LINEAR Convert Temperature to Current with High Linearity using LT3092 – Design Note 484 Current Source Specification Owner's Manual User Guide

Convert Temperature to Current at High Linearity with LT3092 Current Source – Design Note 484 Todd Owen Electronics 101 The fl at temperature coeffi cient of the internal reference

One of the fi rst lessons in a basic electronics course current (highlighted in Figure 2) is as good as many voltage covers the symbols for resistors, capacitors, induc- references. Low TC resistors do not need to be used; the tors, voltage sources and current sources. Although temperature coeffi cients of the external resistors need each symbol represents a functional component of a only match one another for optimum results.

real-world circuit, only some of the symbols have direct

No frequency compensation or supply bypass capacitors

physical counterparts. For instance, the three discrete

are needed. Frequency compensation is internal and the

passive devices—resistors, capacitors, inductors—can

internal reference circuitry is buffered to protect it from

be picked off a shelf and placed on a real board much

line changes.

SE PI CU RR EN (μ A)

as their symbolic analogs appear in a basic schematic.

Likewise, while voltage sources have no direct 2-terminal No input-to-output capacitors are required. While exten- analog, a voltage source can be easily built with an off- sive testing has been done to ensure stable operation the-shelf linear regulator. under the widest possible set of conditions, complex load impedance conditions could provoke instability. As such,

The black sheep of basic electronics symbols has long

testing in situ with fi nal component values is highly rec-

been the 2-terminal current source. The symbol shows

ommended. If stability issues occur, they can be resolved

up in every basic electronics course, but Electronics 101

with small capacitors or series RC combinations placed

instructors must take time to explain away the lack of

on the input, output, or from input to output.

a real-world equivalent. The symbol presents a simple

electronics concept, but building a current source has, The LT3092 offers all the protection features expected until now, been a complex undertaking. from a high performance product: thermal shutdown, overcurrent protection, reverse-voltage and reverse-

A Real 2-Terminal Current Source

current protection. Because a simple resistor ratio sets

With the introduction of the LT®3092, it is now as easy the current, a wide variety of techniques can be utilized to produce a 2-terminal current source as it is to create a to adjust the current on the fl y. The LT3092 can also be voltage source. Figure 1 shows how the LT3092 uses an confi gured as a linear regulator without output capacitors internal current source and error amplifi er, together with for use in intrinsic safety environments. the ratio of two external resistors, to program a constant

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of output current at any level between 0.5m A and 200m A. Linear Technology Corporation. All other trademarks are the property of their respective owners.

VIN – VOUT = 1.2V TO 40V

LT3092 IN

SET OUT

ROUTRSET

DN484 F01

ISOURCE = 10μA • 9.900

TEMPERATURE (°C) DN484 F02

Figure 1. 2-Terminal Current Source Requires Only Two Resistors to Program

Figure 2. SET Pin Current vs Temperature