AAT3110 Datasheet, PDF(12/18 Page) Advanced Analogic Technologies

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AAT3110 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

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AAT3110

MicroPowerâ¢ Regulated Charge Pump

Applications Information

External Capacitor Selection

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 (<100mâ¦) ceramic capaci-

tors are used for CIN and COUT and CFLY. In gener-

al, low ESR may be defined as less then 200mâ¦ . 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.

Typically as a starting point, a capacitor value of

10ÂµF should be used for CIN and COUT with 1ÂµF

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 1ÂµF and a CFLY value of 0.1ÂµF. CIN

and COUT may range from 1ÂµF for light loads to

10ÂµF or more for heavy output load conditions.

CFLY may range from 0.01ÂµF to 2.2ÂµF 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 = 10ÂµF and CFLY = 1ÂµF, the AAT3110

output performance should be evaluated to assure

the device meets the application requirements.

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

is high, or if the value of CIN is too low. This situa-

tion can be remedied by increasing the value of CIN.

Capacitor Characteristics

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.

Ceramic Capacitor Materials: Ceramic capacitors

less then 0.1ÂµF 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 2.2ÂµF 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 Â±50%

over the operating temperature range of the device.

A 10ÂµF Y5V capacitor could be reduced to less than

5ÂµF 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 Â±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.

3110.2001.9.0.92

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