LP2952-N, LP2952A, LP2953, LP2953A

SNVS095F – MAY 2004 – REVISED MARCH 2015

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Application Information (continued)

8.1.1 External Capacitors

A 2.2-

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

set to 5 V. Without this capacitor, the device will oscillate. Most types of tantalum or aluminum electrolytic

capacitors will work here. Film types will work, but are more expensive. Many aluminum electrolytic capacitors

contain electrolytes which freeze at

−30°C, which requires the use of solid tantalum capacitors below −25°C. The

important parameters of the capacitor are an equivalent series resistance (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.

Programming the output for voltages below 5 V runs the error amplifier at lower gains requiring more output

capacitance for stability. At 3.3-V output, a minimum of 4.7

μF is required. For the worst-case condition of 1.23-V

output and 250 mA of load current, a 6.8-

μF (or larger) capacitor should be used.

A 1-

μF capacitor should be placed from the IN pin to ground if there is more than 10 inches of wire between the

IN pin and the ac filter capacitor or if a battery input is used.

Stray capacitance to the FEEDBACK pin 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 OUT and

FEEDBACK pins and increasing the output capacitance to 6.8

μF (or greater) will cure the problem.

8.1.2 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 Characteristics: All Voltage

Options test conditions, so this value must be used to obtain correlation on these tested limits.

8.1.3 Programming the Output Voltage

The regulator may be pin-strapped for 5-V operation using its internal resistive divider by tying the OUTPUT and

SENSE pins together and also tying the FEEDBACK and VTAP pins together.

Alternatively, it may be programmed for any voltage between the 1.23-V reference and the 30-V maximum rating

using an external pair of resistors (see Figure 31). The complete equation for the output voltage is:

where

•

(1)

The minimum recommended load current of 1

μA sets an upper limit of 1.2 MΩ on the value of R2 in cases

which can be eliminated at room temperature by trimming R1. For better accuracy, choosing R2 = 100 k

Ω will

reduce this error to 0.17% while increasing the resistor program current to 12

μA. Because the typical quiescent

current is 120

μA, this added current is negligible.

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