DC-DC converter uses spread spectrum clock to reduce EMI noise

DC-DC converter uses spread spectrum clock to reduce EMI noise

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By Nick Flaherty

The R1275 allows connecting to a variety of input sources, such as battery-powered portable devices, industrial supplies and automotive batteries. It uses a Spread Spectrum Clock Generator (SSCG) to modulate the clock frequency, spreading the EMI energy amongst many frequencies instead of concentrated at one point. This reduces the potential interference of the base frequency and its harmonics.

The switching frequency is internally set to 2MHz, but it is also possible to synchronise with another DC/DC converter by applying an external clock signal from 1.8MHz to 2.2MHz. Especially in case the power supply consists of multiple DC/DC converters, the individual switching frequencies can result in a significant noise in common ground wiring. By synchronizing all DC/DC converters to one frequency, the resulting noise is easier to filter out and will not interfere with the control of the converters.

The R1275 has a special protection for cold cranking conditions. A power supply of automotive electronic systems can be subject to a significant input voltage drop when the engine starts, so to ensure that the R1275 is able to maintain its output voltage during such event, the clock signal is lowered linearly to a quarter of the normal frequency. This will make the on-duty larger than the normal maximum duty, which reduces the voltage difference between input and output and provides continuous operation in cold cranking condition.

Additional safety features include input under- and over-voltage lockout, hiccup overcurrent protection, internal soft-start circuitry, peak current limit, thermal shutdown, output over-voltage and under-voltage protection. A Power-Good pin provides a fault signal when the chip is not ready for normal operation or any of the protection circuits is active.

The R1275 is available in a thermally enhanced 5.2 x 6.2 x 1.5mm HSOP-18 package for consumer, industrial and automotive applications.

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