App Note 32

"High efficiency linear regulators." 12 pages.

This app note discusses tricks to improve the efficiency of linear regulators by decreasing the input-to-output voltage drop. For example, Figure 5 shows a SCR preregulator for an AC-to-DC regulator, similar to Figure 7 in App Note 2. The SCR circuit keeps the voltage drop across the LT1086 around 2V, thus improving efficiency. (There are a lot of circuits here that are improved versions of circuits from App Note 2.)

Most of these applications involve using a DC-to-DC switching regulator in front of the linear regulator to control the voltage drop. We've also seen this approach previously (see App Note 2 Figure 5 and App Note 29 Figure 46). Figure 8a shows a DC-to-DC regulator scheme. Figure 8a has two switching regulators, one to control the voltage (6.75V) at the input of the LT1083, and another to provide 30V to overdrive the gate of the main switch. This application has a fixed output voltage, so the main switching regulator controls the input voltage of the LT1083. Figure 8b is used where the output is variable, and the feedback path of the main switching regulator measures the voltage drop across the LT1083.

Figure 11 is a linear regulator design with only 400-mV drop out. The switching regulator in this schematic provides a large voltage only for the gate of the pass transistor. The best circuit, shown in Figure 13, combines Figures 8a and 11 into a regulator with a linear output and an efficiency between 76% and 86%. The last circuit, shown in Figure 15, is a micropower version of the previous circuits, using the LT1020 linear regulator and using its integrated comparator to implement the switching regulator loop.

Appendix A, Achieving Low Dropout, discusses the tradeoffs involved in choosing the right device for the pass element in a linear regulator. The final sentence on page AN32-10 is a close runner-up for best quote: "Readers are invited to submit results obtained with our emeritus thermionic friends, shown out of respectful courtesy." (For an example, see Figure 11A in App Note 2.)

Appendix B discusses the LT1083 family of low drop out regulators. Appendix C discusses the measurement of power consumption and shows a circuit for measuring the instantaneous power in a 120V line. This circuit is a very useful instrumentation scheme. Unfortunately, the Analog Devices 286J isolation amplifier is no longer available (and the link to the datasheet on Analog Devices' website is broken). However, 286J amplifiers are still available from second sources, but I don't have any experience with them.

The best quote is actually a Freudian slip on pages AN32-8 and 9: "A drop at the pre-regulator's output (Pin 3 of the LT1020 regulator, Trace A, Figure 16) causes the LT1020's comparator to go high. The 74C04 inverter chain switches, biasing the P-channel MOSFET switch's grid (Trace B)." Of course, he meant "gate" instead of "grid", but I think we can forgive him for having vacuum tubes on the brain. (I sincerely hope that Linear Technology never fixes this typo!)