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AAT3111IJS-33-T1 Datasheet(PDF) 10 Page - Advanced Analogic Technologies |
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AAT3111IJS-33-T1 Datasheet(HTML) 10 Page - Advanced Analogic Technologies |
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10 / 16 page  AAT3111 MicroPower™ Regulated Charge Pump 10 3111.2002.3.0.91 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 (<100m Ω) ceramic capaci- tors are used for CIN and COUT and CFLY. In gener- al, low ESR may be defined as less than 100m Ω. 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 AAT3111 due to their inherent high ESR characteristic. Typically as a starting point, a capacitor value of 10µF should be used for CIN and COUT with 1µF for CFLY when the AAT3111 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 be approximately 10 to 1. The compromise for lowering the value of CIN, COUT and the flying 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 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 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 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): 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 than several ohms for tan- talum or aluminum electrolytic capacitors. Ceramic Capacitor Materials: Ceramic capacitors less than 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 ±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. |
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