Selecting the Right Power Supply Regulators for Automotive Secondary Rail Applications

June 02, 2016 // By Jerome Johnston, Intersil Corporation
Today’s car manufacturers are focusing their innovations on the sophisticated cockpit electronics that improve driver safety and the overall driving experience. More and more consumers are factoring a car’s advanced driver assistance systems (ADAS) and infotainment features into their buying decisions. These systems combine several features that require heavy-duty signal processing, such as forward-looking smart cameras for detecting and classifying objects, back-up camera electronic control units (ECUs), and head-unit center information displays, to name a few. As a result, they require higher current power supply regulation at low voltages.

The buck regulators in these systems must generate point-of-load (POL) voltages as low as 0.6V for the GPUs, FPGAs, DSPs and other higher current devices receiving their power from a 5V or 3.3V primary supply rail as shown in Figure 1. Several clever IC design choices have yielded a new generation of 3A, 4A and 5A synchronous buck regulators able to address the varying load requirements from entry-level to luxury automobiles. To help system designers understand their benefits, it is useful to understand the architectural choices made when developing these fully integrated devices, as there are many different ways to implement a buck regulator.

This article examines the asynchronous buck versus synchronous buck configuration. We’ll also discuss the tradeoffs N-channel or P-channel transistors used for the switches in the synchronous buck configuration. We’ll then highlight a family of fully optimized 3A, 4A and 5A sync buck regulators, and show how their wettable flank QFN packages pass visual inspection during the printed circuit board (PCB) assembly process.


Figure 1. Typical automotive power supply architecture.
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The Asynchronous Buck Regulator


As you can see in Figure 2, the asynchronous buck DC/DC converter has one switch (S1) that is driven on and off to control the duty cycle ratio. The circuit includes a diode that acts as a secondary switch when the potential across it causes forward biasing. When switch S1 is on, the input voltage is connected to the inductor, causing current to build up in the inductor until switch S1 is shut off. When S1 is switched off, the current flowing through the switch to the inductor is interrupted. 


Figure 2. Asynchronous Buck Implementation


However, due to the nature of the inductor, the current flowing through it wants to continue flowing in the same direction. For this to happen, the voltage polarity across the inductor changes, allowing the current to flow

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