By Frank Kolanko 

People have been predicting the demise of low-drop-out linear regulators in cars for many years. Yet low-drop-out (LDO) regulators continue to survive and even thrive because they're cheap and easy to use. In this article I'll highlight the complexity of LDO regulators, look at what's new in the market (yes, there are some advances), and examine the move toward switching regulators as automotive power requirements continue to rise.

New linear regulators
What's new with linear regulators? Let's start with the output capacitors. Ceramic capacitors in the familiar 0402 package are the capacitors of choice these days. That's mostly because improved materials have raised their temperature range from 125°C (257°F) to 150°C (302°F) and improved mounting methods reduce thermal shock and improve vibration resistance. These caps' small size reduces their inductive component, for better high-frequency performance. But the key trait of concern for ceramic capacitors is their low equivalent series resistance, or ESR.

The basic closed-loop linear regulator system consists of an error amplifier, output driver, and loads. Figures 1 through 3 detail the closed-loop frequency response of a bipolar linear regulator, highlighting the effect of changing the output capacitor ESR while keeping system setup and output capacitance values the same. A capacitor with a 1 ohm ESR (Figure 1) is stable, while one with a very low 0.01 ohm ESR (Figure 2) is unstable; the larger ESR of 3 ohms (Figure 3) is also unstable.

A rule of thumb for switching power supplies--that whenever the closed-loop gain is greater than or equal to 1, the closed-loop phase will never come to within 30 degrees of 360--is also very applicable here.

Most linear regulators don't give you any accessible points to measure stability curves. Instead, chip manufacturers include graphs that show where the expected areas of stability are versus the output capacitor's ESR value. Figure 4 shows the typical difference in unstable and stable regions for the output capacitor ESR, dependent on the output regulator voltage over a variation in output current. Figure 5 shows the difference in unstable and stable regions based on the output capacitor value.

Load response times will typically vary inversely with IC stability product regions. The loop response time has been slowed down to provide better stability. An external output capacitor will compensate for most transient requirements. Be sure to provide a capacitor large enough for your requirements. Use your typical capacitor equation:

to calculate the capacitor value based on the system load magnitude, transient time, and system allowable drop in output voltage.