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LTC2995

2995f

OPERATION

Overview

The LTC2995 combines the functionality of a temperature

measurement and monitor device with a dual voltage

supervisor. It provides a buffered voltage proportional to

the absolute temperature of either an internal or a remote

diode (VPTAT) and compares this voltage to thresholds that

can be set by external resistor dividers from the on-board

reference (VREF).

The LTC2995 also provides four voltage threshold

inputs that are continuously compared to an internal 0.5V

reference allowing two systems voltages to be monitored

for undervoltage and overvoltage conditions.

Diode Temperature Sensor

Temperature measurements are conducted by measuring

the voltage of either an internal or an external diode with

multiple test currents. The relationship between diode

voltage VD and diode current ID can be solved for absolute

Temperature in degrees Kelvin T:

⎛

⎝⎜

⎞

⎠⎟

η k

where IS is a process dependent factor on the order of

10–13A,

η is the diode ideality factor, k is the Boltzmann

constant and q is the electron charge. This equation shows

a relationship between temperature and voltage dependent

on the process depended variable IS. Measuring the same

diode (with the same value IS) at two different currents

(ID1 and ID2) yields an expression independent of IS:

ID2

ID1

⎛

⎝⎜

⎞

⎠⎟

η k

VD2 –VD1

Series Resistance Cancellation

Resistance in series with the remote diode causes a positive

temperature error by increasing the measured voltage at

each test current. The composite voltage equals:

⎛

⎝⎜

⎞

⎠⎟

VD + VERROR =η

+ RS ID

The LTC2995 removes this error term from the sensor

signal by subtracting a cancellation voltage VCANCEL. A

resistance extraction circuit uses one additional current

measurement to determine the series resistance in the

measurement path. Once the correct value of the resistor is

determined, VCANCEL equals VERROR. Now the temperature

to voltage converter input signal is free from errors due

to series resistance.

LTC2995 can cancel series resistances up several hundred

ohms (see Typical Performance Characteristics curves).

Higher series resistances cause the cancelation voltage

to saturate.

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