Introduction
This article describes a BUCK converter design using the AD780AN to generate regulated outputs. The example circuits step 15 V down to 2.5 V or 3.0 V.
Circuit Design
15 V to 2.5 V design
15 V to 3.0 V design
AD780 Overview
AD780 is a high-precision bandgap reference that provides either 2.5 V or 3.0 V output from input voltages between 4.0 V and 36 V. The output level (2.5 V or 3.0 V) is selected by tying pin 8 to ground or leaving it open, according to the device datasheet.
The device exhibits low initial error, low temperature drift, and low output noise, and it can drive capacitive loads. These characteristics make it appropriate for improving the performance of high-resolution ADCs and DACs and for general precision reference applications. The low headroom design allows a 3.0 V output from a 5.0 V ±10% supply, which can increase ADC dynamic range compared with a 2.5 V reference.
AD780 can source or sink up to 10 mA and can operate in series or shunt configurations to provide positive or negative outputs without external components. When a 1 μF bypass capacitor is placed on the supply pin, the device remains stable across load conditions and does not exhibit a "potentially unstable" operating region reported for some competing parts.
A comparator is included downstream of the reference in this design to help hold the output stable and protect downstream circuitry from overvoltage conditions.
Design Results
One notable feature of AD780 is the low headroom requirement of its internal high-gain amplifier, which enables a precise 3.0 V output from an input as low as 4.5 V (or a 2.5 V output from a 4.0 V input).
