TPS799 datasheet(14/49 Pages) TI2

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8.2.2 Detailed Design Procedure

8.2.2.1 Input and Output Capacitor Requirements

Although an input capacitor is not required for stability, good analog design practice is to connect a 0.1-μF to

1-μF low 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 located several inches from

the power source. If source impedance is not sufficiently low, a 0.1-μF input capacitor may be necessary to

ensure stability.

The TPS799 is designed to be stable with standard ceramic capacitors with values of 2.2 μF or greater. X5R-

and X7R-type capacitors are best because they have minimal variation in value and ESR over temperature.

Maximum ESR must be less than 1.0 Ω.

8.2.2.2 Output Noise

In most LDOs, the band gap is the dominant noise source. If a noise-reduction capacitor (CNR) is used with

the TPS799, the band gap 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 0.01-μF noise

reduction capacitor. To further optimize noise, equivalent series resistance of the output capacitor can be set to

approximately 0.2 Ω. This configuration maximizes phase margin in the control loop, reducing total output noise

by up to 10%.

Noise can be referred to the feedback point; with CNR = 0.01 μF total noise is approximately given by Equation 1:

V =xV

OUT

10.5 V

RMS

(1)

8.2.2.3 Dropout Voltage

The TPS799 uses a PMOS pass transistor to achieve a low dropout voltage. When (VIN – VOUT) is less than

the dropout voltage (VDO), the PMOS pass device is in its linear region of operation and rDS(on) of the PMOS

pass element is the input-to-output resistance. Because the PMOS device behaves like a resistor in dropout,

VDO approximately scales with the output current.

As with any linear regulator, PSRR degrades as (VIN – VOUT) approaches dropout. This effect is illustrated in

Figure 6-10 through Figure 6-18 in the Typical Characteristics section.

8.2.2.4 Transient Response

As with any regulator, increasing the size of the output capacitor reduces over- and undershoot magnitude, but

increases the duration of the transient response. The transient response of the TPS799 is enhanced by an active

pulldown device that engages when the output overshoots by approximately 5% or more when the device is

enabled. When enabled, the pulldown device behaves like a 350-Ω resistor to ground.

8.2.2.5 Minimum Load

The TPS799 is stable with no output load. To meet the specified accuracy, a minimum load of 500 μA is required.

With loads less than 500 μA at junction temperatures near 125°C, the output can drift up enough to cause

the output pulldown device to turn on. The output pulldown device limits voltage drift to 5% typically; however,

ground current can increase by approximately 50 μA. In typical applications, the junction cannot reach high

temperatures at light loads because there is no noticeable dissipated power. The specified ground current is then

valid at no load in most applications.

8.2.2.6 Feedback Capacitor Requirements (TPS79901 Only)

The feedback capacitor, CFB, shown in Figure 8-2 is required for stability. For a parallel combination of R1 and

R2 equal to 250 kΩ, any value from 3 pF to 1 nF can be used. Fixed voltage versions have an internal 30-pF

feedback capacitor that is quick-charged at start up. The adjustable version does not have this quick-charge

circuit, so use values below 5 pF to ensure fast start up; values above 47 pF can be used to implement an output

TPS799

SBVS056L – JANUARY 2005 – REVISED FEBRUARY 2022

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