LP2954AISX National Semiconductor, LP2954AISX Datasheet - Page 7

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LP2954AISX

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

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Typical Performance Characteristics
Maximum Power Dissipation
(TO-263) (See (Note 2) )
Application Hints
EXTERNAL CAPACITORS
A 2.2 µF (or greater) capacitor is required between the out-
put pin and the ground to assure stability (refer to Figure 1 ).
Without this capacitor, the part may oscillate. Most types 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.
Programming the output for voltages below 5V runs the error
amplifier at lower gains requiring more output capacitance
for stability. At 3.3V output, a minimum of 4.7 µF is required.
For the worst case condition of 1.23V output and 250 mA of
load current, a 6.8 µF (or larger) capacitor should be used.
Stray capacitance to the Feedback terminal can cause insta-
bility. This problem is most likely to appear when using high
value external resistors to set the output voltage. Adding a
100 pF capacitor between the Output and Feedback pins
and increasing the output capacitance to 6.8 µF (or greater)
will cure the problem.
MINIMUM LOAD
When setting the output voltage using an external resistive
divider, a minimum current of 1 µA is recommended through
the resistors to provide a minimum load.
It should be noted that a minimum load current is specified in
several of the electrical characteristic test conditions, so this
value must be used to obtain correlation on these tested lim-
its. The part is parametrically tested down to 100 µA, but is
functional with no load.
or less
(Continued)
7
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 rectified AC source with a
capacitive filter, the minimum AC line voltage and maximum
load current must be used to calculate the minimum voltage
at the input of the regulator. The minimum input voltage, in-
cluding AC ripple on the filter capacitor , must not drop
below the voltage required to keep the LP2954 in regulation.
It is also advisable to verify operating at minimum operating
ambient temperature, since the increasing ESR of the filter
capacitor makes this a worst-case test for dropout voltage
due to increased ripple amplitude.
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2954 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). Fig-
ure 1 shows the voltages and currents which are present in
the circuit. The formula for calculating the power dissipated
in the regulator is also shown in Figure 1 .
DS011128-11
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