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Step-Down DC-DC Converter Series Compatible with Stringent FPGA Power Supply RequirementsCoping with FPGA Power Supply Requirements


- You’ve explained the requirements for an FPGA power supply, but now I’d like to know the reasons why this DC-DC converter series is “suitable for FPGAs”. First of all, what kind of DC-DC converter ICs are included in the series?

At present there are eight models. This product lineup covers the voltages and currents needed for FPGA power supplies. Individual explanations would be burdensome, so let’s look at a table of the product lineup.

Categorized broadly, there are devices that incorporate power transistors, and a controller-type device with external power transistors; all are single-channel synchronous-rectifier devices. Input voltages assume cases in which the system voltage of 5 V is used and cases in which 12 V is taken from a bus power; rated voltages are 7 V and 15.2 V, and 28 V for the controller type. The output current is from 1 A to 6 A for devices with internal power transistors, and the output current of the controller type can be set over a wide range using an external MOSFET. Output voltages assume voltages of around 1 to 1.8 V, and so the minimum output voltage is 0.8 V, and the controller type supports 0.75 V. Various protection functions commonly found in general-use power supplies, such as a power-good output, a soft start function and the like, are provided.

- The requirements relating to power supplies for FPGAs, summarized earlier, were 1) numerous power supply voltages, 2) power supply sequencing, 3) low voltages and high currents, 4) strict voltage accuracy (including such phenomena as ripples, fluctuations due to load transients, voltage drops due to board wiring resistance), and 5) low noise. These are issues concerning power supplies; how does this series of DC-DC converters satisfy these requirements?

First of all, regarding 1) numerous power supply voltages, by using one DC-DC converter for each power supply, optimal conditions can be set. The requirement for 2) power supply sequencing can be satisfied by using the soft start function and external control. Important points here are support for 3) low voltages and high currents, and 4) voltage accuracy. These issues are also related to 5) low noise.

- Looking at your explanation and the tables, I see that the minimum voltage is low at 0.8 V/0.75 V, with output currents from 1 A to 6 A, and greater currents possible for the controller type. But, I think recent general-purpose DC-DC converters have similar specifications.

What is important here is 4) voltage accuracy. The DC voltage accuracy is determined by the accuracy of an internal reference voltage, for which a maximum value of ±1% is guaranteed (±1.5% for the controller type). This makes these devices among the highest accuracy devices of their kind. In other words, if this were the only factor affecting the output voltage accuracy, then the example requirement previously mentioned of a 1 V ±3% output voltage could easily be met. However, when the factors that I had mentioned parenthetically are included as factors affecting voltage accuracy–that is, ripple, fluctuations caused by load transients, voltage drops due to board wiring resistance, and the like–then these effects further detract from voltage accuracy.

- Well, how to we deal with these added effects?

Measures to deal with voltage ripple and load transients as factors affecting voltage accuracy are separate from measures to address voltage drops. First I’ll explain ripple and load transients. Where ripple is concerned, it can be reduced by speeding up the response to the feedback voltage from the output and through the control of switching within a narrow range relative to a reference voltage. With respect to load transients also, if rapid response is possible when there has been a sudden load fluctuation to return the changed voltage back to the voltage setting, then the output fluctuation can be kept small and will attenuate within a short time. For this purpose, these DC-DC converters are all provided with high-speed control modes, which are current mode, hysteresis mode, and H3Reg™ mode.

- Current mode and hysteresis mode are well-known as basic control modes, but what kind of mode is H3Reg?

H3Reg is ROHM’s proprietary fast transient response control. The block diagram shown below illustrates the のH3Reg control loop of the BD95601MUV-LB. H3Reg is a control method positioned as an advanced version of fixed on-time control, in which a voltage comparator rapidly compares a reference voltage with a feedback voltage, and rapidly switches an output switch. This is fast response that is independent of the switching frequency.

Waveforms during normal operation are as follows.

When the feedback voltage FB (obtained by dividing the output voltage for use in comparison) falls below the reference voltage REF, the comparator immediately turns on HG (a high-side output power transistor), and by supplying current to the output for a time determined by the equation on the right, raises Vout before turning off the transistor. Then, LG (a low-side output power transistor) is turned on until FB = REF to lower Vout.

If the load current (Io) momentarily increases, the output voltage drops considerably, and there are cases in which the voltage cannot rise to the preset voltage even when the time tON of the equation has elapsed. When this is detected, H3Reg control extends the time tON, as in the waveform on the left, to promote the recovery of Vout, that is, to hasten the transient response. Thereafter, if Vout has recovered, normal operation is resumed.

There are additional details, but fast transient response control, of which H3Reg is representative, is vital for satisfying the demands imposed on FPGA power supplies of providing low voltages and large currents with strict output voltage accuracy.

- How should I deal with voltage drops caused by board wiring, which is another problem relating to voltage accuracy?

There is a remote sensing method in which the voltage of the FPGA power supply pin is fed back, and there is also a POL (Point-of-Load) approach, in which the output of the DC-DC converter is placed on the side of the FPGA power supply pin insofar as possible.

- Alright, I understand that in addition to their basic functions, this DC-DC converter series can meet the requirements imposed on FPGA power supplies by virtue of their excellent fast transient response characteristic. However, during actual design, I expect that practical knowledge relating to component selection and board layout will be necessary to make full use of the performance of these ICs.

This is an extremely important consideration. In this interview, we have limited discussion to important aspects of IC functions and performance. But in order to actually address the requirements imposed on a power supply circuit for an FPGA, there are other matters that demand attention, such as selection of constituent components and layout. To address these matters, ROHM provides reference designs using the devices of this DC-DC converter series, and also provides separate design support. Below is an example of a reference design.

- I think that such examples and support will invaluable for designers.

As their name implies, FPGAs are programmable devices, and have different configurations and operation; hence their power supply requirements are also different. For this reason, I think design support is essential.

- Is there anything else you’d like to add?

As a matter of fact, we have not talked about efficiency, which is always important when discussing power supplies. It is certainly not the case that FPGAs do not require high efficiency; they are devices that use relatively large amounts of power, and so a high maximum efficiency is expected from the power supply being used. This DC-DC converter series achieves maximum efficiencies of around 90% through what is essentially a synchronous rectifier design, and under light loading have light-load efficiency-maintaining modes, of which Deep-SLLM™ (Simple Light Load Mode) is representative. The device specifications make it possible to satisfy demands relating to FPGA efficiency as well.

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Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

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