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RT9182IGES Datasheet(PDF) 8 Page - Richtek Technology Corporation |
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RT9182IGES Datasheet(HTML) 8 Page - Richtek Technology Corporation |
8 / 9 page RT9182 8 DS9182-17 March 2007 www.richtek.com of larger capacitance can reduce noise and improve load- transient response, stability, and PSRR. The output capacitor should be located not more than 0.5" from the VOUT pin of the RT9182 and returned to a clean analog ground. Note that some ceramic dielectrics exhibit large capacitance and ESR variation with temperature. It may be necessary to use 2.2 μF or more to ensure stability at temperatures below -10 °C in this case. Also, tantalum capacitors, 2.2 μF or more may be needed to maintain capacitance and ESR in the stable region for strict application environment. Tantalum capacitors maybe suffer failure due to surge current when it is connected to a low-impedance source of power (like a battery or very large capacitor). If a tantalum capacitor is used at the input, it must be guaranteed to have a surge current rating sufficient for the application by the manufacture. Load-Transient Considerations The RT9182 load-transient response graphs show two components of the output response: a DC shift from the output impedance due to the load current change, and the transient response. The DC shift is quite small due to the excellent load regulation of the IC. Typical output voltage transient spike for a step change in the load current from 0mA to 50mA is tens mV, depending on the ESR of the output capacitor. Increasing the output capacitor's value and decreasing the ESR attenuates the overshoot. Input-Output (Dropout) Voltage A regulator's minimum input-output voltage differential (or dropout voltage) determines the lowest usable supply voltage. In battery-powered systems, this will determine the useful end-of-life battery voltage. Because the RT9182 uses a P-Channel MOSFET pass transistor, the dropout voltage is a function of drain-to-source on- resistance [RDS(ON)] multiplied by the load current. Reverse Current Path The power transistor used in the RT9182 has an inherent diode connected between each regulator input and output (see Figure2). If the output is forced above the input by more than a diode-drop, this diode will become forward biased and current will flow from the VOUT terminal to VIN. This diode will also be turned on by abruptly stepping the input voltage to a value below the output voltage. To prevent regulator mis-operation, a Schottky diode could be used in the applications where input/output voltage conditions can cause the internal diode to be turned on (see Figure3). As shown, the Schottky diode is connected in parallel with the internal parasitic diode and prevents it from being turned on by limiting the voltage drop across it to about 0.3V < 100mA to prevent damage to the part. VIN VOUT VIN VOUT Figure 2 Figure 3 Figure 1 Region of Stable Cout ESR v.s Load Current 0.001 0.01 0.1 1 10 100 0 40 80 120 160 200 Load Current (mA) COUT = 1uF COUT = 4.7uF Instable Instable Stable |
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