8.2.2.1 Input and Output Capacitor Requirements

Although an input capacitor is not required for stability, it is good analog design practice to connect a 0.1-μF to 1.0-μF low equivalent series resistance (ESR) capacitor across the input supply near the regulator. This capacitor counteracts reactive input sources and improves transient response, noise rejection, and ripple rejection. A higher-value capacitor may be necessary if large, fast rise-time load transients are anticipated, or if the device is not located near the power source. If source impedance is not sufficiently low, a 0.1-μF input capacitor may be necessary to ensure stability.

The TPS782 series is designed to be stable with standard ceramic capacitors with values of 1.0 μF or larger at the output. X5R- and X7R-type capacitors are best because they have minimal variation in value and ESR over temperature. Maximum ESR should be less than 1.0 Ω. With tolerance and dc bias effects, the minimum capacitance to ensure stability is 1 μF.

8.2.2.2 Dropout Voltage

The TPS782 uses a PMOS pass transistor to achieve low dropout. When (V_IN – V_OUT) is less than the dropout voltage (V_DO), the PMOS pass device is the linear region of operation and the input-to-output resistance is the R_DS(ON) of the PMOS pass element. V_DO approximately scales with output current because the PMOS device behaves like a resistor in dropout. As with any linear regulator, PSRR and transient response are degraded as (V_IN – V_OUT) approaches dropout. This effect is shown in the Typical Characteristics section. Refer to application report SLVA207,Understanding LDO Dropout, available for download from www.ti.com.

8.2.2.3 Transient Response

As with any regulator, increasing the size of the output capacitor reduces over/undershoot magnitude but increases duration of the transient response. For more information, see Figure 19.

8.2.2.4 Minimum Load

The TPS782 series is stable with no output load. Traditional PMOS LDO regulators suffer from lower loop gain at very light output loads. The TPS782 employs an innovative, low-current circuit under very light or no-load conditions, resulting in improved output voltage regulation performance down to zero output current. See Figure 19 for the load transient response.