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DC/DC Converter IC Capable of Direct Step-Down from 48 V to 3.3 V

Simplifying the Power Supplies of 48V Hybrid Systems to Cut Losses

Part 2

Keyword
  • Two steps
  • Single step
  • Efficiency
  • Footprint
  • Number of components
  • Loss
  • Reliability
  • Man-hours
  • AEC-Q100
  • Incorporating N-ch MOSFET
  • Boost diode

- What is it about power supply requirements for the current 48 V system that nearly all conventional converter ICs cannot meet?

In nearly all cases, a two-step step-down design is used. For example, first 48 V is stepped down to 12 V, and then the 12 V is further stepped down to a low voltage such as 3.3 V. If this method is used, the ICs of the other three companies cited above as examples can be used to step down from 48 V to 12 V, and to step down from 12 V, any existing automotive-grade converter IC that supports 12 V systems can be used.

- Well then, why did ROHM go to the trouble of directly stepping down to lower voltages from 48 V?

There are various advantages when two steps are replaced by a single step. First of all, efficiency is improved. And, a single power supply circuit takes the place of two circuits, so the footprint and the number of components can both be reduced.

- I can immediately see how the number of components and the footprint would be reduced, but by about how much would efficiency be improved?

Here is an example.

In this example, using the two-step design, there is a loss of 20% in the first step. The loss in the second step is a further 15%, and so the efficiency is 80%×85% = 69%. In contrast, in the single-step design, while there is a tendency for the efficiency to be lower when the step-down ratio is high, nevertheless the efficiency can be well expected to be higher than for the two-step design.

- Are there any other advantages?

There is one less circuit to be designed, and so design and evaluation man-hours are reduced, and development time can be shortened. Of course, this product should also be able to contribute to reduce costs. And as another important consideration, a smaller number of components means enhanced reliability. This should be particularly welcome in automotive applications.

- Since you mentioned design, could you please describe the specifications of the BD9V100MUF-C.

Certainly. However, these numerical values and the like have not been finalized, and so should be used only for reference. Features and key specifications are summarized below.

Features

  • ・AEC-Q100 compliant
  • ・Internal N-ch POWER MOSFET
  • ・Soft start function
  • ・Current mode control
  • ・Over-current protection function
  • ・Under-voltage lockout function
  • ・Thermal shutdown function
  • ・Input over-voltage protection function
  • ・Output over-voltage protection function
  • ・Short circuit protection function
  • ・Compact, high-power surface
       mount package

Key Specifications

  • ・Input voltage range: 16 V to 60 V
  • ・Minimum on-time: 9 ns
  • ・Output voltage range: 0.8 V to 5.5 V
  • ・Output current: 1 A (Max)
  • ・Operating frequency: 2.1 MHz (Typ)
  • ・Reference voltage: 0.8 V±2 %
       (over temperature)
  • ・Shutdown circuit current: 0 µA (Typ)
  • ・Operating temperature range: -40°C to
       +125°C

Package W(Typ)×D(Typ)×H(Max)
VQFN24SV4040 4.00mm×4.00mm×1.00mm

Features include, first of all, compliance with AEC-Q100, for automotive applications. This can be called an essential requirement for automotive uses. A power switch is integrated. Many high-voltage DC/DC converter ICs require an external switch, but the BD9V100MUF-C incorporates a high-voltage N-ch MOSFET to minimize the number of external components and simplify design.

- I think that this circuit diagram is a basic application circuit example. Indeed, there aren't many external components.

In essence, the only required external components are the input and output capacitors, an inductor, resistors R1 and R2 to set the output voltage, R3 and C1 for phase compensation, a resistor RRT to set the oscillation frequency, a capacitor for the internal regulator, and the bootstrapping components RBOOT and CBOOT. In addition to the MOSFET switch, the boost diode for bootstrapping is also integrated. Thus nearly all the required protective functions are integrated. Even given the high rated voltage and short minimum on-time, absolutely no special circuit configurations or components are required.

Current mode control is used. Current mode is well suited to fast operation, and phase compensation is simple.

The output current is 1 A, which I think covers a broad range of applications. The input voltage range is 16 V to 60 V, and the output voltage range is 0.8 V to 5.5 V. From these specs, it should be clear that a high input voltage can be directly stepped down to a low voltage below 5 V.

- Finally, could you explain how the industry's shortest on-time was achieved?

Due to patent issues and other matters, I cannot talk about this too much. Everyone knows that in order to support a high step-down ratio, the minimum on-time must be shortened, but using conventional control methods, we were approaching limits imposed by the effects of noise, circuit delays, and other issues. Allow me to simply state that ROHM used its own idea and technology to move past these limits.

- Well then, could you please give us a summary.

The essential thing to understand is that this is a step-down DC/DC converter IC that was developed with very heavy emphasis on 48 V hybrid systems, and it is capable of directly stepping down to low voltages below 5 V, enabling use in high voltage applications such as industrial equipment and telecom.

Until now, I think there have been many cases in which single-step conversion was abandoned when a high step-down ratio was required, and the designer settled for conversion in two steps. And in automotive equipment, if a switching frequency of 2 MHz or higher is required in order to avoid interference with the radio, many engineers probably regarded two-step conversion to be pretty much the standard approach in 48 V systems.

The BD9V100MUF-C achieves a minimum on-time of 9 ns through ROHM's own control technology, and makes possible single-step conversion from 60 V to under 5 V using high-speed switching at 2 MHz and above. Given all the advantages that this IC offers, such as improved efficiency, smaller size, reduced number of components, and enhanced reliability, I think it will redefine what is common knowledge in this area.

- Thank you.

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