LP2950/LP2951
ADJUSTABLE MICROPOWER VOLTAGE REGULATORS
WITH SHUTDOWN
FEATURES
•
•
•
•
•
•
•
•
•
•
•
Wide Input Range…up to 30 V
Rated Output Current of 100 mA
Low Dropout…380 mV (Typ) at 100 mA
Low Quiescent Current…75 µA (Typ)
Tight Line Regulation…0.03% (Typ)
Tight Load Regulation…0.04% (Typ)
High VO Accuracy
– 1.4% at 25°C
– 2% Over Temperature
Can Be Used as a Regulator or Reference
LP2950...LP (TO-226/TO-92 PACKAGE
(BOTTOM VIEW)
OUTPUT
GND
INPUT
Stable With Low ESR (>12 mΩ) Capacitors
Current- and Thermal-Limiting Features
LP2950 Only (3-Pin Package)
– Fixed-Output Voltages of 5 V, 3.3 V,
and 3 V
LP2951 Only (8-Pin Package)
– Fixed- or Adjustable-Output Voltages:
5 V/ADJ, 3.3 V/ADJ, and 3 V/ADJ
– Low-Voltage Error Signal on Falling Output
– Shutdown Capability
– Remote Sense Capability for Optimal
Output Regulation and Accuracy
•
LP2951...D (SOIC) PACKAGE
(TOP VIEW)
OUTPUT
SENSE
SHUTDOWN
GND
1
8
2
7
3
6
4
5
INPUT
FEEDBACK
VTAP
ERROR
DESCRIPTION
The LP2950 and LP2951 devices are bipolar, low-dropout voltage regulators that can accommodate a wide input
supply-voltage range of up to 30 V. The easy-to-use, 3-pin LP2950 is available in fixed-output voltages of 5 V,
3.3 V, and 3 V. However, the 8-pin LP2951 is able to output either a fixed or adjustable output from the same
device. By tying the OUTPUT and SENSE pins together, and the FEEDBACK and VTAP pins together, the
LP2951 outputs a fixed 5 V, 3.3 V, or 3 V (depending on the version). Alternatively, by leaving the SENSE and
VTAP pins open and connecting FEEDBACK to an external resistor divider, the output can be set to any value
between 1.235 V to 30 V.
The 8-pin LP2951 also offers additional functionality that makes it particularly suitable for battery-powered
applications. For example, a logic-compatible shutdown feature allows the regulator to be put in standby mode
for power savings. In addition, there is a built-in supervisor reset function in which the ERROR output goes low
when VOUT drops by 6% of its nominal value for whatever reasons – due to a drop in VIN, current limiting, or
thermal shutdown.
The LP2950 and LP2951 are designed to minimize all error contributions to the output voltage. With a tight
output tolerance (0.5% at 25°C), a very low output voltage temperature coefficient (20 ppm typical), extremely
good line and load regulation (0.3% and 0.4% typical), and remote sensing capability, the parts can be used as
either low-power voltage references or 100-mA regulators.
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1
2014 OCT
LP2950/LP2951
LP2950 FUNCTIONAL BLOCK DIAGRAM
Unregulated DC
+
INPUT
VOUT
IL 3 100 mA
OUTPUT
+
−
+
ERROR
Amplifier
+
See Application
Information
1.23-V
Reference
GND
LP2951 FUNCTIONAL BLOCK DIAGRAM
Unregulated DC
+
VOUT
IL 3 100 mA
7
8
1
INPUT
FEEDBACK
OUTPUT
2
SENSE
+
−
ERROR
Amplifier
3
From
CMOS
or TTL
SHUTDOWN
6
VTAP
330 kW
5
+
+
ERROR
60 mV
−
+
See Application
Information
See Application Information
To CMOS
or TTL
+
1.235-V
Reference
4
GND
ERROR Detection Comparator
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2014 OCT
LP2950/LP2951
Absolute Maximum Ratings (1)
over operating free-air temperature range (unless otherwise noted)
MIN
MAX
UNIT
VIN
Continuous input voltage range
–0.3
30
V
VSHDN
SHUTDOWN input voltage range
–1.5
30
V
ERROR comparator output voltage range (2)
–1.5
30
V
30
V
VFDBK
FEEDBACK input voltage
range (2) (3)
θJA
Package thermal impedance (4) (5)
TJ
Operating virtual junction temperature
Tstg
Storage temperature range
(1)
(2)
(3)
(4)
(5)
–1.5
D package
97
LP package
140
–65
°C/W
150
°C
150
°C
Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under "recommended operating
conditions" is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
May exceed input supply voltage
If load is returned to a negative power supply, the output must be diode clamped to GND.
Maximum power dissipation is a function of TJ(max), θJA, and TA. The maximum allowable power dissipation at any allowable ambient
temperature is PD = (TJ(max) – TA)/θJA. Operating at the absolute maximum TJ of 150°C can affect reliability.
The package thermal impedance is calculated in accordance with JESD 51-7.
Recommended Operating Conditions
MIN
VIN
Supply input voltage
TJ
Operating virtual junction temperature
(1)
MAX
UNIT
(1)
30
V
–40
125
°C
Minimum VIN is the greater of:
a. 2 V (25°C), 2.3 V (over temperature), or
b. VOUT(MAX) + Dropout (Max) at rated IL
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2014 OCT
LP2950/LP2951
Electrical Characteristics
VIN = VOUT (nominal) + 1 V, IL = 100 µA, CL = 1 µF (5-V versions) or CL = 2.2 µF (3-V and 3.3-V versions),
8-pin version: FEEDBACK tied to VTAP, OUTPUT tied to SENSE, VSHUTDOWN ≤ 0.7 V
PARAMETER
TEST CONDITIONS
TJ
MIN
TYP
MAX
UNIT
3-V VERSION (LP295x-30)
VOUT
Output voltage
IL = 100µA
25°C
2.970
3
3.030
–40°C to 125°C
2.940
3
3.060
V
3.3-V VERSION (LP295x-33)
VOUT
Output voltage
IL = 100µA
25°C
3.267
3.3
3.333
–40°C to 125°C
3.234
3.3
3.366
25°C
4.950
5
5.050
–40°C to 125°C
4.900
5
5.100
V
5-V VERSION (LP295x-50)
VOUT
Output voltage
IL = 100µA
V
ALL VOLTAGE OPTIONS
Output voltage temperature
coefficient (1)
IL = 100 µA
Line regulation (2)
VIN = [VOUT(NOM) + 1 V] to 30 V
Load regulation (2)
IL = 100 µA to 100 mA
–40°C to 125°C
0.03
–40°C to 125°C
25°C
0.04
–40°C to 125°C
50
380
75
–40°C to 125°C
8
–40°C to 125°C
Dropout ground current
VIN = VOUT(NOM) – 0.5 V,
IL = 100 µA
Current limit
VOUT = 0 V
25°C
110
–40°C to 125°C
120
12
170
200
25°C
160
200
220
25°C
0.05
CL = 1 µF (5 V only)
Output noise (RMS),
10 Hz to 100 kHz
450
14
–40°C to 125°C
IL = 100 µA
%
80
140
25°C
IL = 100 mA
%/V
mV
600
25°C
GND current
0.2
150
–40°C to 125°C
IL = 100 µA
0.2
0.3
25°C
IL = 100 mA
100 ppm/°C
0.4
–40°C to 125°C
VIN – VOUT Dropout voltage (3)
Thermal regulation (4)
25°C
25°C
IL = 100 µA
IGND
20
0.2
µA
mA
µA
mA
%/W
430
CL = 200 µF
160
LP2951-50: CL = 3.3 µF,
CBypass = 0.01 µF between pins 1
and 7
µV
25°C
100
(LP2951-xx) 8-PIN VERSION ONLY ADJ
Reference voltage
VOUT = VREF to (VIN – 1 V),
VIN = 2.3 V to 30 V,
IL = 100 µA to 100 mA
Reference voltage
temperature coefficient (1)
(1)
(2)
(3)
(4)
25°C
1.218
1.235
1.252
–40°C to 125°C
1.212
1.257
–40°C to 125°C
1.200
1.272
V
25°C
20
ppm/°C
Output or reference voltage temperature coefficient is defined as the worst-case voltage change divided by the total temperature range.
Regulation is measured at constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage due to
heating effects are covered under the specification for thermal regulation.
Dropout voltage is defined as the input-to-output differential at which the output voltage drops 100 mV, below the value measured at 1-V
differential. The minimum input supply voltage of 2 V (2.3 V over temperature) must be observed.
Thermal regulation is defined as the change in output voltage at a time (T) after a change in power dissipation is applied, excluding load
or line regulation effects. Specifications are for a 50-mA load pulse at VIN = 30 V, VOUT = 5 V (1.25-W pulse) for t = 10 ms.
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2014 OCT
LP2950/LP2951
Electrical Characteristics (continued)
VIN = VOUT (nominal) + 1 V, IL = 100 µA, CL = 1 µF (5-V versions) or CL = 2.2 µF (3-V and 3.3-V versions),
8-pin version: FEEDBACK tied to VTAP, OUTPUT tied to SENSE, VSHUTDOWN ≤ 0.7 V
PARAMETER
TEST CONDITIONS
TJ
MIN
25°C
FEEDBACK bias current
TYP
MAX
20
40
–40°C to 125°C
FEEDBACK bias current
temperature coefficient
60
25°C
0.1
25°C
0.01
UNIT
nA
nA/°C
ERROR COMPARATOR
Output leakage current
VOUT = 30 V
Output low voltage
VIN = VOUT(NOM) – 0.5 V,
IOL = 400 µA
–40°C to 125°C
Upper threshold voltage
(ERROR output high) (5)
2
25°C
150
–40°C to 125°C
25°C
40
–40°C to 125°C
25
60
75
–40°C to 125°C
Hysteresis (6)
250
400
25°C
Lower threshold voltage
(ERROR output low) (5)
1
mV
mV
95
140
25°C
µA
15
mV
mV
SHUTDOWN INPUT
Input logic voltage
Low (regulator ON)
–40°C to 125°C
High (regulator OFF)
25°C
VTAP = 2.4 V
25°C
VTAP = 30 V
(5)
(6)
30
50
450
600
–40°C to 125°C
SHUTDOWN input current
Regulator output current
in shutdown
0.7
2
100
–40°C to 125°C
VSHUTDOWN ≥ 2 V,
VIN ≤ 30 V, VOUT = 0,
FEEDBACK tied to VTAP
25°C
–40°C to 125°C
V
µA
750
3
10
20
µA
Comparator thresholds are expressed in terms of a voltage differential equal to the nominal reference voltage (measured at
VIN – VOUT = 1 V) minus FEEDBACK terminal voltage. To express these thresholds in terms of output voltage change, multiply by the
error amplifier gain = VOUT/VREF = (R1 + R2)/R2. For example, at a programmed output voltage of 5 V, the ERROR output is specified to
go low when the output drops by 95 mV × 5 V/1.235 V = 384 mV. Thresholds remain constant as a percentage of VOUT (as VOUT is
varied), with the low-output warning occurring at 6% below nominal (typ) and 7.7% (max).
Comparator thresholds are expressed in terms of a voltage differential equal to the nominal reference voltage (measured at
VIN – VOUT = 1 V) minus FEEDBACK terminal voltage. To express these thresholds in terms of output voltage change, multiply by the
error amplifier gain = VOUT/VREF = (R1 + R2)/R2. For example, at a programmed output voltage of 5 V, the ERROR output is specified to
go low when the output drops by 95 mV × 5 V/1.235 V = 384 mV. Thresholds remain constant as a percentage of VOUT (as VOUT is
varied), with the low-output warning occurring at 6% below nominal (typ) and 7.7% (max).
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5
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS
QUIESCENT CURRENT
vs
LOAD CURRENT
INPUT CURRENT
vs
INPUT VOLTAGE
100
10
RL = ∞
80
1
Input Current – µA
Quiescent Current – mA
90
0.1
70
60
50
40
30
20
10
0.01
0.0001
0.001
0.01
0.1
0
IL – Load Current – A
0
1
2
3
4
5
6
7
8
9
10
8
9
10
VIN – Input Voltage – V
INPUT CURRENT
vs
INPUT VOLTAGE
200
INPUT CURRENT
vs
INPUT VOLTAGE
120
RL = 50 kΩ
RL = 50 Ω
110
180
100
160
Input Current – mA
Input Current – µA
90
140
120
100
80
60
80
70
60
50
40
30
40
20
20
10
0
0
1
2
3
4
5
6
7
8
9
0
10
0
VIN – Input Voltage – V
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1
2
3
4
5
6
7
VIN – Input Voltage – V
6
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
OUTPUT VOLTAGE
vs
TEMPERATURE
QUIESCENT CURRENT
vs
INPUT VOLTAGE
5.100
120
110
IL = 0
5.075
5.050
5.025
Quiescent Current – µA
VOUT – Output Voltage – V
100
IL = 100 µA
5.000
IL = 100 m A
4.975
4.950
90
80
70
60
50
40
30
20
4.925
10
4.900
-40 -25 -10 5
0
0
20 35 50 65 80 95 110 125
1
2
3
4
5
TA – Temperature – °C
VIN – Input Voltage – V
QUIESCENT CURRENT
vs
INPUT VOLTAGE
QUIESCENT CURRENT
vs
INPUT VOLTAGE
6
7
8
6
7
8
8
120
IL = 100 mA
IL = 1 mA
110
7
90
Quiescent Current – µA
Quiescent Current – µA
100
80
70
60
50
40
30
20
6
5
4
3
2
1
10
0
0
0
1
2
3
4
5
6
7
0
8
2
3
4
5
VIN – Input Voltage – V
VIN – Input Voltage – V
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1
7
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
QUIESCENT CURRENT
vs
TEMPERATURE
QUIESCENT CURRENT
vs
TEMPERATURE
100
10
9.5
IL = 100 m A
V IN = 6 V
95
90
Quiescent Current – µA
Quiescent Current – mA
9
8.5
8
7.5
7
6.5
85
80
75
70
65
6
60
5.5
55
5
-40 -25 -10
5
20
35
50
65
80
50
-40 -25 -10
95 110 125
5
20
35 50
65
80
TA – Temperature – °C
TA – Temperature – °C
SHORT-CIRCUIT CURRENT
vs
TEMPERATURE
DROPOUT VOLTAGE
vs
TEMPERATURE
250
450
(V IN – V OUT ) – Dropout Voltage – mV
200
175
150
125
100
75
50
-40 -25 -10
95 110 125
500
225
Short-Circuit Current – A
IL = 100 µA
V IN = 6 V
5
20
35 50
65
80
350
RL = 100 m A
300
250
200
150
100
RL = 100 µA
50
0
-40 -25 -10
95 110 125
TA – Tem perature – °C
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400
5
20 35
50 65
80 95 110 125
TA – Temperature – °C
8
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
LP2951 MINIMUM OPERATING VOLTAGE
vs
TEMPERATURE
400
2
350
1.95
Minimum Operating Voltage – V
(V IN – V OUT ) – Dropout Voltage – mV
DROPOUT VOLTAGE
vs
OUTPUT CURRENT
300
250
200
150
100
50
1.9
1.85
1.8
1.75
1.7
1.65
0
0.0001
0.001
0.01
1.6
-40 -25 -10
0.1
30
8
25
7
20
6
15
10
5
0
-5
4
3
1
-15
0
50-kW resistor
to VOUT
0
45
70
95
95 110 125
5
-10
20
65 80
50-kW resistor to
external 5-V supply
2
-5
35 50
LP2951 ERROR COMPARATOR OUTPUT
vs
INPUT VOLTAGE
ERROR Output – V
FEEDBACK Bias Current – nA
LP2951 FEEDBACK BIAS CURRENT
vs
TEMPERATURE
-30
20
TA – Temperature – °C
IO – Output Current – A
-20
-55
5
1
2
3
4
5
6
7
8
V IN – Input Voltage – V
120 145
TA – Temperature – °C
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9
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
LINE TRANSIENT RESPONSE
vs
TIME
LP2951 ERROR COMPARATOR SINK CURRENT
vs
OUTPUT LOW VOLTAGE
2
ISINK – Sink Current – mA
1.75
TA = 125
Input Voltage
2 V/div
1.5
1.25
TA = 25
1
Output Voltage
80 mV/div
0.75
TA = –40
0.5
0.25
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
VOL – Output Low Voltage – V
LOAD TRANSIENT RESPONSE
vs
TIME
(VOUT = 5 V, CL = 1 µF)
LOAD TRANSIENT RESPONSE
vs
TIME
(VOUT = 5 V, CL = 10 µF)
Output Voltage
100 mV/div
Output Voltage
100 mV/div
Output Load
100 mA/div
Output Load
100 mA/div
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10
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
ENABLE TRANSIENT RESPONSE
vs
TIME
(CL = 1 µF, IL = 1 mA)
ENABLE TRANSIENT RESPONSE
vs
TIME
(CL = 10 µF, IL = 1 mA)
OUTPUT IMPEDANCE
vs
FREQUENCY
RIPPLE REJECTION
vs
FREQUENCY
100
90
Power-Supply Ripple Rejection – dB
Ω
Output Impedance – Ohm
IL = 100 µA
10
1
IL = 1 m A
0.1
IL = 100 m A
0.01
10
1.E+01
100
1.E+02
1k
1.E+03
10k
1.E+04
100k
1.E+05
80
IL = 0
70
60
50
IL = 100 µA
40
30
V IN = 6 V
CL = 1 µF
20
10
1.E+01
1M
1.E+06
100
1.E+02
1k
1.E+03
10k
1.E+04
100k
1.E+05
1M
1.E+06
f – Frequency – Hz
f – Frequency – Hz
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11
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
RIPPLE REJECTION
vs
FREQUENCY
RIPPLE REJECTION
vs
FREQUENCY
100
100
VIN = 6 V
90
Power-Supply Ripple Rejection – dB
Power-Supply Ripple Rejection – dB
VIN = 6 V
CL = 1 µF
90
80
70
60
50
IL = 1 mA
40
30
20
80
CL = 1 µF
IL = 50 mA
70
60
50
40
IL = 100 mA
30
20
IL = 10 mA
10
1.E+01
10
1.E+02
100
1.E+03
1k
1.E+04
10k
1.E+05
100k
10
10
1.E+01
1.E+06
1M
100
1.E+02
1k
1.E+03
f – Frequency – Hz
100k
1.E+05
1M
1.E+06
f – Frequency – Hz
LP2951 OUTPUT NOISE
vs
FREQUENCY
LP2951 DIVIDER RESISTANCE
vs
TEMPERATURE
400
RP2P4 – Pin 2 to Pin 4 Resistance – k W
6
5
Output Noise – µV
10k
1.E+04
4
CL = 200 µF
3
CL = 1 µF
2
1
350
300
250
200
150
100
50
CL = 3.3 µF
0
1.E+01
10
1.E+02
100
1.E+03
1k
1.E+04
10k
0
-40 -25 -10
1.E+05
100k
20
35 50
65 80 95 110 125
TA – Temperature – °C
f – Frequency – Hz
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5
12
2014 OCT
LP2950/LP2951
TYPICAL CHARACTERISTICS (continued)
SHUTDOWN THRESHOLD VOLTAGE (ON TO OFF)
vs
TEMPERATURE
1.7
1.7
1.6
1.6
Input Logic Voltage (ON to OFF) – V
Input Logic Voltage (OFF to ON) – V
SHUTDOWN THRESHOLD VOLTAGE (OFF TO ON)
vs
TEMPERATURE
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
-40 -25 -10
5
20
35
50
65
80
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
-40 -25 -10
95 110 125
TA – Temperature – °C
5
20
35
50
65
80
95 110 125
TA – Temperature – °C
LINE REGULATION
vs
INPUT VOLTAGE
6
Output Voltage Change – mV
5
4
3
2
1
0
-1
-2
0
5
10
15
20
25
30
VIN – Input Voltage – V
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13
2014 OCT
LP2950/LP2951
APPLICATION INFORMATION
Input Capacitor (CIN)
A 1-µF (tantalum, ceramic, or aluminum) electrolytic capacitor should be placed locally at the input of the
LP2950 or LP2951 if there is, or will be, significant impedance between the ac filter capacitor and the input; for
example, if a battery is used as the input or if the ac filter capacitor is located more than 10 in away. There are
no ESR requirements for this capacitor, and the capacitance can be increased without limit.
Output Capacitor (COUT)
As with most PNP LDOs, stability conditions require the output capacitor to have a minimum capacitance and an
ESR that falls within a certain range.
Capacitance Value
For VOUT ≥ 5 V, a minimum of 1 µF is required. For lower VOUT, the regulator ’sloop gain is running closer to
unity gain and, thus, has lower phase margins. Consequently, a larger capacitance is needed for stability. For
VOUT = 3 V or 3.3 V, a minimum of 2.2 µF is recommended. For worst case, VOUT = 1.23 V (using the ADJ
version), a minimum of 3.3 µF is recommended. COUT can be increased without limit and only improves the
regulator stability and transient response. Regardless of its value, the output capacitor should have a resonant
frequency less than 500 kHz.
The minimum capacitance values given above are for maximum load current of 100 mA. If the maximum
expected load current is less than 100 mA, then lower values of COUT can be used. For instance, if IOUT < 10
mA, then only 0.33 µF is required for COUT. For IOUT < 1 mA, 0.1 µF is sufficient for stability requirements. Thus,
for a worst-case condition of 100-mA load and VOUT = VREF = 1.235 V (representing the highest load current and
lowest loop gain), a minimum COUT of 3.3 µF is recommended.
For the LP2950, no load stability is inherent in the design — a desirable feature in CMOS circuits that are put in
standby (such as RAM keep-alive applications). If the LP2951 is used with external resistors to set the output
voltage, a minimum load current of 1 µA is recommended through the resistor divider.
ESR Range
The regulator control loop relies on the ESR of the output capacitor to provide a zero to add sufficient phase
margin to ensure unconditional regulator stability; this requires the closed-loop gain to intersect the open-loop
response in a region where the open-loop gain rolls off at 20 dB/decade. This ensures that the phase always is
less than 180° (phase margin greater than 0°) at unity gain. Thus, a minimum-maximum range for the ESR must
be observed.
The upper limit of this ESR range is established by the fact that too high an ESR could result in the zero
occurring too soon, causing the gain to roll off too slowly, which, in turn allows a third pole to appear before unity
gain and introduce enough phase shift to cause instability. This typically limits the max ESR to approximately
5 Ω.
Conversely, the lower limit of the ESR is tied to the fact that too low an ESR shifts the zero too far out (past
unity gain) and, thus, allows the gain to roll off at 40 dB/decade at unity gain, with a resulting phase shift of
greater than 180°. Typically, this limits the minimum ESR to approximately 20 mΩ to 30 mΩ.
For specific ESR requirements, see Typical Characteristics .
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2014 OCT
LP2950/LP2951
APPLICATION INFORMATION (continued)
Capacitor Types
Most tantalum or aluminum electrolytics are suitable for use at the input. Film-type capacitors also work, but at
higher cost. When operating at low temperature, care should be taken with aluminum electrolytics, as their
electrolytes often freeze at –30°C. For this reason, solid tantalum capacitors should be used at temperatures
below –25°C.
Ceramic capacitors can be used, but due to their low ESR (as low as 5 mΩ to 10 mΩ), they may not meet the
minimum ESR requirement previously discussed. If a ceramic capacitor is used, a series resistor between 0.1 Ω
to 2 Ω must be added to meet the minimum ESR requirement. In addition, ceramic capacitors have one glaring
disadvantage that must be taken into account — a poor temperature coefficient, where the capacitance can vary
significantly with temperature. For instance, a large-value ceramic capacitor (≥2.2 µF) can lose more than half of
its capacitance as temperature rises from 25°C to 85°C. Thus, a 2.2-µF capacitor at 25°C drops well below the
minimum COUT required for stability as ambient temperature rises. For this reason, select an output capacitor
that maintains the minimum 2.2-µF required for stability for the entire operating temperature range.
CBYPASS: Noise and Stability Improvement
In the LP2951, an external FEEDBACK pin directly connected to the error amplifier noninverting input can allow
stray capacitance to cause instability by shunting the error amplifier feedback to GND, especially at high
frequencies. This is worsened if high-value external resistors are used to set the output voltage, because a high
resistance allows the stray capacitance to play a more significant role; i.e., a larger RC time delay is introduced
between the output of the error amplifier and its FEEDBACK input, leading to more phase shift and lower phase
margin. A solution is to add a 100-pF bypass capacitor (CBYPASS) between OUTPUT and FEEDBACK; because
CBYPASS is in parallel with R1, it lowers the impedance seen at FEEDBACK at high frequencies, in effect
offsetting the effect of the parasitic capacitance by providing more feedback at higher frequencies. More
feedback forces the error amplifier to work at a lower loop gain, so COUT should be increased to a minimum of
3.3 µF to improve the regulator ’sphase margin.
CBYPASS can be also used to reduce output noise in the LP2951. This bypass capacitor reduces the closed loop
gain of the error amplifier at the high frequency, so noise no longer scales with the output voltage. This
improvement is more noticeable with higher output voltages, because loop gain reduction is greatest. A suitable
CBYPASS is calculated as shown in Equation 1:
1
f (CBYPASS) ] 200 Hz ³ CBYPASS +
2p R1 200 Hz
(1)
On the 3-pin LP2950, noise reduction can be achieved by increasing the output capacitor, which causes the
regulator bandwidth to be reduced, therefore, eliminating high-frequency noise. However, this method is
relatively inefficient, as increasing COUT from 1 µF to 220 µF only reduces the regulator ’soutput noise from
430 µV to 160 µV (over a 100-kHz bandwidth).
ERROR Function (LP2951 Only)
The LP2951 has a low-voltage detection comparator that outputs a logic low when the output voltage drops by
≈6% from its nominal value, and outputs a logic high when VOUT has reached ≈95% of its nominal value. This
95% of nominal figure is obtained by dividing the built-in offset of ≈60 mV by the 1.235-V bandgap reference,
and remains independent of the programmed output voltage. For example, the trip-point threshold (ERROR
output goes high) typically is 4.75 V for a 5-V output and 11.4 V for a 12-V output. Typically, there is a hysteresis
of 15 mV between the thresholds for high and low ERROR output.
A timing diagram is shown in Figure 1 for ERROR vs VOUT (5 V), as VIN is ramped up and down. ERROR
becomes valid (low) when VIN ≈ 1.3 V. When VIN ≈ 5 V, VOUT = 4.75 V, causing ERROR to go high. Because the
dropout voltage is load dependent, the output trip-point threshold is reached at different values of VIN, depending
on the load current. For instance, at higher load current, ERROR goes high at a slightly higher value of VIN, and
vice versa for lower load current. The output-voltage trip point remains at ≈4.75 V, regardless of the load. Note
that when VIN ≤ 1.3 V, the ERROR comparator output is turned off and pulled high to its pullup voltage. If VOUT is
used as the pullup voltage, rather than an external 5-V source, ERROR typically is ≈1.2 V. In this condition, an
equal resistor divider (10 kΩ is suitable) can be tied to ERROR to divide down the voltage to a valid logic low
during any fault condition, while still enabling a logic high during normal operation.
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15
2014 OCT
LP2950/LP2951
APPLICATION INFORMATION (continued)
Output
Voltage
4.75 V
ERROR
5V
Input
Voltage
1.3 V
Figure 1. ERROR Output Timing
Because the ERROR comparator has an open-collector output, an external pullup resistor is required to pull the
output up to VOUT or another supply voltage (up to 30 V). The output of the comparator is rated to sink up to
400 µA. A suitable range of values for the pullup resistor is from 100 kΩ to 1 MΩ. If ERROR is not used, it can
be left open.
Programming Output Voltage (LP2951 Only)
A unique feature of the LP2951 is its ability to output either a fixed voltage or an adjustable voltage, depending
on the external pin connections. To output the internally programmed fixed voltage, tie the SENSE pin to the
OUTPUT pin and the FEEDBACK pin to the VTAP pin. Alternatively, a user-programmable voltage ranging from
the internal 1.235-V reference to a 30-V max can be set by using an external resistor divider pair. The resistor
divider is tied to VOUT, and the divided-down voltage is tied directly to FEEDBACK for comparison against the
internal 1.235-V reference. To satisfy the steady-state condition in which its two inputs are equal, the error
amplifier drives the output to equal Equation 2:
V OUT + V REF
ǒ 1 ) R1
Ǔ*I
R2
FB
R1
(2)
Where:
VREF = 1.235 V applied across R2
IFB = FEEDBACK bias current, typically 20 nA
A minimum regulator output current of 1 µA must be maintained. Thus, in an application where a no-load
condition is expected (for example, CMOS circuits in standby), this 1-µA minimum current must be provided by
the resistor pair, effectively imposing a maximum value of R2 = 1.2 MΩ (1.235 V/1.2 MΩ ≈ 1 µA).
IFB = 20 nA introduces an error of ≈0.02% in VOUT. This can be offset by trimming R1. Alternatively, increasing
the divider current makes IFB less significant, thus, reducing its error contribution. For instance, using R2 = 100
kΩ reduces the error contribution of IFB to 0.17% by increasing the divider current to ≈12 µA. This increase in the
divider current still is small compared to the 600-µA typical quiescent current of the LP2951 under no load.
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2014 OCT
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