LP2954, LP2954A

SNVS096D – JUNE 1999 – REVISED MARCH 2013

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APPLICATION HINTS

EXTERNAL CAPACITORS

A 2.2

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

Figure 23). 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 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.

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 instability. 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 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 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, including 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 temperature of the application. Under all possible operating conditions, 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 produces the maximum DC input

voltage to the regulator). Figure 23 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 23.

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