Current Source Reference Linear Regulators

In many low noise applications, a linear regulator is an ideal solution for the power conversion due to its very low output voltage noise and low electromagnetic noise generation (compared to a charge pump or a switching regulator). The LT308x linear regulator family is ideal for these applications. This popular family of positive output linear regulators offers output currents from 200mA to 3A. Attractive features for this family include, a 1% initially accurate current source reference (the current source reference only requires a single external resistor to set the output voltage, allows the output voltage to go down to 0V, and makes paralleling multiple linear regulator outputs easy), a wide input voltage range (up to 36V or 40V depending on the device) and low output voltage noise (as low as 27µV_RMS, measured from 10Hz to 100kHz).

This new linear regulator architecture (employing a current source reference and a power buffer) offers several benefits over the historical voltage-reference based linear regulator. The most important is that this new architecture always operates in unity-gain mode. Therefore, bandwidth, transient response, load regulation, output voltage noise and PSRR are all virtually independent of output voltage. In voltage-reference based linear regulators, these parameters all scale up or down with output voltage changes, compromising performance. Some applications require the linear regulator to be disabled, removing power from the output and disabling the device to minimize power dissipation. Other applications require the linear regulator output to be at 0V to remove power from sensitive circuitry when desired, but a lower power state is not required.The LT308x family offers solutions for both cases.

For micropower shutdown needs, the 2.1A LT3086 is available. It has a shutdown control pin that disables the internal circuitry, resulting in a shutdown current less than 1µA over temperature and 0V at the output. 

 

 

The other devices in the LT308x family are classic-style 3-terminal regulators; they have an input pin, an output pin and a reference pin, but do not have a shutdown pin. For these devices, the shutdown state requires external circuitry. Shutdown forces the output to 0V but does not disable the internal circuitry, resulting in power dissipation that is not micropower. The figure below shows the LT3080 with shutdown circuitry added. In this circuit, the output goes to 0V when the gates of MOSFETs Q1 and Q2 are driven high. MOSFET Q1 (across the R_SET resistor) forces the output voltage down to 0V. MOSFET Q2 at the output clamps V_OUT to ground, ensuring the output remains at 0V at all times.

 

 

If a micropower shutdown state is desired, another option is to simply use a switch in series with the power supply input. However, the linear regulator’s output voltage may remain high for a considerable time while other system voltages have turned off (depending on the output load), defeating the goal of removing power from sensitive analog circuitry in a timely manner.

Negative Output Linear Regulators and Shutdown

For negative output linear regulators, the LT3090 and LT3091 (–36V/600mA and –36V/1.5A respectively) have a unique bidirectional shutdown feature that accepts either a positive or a negative voltage on the shutdown control pin to disable the device. This prevents having to generate a negative control signal if one is not readily available. Our complete family of voltage reference-based and current source reference-based bidirectional shutdown negative linear regulators can be found here.

 

 

The following table is a complete list of our current source reference linear regulators. These low noise, wide input voltage range devices offer either true shutdown or provide an easy way to have 0V at the output with a few external components.