LP2957, LP2957A

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SNVS102C – JUNE 1998 – REVISED APRIL 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

A 2.2 µF (or greater) capacitor is required between the output pin and ground to assure stability (refer to

Figure 30). 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 electrolytics contain electrolytes which

freeze at

−30°C, which requires 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 increase 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

increased 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 input 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 impedance is high (see Snap-On/Snap-Off

Operation section).

SHUTDOWN INPUT

A logic-level signal will shut off the regulator output when a “LOW” (< 1.2V) is applied to the Shutdown input.

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 recommended) 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 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

regulator 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 minimum input-to-output voltage differential required for the

output voltage to stay within 100 mV of the output voltage measured with a 1V differential. The dropout voltages

for various values of load current are listed under Electrical Characteristics.

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 capacitor . 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 increased ripple amplitude.

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