Low EMI automotive power designs: Page 5 of 7

February 05, 2015 //By Jeff Gruetter
Low EMI automotive power designs
Jeff Gruetter, Product Marketing Engineer, Linear Technology Corporation considers low EMI automotive power design issues.
always-on processors operating at once, there is a significant power demand on the battery even when the ignition is turned off.  Collectively, hundreds of milliamps (mA) of supply current can be required to power these always-on processors, which could completely drain a battery in a matter of days.
 
Therefore, the quiescent current of these power ICs needs to be drastically reduced in order to preserve battery life without increasing the size or complexity of the electronic systems.  Until recently, the requirement of high input voltage capability and low quiescent currents were mutually exclusive parameters for a DC/DC converter.  About a decade ago, several automotive manufacturers created a low quiescent current target of <100µA for each always-on DC/DC converter, but today lower than 10µA is preferred. Luckily, a new generation of power ICs is now available which offers quiescent currents of less than 2.5µA in standby mode.  

New Alternatives

Until now, there was no sure way to guarantee that EMI could be suppressed and efficiency requirements attained via power IC selection. However, the LT8640 Silent Switcher® regulator makes this possible. The LT8640 is the second device in a family of Silent Switcher high voltage synchronous buck regulators. It is a 5A (continuous current with 7A peaks), 42V input capable synchronous step-down switching regulator. As can be seen in Figure 3, the EMI emissions are 10dB to 30dB below the automotive CISPER 25, Class 5 peak limits without spread spectrum enabled. Spread spectrum lowers these levels by another 5dB to 10dB across the most critical automotive frequency band.  This combination reduces EMI emissions by more than 25dB when compared to current state-of-the-art switching regulators. The LT8640 in the graph below was switching at 2MHz with a load current of 4A and no external EMI shielding was required.

Figure 3. LT8640 Radiated EMI Performance With/Without Spread Spectrum (fSW=2MHz, ILOAD=4A)
 
The schematic for the LT8640 is shown in Figure 4.  Synchronous

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