AS1302
Datasheet - Application Information
Flying Capacitor Selection
To ensure the required output current and avoid high peak currents the values of the flying capacitors CFLY1 and CFLY2
are very critical. A 220nF capacitor is sufficient for most applications. Dependent on the operation mode the AS1302
alternately charges and discharges the CFLY1/2 . While the ESR of the output capacitor produces a part of the output
voltage ripple, the ESR of the flying capacitors directly adds to the charge pump’s output source resistance. Therefore
low ESR capacitors, e.g. tantalum or ceramic, are recommended for the flying capacitors as well.
Due to different materials for ceramic capacitors the on the material depending temperature and voltage coefficients
have to be considered. The capacitance of a X7R ceramic capacitor is more stable than a Z5U or Y5V ceramic
capacitor over the whole temperature range from -40°C to +85°C. As an additional effect a Z5U or Y5V ceramic
capacitor will loose about the half of his nominal capacitance when the rated voltage is applied.
It is important to choose the ceramic capacitor according to the minimum available capacitance over the operating
voltage and the bias voltage. This information is stated in the datasheets of the capacitor manufacturer.
Table 5. Recommended Flying Capacitors
Part Number
C
TC Code Rated Voltage
Dimensions
0603
Manufacturer
Murata
www.murata.com
GRM188R71E224KA88
GRM155R61A224KE19
220nF
220nF
X7R
X5R
25V
10V
0402
Layout Consideration
To achieve the best performance of the AS1302 a careful board layout is necessary to reduce the impact of the high
switching frequency and the high transient currents which are produced by the device. For a proper regulation under all
conditions a true ground plane and short connections to all external capacitors are needed.
www.austriamicrosystems.com/DC-DC_Step-Up/AS1302
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