www.ti.com

APPLICATION INFORMATION

TPS794xx

GND

EN

NR

IN

OUT

V

IN

V

OUT

0.1

µF

0.01

µF

2.2

µF

External Capacitor Requirements

TPS79401, TPS79418

TPS79425, TPS79428

TPS79430, TPS79433

SLVS349D – NOVEMBER 2001 – REVISED OCTOBER 2004

The TPS794xx family of low-dropout (LDO) regulators has been optimized for use in noise-sensitive equipment.

The device features extremely low dropout voltages, high PSRR, ultralow output noise, low quiescent current,

and enable input to reduce supply currents to less than 1 µA when the regulator is turned off.

A typical application circuit is shown in Figure 22.

Figure 22. Typical Application Circuit

A 0.1-µF or larger ceramic input bypass capacitor, connected between IN and GND and located close to the

TPS794xx, is required for stability. It 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 TPS794xx 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.

The internal voltage reference is a key source of noise in an LDO regulator. The TPS794xx has an NR pin which

is connected to the voltage reference through a 250-k

Ω 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 NR 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 diagram.

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-k

Ω resistor and external

capacitor.

9