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ROHM BD9S Series of Automotive Buck Switching Regulators

  1. Introduction

In recent years, as automotive electronics has advanced, vehicle infotainment, including onboard navigation and audio systems, along with ADAS (Advanced Driver Assist System), have become standard equipment in the latest cars. Developing electronic products that provide greater safety when driving such as collision prevention and lane assist – achieved using onboard cameras and sensors – is attracting keen interest, demanding power supply ICs that support increased functionality.

This increasing sophistication of ADAS and onboard systems is requiring SoC (System-on-Chip) capable of enormous processing power, which typically results in higher power consumption. Consequently, power supplies used for these applications must support larger currents, and linear regulators that are widely used require thermal countermeasures for heat generated during voltage supply, making it advantageous to transition to switching regulators that provide higher power conversion efficiency. However, when switching regulators are used as electrical components in vehicle applications, the switching frequency may interfere with the AM radio band (526.5kHz to 1620.0kHz), causing noise. This requires the use of a coil for output control, which leads to another problem in the form of larger mounting area. And as always, ensuring high reliability is particularly important for automotive components.

The characteristics required for the switching regulator side in order to solve technical challenges arising from the increasing functionality in automotive systems are summarized as follows.

  • High frequency switching operation that does not interfere with the AM radio band
  • Reduced mounting area (including peripheral applications)
  • Large current compatibility with low heat generation
  • Multiple protection circuits that improve reliability

ROHM has developed, commercialized, and began sample shipment of the BD9S series of high efficiency, high frequency current mode buck switching regulators that meets these requirements. (Figure 1)


Figure 1. ROHM BD9S Series of Automotive Secondary Buck Switching Regulators

The following is ROHM’s approach for utilizing these products to support the demands of the automotive market (i.e. ADAS).

2. Market Requirements and ROHM Solutions

  • High frequency switching via current mode control

In general, with switching regulators the settable switching frequency range is 1MHz or less, and when used in vehicle applications the frequency is restricted to below the AM radio band. However, in this case a large coil size is necessary, so to meet the demand for greater miniaturization higher switching frequency is required. In response to this need, the BD9S series achieves a switching frequency of 2.2MHz while maintaining high power conversion efficiency.

Switching regulator control methods used to achieve high frequency switching can be classified into 3 types: voltage mode, current mode, and constant ON time mode. ROHM switching regulators utilize current mode for the following 2 reasons.

The first is superior phase characteristics when using low ESR capacitors (i.e. ceramic) at the output. When increasing the switching frequency, if an electrolytic capacitor is used at the output, in addition to larger size and decreased capacitance at low temperatures, losses due to the rise in ESR cannot be ignored, making it necessary to use a compact, low ESR ceramic capacitor. However, with voltage mode products, when the ESR decreases it becomes difficult to secure the phase margin in the high frequency region – unlike current mode products which can provide stable operation using simple phase compensation.

The second reason is small switching frequency fluctuation. In contrast to constant ON time control in which the switching frequency varies in response to the magnitude of the load current, in current mode products switching control is performed using a constant frequency clock generated from the built-in oscillation circuit, allowing users to set the switching frequency. This makes it easy to obtain the frequency component of the generated noise and take appropriate countermeasures.

On the other hand, since current mode products captures current information flowing through the output power MOSFETs for each switching cycle, in order to perform PWM control it is necessary to shorten the time required for control, generally making it difficult to achieve high frequency.

In response, by conducting a thorough review of the output current detection circuit and propagation delay of the output power MOSFET drive circuit along with simulations and actual device evaluation, ROHM was able to optimize the delay time to achieve a minimum pulse control time per cycle of 70ns – 50% faster than conventional products. This makes it possible to ensure stable operation even at a switching frequency of 2.2MHz which does not interfere with the AM radio band.

  • Reduced mounting area (including peripheral applications)

Module miniaturization is rapidly progressing, particularly in ADAS equipment such as sensors, camera modules, and wireless communication modules, placing a greater importance on the percentage of board area taken up by the power supply IC. For conventional switching regulators with low frequency setting, when the coil inductance is high and a significant current capacity is required, a larger coil must be used. In contrast, ROHM’s latest products support smaller 1uH coils along with 2012 and 3216 size output capacitors by increasing the switching frequency and integrating output power MOSFETs and phase compensation components in a compact 2mm x 3mm package, making it possible to reduce mounting area by as much as 60% (including peripherals) over conventional products. (Fig. 2)

BD9S Series

Figure 2. Image of the BD9S Series Board Area

ROHM is currently developing a lineup featuring fixed output voltage and built-in resistor for setting the output voltage. In addition, the 3mm square VQFN16FV3030 package supports wettable flanks, significantly improving visibility during solder mounting, which is a major problem with the conventional QFN package. (Fig. 3)

Wettable Flank

Figure 3: Image of Wettable Flank Package Pins

  • Large current compatibility with low heat generation

One major advantage of the BD9S series is the class-leading low ON resistance output power MOSFETs (High Side FET: 35mΩ, Low Side FET: 35mΩ). This ensures high power conversion efficiency even with large load currents of 2A or more, minimizing heat generation. (Fig. 4)

Power Conversion

Figure 4. Power Conversion Efficiency Graph

Although 2A class output current products are considered mainstream, as equipment and systems become more sophisticated the demand for 3A and 4A class specifications is expected to increase. The BD9S series includes pin-compatible models with rated output currents of 2A, 3A, and 4A. This makes it possible to flexibly respond to changes in output current specifications which often occurs during set design, reducing evaluation man-hours.

Part No.Input Voltage RangeOutput VoltageMax. Output CurrentOutput Voltage PrecisionOperating FrequencyOperating TemperaturePackage
BD9S400MUF-C2.7V to 5.5V0.8V to 0.8*Vin4.0A±1.5%2.2MHz ±0.2MHz-40°C to 125°CVQFN16FV3030 (3.0 x 3.0 x 1.0mm)
BD9S100NUX-C0.8V to Vin1.0AVSON008X2020 (2.0 x 2.0 x 0.6mm)

**BD9S110NUX-C and BD9S111NUX-C are available soon.

Table: BD9S Series Lineup

  • Multiple protection circuits improve reliability

ROHM ICs integrate a variety of protection circuits, including ① SCP/OVP (Short-Circuit/Over Voltage Protection) that protect the IC from abnormal voltages during output short-circuits, ② Thermal Shut Down (TSD) to guard against abnormal heat generation, and ③ Over Current

Protection (OCP) in case of abnormal currents (i.e. due to output shorts), ensuring safe IC operation that prevents device damage due to adjacent pin shorts, ground faults and output terminal shorts. A power good function is also built in that monitors the output voltage of the IC itself to ensure it’s within the normal range. This contributes to improved system safety by notifying the MCU of an error flag when the output voltage is outside the set range.

In addition, the BD9S series is compatible with the international AEC-Q100 (Grade 1) standard for automotive ICs. ROHM has already acquired certification under the IATF16949 standard for quality management systems for the automotive industry and has sufficiently cleared reliability evaluation criteria for use under severe environments, including high quality and temperature conditions required for automotive development. These features and advantages are a testament to our track record of ensuring long-term supply for over 10 years to the automotive sector.

3. Future Prospects

In the future, the diversification and increasing sophistication of vehicle systems such as ADAS is expected to strengthen the demand for power supply ICs that support larger currents and smaller footprints. In addition to these latest ICs, ROHM proposes a broad range of power supply solutions ranging from high voltage switching regulators compatible with battery voltages and linear regulators such as LDOs to complex power management ICs, and will continue to expand our lineup of high quality, high reliability products for the automotive market while developing products in anticipation of customer and market needs.

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BiS Team

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