Datasheet

LTC6655
11
6655fd
For more information www.linear.com/LTC6655
applicaTions inForMaTion
Bypass and Load Capacitors
The LTC6655 voltage references require a 0.1µF or larger
input capacitor located close to the part to improve power
supply rejection. An output capacitor with a value between
2.7µF and 100µF is also required.
The output capacitor has a direct effect on the stability,
turn-on time and settling behavior. Choose a capacitor
with low ESR to insure stability. Resistance in series with
the output capacitor (ESR) introduces a zero in the output
buffer transfer function and could cause instability. The
2.7μF to 100μF range includes several types of capacitors
that are readily available as through-hole and surface mount
components. It is recommended to keep ESR less than or
equal to 0.1Ω. Capacitance and ESR are both frequency
dependent. At higher frequencies capacitance drops and
ESR increases. To insure stable operation the output ca
-
pacitor should have the required values at 100kHz.
In order
to achieve the best performance, caution should
be used when choosing a capacitor. X7R ceramic ca
-
pacitors are small, come in appropriate values and are
relatively stable over a wide temperature range. However,
for a low noise application X7R capacitors may not be
suitable since they may exhibit a piezoelectric effect. The
mechanical vibrations cause a charge displacement in the
ceramic dielectric and the resulting perturbation can look
like noise. If X7R capacitors are necessary, a thorough
bench evaluation should be completed to verify proper
performance.
For very low noise applications where every nanovolt
counts, film capacitors should be considered for their
low noise and lack of piezoelectric effects. Film capaci
-
tors such as polyester, polystyrene, polycarbonate, and
polypropylene have good temperature stability
. Additional
care must be taken as polystyrene
and polypropylene have
an upper temperature limit of 85°C to 105°C. Above these
temperatures, the working voltages need to be derated
according to manufacturer’s specifications. Another type
of film capacitor is polyphenylene sulfide (PPS). These
devices work over a wide temperature range, are stable,
and have large capacitance values beyond 1μF. In general,
film capacitors are found in surface mount and leaded
packages. Table 1 is a partial list of capacitor companies
and some of their available products.
In voltage reference applications, film capacitor lifetime
is affected by temperature and applied voltage. When
polyester capacitors are operated beyond their rated
temperatures (some capacitors are not rated for operation
above 85°C) they need to be derated. Voltage derating is
usually accomplished as a ratio of applied voltage to rated
voltage limit. Contact specific film capacitor manufacturers
to determine exact lifetime and derating information.
The lifetime of X7R capacitors is long, especially for
reference applications. Capacitor lifetime is degraded by
operating near or exceeding the rated voltage, at high
temperature, with AC ripple or some combination of these.
Most reference applications have AC ripple only during
transient events.
Table 1. Film Capacitor Companies
COMPANY DIELECTRIC AVAILABLE CAPACITANCE TEMPERATURE RANGE TYPE
Cornell Dublier Polyester 0.5µF to 10µF –55°C to 125°C DME
Dearborn Electronics Polyester 0.1µF to 12µF –55°C to 125°C 218P, 430P, 431P, 442P, and 410P
Tecate Polyester 0.01µF to 18µF –40°C to 105°C 901, 914, and 914D
Wima Polyester 10µF to 22µF –55°C to 100°C MKS 4, MKS 2-XL
Vishay Polyester 1000pF to 15µF –55°C to 125°C MKT1820
Vishay Polycarbonate 0.01µF to 10µF –55°C to 100°C MKC1862, 632P
Dearborn Electronics Polyphenylene Sulfide (PPS) 0.01µF to 15µF –55°C to 125°C 820P, 832P, 842P, 860P, and 880P
Wima Polyphenylene Sulfide (PPS) 0.01µF to 6.8µF –55°C to 140°C SMD-PPS