AAT3111 Datasheet, PDF (10/16 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

AAT3111
MicroPower™ Regulated Charge Pump
External Capacitor Selection
Careful selection of the three external capacitors
C_IN, C_OUT, and C_FLY is very important because they
will affect turn-on time, output ripple, and transient
performance. Optimum performance will be
obtained when low ESR ceramic capacitors are
used. In general, low ESR may be defined as less
than 100mΩ. If desired for a particular application,
low ESR tantalum capacitors may be substituted;
however, optimum output ripple performance may
not be realized. Aluminum electrolytic capacitors are
not recommended for use with the AAT3111 due to
their inherent high ESR characteristic.
Typically as a starting point, a capacitor value of
10µF should be used for C_IN and C_OUT with 1μF for
C_FLY when the AAT3111 is used under maximum
output load conditions. Lower values for C_IN, C_OUT,
and C_FLY may be utilized for light load current appli-
cations. Applications drawing a load current of
10mA or less may use a C_IN and C_OUT capacitor
value as low as 1µF and a C_FLY value of 0.1µF. C_IN
and C_OUT may range from 1µF for light loads to
10µF or more for heavy output load conditions.
C_FLY may range from 0.01µF to 2.2µF or more. If
C_FLY is increased, C_OUT should also be increased
by the same ratio to minimize output ripple. As a
basic rule, the ratio between C_IN, C_OUT, and C_FLY
should be approximately 10 to 1. The compromise
for lowering the value of C_IN, C_OUT, and the flying
capacitor C_FLY is the output ripple voltage may be
increased. In any case, if the external capacitor
values deviate greatly from the recommendation of
C_IN = C_OUT = 10µF and C_FLY = 1µF, the AAT3111
output performance should be evaluated to assure
the device meets application requirements.
In applications where the input voltage source has
very low impedance, it is possible to omit the C_IN
capacitor. However, if C_IN 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
is high, or if the value of C_IN is too low. This situa-
tion can be remedied by increasing the value of C_IN.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3111. 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 is a very
important characteristic to consider when selecting
a capacitor. ESR is a resistance internal to a
capacitor that is caused by the leads, internal con-
nections, size or area, material composition, and
ambient temperature. Typically capacitor ESR is
measured in milliohms for ceramic capacitors and
can range to more than several ohms for tantalum
or aluminum electrolytic capacitors.
Ceramic Capacitor Materials: Ceramic capaci-
tors less than 0.1µF are typically made from NPO
or C0G materials. NPO and C0G materials gener-
ally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually
composed of X7R, X5R, Z5U, or Y5V dielectric
materials. Large ceramic capacitors (i.e., 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 ±50%
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 ±15%.
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.
10
3111.2006.06.1.3