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Output Noise

V

N

+

10.7mV

RMS

V

V

OUT

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Board Layout Recommendations to Improve PSRR and Noise Performance

Internal Current Limit

Shutdown

Dropout Voltage

Startup

TPS799xx

SBVS056E – JANUARY 2005 – REVISED OCTOBER 2005

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 0.01µF noise

reduction capacitor; for the adjustable version, smaller value resistors in the output resistor divider reduce noise.

A parallel combination that gives 2µA of divider current will have the same noise performance as a fixed voltage

version. 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%.

The TPS79901 adjustable version does not have the noise-reduction pin available, so ultra-low noise operation is

not possible. Noise can be minimized according to the above recommendations.

To improve ac performance such as PSRR, output noise, and transient response, it is recommended that the

GND pin of the device. In addition, the ground connection for the bypass capacitor should connect directly to the

GND pin of the device.

The TPS799xx internal current limit helps protect the regulator during fault conditions. During current limit, the

output will source a fixed amount of current that is largely independent of output voltage. For reliable operation,

the device should not be operated in current limit for extended periods of time.

The PMOS pass element in the TPS799xx has a built-in body diode that conducts current when the voltage at

OUT exceeds the voltage at IN. This current is not limited, so if extended reverse voltage operation is

anticipated, external limiting may be appropriate.

The enable pin (EN) is active high and is compatible with standard and low voltage TTL-CMOS levels. When

shutdown capability is not required, EN can be connected to IN.

approximately scale with output current.

This effect is shown in Figure 18 through Figure 20 in the Typical Characteristics section.

Fixed voltage versions of the TPS799xx use a quick-start circuit to fast-charge the noise reduction capacitor,

ceramic capacitors are appropriate in this configuration.

startup will be somewhat slower. Refer to Figure 25 and Figure 26 in the Typical Characteristics section. The

time, a 0.01µF or smaller capacitor should be used.

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