Precision meets power, it’s the regulator

September 25, 2018 // By Michael B. Anderson
Precision analog designers often rely on the quiet humble voltage reference to power their DAC and ADC converters. This is beyond the fundamental role of the reference, which is designed to solely provide a clean precise stable voltage to a converter’s reference input.

With some caveats, this usually works, which is why references are continually expected to address increasingly higher current applications.  After all, if the reference can power the converter, why not the analog signal chain, or another converter and the list grows--and why not?

There are many times in the design process where there is a decision between precision and power. The brute force approach suggests using a reference when precision is demanded, and a regulator whenever milliwatts of power are demanded.  Besides the additional board space and cost, separate signals must be routed, even if their nominal voltages are the same.  And, if a high precision voltage source is required to provide milliwatts of power, the designer is forced to buffer a reference.  The LT6658 solves this dilemma by providing two low noise precision outputs with a combined 200mA output current and world class reference specifications.

 

General overview


Fig. 1: LT6658 typical application.

The LT6658 is a precision low noise, low drift regulator with the accuracy specifications of a reference and the power of a linear regulator. The LT6658 boasts 10ppm/°C drift and 0.05% initial accuracy. On the power side the LT6658 has two outputs providing 150mA and 50mA each with 20mA active sinking capability. To maintain accuracy, load regulation is 0.1ppm/mA. Line regulation is typically 1.4ppm/V when the input voltage supply pins are tied together and less than 0.1ppm/V when the input pins are provided with independent supplies. To get a better understanding of the LT6658 features and how it works, a typical application is shown in Figure 1. The LT6658 consists of a bandgap stage, a noise reduction stage and two output buffers. The bandgap and two output buffers are powered separately to provide exceptional isolation. Each output buffer has a Kelvin sense feedback pin for optimum load regulation.

The noise reduction stage consists of a 400W resistor with a pin provided for an external capacitor. The RC network acts as a low pass filter band-limiting the noise from the bandgap stage. The external capacitor can be arbitrarily large reducing the noise bandwidth to a very low frequency.

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