Increased use of electronic systems in automotive applications is placing greater demands on the power systems that supply them. With pc-board space at a premium, the higher levels of load power necessitate system-level solutions with very high conversion efficiency and high power density.

The LM5010A, which is one of a family of hysteretic parts developed by National Semiconductor, can address these needs while still coping with the tough environment found in automotive systems. The design example system in this article achieves 90% full power efficiency whilst maintaining an IC temperature rise of less than 30°C above ambient.

Although trucks and buses may operate from 24V, most automotive applications still use a 12V battery system. When the engine starts in a 12V system, the high current drawn by the starter motor (>100A) can transiently reduce the battery voltage down to 6V. At the other extreme, voltage surges due to load dump can transiently increase the system voltage to around 70V. Whilst the design should be optimised to operate at highest efficiency at the nominal 12V level, the power-supply system should still be able to provide full power to the electrical systems over the full operating input voltage range.

Converter control

Most requirements for non-isolated step-down conversion can be met with the buck-converter topology (Fig 1).

The output voltage from this converter is approximately the input voltage multiplied by the duty cycle of the buck converter switch. Controlling the switching device's duty cycle can therefore regulate the converter's output voltage. Traditionally, this is achieved with running at a constant switching frequency and then varying the on-time of the device. However, for a buck converter to step 70V down to less than 5V, a duty cycle significantly below 10% is required.

In practice, this duty cycle combined with the trend to ever higher switching frequencies requires that the buck-converter switch is held on for a very short time, typically only a few hundred nanoseconds. This creates a significant problem for traditional high-frequency PWM controllers.

Another way to control the duty cycle is to run the switch with a constant fixed on-time and then vary the switching frequency to maintain regulation (pulse frequency modulation). This mode of operation is used with hysteretic converters.

As the equation shows, when a traditional hysteretic buck converter runs in continuous conduction mode with a constant on-time, the switching frequency F_S will vary significantly as input voltage changes in order to maintain the output voltage at a constant level. In this equation, V_D represents the forward conduction drop of the buck converter diode. For an automotive application running with an input voltage range of 6V to 70V, the variation in switching frequency will vary over a range of around 10:1.

As a result, the hysteretic operation simultaneously allows for a high step-down ratio and high switching frequency, both of which are essential for automotive solutions with restrictions on power-supply-unit (PSU) area. The only problem here is that the wide switching-frequency range can sometimes lead to unpredictable EMI and interference.