AAT3110
MicroPower™ Regulated Charge Pump
is high, or if the value of CIN is too low. This situa-
tion can be remedied by increasing the value of CIN.
Applications Information
External Capacitor Selection
Capacitor Characteristics
Careful selection of the three external capacitors
CIN, COUT and CFLY is very important because they
will affect turn on time, output ripple and transient
performance. Optimum performance will be
obtained when low ESR (<100mΩ) ceramic capaci-
tors are used for CIN and COUT and CFLY. In gener-
al, low ESR may be defined as less then 200mΩ . If
desired for a particular application, low ESR
Tantalum capacitors may be substituted; however
optimum output ripple performance may not be real-
ized. Aluminum Electrolytic capacitors are not rec-
ommended for use with the AAT3110 due the their
inherent high ESR characteristic.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3110. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. A ceramic capacitor typically
has very low ESR, is lower cost, has a smaller PCB
footprint and is non-polarized. Low ESR ceramic
capacitors help maximize charge pump transient
response. Since ceramic capacitors are non-polar-
ized, they are not prone to incorrect connection
damage.
Equivalent Series Resistance (ESR): ESR is a
very important characteristic to consider when
selecting a capacitor. ESR is a resistance internal
to a capacitor, which is caused by the leads, inter-
nal connections, size or area, material composition
and ambient temperature. Typically capacitor ESR
is measured in milliohms for ceramic capacitors
and can range to more then several ohms for tan-
talum or aluminum electrolytic capacitors.
Typically as a starting point, a capacitor value of
10µF should be used for CIN and COUT with 1µF
CFLY when the AAT3110 is used under maximum
output load conditions. Lower values for CIN, COUT
and CFLY may be utilized for light load current appli-
cations. Applications drawing a load current of
10mA or less may use a CIN and COUT capacitor
value as low as 1µF and a CFLY value of 0.1µF. CIN
and COUT may range from 1µF for light loads to
10µF or more for heavy output load conditions.
CFLY may range from 0.01µF to 2.2µF or more. If
CFLY is increased, COUT should also be increased
by the same ratio to minimize output ripple. As a
basic rule, the ratio between CIN, COUT and CFLY
should not exceed a ratio of 10 to 1. The compro-
mise for lowering the value of CIN, COUT and the fly-
ing capacitor CFLY is the output ripple voltage may
be increased. In any case, if the external capacitor
values deviate greatly from the recommendation of
CIN = COUT = 10µF and CFLY = 1µF, the AAT3110
output performance should be evaluated to assure
the device meets the application requirements.
Ceramic Capacitor Materials: Ceramic capacitors
less then 0.1µF are typically made from NPO or
COG materials. NPO and COG materials typically
have tight tolerance and are very stable over tem-
perature. Large capacitor values are typically com-
posed of X7R, X5R, Z5U or Y5V dielectric materi-
als. Large ceramic capacitors, typically greater
than 2.2µF are often available in low cost Y5V and
Z5U dielectrics. If these types of capacitors are
selected for use with the charge pump, the nominal
value should be doubled to compensate for the
capacitor tolerance which can vary more than 50ꢀ
over the operating temperature range of the device.
A 10µF Y5V capacitor could be reduced to less than
5µF over temperature, this could cause problems
for circuit operation. X7R and X5R dielectrics are
much more desirable. The temperature tolerance
of X7R dielectric is better than 15ꢀ.
In applications where the input voltage source has
very low impedance, it is possible to omit the CIN
capacitor. However, if CIN is not used, circuit per-
formance should be evaluated to assure desired
operation is achieved. Under high peak current
operating conditions that are typically experienced
during circuit start up or when load demands create
a large inrush current, poor output voltage regula-
tion can result if the input supply source impedance
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material
smaller capacitor. These larger devices can improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
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3110.2001.9.0.92
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