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DC-DC Converter IC Capable of Direct Step-Down from 48 V to 3.3 V : Is Direct Step-Down from 48 V to 3.3 V Possible?


ROHM has announced the “BD9V100MUF-C” DC-DC converter IC, capable of directly stepping down to a low 3.3 V from 48 V or other high input voltages, and demand for which has been mounting in the automotive, industrial equipment, and other industries. Existing high-voltage DC-DC converter ICs were able to accept high input voltages exceeding 48 V, but nearly all of these could not easily step down to 5 V, let alone 3.3 V, due to limitations on step-down ratios. The BD9V100MUF-C accomplishes this feat by reducing the minimum on-time of the power switch, which limits the step-down ratio, to an industry-shortest 9 ns. In succession to TECHNO-FRONTIER 2016, the BD9V100MUF-C was also exhibited at the recently held CEATEC JAPAN 2016, attracting intense interest. We therefore asked Mr. Takanobu Shibako, application engineer at ROHM, about the background of development and features of the IC.

- The BD9V100MUF-C has been marketed as a “DC-DC Converter IC Capable of Direct Step-Down from 48 V to 3.3 V”. It caught the eye of many visitors at CEATEC the other day.

Thank you. Since our press release in April of this year, we have received many inquiries.

- To begin with, I am wondering about the “from 48 V” voltage value. What kind of circumstances resulted in the marketing of this IC with this voltage value emphasized?

It should be said first that there should be no mistaken impressions that this device is for use only at 48 V. The maximum rated input voltage of the BD9V100MUF-C is +70 V, and the recommended operation voltage is +60 V. It is important to understand that the minimum on-time is an industry-low 9 ns. Judging only from the device specifications, “60 V input” might make more sense; but one aspect of the development background of the BD9V100MUF-C was the concentrated interest in hybrid automobiles with the 48 V power supply system in the automotive industry.

- Personally, I had associated 48 V with the telecom industry, but the aim here was automotive applications.

Of course it also supports telecom 48 V systems, which are another of its target applications. However, automotive applications are one specialty area of ROHM, and in particular, the company is highly interested in energy efficiency in automobiles.

- I see. I don’t want our discussion to wander, so I’d like to ask about the relationship with the 48 V hybrid system of automobiles, given that this is a high-voltage DC-DC converter IC.

There is a lot of interest in 48 V hybrid systems, because they are thought to achieve greater fuel economy than the conventional 12 V systems. In these systems, 48 V is the main power supply voltage for the onboard equipment. As you know, the power supplies for electronic circuits in the automotive equipment use low voltages such as 5 V and 3.3 V, and so in a conventional 12 V system, the nominal 12 V is stepped down to 5 V or 3.3 V. I think that many of the DC-DC converters used in such systems have voltage rating of around 40 V or so, intended for automotive applications. In contrast, in a 48 V system, the voltage is stepped down from 48 V to 5 V and 3.3 V.

- Then, the DC-DC converters with a voltage rating of 40 V or so, which had been used in the past, can no longer be adopted. Still, although there aren’t so many different types, there are step-down converters with a voltage rating of 60 V or more. And I think ROHM released a step-down converter with a rated voltage of 80 V a little while ago.

Yes, that would be the BD9G341AEFJ. And speaking only in terms of the rated input voltage, there certainly are step-down converter ICs that can accept 48 V input. But when it comes to stepping down from 48 V to 3.3 V in particular, due to the step-down ratios supported by these ICs, it turns out that in essence, conventional DC-DC converter ICs cannot directly step down 48 V to 3.3 V at an oscillation frequency of 2 MHz.

- I’m not sure I understand what you mean by “supported step-down ratios”.

A step-down DC-DC converter steps down a voltage by transferring an input to the output only while a switch is turned on during one switching period. To put things simplistically, if a 10 V input has a 50% duty cycle, meaning it is switched with an on-time of 50% and an off-time of 50%, then the output is half the input, or 5 V. Are you with me so far?

- Yes, I follow you.

Well then, you will understand that to drop 10 V down to 3.3 V, we need to perform switching with a duty cycle having an on-time of 33%. This ratio of the output voltage to the input voltage is called the step-down ratio, and the higher the voltage that is being stepped down, the higher is the step-down ratio. Conversely, the lower the output voltage, the higher is the step-down ratio. The step-down from 48 V to 3.3 V that we have been discussing would require a duty cycle of 3.3 V / 48 V, or about 6.9%, and so the voltage drop is about 93%. In general, this can be described as a very high step-down ratio.

- I see. Well then, is it the case that conventional step-down converter ICs cannot operate at such a step-down ratio, that is, with such a duty cycle?

In effect, that is correct. In general, the lowest voltage to which an input can be stepped down can be represented by the following equation:

Minimum duty cycle = switching frequency × minimum on-time
Lowest possible step-down output voltage = input voltage × minimum duty cycle

Among these parameters, the “minimum on-time” is the shortest on-time that can be controlled by the step-down converter IC. The power switch is a transistor, and so time is required for on/off operation. Moreover, there are limits, imposed by delays, noise and the like, to control by internal feedback and control circuits, which determine the switching duty cycle. These various factors determine the actual minimum on-time, that is, the actual possible minimum on-time. Put another way, the minimum on-time of a step-down converter IC is one performance factor of the IC, and is often but not always indicated on the data sheet for the device.

Let’s try inserting example numbers into the equations. If the switching frequency is 600 kHz and the minimum on-time is 200 ns, then:

Minimum duty cycle = 600 kHz × 200 ns = 0.12 (12%)
Lowest possible step-down output voltage = 48 V × 0.12 = 5.76 V

For a converter with this performance, given a 48 V input, voltage step-down would be limited to a little under 5.8 V. It should be noted that for a 12 V input, voltage step-down to 1.44 V is possible, so that even given these specifications, nearly all output voltage requirements can be supported.

- And “9 ns minimum on-time”, another watchword for the BD9V100MUF-C, emphasizes that the minimum duty cycle can be reduced, so that higher voltage drops can be achieved.。

Let’s try applying that 9 ns to the above-mentioned equations.

Minimum duty cycle = 600 kHz × 9 ns = 0.0054 (0.54%)
Lowest possible step-down output voltage = 48 V × 0.0054 = 0.2592 V

This is how the calculations work out. In actuality, an output voltage cannot be generated that is lower than the internal reference voltage of the converter IC, such as for example 0.8 V. However, step-down to the minimum output voltage of the converter IC is possible.

-I noticed, while studying the equations, that when the switching frequency is below 300 kHz, for example, even under these conditions 3.3 V becomes possible. Is that correct?

Minimum duty cycle = 300 kHz × 200 ns = 0.6 (6%)
Lowest possible step-down output voltage = 48 V × 0.06 = 2.88 V

That is correct. Lowering the frequency is one method for coping when the step-down ratio is high.

- Well then, why wouldn’t this work?

Here, the fact of the “automotive” environmental conditions is important. Cars are equipped with radios, and the AM band uses frequencies up to 1.84 MHz. If the oscillation frequency of a switching power supply is below this, there is interference with the AM band, and so switching frequencies of 2 MHz or higher are required. As mentioned at the beginning, a major target of the BD9V100MUF-C is the automotive industry, and what ROHM wanted to achieve was “direct step-down from 48 V to 3.3 V, with a switching frequency of 2 MHz or higher“.

Actual calculations give us the following.

Minimum duty cycle = 2 MHz × 9 ns = 0.018 (1.8%)
Lowest possible step-down output voltage = 48 V × 0.018 = 0.864 V

Moreover, 48 V is the nominal voltage, but of course a margin must be provided. In the 48 V hybrid market, 60 V is required as the maximum input voltage, and applying this to the above, we have the following.

Lowest possible step-down output voltage = 60 V × 0.018 = 1.08 V

Moreover, if we ask up to what voltage can be input and still result in a 3.3 V output, calculating backwards gives us 3.3 V / 0.018 = 183 V. However, the BD9V100MUF-C has a rated input voltage of up to 70 V, and a recommended operation voltage of up to 60 V, and so the actual maximum turns out to be 60 V.

- Approximately what is the minimum on-time for conventional converter ICs?

I have some data here, so please have a look. Other than the minimum on-times, the figures are comparisons of similar step-down DC-DC converter ICs.

Min. on-time 9ns 30ns 80ns 80ns
Min. duty cycle (at fsw=2 MHz) 0.018(1.8%) 0.06(6%) 0.16(16%) 0.16(16%)
Lowest possible step-down output voltage (at Vin=60 V) 1.08V 3.6V 9.6V 9.6V
Max. input voltage (at Vout=3.3 V)
market demands:60V
(Max. operation voltage)
55V 20.6V 20.6V

From the results of this survey, the shortest on-time of the ICs of other manufacturers is 30 ns, and calculations indicate that this falls slightly short of market demands. Some may judge that such devices are barely usable, but considering safety margins, such use would be risky. In contrast, the BD9V100MUF-C offers performance sufficient to satisfy demands with a comfortable margin.

(Continued to Part 2)

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