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

Ω or less

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.

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.

Maximum Power Dissipation

(TO-263) (See (Note 2) )

DS011128-11

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