Table 2. Design Parameters

8.2.2.1 Capacitor Recommendations

Low equivalent series resistance (ESR) capacitors should be used for the input, output, noise reduction, and bypass capacitors. Ceramic capacitors with X7R and X5R dielectrics are preferred. These dielectrics offer more stable characteristics. Ceramic X7R capacitors offer improved over-temperature performance, while ceramic X5R capacitors are more cost-effective and are available in higher values.

8.2.2.2 Input and Output Capacitor Requirements

A 0.1-μF or larger ceramic input bypass capacitor, connected between IN and GND and located close to the TPS793, is required for stability and improves transient response, noise rejection, and ripple rejection. A higher-value input capacitor may be necessary if large, fast-rise-time load transients are anticipated or the device is located several inches from the power source.

Like most low-dropout regulators, the TPS793 requires an output capacitor connected between OUT and GND to stabilize the internal control loop. The minimum recommended capacitance is 2.2 μF. Any 2.2-μF or larger ceramic capacitor is suitable, provided the capacitance does not vary significantly over temperature. If load current is not expected to exceed 100 mA, a 1.0-μF ceramic capacitor can be used. If a feed-forward capacitor is not used (such as in a unity-gain configuration) or if an output voltage less than 1.8 V is chosen, then the minimum recommended output capacitor is 4.7 μF instead of 2.2 μF. Table 3 lists the recommended output capacitor sizes for several common configurations.

8.2.2.3 Noise Reduction and Feed-Forward Capacitor Requirements

The internal voltage reference is a key source of noise in an LDO regulator. The TPS793 has an NR pin which is connected to the voltage reference through a 250-kΩ internal resistor. The 250-kΩ 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 order for the regulator to operate properly, the current flow out of the NR pin must be at a minimum, because any leakage current creates an IR drop across the internal resistor, thus creating an output error. Therefore, the bypass capacitor must have minimal leakage current. The bypass capacitor should be no more than 0.1 μF to ensure that it is fully charged during the quickstart time provided by the internal switch shown in the Functional Block Diagrams.

As an example, the TPS79328 exhibits only 32 μV_RMS of output voltage noise using a 0.1-μF ceramic bypass capacitor and a 2.2-μF ceramic output capacitor. Note that the output starts up slower as the bypass capacitance increases due to the RC time constant at the NR pin that is created by the internal 250-kΩ resistor and external capacitor. This RC time constant is affected by the quick-start circuit, especially for values near or below 10 nF. See Figure 13 for a comparisons of C_NR capacitors and startup times.

A feed-forward capacitor is recommended to improve the stability of the device. If R₂ = 30.1 kΩ, set C₁ to 15 pF for optimal performance. For voltages less than 1.8 V, the value of this capacitor should be 100 pF. For voltages greater than 1.8 V, the approximate value of this capacitor can be calculated as shown in Equation 3.