LP2951A Datasheet, PDF (3/5 Pages) Semtech Corporation – 150mA ADJUSTABLE VOLTAGE REGULATOR

150mA ADJUSTABLE
VOLTAGE REGULATOR
LP2951A
March 17, 1998
PRINCIPLES OF OPERATION (LP2951 AND LP2951A)
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:
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.
V_OUT =
êëéçèæ
1.235
R2
+ 20 x10 ^−9 ÷ö
ø
x
R1^ù + 1.235 V
An upper limit of values for R2 occurs at ~1.2MΩ 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 1.2MΩ 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 120KΩ
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
higher
co_êëéau_çèæp1_tpa.R2_uc32_tit5_{voor+lvt2aa0grx}ie₁e_0ss)−9wa_÷øöi_xntRhd₁o^ù_ou+utt1pp_.u_2u3tt ₅vco_Vulrtraegnet
(smaller
(smaller
at
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 5Ω 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.
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.
Theoretically, it is also possible for the regulator to be-
come unstable if very large capacitances (>10,000µF)
© 1998 SEMTECH CORP.
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