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Synchronous rectification step-down DC/DC converters developed as automotive secondary power supplies

Advantages of Secondary Power Supplies in Automotive Equipment

BD905xx-C series: Part 1

  • Secondary power supplies
  • Input voltage rating
  • Automotive use
  • Step-down ratio

The BD905xx-C series is developed to save power and promote compactness in automotive electronic equipment, and with a number of components and footprint comparable to those of LDO regulators, can result in significantly improved efficiency and power consumption.
We asked Mr. Shoichi Harada, a field application engineer at ROHM, about key points of these devices.

- I'd like you to summarize the BD905xx-C series, but first, please explain the meaning of "automotive secondary power supplies".

To begin with the "automotive" part, this series supports the AEC-Q100, which in recent years has effectively become the industry standard for automotive ICs. The rated operating temperature range is from 40°C to +125°C, accommodating demands for automotive applications.
Moreover, production processes, quality control, and supply aspects are all based on ROHM's automotive specifications, and so they are specified as "for automotive use". Of course they can also be used in non-automotive applications.


It is called "secondary" because the specifications assume that, rather than using the vehicle battery directly as an input, a voltage that has been stepped-down by a separate regulator is used as the input. There are various terms used, I think, but in essence this is a power supply that is at least two stages removed from the car battery. I would like to explain this in more detail later.


- Now, please tell me the descriptions and basic specifications.

The BD905xx-C series is a synchronous rectification step-down DC/DC converter with a switching frequency of 2.25 MHz.
Circuits and components that can be integrated on a chip are incorporated to the extent possible. Of course output transistors are incorporated, with a p-channel top and an n-channel bottom. Output voltages are fixed at 1.2 V, 1.5 V, or 1.8 V.
For each output voltage there are two models with different output currents, 2 A and 3 A, and in addition a model with a 1 A output has been added to the 1.2 V models, so that at present there are seven models in all. And although it is still in development, there are plans to add adjustable-output products as well.

- What are the input voltage specifications?

The input voltage range for the BD905xx-C series is 2.69 V to 5.5 V. This is directly related to their status as "secondary" devices explained at the beginning. Automotive electronic equipment often uses a 5 V or 3.3 V as the main power supply, and in automobiles, the voltage is generally converted directly to 5 V or to 3.3 V from the 12-V battery.

Regardless of whether we should call this voltage converted from the battery voltage the primary voltage, this first-stage DC/DC converter uses the 12-V battery, with its large fluctuations, as the input power supply, and so generally a converter with an input voltage rating of 40 V or so is used.

On the other hand, the input voltage range of the BD905xx-C series is from 2.69 V to 5.5 V, so that the 5.5 V or 3.3 V obtained by conversion in the first stage is converted to a still lower voltage of 1.2 V, 1.5 V, or 1.8 V. These devices are intended for use as this second-stage DC/DC converter.

- This is a little off the subject, but why are two stages necessary?
I think it should be possible to general a low voltage like 1.2 V directly from the battery voltage.

Of course it is possible, but it has a number of disadvantages.
As I explained above, in order to connect a DC/DC converter directly to a battery, if the battery is a 12-V battery, an input voltage rating of about 40 V is necessary. In other words, a power supply IC with a high input voltage rating must be used, and a suitable external component is necessary, so that increases in size and cost are unavoidable. And from an engineering standpoint, if the step-down ratio is high, limits relating to control and to heat generation may appear. For these reasons, it is often advantageous to use a secondary power supply to step down from 5 V to obtain a main power supply voltage below 5 V.


- Next I'd like to ask about output voltages.
To what kinds of devices do you expect the low output voltage, set to a little over 1 V, to be supplied?

The 1.2 V, 1.5 V and 1.8 V values that are set as fixed outputs correspond to the power supplies of, for example, MCU and DDR memory that are driven at low voltages. Output currents of 1 A to 3 A are the current values generally required by such devices.
And although they are still in development, we are planning to add a lineup of adjustable-output voltage products, so that the range of supported applications will further broaden.

BD905xx-C series: Part 2

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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