The A6985F and A6986F, Two New Efficient Synchronous DC/DC Buck Converters for Automotive Applications
March 17, 2016 by John Day
By Domenico Attianese, DC-DC Converters Senior Application Engineer
Fulvio Ottavio Lissoni, DC-DC Converters Application Manager
The latest car generation includes more and more functionalities in body control, car infotainment / connectivity (including Advanced driver assistance systems — ADAS), and clusters. For example, onboard car multimedia is required to be compatible with everyday portable equipment and to play audio/video files from external storage media. Navigation and ADAS are options widely diffused in the automotive market.
Body control applications that are always connected to the battery require high efficiency in light load conditions to limit the overall current consumption, below 100 A typically, and so preserve the battery charge while the engine is off.
The automotive environment is generally sensitive for noise minimization to satisfy EMC specifications, but car infotainment applications, in addition, also demand a high switching frequency to prevent noise injection into the tuned radio bandwidth.
Specifications for extreme power bus conditions, like load dump and cold crank, require sustaining a wider input voltage range than the nominal battery voltage.
This paper deals with the key features required of the new DC-DC converters specifically developed for the automotive market and describes their implementation inside the new product family from STMicroelectronics: the A6985F and A6986F, both qualified to AECQ100 specifications.
In most car body applications, the switching regulator is directly connected to the car battery and the output voltage must also be regulated while the engine is off for an extended period. In this application condition, current consumption is minimal while supplying at least a CAN transceiver and a microcontroller in sleep mode, as shown in Figure 1.
As soon as a valid word, or sequence of bits, from the central body controller awakes the local application, current consumption rises to the nominal load, which can be higher than 1 A. When a car is parked with the engine off for a long time, the current drawn by the total application must be very low to prevent the battery discharge. This specification poses a strong limit on the DC-DC quiescent current, i.e. the current absorbed by the converter to regulate the output voltage at zero output load. Car manufacturers’ typical requirement for a single electronic module is a total current consumption lower than 100A, including the active load current consumption and DC-DC quiescent current.
In noise sensitive systems, electronic devices must respect stringent electromagnetic emissions limits for system compatibility purpose. Since all switching converters have emission peaks located at multiples of the switching frequency, those applications require constant switching frequency located above the noise sensitive band over the application conditions.
In addition, for car infotainment applications, car manufacturers usually request that electromagnetic emission peaks are located beyond the extended AM band, above 1710 kHz, to prevent noise coupling on the car radio. A constant 2MHz switching frequency is the standard request for noise sensitive automotive applications, thus guaranteeing full electromagnetic compatibility with the car radio operation and reducing the application’s size and cost.
The load dump and cold crank extreme operating conditions require a wide DC-DC converter input voltage range.
The load dump event represents a sudden load disconnection from the generator that produces overshoot on the power bus due to the power distribution parasitic inductive path. In automotive systems, this happens when the battery in charge is disconnected from the alternator that generates an overvoltage profile in the 12 V bus like that shown in Figure 2A. The standard profile has voltage peak up to 87 V that is usually clamped below 35 V.
The cold crank event happens at low temperature engine start-up when the output battery voltage drops because of the decreased battery capacity to provide the nominal starter motor current. Figure 2B shows the typical input voltage profile in cold crank conditions to test the output voltage regulation of the power supply. The minimum input voltage level depends on the car manufacturer.
The simplified block diagram of A6986F and A6985F is shown in Figure 3. Both devices share a common internal architecture with different current capabilities: A6986F is rated for 1.5A DC and A6985F for 0.5A DC, and there are related optimizations.
As the block diagram shows, both devices are synchronous monolithic DC-DC buck converters, with internal High Side PMOS and Low Side NMOS switches, based on a Peak Current Mode control scheme with an internal high precision Error Amplifier.
The synchronous rectification guarantees low conduction losses, while reducing the size and the cost of the final application. Moreover, the internal Low Side switch allows the inversion of the current in the inductor, guaranteeing the requested forced PWM operation at fixed switching frequency in noise sensitive applications.
The Peak Current Mode Control features excellent dynamic line transient response, and due to the internal High Side PMOS switch, the device is able to operate in Low Drop-Out mode (High Side always on) with the output voltage equal to the input in defect of the voltage drop across the power element.
The A6986F/A6985F output voltage is adjustable between 0.85V and VIN with an external resistive divider connected at the VOUT pin. Moreover, there are dedicated part numbers with internal matched voltage dividers that feature precise 3.3V and 5V output voltages, thus simplifying the BOM and ensuring higher precision and lower current consumption.
Low Noise Mode/Low Consumption Mode
The A6986F/A6985F can be programmed either in Low Consumption Mode or in Low Noise Mode through an external resistor connected at the MLF pin. The best suited configuration for the final application is selected at the power-up stage.
Low Consumption Mode is mainly used to serve car body applications, because of the minimal quiescent current of just 30µA with VIN=12V and VOUT=3.3V. To reduce the current consumption, the regulator minimizes the switching activity when the output current request is low, in order to maximize the efficiency at light-load. Therefore, the A6986F/A6985F works in bursts at constant peak current (equal to ISKIP internal programmable current threshold) and minimizes the current request between the switching bursts. When working in bursts, the output voltage ripple slightly increases, but the trade-off between current consumption and voltage ripple can be optimized by dynamically changing the ISKIP current level by driving the SYNCH/ISKIP pin. The ISKIP current levels for A6986F and A6985F are summarized in Table 1.
The Low Noise Mode, because of its constant switching frequency over the application range, represents the ideal configuration for noise sensitive applications in car infotainment. The device always operates in forced PWM, so never skips pulses in steady loading condition. With the continuous conduction mode (CCM), the output voltage ripple is minimized and constant over the loading conditions.
The device automatically derives the main contribution to supply the internal circuitry from the VBIAS pin, which reduces the battery current consumption and therefore increases the light load efficiency. The VBIAS pin is typically connected to the output of the regulator if 3.3V<VOUT<5V. Therefore, the switchover feature drastically reduces the quiescent current of the A6986F/A6985F, which is crucial in car body applications, and increases the efficiency of the converter.
Programmable switching frequency
The switching frequency can be programmed in the range 250kHz – 2MHz, thus offering a wide configurability for power distribution designers to choose from. The switching frequency is selected at the power-up stage through a resistor connected to the FSW pin. After the start-up procedure, the pin is left floating in order to minimize the quiescent current from VIN. The wide programmable range includes a 2MHz operation to prevent noise injection in the radio-tuned band for car infotainment applications.
Synchronization in Low Noise Mode
Beating frequency noise is an issue when multiple switching regulators populate the same application board. Therefore, the A6986F/A6985F can be synchronized in Low Noise Mode operation to an external clock or to another A6986F/A6985F with the pin, SYNCH. This pin is an I/O pin able to deliver or recognize a frequency signal, so that two or more regulators can be synchronized by simply connecting their SYNCH pins together. Once synchronized, the master will provide the synchronization signal so that the regulators will switch at the same frequency with a 180° phase shift, and minimize the RMS current flowing through the input filter.
Start-Up and Soft-Start
The soft-start and inhibit features are multiplexed on the same pin, and the soft-start time is adjusted by connecting an external capacitor to the SS/INH pin. The output soft-start feature minimizes the input inrush current and decreases the stress of the power components during the power-up phase.
An external open collector can inhibit the device and limit the current drawn from the battery to a few A whenever the output voltage regulation is not required.
The embedded voltage supervisor monitors the regulated output voltage and drives the RST pin in low impedance as long as the DC-DC output voltage is out of regulation. The threshold voltage at which the supervisor releases the RST open collector is programmed at the startup by connecting a resistor to the MLF pin. It can be selected to be 80%-86%-93%-96% of the target output voltage.
In order to ensure a proper reset of digital devices with a valid power supply, the A6986F/A6985F can also delay the RST assertion with a programmable time using a capacitor connected to the DELAY pin. If the DELAY pin is left floating, the RST output acts as a power good without delay, useful to implement output voltage sequencing for cascaded regulators. The RST pin is driven in low impedance in case the thermal protection is triggered or in case of Under Voltage Lock Out event, it is also very useful as a fault monitor.
The pulse by pulse current sensing implements a constant current protection that limits the output current in overload conditions. Because both power elements have active current sensing, the device implements an effective current protection against heavy short circuit over the different application conditions.
A6986F/A6985F EVALUATION BOARDs
Table 2 summarizes the starter kits for the different A6986F and A6985F part numbers available. All the evaluation kits are based on the same printed circuit board, shown in Figure 4, assembled with different BOMs for dedicated device optimization. The next paragraphs highlight relevant measurements for essential device operations performed on the evaluation boards.
Low Consumption Mode/ Low noise Mode
The evaluation board can be easily configured for LNM or LCM device evaluations with a suitable MLF pin connection.
Figure 5 shows LNM operation at zero output load in Continuous Conduction Mode. The forced PWM minimizes the output voltage ripple, shown in the purple color scope trace, over the entire input voltage and output current range affecting the input current consumption, and therefore affecting the light load efficiency.
Figure 6 shows the LCM mode operation at zero output load. The burst operation at constant peak inductor current, equal to the ISKIP internal current threshold, implies a higher output voltage ripple than that shown in Figure 5 for LNM. Consequently, the device features reduced switching activity and decreased input current consumption to maximize the light load efficiency. The frequency of the burst increases with the load current request approaching the programmed switching frequency towards CCM operation at adequate output current.
Because the ISKIP is programmable, the designer can optimize the output voltage ripple vs. the light load efficiency performance.
This operation guarantees very low quiescent currents down to 30A in a 12V—>3.3V conversion.
Figure 7 shows the efficiency measurements and highlights the improved LCM performance in a light load condition. As soon as the output current is adequate to sustain CCM operation, LCM and LNM curves overlap.
It can be noted, as described above, that the dynamically programmable ISKIP level influences the light load efficiency and the switchover (VBIAS pin connected to VOUT) improves the input current consumption.
Figure 8 shows the RST output behavior after the soft-start time with a programmed 6ms time delay. The RST output is asserted with programmed delay time after the output voltage reaches the programmed internal threshold. The RST signal can be used to properly reset other devices or to implement complex power sequencing schemes.
Figure 9 shows the A6986F5V’s excellent output voltage response in a cold crank test condition at 0.8 A DC output current because of the control loop dynamic performance.
A698xFx EVALUATION SOFTWARE
The eDesignSuite on-line simulation tool is primarily oriented to power management applications: Power Supply (AC-DC and DC-DC), LED lighting (AC-DC and DC-DC), Photovoltaic, Battery Charger, Filter, and Antenna design.
The tool supports designs based on the A6986F and A6985F by inserting the required electrical specifications of the final application. The interface is based on a fully annotated and interactive schematic, and the output provides a complete set of analysis diagrams to estimate the electrical, thermal, and efficiency performances (e.g. the main current and voltage simulations, the efficiency graphs, the Bode diagrams, and the power losses). Moreover, it is possible to design the dynamic performance of the regulator and change all the device settings, such as switching frequency, LCM or LNM, skip current level, for example, supporting any external power component in order to personalize the design and achieve the best performance for the final application.
Once connected to ST’s web site, www.st.com/edesignsuite after simple online registering, it is easy to access the software tool (below is an example of the eDesignSuite design view based on the A6986F3V3).
The A6986F and A6985F are a new generation of DC-DC converters from STMicroelectronics, specifically designed to meet the latest specifications of the automotive market.
The available evaluation kits support all the device performances that meet both car body and car infotainment systems requirements in order to achieve the best trade-off between efficiency / consumptions and noise / output voltage in respect of the AEC-Q100 qualification standard. Definitely, the A6986F and A6985F offer automotive market system designers a wide range of solutions to obtain the best performance for their design.
 A6986F Datasheet: “38 V 1.5 A synchronous step-down switching regulator with 30 µA quiescent current”: available on www.st.com
 A6985F Datasheet: “38 V 500 mA synchronous step-down switching regulator with 30 µA quiescent current”: available on www.st.com .
 ISO 7637-2:2004(E): “Road vehicles-Electrical disturbances from conduction and coupling-Electrical transient conduction along supply lines only”
 ISO 16750-2:2003(E): “Road vehicles-Environmental conditions and testing for electrical and electronic equipment-Electrical loads”
 eDesignSuite tool available for A6986F and A6985F on www.st.com/edesignsuite.