Analog Devices Ltd

A Voltage Reference with the Power of a Regulator

The LT6658 combines a performance voltage reference with 2 high-current output buffers.  The LT6658 offers the performance of a precision reference while being able to source up to 150mA and 50mA.  In parallel, the buffer outputs can be combined for 200mA of drive current.

The LT6658 is currently in production with a 2.5V version. Each output of this option can be separately configured for a voltage between 2.5V and 6V using 2 external resistors.  The complete LT6658 family will include versions with 1.2V, 1.8V, 3V, 3.3V, and 5V references to be available in the first half of 2017.  Prices start at $2.35 each in 1,000-piece quantities.

The LT6658 was designed to serve as both a reference and a regulator combined.  Both outputs have 0.05% initial accuracy, 10ppm/°C temperature drift and only 1.5ppm of low frequency noise.  The buffered outputs reduce the typical load regulation error to only 0.1ppm/mA with full load current.  The outputs have excellent supply rejection and are stable with up to 50μF over the entire specified operating range.  A typical application would be for the LT6658 to drive the reference input for a high resolution ADC or DAC using one output, while simultaneously supplying power to other devices using the second output.

What is Unique about the LT6658? 

Offers Very High Precision with High Current Capability

Allows the ability to drive multiple circuits with a single reference

Allows the ability to operate as a precision voltage regulator

Offers significantly better load regulation than most voltage references

Offers Excellent AC Performance

Offers significant improvement over an op amp buffer that can become unstable

Can drive large capacitive loads (such as ADCs) without ringing or instability

Includes Two Isolated Outputs with Separate Supply Pins

The channel-to-channel isolation ensures noise on one output does not affect other

Ensures outstanding Power Supply Rejection (PSRR)

Tracking

Includes Two Functions in a Single Package

Eliminates a regulator (includes supporting components, board space and routing)

Localizes the power distribution to a single IC and simplifies board layout

Offers cost savings

Without an LT6658, achieving LT6658-like performance requires the combination of a precision voltage reference with a dual buffer, or the combination of a voltage regulator with a precision reference.  Such discrete circuits have performance drawbacks, higher cost and a larger PCB footprint.  Since most amplifiers are not designed for precision reference-type applications, discrete buffers will often contribute significant errors such as line regulation.  Similarly, an LDO with a reference will exhibit combined errors such as offset and temperature drift.

High Fanout

The LT6658 “Refulator” is ideal for high fanout precision reference applications for ADCs and DACs.  With excellent AC performance, the LT6658 exhibits fast settling, even when used with SAR and delta-sigma ADCs, which commonly appear as a switched capacitive load.  The LT6658 avoids excessive ringing or instability which is typical in an op amp buffer, in the presence of switching transients.  For most voltage references, supplying appreciable current can generate significant errors. With only 0.1ppm/mA of load regulation and its ability to comfortably source 150mA and 50mA, variation in loads will not affect each other.

High Capacitive Loads

The LT6658 is designed to drive loads from 1uF to 50uF.  When driving a capacitor switching circuit, such as a SAR

or delta-sigma converter, it remains stable and settles quickly.  The buffered outputs ensure that capacitive glitching on one output will not affect the other output. Both LT6658 outputs are ideally suited to drive separate ADCs.

Regulator Applications

The LT6658 is ideal as a regulated power source for microcontrollers and precision analog circuits.  Whether the application requires very high precision power, which can be large and costly, or simply a low noise regulator, either output of the LT6658 can handle the job. With combined capability, the LT6658 can eliminate the need for a separate regulator, supporting components, board space and routing.

Tracking Applications

The LT6658 outputs track each other over temperature and load, making them suitable as ratiometric supply voltages.  This is valuable in difference amplifiers, bridge circuits and other sensor circuits.  If the 2 LT6658 outputs are used for separate ADCs, the tracking feature maintains consistent conversion results between both.

Isolation Applications

The LT6658 outputs are buffered separately and exhibit excellent channel-to-channel isolation, especially if they are powered separately.  Combined with their ability to supply

appreciable load current, corruption on one output will not affect the other.

Low Noise Applications

The LT6658 offers very low noise performance, which can be reduced even further using the noise reduction pin. By placing a capacitor on the NR pin, the noise bandwidth of the internal reference is limited; with no capacitor on the NR pin, the voltage noise density is around 150nV/rt(Hz) until the bandwidth is limited by the output buffer.  Adding a 10uF capacitor to the NR pin will roll off the noise bandwidth to about 8nV/rt(Hz).  This can be reduced further by increasing the size of the capacitor on the NR pin.

Summary:

Customer demand for precision voltage references with regulator-like drive has given rise to a new type of reference, called the “refulator.”  With its outstanding features, the LT6658 will serve a broad market for small, precision and low power applications.

Two Outputs
Source Current..............................................150mA and 50mA
Sink Current...................................................20mA
Temperature Drift ....................................................10ppm/°C
Accuracy...................................................................0.05%
Load Regulation.......................................................0.1ppm/mA
Output Tracking ......................................................±150uV
PSRR.........................................................................> 100dB at 10Hz
Low Frequency Noise (0.1Hz to 10Hz)..................1.5ppm
Maximum Supply.....................................................36V

Noise Reduction......................................................YES
Output Disable.........................................................YES
Current Protection...................................................YES
Thermal Protection..................................................YES
Small Footprint.........................................................YES
Temperature Range.................................................-40°C to 125°C