Fixed 5.0V or Adjustable
100mA Voltage Regulator
POWER MANAGEMENT
Description
The LP2951 low power voltage regulator has low quies-
cent 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 in-
cludes 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 1.235 volts and 29 volts using
external resistor pairs.
LP2951
Features
Guaranteed 100mA current
Adjustable output voltage - 1.235V to 29V
Accurate 1.235V reference
Internally set 5V output optional
Low dropout voltage - 300mV @ 100mA
Regulator or reference functions
Full industrial temperature range
SO-8 package
Applications
Microcontroller supplies
Linear regulators
Adjustable Supplies
Switching power supplies - post-regulation
Portable modems
Battery powered systems
Cellular telephones
Voltage references
Typical Application Circuits
C1
10nF
R1
U1
VOUT = 1.24V to 30V
1
2
SHUTDOWN INPUT
3
4
OUT
LP2951
IN
FB
TAP
ERRORB
R3
8
7
6
5
R2
ERROR OUTPUT
VIN
SENSE
SHDN
GND
R1
V
OUT
=
V
REF
1
+
R2
+
C2
3.3uF
100k
VOUT = 5V
1
2
U1
OUT
LP2951
IN
FB
TAP
ERRORB
R1
8
7
6
5
ERROR OUTPUT
VIN
SENSE
SHDN
GND
SHUTDOWN INPUT
+
C1
3.3uF
3
4
100k
Revision: July 21, 2004
1
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LP2951
POWER MANAGEMENT
Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified
in the Electrical Characteristics section is not implied.
Parameter
Supply Voltage
Shutdown Input Voltage
Error Comp. Output Voltage
Power Dissipation
Thermal Resistance Junction to Case
Thermal Resistance Junction to Ambient
(1)
Operating Junction Temperature Range
Storage Temperature Range
Lead Temperature (Soldering) 5 Sec.
Symbol
V
IN
V
SHDN
Maximum
-0.3 to 30
-0.3 to 30
-0.3 to 30
Units
V
V
V
W
°C/W
°C/W
°C
°C
°C
P
D
θ
J C
θ
JA
T
J
T
STG
T
LEAD
Internally Limited
47
65
-40 to 125
-65 to 150
300
Note:
(1) 2 inch square of 1/16” FR4, double sided, 1oz. minimum copper weight.
Electrical Characteristics
(3)
Unless specified: (V
OUT(NOM)
+ 1V)
≤
V
IN
≤
30V, 100µA
≤
I
OUT
≤
100mA, C
OUT
= 3.3µF, T
A
= 25°C.
Values in
bold
apply over full operating temperature range.
Parameter
Output Voltage
Line Regulation
Load Regulation
Dropout Voltage
Symbol
V
OUT
REG
(LINE)
REG
(LOAD)
V
D
Conditions
V
IN
= 6V, I
OUT
= 100µA
V
IN
= 2.235V to 30V, I
OUT
= 100µA
V
IN
= 2.235V, I
OUT
= 100µA to 100mA
I
OUT
= 100µA
I
OUT
= 100mA
Min
Typ
5.000
0.1
0.15
50
300
130
8
175
Max
Units
V
0.8
0.40
150
600
200
20
250
250
%
%
mV
Ground Pin Current
I
GND
V
IN
= 2.235V, I
OUT
= 100µA
V
IN
= 2.235V, I
OUT
= 100mA
µA
mA
µA
mA
V
Dropout Ground Pin Current
Short Circuit Current Limit
Reference Voltage
I
GND(D)
I
SC
V
REF
V
IN
= (V
OUT(NOM)
- 0.5V), I
OUT
= 100µA
V
OUT
= 0V
V
IN
= 2.235V, I
OUT
= 100µA
-1%
-2.5%
1.2350
+1%
+2.5%
Temperature Coefficient
(1)(2)
Feedback Bias Current
(1)
T
C(REF)
I
FB
V
IN
= 2.235V, I
OUT
= 100µA
20
5
120
60
ppm/°C
nA
2004 Semtech Corp.
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LP2951
POWER MANAGEMENT
Applications Information
Setting the Output Voltage
The LP2951 can be set to deliver any output voltage from
1.235V 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
Figure 1). When using an external voltage divider,
resistances can be calculated from the following formula:
−
9
V
OUT
=
R 2
+
20 x10
x R1
+
1 .235 V
1 .235
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.
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. 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 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
equivalent series resistance (ESR) should be held to 5Ω
or less. For small load currents the capacitance can be
reduced, for example, 1µF will be satisfactory for output
currents of 30mA or less. Care should be taken to
ensure that the minimum capacitance value is maintained
over the entire operating temperature range.
2004 Semtech Corp.
5
Figure 1: Adjustable Regulator
If the lead inductance between the input of the LP2951
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 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 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
eventually, 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.
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