Application Hints

EXTERNAL CAPACITORS

Bypass capacitors on the input and output of the LP2960 are

required: without these capacitors,

the part will oscillate.

A capacitor (whose value is

at least 4.7 µF) must be con-

nected from the V

cated more than one inch away from the LP2960, the capaci-

tor may have to be increased to 22 µF to assure stability. A

capacitor is also required between V

the minimum amount of capacitance required here depends

on output voltage.

If the output voltage of the LP2960 is set to 5V, a minimum of

10 µF is needed in output capacitance. At 3.3V output, at

least 22 µF is required to assure stability.

ESR LIMIT: The ESR of the capacitor used on the LP2960

must be less than 0.7

Ω throughout the entire operating tem-

perature range to assure stability.

The ESR of an aluminum eIectroIytic capacitor is typically

only specified at 25˚C, and does not reflect the maximum

ESR that can be expected to occur over the entire tempera-

ture range of the capacitor.

Aluminum electrolytics show a marked increase in ESR at

low temperatures (ESR can increase by a factor of 30 or

more when going from 25˚C to −30˚C) which could lead to

oscillation probIems in applications with very low ambient

temperatures. Solid tantalum capacitors are recommended

for use in such cases.

Regulator instability can be caused by stray (board layout)

capacitance appearing at the Feedback terminal. Oscilla-

tions from this effect are most Iikely to occur when very high

value resistors are used to set the output voltage.

Adding a 100 pF capacitor between the Output and Feed-

back pins and increasing the output capacitor to at least

22 µF will stop the osciIIations.

MINIMUM LOAD

The internal resistive divider in the LP2960 provides suffi-

cient output loading for proper regulation. If externaI resis-

tors are used to set the LP2960 output voltage, a minimum

current of 5 µA through the externaI resistive divider is rec-

ommended.

It should be noted that a minimum load current is specified in

several of the test conditions listed under

Electrical Charac-

teristics, and this value of load current must be used to get

correlation on these test limits.

PROGRAMMING THE OUTPUT VOLTAGE

The LP2960 regulator may be pin-strapped for operation at

the nominal output voltage using its internal resistive divider

by tying the Output and Sense pins together and also tying

the Feedback and V

Alternatively, it may be programmed for any voltage between

the 1.23V reference and the 30V maximum rating using an

external pair of resistors (see Basic Application Circuit).

The complete equation for the output voltage is:

V

The term V

back pin bias current (−20 nA typical). The minimum recom-

mended load current of 5 µA sets an upper limit of 240 k

Ω on

the value of R2 in cases where the regulator must work with

no load (see

Minimum Load).

For best output accuracy, choosing R2 = 100 k

Ω will reduce

the error resulting from I

sistive divider current to 12 µA. Since the typicaI quiescent

current of the LP2960 is 450 µA, this added current through

R2 is negligible.

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 voltage is

independent of the programmed output voltage.

OUTPUT ISOLATION

If the LP2960 output is connected to an active voltage

source (such as a battery) the regulator input should not be

shorted to ground, as this will cause a large current to flow

from the battery into the LP2960 output lead.

If the LP2960 input is

left floating with the output connected

to a battery, a small current (a few mA) will flow into the out-

put lead.

The “reverse” current flowing from the battery into the

LP2960 output can be prevented by using a blocking diode

between the output and the battery.

REDUCING OUTPUT NOISE

In reference applications it may be desirabIe to reduce the

AC noise present on the output. One method is to reduce

regulator bandwidth by increasing output capacitance. This

is relatively inefficient, since large increases in capacitance

are required to get significant improvement.

Noise can be reduced more effectively by a bypass capacitor

placed across R1 (refer to

Basic Application Circuit).

A 0.1 µF capacitor connected across R1 will reduce the high

frequency gain of the circuit to unity, lowering the RMS out-

put noise voltage from 210 µV to 130 µV (typical) using a 10

Hz–100 kHz bandwidth test measurement.

Also, output noise is no longer proportional to the output volt-

age, so improvements are more pronounced at higher output

voltages.

IMPORTANT: Since the 0.1 µF capacitor reduces the AC

gain of the LP2960 to unity, the output capacitance must be

increased to at least 33 µF to assure regulator stability.

DROPOUT DETECTION COMPARATOR

The dropout detection comparator produces a logic “LOW”

on the Error output whenever the LP2960 output drops out of

regulation by more than about 5%. This figure results from

the comparator’s built-in offset of 60 mV divided by the 1.23V

reference (refer to block diagram).

The “5% below nominal” trip level remains constant regard-

less of the programmed output voltage. An out-of-regulation

condition can result from low input voltage, current limiting,

or thermal limiting.

The figure below gives a timing diagram showing the rela-

tionship between the output voltage, the Error output, and in-

put voltage as the input voltage is ramped up and down to a

regulator programmed for 5V output.

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