1 www.semtech.com
LP2951A
150mA Adjustable
Voltage Regulator
POWER MANAGEMENT
Revision 1, January 2001
uGuaranteed 150mA current
uAdjustable output voltage - 1.24V to 29V
uAccurate 5V output @ 150mA
uLow dropout voltage - 350 to 400mV @ 150mA
uRegulator or reference functions
uDirect replacement for LP2951AC, MIC2951-02,
AS2951AC
uFull industrial temperature range
uSO-8 package
The LP2951A is an improved version of the LP2951 and
features tighter tolerance on the output voltage and
reference voltage specifications. Both the LP2951 and
LP2951A feature 150mA output current capability.
The LP2951 series of low power voltage regulators have
low quiescent current and low dropout voltage. The
quiescent current increases minimally during dropout
conditions thereby extending battery life.
Available in the 8 lead SOIC package, the LP2951 series
includes features such as shutdown and low output
voltage detect (typically due to low battery conditions).
This function may also be used as a power on reset
function when triggered by CMOS or TTL inputs.
The circuit can be used as a fixed voltage 5 volt regulator
or adjusted between 1.24 volts and 29 volts using
external resistor pairs.
uMicrocontroller supplies
uLinear regulators
uAdjustable Supplies
uSwitching power supplies - post-regulation
uPortable modems
uBattery powered systems
uCellular telephones
uVoltage references
ERROR OUTPUT
VOUT = 1.24V to 30V VIN
SHUTDOWN INPUT
R3
100k
U1 LP2951A
1
2
3
45
6
7
8
OUT
SENSE
SHDN
GND ERRORB
TAP
FB
IN
R2
R1
C1 10nF
+
C2
3.3uF
+= 2R1R
1VV REFOUT
Description Features
Applications
Typical Application Circuits
2ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
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Unless specified: VIN = VOUT(NOM) + 1V, IOUT = 100µA, COUT = 1µF, limits are over operating temperature range (TJ = TA).
Absolute Maximum Ratings
Note:
(1) 2 inch square of 1/16 FR4, double sided, 1oz. minimum copper weight.
Electrical Characteristics
3ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
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Note:
(1) Temperature coefficient is defined as the worst case voltage change divided by total temperature range.
Electrical Characteristics (Cont.)
Unless specified: VIN = VOUT(NOM) + 1V, IOUT = 100µA, COUT = 1µF, limits are over operating temperature range (TJ = TA).
4ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
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Notes:
(1) Only available in tape and reel packaging. A reel
contains 2500 devices.
Pin Configuration Ordering Information
Block Diagram
5ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
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
(or 3.3V for LP2951-3.3) output is desired. To use the
internal voltage divider, 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 output to ground, with its center connected to the
feedback pin (see Adjustable Regulator figure below).
When using an external voltage divider, resistances can
be calculated from the following formula:
VOUT = V235.11Rx10x20
2R
235.1 9+
+−
An upper limit of values for R2 occurs at ~1.2MW 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
attention must be paid to the -20nA (typical) bias current
required by the feedback pin. Using a 1.2MW 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 120kW
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 sufficient. For
regulator operation at minimum output voltage (1.24V)
and output currents of 100mA, the required filter
increases to 3.3µF. Any type of capacitor may be used,
although if aluminum electrolytics are chosen, the equiva-
lent series resistance (ESR) should be held to 5W or less.
For small load currents the capacitance 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
become unstable if very large capacitances (>10,000µF)
are connected to the output, but this has not been
observed in practice. It is also important that the
capacitance be mounted close (1cm or less) to the
output pin of the regulator.
Figure 1: 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 between
the input terminal and ground. A 1µF tantalum or
aluminum electrolytic capacitor is usually sufficient. Lower
values can be used if load currents are small. Noise
injection into the feedback terminal of the LP2951(A)
from nearby noise sources can also upset the output.
Generally this can be cured by the addition of 100pF or
so from the feedback terminal to the output.
Reducing Output Noise
In ultra-quiet systems, or when the LP2951(A) is being
used as a reference, it may be desirable to perform
additional output filtering to reduce noise. While this can
be done by simply using larger capacitors on the output,
that solution tends to be bulky and expensive, and even-
tually, with huge capacitors (>1,000µF) may cause
instability in the regulator. Generally, it is more cost-
effective to let the regulator regulate output noise away.
Applications Information (LP2951(A))
6ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
This can be done by bypassing the upper resistor in the
feedback divider with a small capacitor to provide a more
direct path for AC feedback. The size of this capacitor
can be calculated from the formula:
CBYPASS = corner1fR2 1
π
where R1 is the upper resistor of the feedback divider
and fcorner is the frequency above which the increased AC
feedback is to become active. Because the gain of the
error amplifier in the LP2951(A) begins to roll off at about
300Hz, this is generally an optimum choice for corner
frequency.
The reduction of the output noise will be proportional to
the ratio of the two resistors in the feedback divider:
2R1R 1R
+
and will increase at a rate of 20 dB per decade at
frequencies above the corner frequency chosen, up to
the frequency where the error amplifiers gain has rolled
off to 1 (~100kHz). In order to maintain regulator
stability when using a noise-reducing bypass capacitor, it
will also be necessary to increase the size of the output
filter capacitor by the ratio:
Applications Information (Cont.)
2R1R 1R
+
7ã 2001 Semtech Corp. www.semtech.com
LP2951A
POWER MANAGEMENT
Outline Drawing - SO-8
Land Pattern -SO-8
Semtech Corporation
Power Management Products Division
652 Mitchell Rd., Newbury Park, CA 91320
Phone: (805)498-2111 FAX (805)498-3804
Contact Information
