LT6657
applicaTions inForMaTion
Figure14showsthemaximumambienttemperaturelimits
Position the input and load capacitors close to the part.
AlthoughtheLT6657has130dBDCPSRR,thepowersup-
ply should be as stable as possible to guarantee optimal
performance. A plot of the 0.1Hz to 10Hz low frequency
noise is shown in the Typical Performance Characteristics
section. Noise performance can be further improved by
wiring several LT6657s in parallel as shown in the Typical
Applications Section. With this technique the noise is
reduced by √N, where N is the number of LT6657s used.
for different V and load condition using a maximum
IN
junction temperature of 125°C in the MSOP-8 package.
If the load current exceeds 10mA the parts could begin to
current limit. In this case, the output voltage is no longer
regulated and the part could dissipate much more power
and operate hotter than the graph shows.
125
V
= 2.5V
OUT
115
105
95
Noise in any frequency band is a random function based
on physical properties such as thermal noise, shot noise,
andflickernoise.Themostprecisewaytospecifyarandom
error such as noise is in terms of its statistics, for example
asanRMSvalue.Thisallowsforrelativelysimplemaximum
error estimation, generally involving assumptions about
noise bandwidth and crest factor. Unlike wideband noise,
low frequency noise, typically specified in a 0.1Hz to 10Hz
band, has traditionally been specified in terms of expected
error, illustrated as peak-to-peak error. Low frequency
noise is generally measured with an oscilloscope over a
10secondtimeframe. Thisisapragmaticapproach, given
that it can be difficult to measure noise accurately at low
frequencies, and that it can also be difficult to agree on the
statistical characteristics of the noise, since flicker noise
dominates the spectral density. While practical, a random
sampling of 10 second intervals is an inadequate method
for representation of low frequency noise, especially for
systems where this noise is a dominant limit of system
performance.Giventherandomnatureofnoise,theoutput
voltage may be observed over many time intervals, each
giving different results. Noise specifications that were
determined using this method are prone to subjectivity,
and will tend toward a mean statistical value, rather than
the maximum noise that is likely to be produced by the
device in question.
85
75
65
55
10mA SINK MS8
10mA SOURCE MS8
45
0
5
10 15 20 25 30 35 40
(V)
V
IN
6657 F14
Figure 14. Maximum Ambient Operating Temperature
Withalargeinputvoltageandsourcingcurrent,aninternal
thermal shutdown protection circuit limits the maximum
power dissipation. When sinking current, there is no need
for thermal shutdown protection because the power dis-
sipation is much smaller and the sinking current limit will
give some load protection.
Noise Performance and Specification
The LT6657 offers exceptional low noise for a bandgap
reference;only0.5ppm inthe0.1Hzto10Hzbandwidth.
P-P
As a result system noise performance may be dominated
by system design and physical layout. Care is required to
achieve the best possible noise performance. The use of
dissimilar metals in component leads and PC board traces
creates thermocouples. Variations in thermal resistance,
caused by uneven air flow over the circuit board create dif-
ferentialleadtemperature,therebycreatingathermoelectric
voltagenoiseattheoutputofthereference.Minimizingthe
number of thermocouples, as well as limiting airflow, can
substantially reduce these errors. Additional information
can be found in Linear Technology Application Note 82.
Because the majority of voltage reference data sheets
express low frequency noise as a typical number, and as
it tends to be illustrated with a repeatable plot near the
mean of a distribution of peak-to-peak values, the LT6657
datasheetprovidesasimilarlydefinedtypicalspecification
in order to allow a reasonable direct comparison against
similarproducts.Dataproducedwiththismethodgenerally
6657fb
13
For more information www.linear.com/LT6657
Linear Systems [ Linear Systems ]
