LP2954, LP2954A

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SNVS096D – JUNE 1999 – REVISED MARCH 2013

* In shutdown mode, ERROR will go high if it has been pulled up to an external supply. To avoid this invalid

response, pull up to regulator output.

** Exact value depends on dropout voltage. (See Application Hints)

Figure 25. ERROR Output Timing

OUTPUT ISOLATION

The regulator output can be left connected to an active voltage source (such as a battery) with the regulator input

power turned off, as long as the regulator ground pin is connected to ground . If the ground pin is left

floating, damage to the regulator can occur if the output is pulled up by an external voltage source.

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, since 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 24). The formula

for selecting the capacitor to be used is:

(5)

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.

SHUTDOWN INPUT

A logic-level signal will shut off the regulator output when a “LOW” (<1.2V) 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 pull-up resistor (20 k

Ω to 100 kΩ recommended) should 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 op-amp), the pull-up resistor

is not required, but may be used.

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

Shutdown input directly to the regulator input.

IMPORTANT: Since the Absolute Maximum Ratings state that the Shutdown input can not go more than 0.3V

below ground, the reverse-battery protection feature which 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 pull-up resistor between the Shutdown input and the

regulator input must be used.

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