BD905xx-C series synchronous rectifier converter for automotive secondary power supplies : Increasing Requirements Not Met by LDO

There have been ever growing numbers of automotive equipment coming into circulation, with an increasing trend toward the diversification of electronic devices that are used, including MCU and memory devices. This translates to a diversification of power supply specifications. In addition to the 5V and 3.3V main power supply units, subsidiary units supplying 1.8V, 1.5V, and 1.2V are also coming into increasing popularity. In many cases, for these low voltages it makes more sense to step down from 5V and 3.3V instead of directly reducing the voltage from an automotive 12V battery. These second-stage power supply units are now referred to as secondary power supplies.

Newer automotive equipment contains a great diversity of electronic components. As in the case of other varieties of electronic components, the current consumption of the CPU and MUC is on an increasing trend with lower voltages. In addition, the inclusion of DDR memory used for image processing requires multiple low-voltage power supplies.

Previously, for such local power supplies the LDO linear regulator was a preferred solution because of its simplicity and compactness. The trend in increasing current consumption, however, exerts a penalty of increases in heat dissipation, resulting in the need for a heat sink and other countermeasures, so much so that, depending on the circumstances, the LDO becomes unsuitable.

If an LDO is used to create 1.8 V/2A power supply from 3.3 V, a minimum of 1.5V × 2A＝3W loss would result due to a 1.5 V input/output difference. Even in a surface-mount package with a high heat dissipation effect, with a four-layer PCB configuration for increased heat dissipation, the maximum heat resistance that could be attained would be 30℃/W. In such a case, if the amount of heat generated is 90℃ with a Tj max value of 150℃, the allowable ambient temperature would be about 60℃. Given the operating temperature range required of automotive equipment, normally such performance would be deemed unacceptable.

Although the addition of a heat sink can be considered, the issues of footprint, the height, and the costs present daunting challenges. Consequently, a substitution by a switching regulator high in efficiency and low in heat dissipation can be considered; however, there are several hurdles that must be overcome.

Compared with the LDO, the switching regulator is considered to require larger numbers of components; in addition, it includes a magnetic component in the form of an inductor, resulting in an increased footprint and requiring greater amount of time and effort in the design and evaluation processes. Because of these issues, there is no denying that a search for a solution by means of the LDO is a worthwhile effort. However, given the demand for reliability, we would like to keep heat dissipation as low as possible, and above all, in terms of power savings there is no denying that the LDO is at a disadvantage in terms of efficiency.