DC-DC conversion from car battery meets stringent EMI standards: Page 3 of 5

September 23, 2020 //By Zhongming Ye, Analog Devices
DC-DC conversion from car battery meets stringent EMI standards
Noise-sensitive applications in harsh automotive and industrial environments require low noise, high efficiency buck regulators that can fit into tight spaces. This article presents state-of-the-art solutions that save space while also achieving low EMI and excellent thermal performance.

To improve the EMI performance, the circuit is set to operate in spread spectrum mode: SYNC/MODE = INTVCC. A triangular frequency modulation is used to vary the switching frequency between the value programmed by RT to approximately 20% higher than that value—that is, when the LT8636 is programmed to 2 MHz, the frequency will vary from 2 MHz to 2.4 MHz at a 3 kHz rate.

From the conducted EMI spectrum, it is obvious the peak harmonic energy is spread out, reducing the peak magnitude at any particular frequency—noise is reduced due to the spread spectrum function by at least 20 dBµV/m. From the radiated EMI spectrum, it is also obvious that spread spectrum mode reduces the radiated EMI as well. This particular circuit meets the stringent automotive CISPR 25 class 5 radiated EMI specification with a simple EMI filter at the input side.

Figure 4. CISPR 25 radiated EMI emission with and without spread spectrum mode.

High Efficiency Over the Entire Load Range

The number of electronics devices in the automotive applications is only increasing, with most devices demanding more supply current with each design iteration. With active load currents so high, heavy load efficiency and proper thermal management are top priorities—robust operation depends on thermal management, with unmitigated heat production possibly resulting in costly design problems.

System designers are also concerned with light load efficiency, which is arguably just as important as heavy load efficiency, since battery life is mostly determined by the quiescent current at light load or no load. Trades-off in the silicon, as well as system level design, have to be made among full load efficiency, no load quiescent current, and light load efficiency.

It might seem straightforward that in order to achieve high efficiency at full load, the RDS(ON) of the FET, especially the bottom FET, should be minimized. However, a transistor with low RDS(ON) usually has a relatively high capacitance, with an associated increase in switching and gate drive losses, plus a larger die size and cost. In contrast, the LT8636 monolithic regulator has very low MOSFET conduction resistances, enabling exceptional efficiency in full load conditions. The maximum output current for the LT8636 is 5 A continuous and 7 A peak in still air without any additional heat sink, simplifying robust design.

To enhance light load efficiency, regulators that operate in low ripple Burst Mode® keep the output capacitor charged to the desired output voltage while minimizing the input quiescent current while minimizing output voltage ripple. In Burst Mode operation, current is delivered in short pulses to the output capacitor, followed by relatively long sleep periods, where most of the control (logic) circuits are shutdown.

In order to achieve higher light load efficiency, a larger value inductor is preferred since more energy can be delivered to the output during the short pulses and the buck regulator can remain longer in sleep mode between each pulse. By maximizing the time between pulses, and minimizing the switching loss of each short pulse, monolithic buck converter quiescent current can approach 2.5 μA in a monolithic regulator, such as the LT8636. This number is compared with tens of µA or hundreds of µA of the typical parts on the market.

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