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LP2954, LP2954A

SNVS096E – JUNE 1999 – REVISED JULY 2016

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Copyright © 1999–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.4 Reducing Output Noise

In reference applications it may be advantageous to reduce the AC noise present on the output. One method is

to reduce regulator bandwidth by increasing output capacitance. This is relatively inefficient, because large

increases in capacitance are required to get significant improvement.

Noise can be reduced more effectively by a bypass capacitor placed across R1 (refer to Figure 19). The formula

for selecting the capacitor to be used is:

(1)

This gives a value of about 0.1 μF. When this is used, the output capacitor must be 6.8 μF (or greater) to

maintain stability. The 0.1-μF capacitor reduces the high frequency gain of the circuit to unity, lowering the output

noise from 260 μV to 80 μV using a 10-Hz to 100-kHz bandwidth. Also, noise is no longer proportional to the

output voltage, so improvements are more pronounced at high output voltages.

7.4 Device Functional Modes

7.4.1 Shutdown Input

A logic-level signal shuts off the regulator output when a LOW (< 1.2 V) is applied to the SHUTDOWN input.

To prevent possible mis-operation, the SHUTDOWN input must be actively terminated. If the input is driven from

open-collector logic, a pullup resistor (TI recommends 20 kΩ to 100 kΩ) must be connected from the

SHUTDOWN input to the regulator input.

If the SHUTDOWN input is driven from a source that actively pulls high and low (like an operational amplifier),

the pullup resistor is not required, but may be used.

If the shutdown function is not to be used, the cost of the pullup resistor can be saved by simply tying the

SHUTDOWN input directly to the regulator input.

IMPORTANT: Because the Absolute Maximum Ratings state that the SHUTDOWN input cannot go more than

0.3 V below ground, the reverse-battery protection feature that protects the regulator input is sacrificed if the

SHUTDOWN input is tied directly to the regulator input.

If reverse-battery protection is required in an application, the pullup resistor between the SHUTDOWN input and

the regulator input must be used. The recommended 20 kΩ to 100 kΩ provides adequate protection of the

SHUTDOWN pin during negative voltage transitions at the IN pin.