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RT9178-31GBR Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT9178-31GBR Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 11 page  RT9178 9 DS9178-15 March 2007 www.richtek.com Tantalum: Solid tantalum capacitors are recommended for use on the output because their typical ESR is very close to the ideal value required for loop compensation. They also work well as input capacitors if selected to meet the ESR requirements previously listed. Tantalums also have good temperature stability: a good quality tantalum will typically show a capacitance value that varies less than 10 to 15% across the full temperature range of 125 °C to −40°C. ESR will vary only about 2X going from the high to low temperature limits. The increasing ESR at lower temperatures can cause oscillations when marginal quality capacitors are used (if the ESR of the capacitor is near the upper limit of the stability range at room temperature). Aluminum: This capacitor type offers the most capacitance for the money. The disadvantages are that they are larger in physical size, not widely available in surface mount, and have poorAC performance (especially at higher frequencies) due to higher ESR and ESL. Compared by size, the ESR of an aluminum electrolytic is higher than either Tantalum or ceramic, and it also varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of as much as 50X when going from 25 °C down to −40°C. It should also be noted that many aluminum electrolytics only specify impedance at a frequency of 120Hz, which indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance specified at a higher frequency (between 20kHz and 100kHz) should be used for the device. Derating must be applied to the manufacturer's ESR specification, since it is typically only valid at room temperature. Any applications using aluminum electrolytics should be thoroughly tested at the lowest ambient operating temperature where ESR is maximum. Thermal Considerations The RT9178 series can deliver a current of up to 200mA over the full operating junction temperature range. However, the maximum output current must be derated at higher ambient temperature to ensure the junction temperature does not exceed 125 °C. With all possible conditions, the junction temperature must be within the range specified under operating conditions. Power dissipation can be calculated based on the output current and the voltage drop across regulator. PD = (VIN − VOUT) IOUT + V IN IGND The final operating junction temperature for any set of conditions can be estimated by the following thermal equation: PD (MAX) = ( TJ (MAX) − TA ) / θJA Where TJ (MAX) is the maximum junction temperature of the die (125 °C) and TA is the maximum ambient temperature. The junction to ambient thermal resistance ( θJA) for SOT-23-5 package at recommended minimum footprint is 250 °C/W (θJA is layout dependent). Visit our website in which “Recommended Footprints for Soldering Surface Mount Packages” for detail. PCB Layout Good board layout practices must be used or instability can be induced because of ground loops and voltage drops. The input and output capacitors MUST be directly connected to the input, output, and ground pins of the device using traces which have no other currents flowing through them. The best way to do this is to layout CIN and C OUT near the device with short traces to the VIN, VOUT, and ground pins. The regulator ground pin should be connected to the external circuit ground so that the regulator and its capacitors have a “single point ground” . |
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