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LP2951A
© 1998 SEMTECH CORP.
150mA ADJUSTABLE
VOLTAGE REGULATOR
March 17, 1998
652 MITCHELL ROAD NEWBURY PARK CA 91320
1
DESCRIPTION
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 eight 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.
FEATURES
•= Guaranteed 150mA current
•= Adjustable output voltage - 1.24V to 29V
•= Accurate 5V output @150mA
•= Low dropout voltage - 450mV @ 150mA
•= Regulator or reference functions
•= Direct replacement for LP2951AC, MIC2951-02,
AS2951AC
APPLICATIONS
•= Microcontroller supplies
•= Linear regulators
•= Adjustable supplies
•= Switching power supplies - post-regulation
•= Portable modems
•= Battery powered systems
•= Cellular telephones
•= Voltage references
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
BLOCK DIAGRAM
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
Parameter Symbol Maximum Units
Supply Voltage V
IN
-0.3 to 30 V
Shutdown Input
Voltage
-0.3 to 30 V
Error Comp. Output
Voltage
-0.3 to 30 V
Power Dissipation
P
D
Internally
Limited
W
Operating Junction
Temperature Range
T
J
-40 to 125 °C
Storage
Temperature Range
T
STG
-65 to 150 °C
Lead Temperature
(Soldering) 5 Sec
T
LEAD
260 °C
ORDERING INFORMATION
Note:
(1) Add suffix ‘TR’ for tape and reel.
DEVICE
(1)
V
OUT
VOLTS PACKAGE
LP2951ACM ADJ SO-8
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LP2951A
© 1998 SEMTECH CORP.
150mA ADJUSTABLE
VOLTAGE REGULATOR
March 17, 1998
652 MITCHELL ROAD NEWBURY PARK CA 91320
2
NOTES:
(1) Temperature coefficient is defined as the worst case voltage change divided by total temperature range.
ELECTRICAL CHARACTERISTICS
Unless specified, limits are over operating temperature range (TJ = TA), VIN = V
OUT(NOM)
+ 1V, IL = 100µA, CL = 1µF
Parameter Symbol Conditions Min Typ Max Units
Output Voltage V
OUT
TJ=25°C, IL = 100µA 4.975 5.000 5.025 V
Temp Coefficient
(1)
T
C
0°C ≤ TJ ≤=70°C
20 120 ppm/°C
Line Regulation REG
(LINE)
6V ≤ V
IN=
≤=30V
0.1 0.5 %
Load Regulation REG
(LOAD)
100µA ≤ IL=≤=150mA
0.1 0.4 %
Dropout Voltage V
D
IL===100µA
80 150 mV
I
L=
==150mA
380 600
Ground Current I
GND
IL===100µA
120 160 µA
I
L=
==150mA
814 mA
Dropout Ground Current I
GND(D)
VIN = 4.5V, IL===100µA
110 250 µA
Current Limit I
CL
V
OUT
= 0 200 250 mA
Reference Voltage V
REF
V
REF
≤ V
OUT
≤ (V
IN
- 1V), TJ ==25°C,
100µA ≤ I
L=
≤=100mA
1.22 1.235 1.25 V
Feedback Bias Current I
FB
20 60 nA
Error Comparator
Output High Leakage
Current
V
OH
= 30V 2 µA
Output Low Voltage V
IN
= 4.5V, IOL = 400µA 150 400 mV
Threshold Voltage Upper 25 60 mV
Lower 75 140
Hysteresis 15 mV
Shutdown Input
Input Logic Voltage V
SD
Low 0.6 V
High 2.0 V
Input Current I
SD
V
SHUTDOWN
= 2.4V 100 µA
V
SHUTDOWN
= 30V 750
Regulator Shutdown
Output Current
I
O(SD)
V
SHUTDOWN
≥ 2V, VIN ≤ 30V,
V
OUT
= 0, Feedback pin to Tap 5V
20 µA
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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)
V235.11Rx10x20
2R
235.1
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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 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:
V
OUT
=
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 attention 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 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 a 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 equivalent series resistance (ESR) should
be held to 5Ω 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.
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 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.
V235.11Rx10x20
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LP2951A
© 1998 SEMTECH CORP.
150mA ADJUSTABLE
VOLTAGE REGULATOR
March 17, 1998
652 MITCHELL ROAD NEWBURY PARK CA 91320
4
PRINCIPLES OF OPERATION (LP2951 and LP2951A) (cont.)
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 simply using larger capacitors on the output, that solution tends to be bulky and expensive, and
eventually, with huge capacitors (>1,000µF) may cause
instability in the regulator. Generally, it is more costeffective to let the regulator regulate output noise
away. 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:
C
BYPASS
=
where R1 is the upper resistor of the feedback divider
and f
corner
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 300 Hz, 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,
and will increase 20 dB per decade at frequencies
above the corner frequency chosen, up to the frequency where the error amplifier’s 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
2R1R1R+
2R1R1R+
corner1
fR21π
Page 5
LP2951A
© 1998 SEMTECH CORP.
150mA ADJUSTABLE
VOLTAGE REGULATOR
March 17, 1998
652 MITCHELL ROAD NEWBURY PARK CA 91320
5
OUTLINE DRAWING SO-8
LAND PATTERN SO-8
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