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TPS79901DDCT Datasheet(PDF) 10 Page - Texas Instruments |
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TPS79901DDCT Datasheet(HTML) 10 Page - Texas Instruments |
10 / 27 page www.ti.com APPLICATION INFORMATION Feedback Capacitor Requirements Output Noise Input and Output Capacitor Requirements TPS79901 GND EN FB IN OUT V IN V OUT R 1 C FB R 2 Optionalinputcapacitor. Mayimprovesource impedance,noise,orPSRR. V = OUT ´ 1.193 (R 1 2 +R ) R 2 V EN 2.2 F m Ceramic TPS799xx GND EN NR IN OUT V IN V OUT Optional input capacitor. May improve source impedance, noise, or PSRR. Optional bypass capacitor to reduce output noise and increase PSRR. 2.2mF Ceramic V EN TPS799xx SBVS056I – JANUARY 2005 – REVISED NOVEMBER 2007 The TPS799xx family of LDO regulators combines the high performance required of many RF and precision analog applications with ultra-low current (TPS79901 only) consumption. High PSRR is provided by a high gain, The feedback capacitor, CFB, shown in Figure 29 is high bandwidth error loop with good supply rejection required for stability. For a parallel combination of R1 at very low headroom (VIN – VOUT). Fixed voltage and R2 equal to 250kΩ, any value from 3pF to 1nF versions provide a noise reduction pin to bypass can be used. Fixed voltage versions have an internal noise generated by the bandgap reference and to 30pF feedback capacitor which is quick-charged at improve PSRR while a quick-start circuit fast-charges start-up. The adjustable version does not have this this capacitor at startup. The combination of high quick-charge circuit, so values below 5pF should be performance and low ground current also make the used to ensure fast startup; values above 47pF can TPS799xx an excellent choice for portable be used to implement an output voltage soft-start. applications. All versions have thermal and Larger value capacitors also improve noise slightly. over-current protection and are fully specified from The TPS79901 is stable in unity-gain configuration –40 °C to +125°C. (OUT tied to FB) without CFB. Figure 28 shows the basic circuit connections for fixed voltage models. Figure 29 gives the connections for the adjustable output version (TPS79901). R1 and R2 can be calculated for any output voltage using the In most LDOs, the bandgap is the dominant noise formula in Figure 29. Sample resistor values for source. If a noise reduction capacitor (CNR) is used common output voltages are shown in Figure 29. with the TPS799xx, the bandgap does not contribute significantly to noise. Instead, noise is dominated by the output resistor divider and the error amplifier input. To minimize noise in a given application, use a Although an input capacitor is not required for 0.01 µF noise reduction capacitor; for the adjustable stability, it is good analog design practice to connect version, smaller value resistors in the output resistor a 0.1 µF to 1µF low ESR capacitor across the input divider reduce noise. A parallel combination that supply near the regulator. This will counteract gives 2 µA of divider current will have the same noise reactive input sources and improve transient performance as a fixed voltage version. To further response, noise rejection, and ripple rejection. A optimize noise, equivalent series resistance of the higher-value capacitor may be necessary if large, fast output capacitor can be set to approximately 0.2 Ω. rise-time load transients are anticipated or the device This configuration maximizes phase margin in the is located several inches from the power source. If control loop, reducing total output noise by up to source impedance is not sufficiently low, a 0.1 µF 10%. input capacitor may be necessary to ensure stability. space The TPS799xx is designed to be stable with standard space ceramic capacitors of values 2.2 µF or larger. X5R and X7R type capacitors are best as they have space minimal variation in value and ESR over temperature. Maximum ESR should be < 1.0 Ω. space Figure 29. Typical Application Circuit for Adjustable Voltage Version Figure 28. Typical Application Circuit for Fixed Voltage Versions 10 Submit Documentation Feedback Copyright © 2005–2007, Texas Instruments Incorporated |
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