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

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.

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

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) 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.

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.

LP2957 Application with Snap-On/Snap-Off Output

DS011340-4

*See Application Hints

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