OUTPUT ISOLATION

The regulator output can be left connected to an active volt-

age source (such as a battery) with the regulator input power

turned off, as long as the regulator ground pin is con-

nected 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 2). The formula for select-

ing the capacitor to be used is:

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 sta-

bility. 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 improve-

ments are more pronounced at high output voltages.

SHUTDOWN INPUT

A logic-level signal will shut off the regulator output when a

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 rec-

ommended) 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 regula-

tor input must be used.

Typical Applications

DS011128-37

* 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.

FIGURE 3. ERROR Output Timing

Typical Application Circuit

DS011128-1

5V Regulator

DS011128-6

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