TABLE OF CONTENTS As mentioned previously, it is often necessary to have an adjustable voltage reference. The term adjustable can be ambiguous though. Some high-performance references have a dedicated TRIM pin, whereas others have an ADJust pin. What's the difference? The TRIM pin works with an external trimmer and allows one to trim the nominal reference voltage, usually over a small range of several hundred millivolts (i.e., 5 volts 300 mV). It allows the designer to trim out any system errors, by deliberately offsetting the reference voltage to a slightly different value from the nominal. Other series references have a dedicated adjustment pin (ADJ), which when used in conjunction with an external resistor network, allows one to adjust the reference voltage in a range from hundreds of millivolts to many volts. This allows the designer to provide a custom reference voltage or to be able to have a wider trim range than is otherwise available. Next we will look at some outstanding adjustable series types.
The first one we will look at is a remarkable new product from Linear Technology, the LT6650. This is a versatile, adjustable micropower 400-mV (sub-bandgap) reference, which includes a rail-to-rail buffer amplifier. Its schematic is shown in Figure 15.18A. It is provided in a low-profile (1-mm) five-lead SOT-23 surface-mount package. Unlike most bandgaps, the LT6650 operates down to a supply rail of 1.4 volts and typically draws less than 6 ?A. It can be configured to work as...
