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BD9P1X5EFV-C 데이터시트(PDF) 32 Page - Rohm |
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32 / 60 page 32/57 TSZ02201-0J1J0AL01480-1-2 © 2019 ROHM Co., Ltd. All rights reserved. 08.Apr.2020 Rev.002 www.rohm.com TSZ22111 • 15 • 001 BD9P1x5EFV-C Series Selection of Components Externally Connected - continued 2. Selection of Output Capacitor COUT The output capacitor is selected based on the ESR that is required from the previous page equation (a). ΔVP-P can be reduced by using a capacitor with a small ESR. The ceramic capacitor is the best option that meets this requirement. It is because not only it has a small ESR but the ceramic capacitor also contributes to the size reduction of the application circuit. Please confirm the frequency characteristics of ESR from the datasheet of the capacitor manufacturer, and consider a low ESR value for the switching frequency being used. It is necessary to consider that the capacitance of the ceramic capacitor changes obviously according to DC biasing characteristic. For the voltage rating of the ceramic capacitor, twice or more the maximum output voltage is usually required. By selecting a high voltage rating, it is possible to reduce the influence of DC bias characteristics. Moreover, in order to maintain good temperature characteristics, the one with the characteristics of X7R or better is recommended. Because the voltage rating of a large ceramic capacitor is low, the selection becomes difficult for an application with high output voltage. In that case, please connect multiple ceramic capacitors. These capacitors are rated in ripple current. The RMS values of the ripple current that can be obtained in the following equation and must not exceed the ripple current rating. ��������������������(������������) = ∆�������� √12 [A] Where: ��������������������(������������) is the value of the ripple electric current [A] ∆�������� is the peak to peak inductor current [A] Next, when the output setting voltage is 3.3 V or more, the output ceramic capacitor COUT is recommended for 22 μF (Typ) or more. When the output setting voltage less than 3.3 V, the output ceramic capacitor COUT is recommended to the following equation. ���������������� ≥ 72.6 ���������������� [ μF] Where: ���������������� is the output voltage [V] When selecting the capacitor ensure that the capacitance COUT_WORST of the following equation is maintained at the characteristics of DC Bias, AC Voltage, temperature and tolerance. The approximate value can be calculated by this formula. ����������������_�������������������� ≥ 54.5 ���������������� [ μF] Where: ���������������� is the output voltage [V] If the capacitance falls below this value, the oscillation may happen. When using the electrolytic capacitor and the conductive polymer hybrid aluminum electrolytic capacitor, please place it in addition to the ceramic capacitors with the capacity described above. Actually, the changes in the frequency characteristic are greatly affected by the type and the condition (temperature, etc.) of parts that are used, the wire routing and layout of the PCB. Please confirm stability and responsiveness in actual application. In addition, for the total value of capacitance in the output line COUT(Max), please choose a capacitance value less than the value obtained by the following equation: ����������������(������������) < ������������(������������)×1.25×(����������������(������������)−����������������_��������������������(������������)) ���������������� [F] Where: ����������������(������������) is the OCP operation current (Min) [A] ������������(������������) is the Soft Start Time (Min) [s] ����������������_��������������������(������������) is the maximum load current during startup [A] ���������������� is the output voltage [V] If the limits from the above-mentioned are exceeded, Startup failure may happen in 7.9 ms after VOUT starts up. If the capacitance value is extremely large, over-current protection may be activated by the inrush current at startup preventing the output to turn on. Please confirm this on the actual application. Also, in case of large changing input voltage and load current, select the capacitance by verifying that the actual application setup meets the required specification. |
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