A precision band-gap reference is used to generate the internal reference voltage, V_ref. This reference is the dominant noise source within the TPS737xx and generates approximately 32 μV_RMS (10 Hz to 100 kHz) at the reference output (NR). The regulator control loop gains up the reference noise with the same gain as the reference voltage, so that the noise voltage of the regulator is approximately given by:

Equation 1.

Because the value of V_R is 1.2 V, this relationship reduces to:

Equation 2.

for the case of no C_NR.

An internal 27-kΩ resistor in series with the noise-reduction pin (NR) forms a low-pass filter for the voltage reference when an external noise-reduction capacitor, C_NR, is connected from NR to ground. For C_NR = 10 nF, the total noise in the 10-Hz to 100-kHz bandwidth is reduced by a factor of approximately 3.2, giving the approximate relationship:

Equation 3.

for C_NR = 10 nF.

This noise reduction effect is shown as RMS Noise Voltage vs C_NR in the Typical Characteristics section.

The TPS73701 adjustable version does not have the NR pin available. However, connecting a feedback capacitor, C_FB, from the output to the feedback pin (FB) reduces output noise and improves load transient performance. Limit this capacitor to 0.1 µF.

The TPS737 uses an internal charge pump to develop an internal supply voltage sufficient to drive the gate of the NMOS pass transistor above V_OUT. The charge pump generates approximately 250 μV of switching noise at approximately 4 MHz; however, charge-pump noise contribution is negligible at the output of the regulator for most values of I_OUT and C_OUT.