LP2954AISX National Semiconductor, LP2954AISX Datasheet - Page 9

• Manufacturer Part Number: LP2954AISX
• Description: 5V and Adjustable Micropower Low-Dropout Voltage Regulators
• Manufacturer: National Semiconductor

Application Hints
* 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.
*
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.
Typical Applications
* Exact value depends on dropout voltage. (See Application Hints)
FIGURE 3. ERROR Output Timing
(Continued)
DS011128-37
Typical Application Circuit
5V Regulator
9
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
“LOW” (
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
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.
<
1.2V) is applied to the Shutdown input.
DS011128-6
DS011128-1
to 100 k
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