conditioning in all applications and operating conditions. A typical application is to enable precision measurements in the vehicle’s onboard charger (OBC). This product family is supplied by a coin-type lithium battery or a regulated voltage in low-power applications.
Maximizing the Op Amps
Nanopower devices exhibit higher impedance nodes and consequently they’re more sensitive to electromagnetic interference (EMI). As such, for the TSU11x series of devices, STMicroelectronics recommends engineers add three capacitors of around 22 pF each between the two inputs, and between each input and ground. These capacitors lower the impedance of the input at high frequencies and therefore reduce the impact of the radiation.
To benefit from the full performance of the TSU11x, the impedances must be maximized so that current consumption isn’t lost where it’s not required. For example, an aluminum electrolytic capacitance can have significantly high leakage. This leakage may be greater than the current consumption of the op amp. For this reason, ceramic-type capacitors are preferred. For the same reason, big resistor values should be used in the feedback loop.
However, there are two main limitations to be considered when choosing a resistor. First, noise generated: a 100-kΩ resistor generates 40 nV/√Hz; a bigger resistor value generates even more noise. Second, leakage on the PCB: leakage can be generated by moisture. This can be improved by using a specific coating process on the PCB.
Layout considerations must be considered as well. For correct operation, it’s advised to add 10-nF decoupling capacitors as close as possible to the power-supply pins.
Minimizing the leakage from sensitive high-impedance nodes on the inputs of the TSU11x can be performed with a guarding technique. The technique consists of surrounding high-impedance tracks by a low impedance track (the ring). The ring is at the same electrical potential as the high impedance node. Therefore, even if some parasitic impedance exists between the tracks, no leakage current can flow through them because they’re at the same potential.
STMicroelectronics offers the STEVAL-CCA058V1 training kit, intended for hands-on training on operational amplifiers and comparator-based analog circuits. Designers can use the kit in an academic context for student and professional training—junior analog engineers, distributors, and customers. It’s also useful for a fast evaluation and prototyping of low-frequency op-amp-based applications.
The STEVAL-CCA058V1 consists of a configurable board, which features the most widely used op-amp-based schematics and applications, and ST op amps mounted on DIP adapters. It should be used with a set of passive components (resistors and capacitors), and a set of lab instruments (power supply, function generator, multimeter, and oscilloscope).
The kit enables the trainee to experiment with the most common op-amp-based schematics. It also instructs on how to choose the most suitable op amp for a custom application.
The TSU111H is in production now, housed in a SOT23-5L package, priced from $1.29 for orders of 1,000 pieces. The device is included in the company’s 10-year longevity program that ensures long-term product availability for automotive and industrial projects. Free samples are available now at the ST eStore.