DC/DC Converter Solutions that
Meet CISPR25 Class 5 Standards

At the Int’l Automotive Electronics Technology Expo (CAR-ELE JAPAN), held at Tokyo Big Sight from January 15 to 17 2019, ROHM exhibited a wide variety of semiconductor devices for use in various automotive applications. Among these products, we here introduce DC/DC converter solutions featuring low noise.

In recent years it has become necessary for electronic and electrical equipment in automobiles to satisfy strict EMC requirements. CISPR25 is an EMC standard for automobiles established by the IEC, and at present is widely adopted by numerous automakers and automotive equipment manufacturers. CISPR25 stipulates EMI regulation values by frequency band. There are five classes; Class 5 stipulates the strictest values.

DC/DC Converter Solutions for Automobiles that Satisfy CISPR25 Class 5
Regulations Without Common Mode Filters

The following are DC/DC converter solutions capable of generating the low voltages required by automotive electronic equipment from a battery voltage (B+). The BD9P233MUF-C, equipped with Nano Pulse Control®, is capable of 1-step step-down with high step-down ratios from a maximum 36 V to 3.3 V at the high switching frequency of 2.2 MHz required by automotive applications, to generate 3.3 V from the battery voltage. From this 3.3 V, other DC/DC converters then step down to 1.8 V, 1.5 V, or 1.0 V.

BD9P233MUF-C（Vo: 3.3 V, Io: 2 A Max.）

• ・ROHM's unique Nano Pulse Control® achieves high step-down ratios
• ・Spread spectrum functions, variable switching frequency
• ・Low-voltage operation from Vin = 3V possible

BD9S200MUF-C / BD9S400MUF-C
（Vo: variable, Io: 2A/4A Max.）

• ・Output voltage, phase compensation adjustable using external R/C
• ・Lineup of pin-compatible devices for 2A/3A/4A

BD9S111NUX-C（Vo: 1.8V, Io: 1A Max.）

• ・Fixed output voltage, internal phase compensation for easy designs
• ・Lineup of pin-compatible devices for 0.6/1.6 A, various output voltages

Another feature of these DC/DC converters is their ability to meet CISPR25 Class 5 standards without a common mode filter. Common mode noise is one type of conductive noise, and common mode filters are necessary to eliminate or reduce such noise.

The figures on the right show the results of CISPR25 conductive noise tests for the BD9P233MUF-C at switching frequencies of 2.2 MHz and 400 kHz, with the spread spectrum function turned on in each case, and with no common mode filter used. It can be seen that the noise levels are below the Class 5 regulation values in each case.

As stated above, automotive electronic and electrical devices, as well as components used in such devices, absolutely must clear CISPR25 requirements. There are limits to measures for dealing with preexisting noise, and such measures are troublesome, entailing both time and added costs. As measures taken in advance, selection of low-noise components and a robust board layout design are important.