LT1461-2.5 Просмотр технического описания (PDF) - Linear Technology

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LT1461-2.5

Micropower Precision Low Dropout Series Voltage Reference Voltage Reference

Linear Technology 

LT1461-2.5

APPLICATIONS INFORMATION

The results of slotting the PC boards of Figures 5a and

5b are shown in Figures 6a and 6b. In this example the

slots can improve the output shift from about 100ppm to

nearly zero.

Long-Term Drift

Long-term drift cannot be extrapolated from acceler-

ated high temperature testing. This erroneous technique

gives drift numbers that are wildly optimistic. The only

way long-term drift can be determined is to measure it

over the time interval of interest. The erroneous tech-

nique uses the Arrhenius Equation to derive an accelera-

tion factor from elevated temperature readings. The

equation is:

The LT1461 long-term drift data was taken with parts that

were soldered onto PC boards similar to a “real world”

application. The boards were then placed into a constant

temperature oven with TA = 30°C, their outputs were

scanned regularly and measured with an 8.5 digit DVM. As

an additional accuracy check on the DVM, a Fluke 732A

laboratory reference was also scanned. Figure 7 shows the

long-term drift measurement system. The long-term drift

is the trend line that asymptotes to a value beyond 2000

hours. Note the slope in output shift between 0 hours and

1000 hours compared to the slope between 1000 hours

and 2000 hours. Long-term drift is affected by differential

stresses between the IC and the board material created

during board assembly.

AF

=

EA

eK





1

T1

–

1

T2

where: EA = Activation Energy (Assume 0.7)

K = Boltzmann’s Constant

T2 = Test Condition in °Kelvin

T1 = Use Condition Temperature in °Kelvin

To show how absurd this technique is, compare the

LT1461 data. Typical 1000 hour long-term drift at 30°C =

60ppm. The typical 1000 hour long-term drift at 130°C =

120ppm. From the Arrhenius Equation the acceleration

factor is:

PCB3

PCB2

PCB1

SCANNER

8.5 DIGIT

DVM

COMPUTER

1461 F07

FLUKE

732A

LABORATORY

REFERENCE

Figure 7. Long-Term Drift Measurement Setup

AF

=

e

0.7  1

0.0000863  303

–

1

403

=

767

The erroneous projected long-term drift is:

120ppm/767 = 0.156ppm/1000 hr

For a 2.5V reference, this corresponds to a 0.39µV shift

after 1000 hours. This is pretty hard to determine (read

impossible) if the peak-to-peak output noise is larger than

this number. As a practical matter, one of the best labora-

tory references available is the Fluke 732A and its long-

term drift is 1.5µV/mo. This performance is only available

from the best subsurface zener references utilizing spe-

cialized heater techniques.

Hysteresis

The hysteresis curves found in the Typical Performance

Characteristics represent the worst-case data taken on 35

typical parts after multiple temperature cycles. As ex-

pected, the parts that are cycled over the wider – 40°C to

125°C temperature range have more hysteresis than those

cycled over lower ranges. Note that the hysteresis coming

from 125°C to 25°C has an influence on the – 40°C to 25°C

hysteresis. The – 40°C to 25°C hysteresis is different

depending on the part’s previous temperature. This is

because not all of the high temperature stress is relieved

during the 25°C measurement.

9

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