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RT9167A-16PB Datasheet(PDF) 8 Page - Richtek Technology Corporation |
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RT9167A-16PB Datasheet(HTML) 8 Page - Richtek Technology Corporation |
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8 / 13 page  RT9167/A 8 DS9167/A-26 March 2007 www.richtek.com Application Information Capacitor Selection and Regulator Stability Like any low-dropout regulator, the external capacitors used with the RT9167/A must be carefully selected for regulator stability and performance. Using a capacitor whose value is > 1 μF on the RT9167/A input and the amount of capacitance can be increased without limit. The input capacitor must be located a distance of not more than 0.5" from the input pin of the IC and returned to a clean analog ground. Any good quality ceramic or tantalum can be used for this capacitor. The capacitor with larger value and lower ESR (equivalent series resistance) provides better PSRR and line-transient response. The output capacitor must meet both requirements for minimum amount of capacitance and ESR in all LDOs application. The RT9167/A is designed specifically to work with low ESR ceramic output capacitor in space-saving and performance consideration. Using a ceramic capacitor whose value is at least 1 μF with ESR is > 5mΩ on the RT9167/A output ensures stability. The RT9167/A still works well with output capacitor of other types due to the wide stable ESR range. Figure 1. shows the curves of allowable ESR range as a function of load current for various output voltages and capacitor values. Output capacitor 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 RT9167/A 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. Use a 10nF bypass capacitor at BP for low output voltage noise. The capacitor, in conjunction with an internal 200k Ω resistor, which connects bypass pin and the band-gap reference, creates an 80Hz low-pass filter for noise reduction. Increasing the capacitance will slightly decrease the output noise, but increase the start-up time. The capacitor connected to the bypass pin for noise reduction must have very low leakage. This capacitor leakage current causes the output voltage to decline by a proportional amount to the current due to the voltage drop on the internal 200k Ω resistor. Figure 2 shows the power on response. Figure 1 Region of Stable COUT ESR vs. Load Current 0.00 0.01 0.10 1.00 10.00 100.00 0 50 100 150 200 250 300 Load Current (mA) 100 10 1 0.1 0.001 0.01 COUT = 1uF Unstable Region Stable Region Unstable Region 10.0 15.0 0 5.0 CBP = 10nF Time (ms) CBP = 1nF VOUT=3.0V 0 5.0 10.0 15.0 CBP = 10nF CBP = 10nF VOUT = 3.0V Figure 2 Time (ms) |
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