LP2951A

© 1998 SEMTECH CORP.

150mA ADJUSTABLE

VOLTAGE REGULATOR

March 17, 1998

652 MITCHELL ROAD NEWBURY PARK CA 91320

3

PRINCIPLES OF OPERATION (LP2951 AND LP2951A)

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Setting the Output Voltage

The LP2951(A) can be set to deliver any output voltage

from 1.24V to 30V by using an external voltage divider.

In addition, an internal voltage divider is provided if a

5V output is desired. To use the internal voltage di-

vider, simply connect the sense pin to the output and

the tap pin to the feedback pin (see block diagram).

When using an external divider the sense and tap pins

are left open, and the divider is installed from the out-

put to ground, with its center connected to the feed-

back pin (see Adjustable Regulator figure below).

When using an external voltage divider, resistances

can be calculated from the following formula:

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An upper limit of values for R2 occurs at ~1.2M

Ω if the

regulator is to be operated when completely unloaded,

as this allows the feedback divider to provide the 1µA

minimum load recommended for the LP2951(A). If the

regulator always has a load of 1µA or more connected

externally, higher resistor values can be used, but at-

tention must be paid to the -20nA (typical) bias current

required by the feedback pin. Using a 1.2M

Ω resistor

for R2, this bias current will already cause a 2% shift in

output voltage between full load and no load. Larger

values of R2 exacerbate the problem. Using a 120K

Ω

resistor for R2 reduces the error caused by feedback

bias current to 0.2% while still only requiring 10µA to

feed the divider string.

Output Filtering

An output filter capacitor is always necessary with the

LP2951(A) in order to assure output stability. The size

of this capacitor varies with output voltage (smaller at

higher output voltages) and output current (smaller at

lower output currents). For 5V operation 1µF is suffi-

cient. For regulator operation at a minimum output volt-

age, (1.24V) and output currents of 100mA, the re-

quired filter increases to 3.3µF. Any type of capacitor

may be used, although if aluminum electrolytics are

chosen, the equivalent series resistance (ESR) should

be held to 5

Ω or less. For small load currents the ca-

pacitance can be reduced. 0.33µF will be satisfactory

for output currents of 10mA or less, and 0.1µF will

work if output current is below 1mA.

Theoretically, it is also possible for the regulator to be-

come unstable if very large capacitances (>10,000µF)

are connected to the output, but this has not been ob-

served in practice. It is also important that the capaci-

tance be mounted close (1cm or less) to the output pin

of the regulator.

Adjustable Regulator

If the lead inductance between the input of the

LP2951(A) and its power source exceeds ~500nH

(approximately 10”/25cm of 0.031”/0.78mm trace) it

may also be necessary to add a filter capacitor be-

tween the input terminal and ground. A 1µF tantalum

or aluminum capacitor is usually sufficient. Lower val-

ues can be used if load currents are small. Noise injec-

tion into the feedback terminal of the LP2951(A) from

nearby noise sources can also upset the output. Gen-

erally this can be cured by the addition of 100pF or so

from the feedback terminal to the output.

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