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TPS79428DCQR Fiches technique(PDF) 8 Page - Texas Instruments |
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TPS79428DCQR Fiches technique(HTML) 8 Page - Texas Instruments |
8 / 23 page www.ti.com APPLICATION INFORMATION GND EN NR IN OUT V IN V OUT 0.01 µF 2.2 µF 1 µF TPS794xx BOARD LAYOUT RECOMMENDATION TO EXTERNAL CAPACITOR REQUIREMENTS REGULATOR MOUNTING TPS794xx SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005 order for the regulator to operate properly, the The TPS794xx family of low-dropout (LDO) current flow out of the NR pin must be at a minimum, regulators has been optimized for use in because any leakage current creates an IR drop noise-sensitive equipment. The device features across the internal resistor, thus creating an output extremely low dropout voltages, high PSRR, ultralow error. Therefore, the bypass capacitor must have output noise, low quiescent current (265 µA minimal leakage current. The bypass capacitor typically), and an enable input to reduce supply should be no more than 0.1- µF in order to ensure currents to less than that it is fully charged during the quickstart time 1 µA when the regulator is turned off. provided by the internal switch shown in the Functional Block Diagram. A typical application circuit is shown in Figure 22. For example, the TPS79430 exhibits only 33 µV RMS of output voltage noise using a 0.1- µF ceramic bypass capacitor and a 10- µF ceramic output capacitor. Note that the output starts up slower as the bypass capacitance increases because of the RC time constant at the bypass pin that is created by the internal 250-k Ω resistor and external capacitor. IMPROVE PSRR AND NOISE Figure 22. Typical Application Circuit PERFORMANCE To improve ac measurements such as PSRR, output noise, and transient response, it is recommended that the board be designed with separate ground A 1- µF or larger ceramic input bypass capacitor, planes for VIN and VOUT, with each ground plane connected between IN and GND and located close connected only at the ground pin of the device. In to the TPS794xx, is required for stability and addition, the ground connection for the bypass improves transient response, noise rejection, and capacitor should connect directly to the ground pin of ripple rejection. A higher-value input capacitor may the device. be necessary if large, fast-rise-time load transients are anticipated and the device is located several inches from the power source. The tab of the SOT223-6 package is electrically Like most low-dropout regulators, the TPS794xx connected to ground. For best thermal performance, requires an output capacitor connected between the tab of the surface-mount version should be OUT and GND to stabilize the internal control loop. soldered directly to a circuit-board copper area. The minimum recommended capacitance is 1 µF. Increasing the copper area improves heat Any dissipation. 1 µF or larger ceramic capacitor is suitable. Solder pad footprint recommendations for the The internal voltage reference is a key source of devices are presented in Application Report noise in an LDO regulator. The TPS794xx has an SBFA015, Solder Pad Recommendations for NR pin which is connected to the voltage reference Surface-Mount Devices, available from the TI web through a 250-k Ω internal resistor. The 250-kΩ site (www.ti.com). internal resistor, in conjunction with an external bypass capacitor connected to the NR pin, creates a low-pass filter to reduce the voltage reference noise and, therefore, the noise at the regulator output. In 8 Submit Documentation Feedback |
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