LP2957AIS National Semiconductor, LP2957AIS Datasheet - Page 7

• Manufacturer Part Number: LP2957AIS
• Description: 5V Low-Dropout Regulator for P Applications
• Manufacturer: National Semiconductor

Typical Application Circuits
*See Application Hints
Application Hints
EXTERNAL CAPACITORS
A 2.2 µF (or greater) capacitor is required between the out-
put pin and ground to assure stability (refer to Figure 1 ).
Without this capacitor, the part may oscillate. Most type of
tantalum or aluminum electrolytics will work here. Film types
will work, but are more expensive. Many aluminum electro-
lytics contain electrolytes which freeze at −30˚C, which re-
quires the use of solid tantalums below −25˚C. The important
parameters of the capacitor are an ESR of about 5
and a resonant frequency above 500 kHz (the ESR may in-
crease by a factor of 20 or 30 as the temperature is reduced
from 25˚C to −30˚C). The value of this capacitor may be in-
creased without limit. At lower values of output current, less
output capacitance is required for stability. The capacitor can
be reduced to 0.68 µF for currents below 10 mA or 0.22 µF
for currents below 1 mA.
A 1 µF capacitor should be placed from the input pin to
ground if there is more than 10 inches of wire between the in-
put and the AC filter capacitor or if a battery input is used.
This capacitor may have to be increased if the regulator
is wired for snap-on/snap-off output and the source im-
pedance is high (see Snap-On/Snap-Off Operation sec-
tion).
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) 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 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 tying the Shutdown input di-
rectly 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.
LP2957 Application with Snap-On/Snap-Off Output
to 100 k
(Continued)
or less
rec-
7
MINIMUM LOAD
It should be noted that a minimum load current is specified in
several of the electrical characteristic test conditions, so the
value listed must be used to obtain correlation on these
tested limits. The part is parametrically tested down to
100 µA, but is functional with no load.
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the mini-
mum input-to-output voltage differential required for the out-
put voltage to stay within 100 mV of the output voltage mea-
sured with a 1V differential. The dropout voltages for various
values of load current are listed under Electrical Characteris-
tics.
If the regulator is powered from a transformer connected to
the AC line, the minimum AC line voltage and maximum
load current must be used to measure the minimum voltage
at the input of the regulator. The minimum input voltage is
the lowest voltage level including ripple on the filter ca-
pacitor . It is also advisable to verify operation at minimum
operating ambient temperature , since the increasing ESR
of the filter capacitor makes this a worst-case test due to in-
creased ripple amplitude.
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2957 depending on
the maximum power dissipation and maximum ambient tem-
perature of the application. Under all possible operating con-
ditions, the junction temperature must be within the range
specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the maximum power
dissipated by the regulator, P(max), must be calculated. It is
important to remember that if the regulator is powered from
a transformer connected to the AC line, the maximum
specified AC input voltage must be used (since this pro-
duces the maximum DC input voltage to the regulator), and
the maximum load current must also be used. Figure 1
shows the voltages and currents which are present in the cir-
cuit. The formula for calculating the power dissipated in the
regulator is also shown in Figure 1 .
DS011340-4
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