LP2950,LP2951
LP2950/LP2951 Series of Adjustable Micropower Voltage Regulators
Literature Number: SNVS764M
LP2950/LP2951
August 20, 2011
Series of Adjustable Micropower Voltage Regulators
General Description
The LP2950 and LP2951 are micropower voltage regulators
with very low quiescent current (75μA typ.) and very low
dropout voltage (typ. 40mV at light loads and 380mV at
100mA). They are ideally suited for use in battery-powered
systems. Furthermore, the quiescent current of the LP2950/
LP2951 increases only slightly in dropout, prolonging battery
life.
The LP2950-5.0 is available in the surface-mount D-Pak
package, and in the popular 3-pin TO-92 package for pin-
compatibility with older 5V regulators. The 8-lead LP2951 is
available in plastic, ceramic dual-in-line, LLP, or metal can
packages and offers additional system functions.
One such feature is an error flag output which warns of a low
output voltage, often due to falling batteries on the input. It
may be used for a power-on reset. A second feature is the
logic-compatible shutdown input which enables the regulator
to be switched on and off. Also, the part may be pin-strapped
for a 5V, 3V, or 3.3V output (depending on the version), or
programmed from 1.24V to 29V with an external pair of re-
sistors.
Careful design of the LP2950/LP2951 has minimized all con-
tributions to the error budget. This includes a tight initial
tolerance (.5% typ.), extremely good load and line regulation
(.05% typ.) and a very low output voltage temperature coef-
ficient, making the part useful as a low-power voltage refer-
ence.
Features
5V, 3V, and 3.3V versions available
High accuracy output voltage
Guaranteed 100mA output current
Extremely low quiescent current
Low dropout voltage
Extremely tight load and line regulation
Very low temperature coefficient
Use as Regulator or Reference
Needs minimum capacitance for stability
Current and Thermal Limiting
Stable with low-ESR output capacitors (10m to 6Ω)
LP2951 versions only
Error flag warns of output dropout
Logic-controlled electronic shutdown
Output programmable from 1.24 to 29V
Block Diagram and Typical Applications
LP2950
854625
© 2011 National Semiconductor Corporation 8546 www.national.com
LP2950/LP2951 Series of Adjustable Micropower Voltage Regulators
LP2951
854601
Connection Diagrams
TO-92 Plastic Package (Z)
854602
Bottom View
Dual-In-Line Packages (N, J)
Surface-Mount Package (M, MM)
854626
Top View
Metal Can Package (H)
854619
Top View
10-Lead Ceramic Surface-Mount Package (WG)
854664
Top View
TO-252 (D-Pak)
854670
Front View
8-Lead LLP
854671
Connect DAP to GND at device pin 4.
Top View
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LP2950/LP2951
Ordering Information
Package Temperature
Range VOUT
Part
Number
Package
Marking
Transport
Media
NSC
Drawing
TO-92
(Z)
−40 < TJ < 125 3.0 LP2950ACZ-3.0 2950A CZ3.0 Bag Z03A
LP2950CZ-3.0 2950 CZ3.0 Bag
3.3 LP2950ACZ-3.3 2950A CZ3.3 Bag
LP2950CZ-3.3 2950 CZ3.3 Bag
5.0 LP2950ACZ-5.0 2950A CZ5.0 Bag
LP2950CZ-5.0 2950 CZ5.0 Bag
TO-252
(D-Pak)
−40 < TJ < 125 3.0 LP2950CDT-3.0 LP2950CDT-3.0 75 Units/Rail TD03B
LP2950CDTX-3.0 2.5k Units Tape and Reel
3.3 LP2950CDT-3.3 LP2950CDT-3.3 75 Units/Rail
LP2950CDTX-3.3 2.5k Units Tape and Reel
5.0 LP2950CDT-5.0 LP2950CDT-5.0 75 Units/Rail
LP2950CDTX-5.0 2.5k Units Tape and Reel
N
(N-08E)
−40 < TJ < 125 3.0 LP2951ACN-3.0 LP2951ACN-3.0 40 Units/Rail N08E
LP2951CN-3.0 LP2951CN-3.0 40 Units/Rail
3.3 LP2951ACN-3.3 LP2951ACN-3.3 40 Units/Rail
LP2951CN-3.3 LP2951CN-3.3 40 Units/Rail
5.0 LP2951ACN LP2951ACN 40 Units/Rail
LP2951CN LP2951CN 40 Units/Rail
M
(M08A)
−40 < TJ < 125 3.0 LP2951ACM-3.0 2951ACM30*
(where * is die rev letter)
95 Units/Rail M08A
LP2951ACMX-3.0 2.5k Units Tape and Reel
LP2951CM-3.0 2951CM30*
(where * is die rev letter)
95 Units/Rail
LP2951CMX-3.0 2.5k Units Tape and Reel
3.3 LP2951ACM-3.3 2951ACM33*
(where * is die rev letter)
95 Units/Rail
LP2951ACMX-3.3 2.5k Units Tape and Reel
LP2951CM-3.3 2951CM33*
(where * is die rev letter)
95 Units/Rail
LP2951CMX-3.3 2.5k Units Tape and Reel
5.0 LP2951ACM 2951ACM*
(where * is die rev letter)
95 Units/Rail
LP2951ACMX 2.5k Units Tape and Reel
LP2951CM 2951CM*
(where * is die rev letter)
95 Units/Rail
LP2951CMX 2.5k Units Tape and Reel
MM
(MUA08A)
−40 < TJ < 125 3.0 LP2951ACMM-3.0 L0BA 1k Units Tape and Reel MUA08A
LP2951ACMMX-3.0 3.5k Units Tape and Reel
LP2951CMM-3.0 L0BB 1k Units Tape and Reel
LP2951CMMX-3.0 3.5k Units Tape and Reel
3.3 LP2951ACMM-3.3 L0CA 1k Units Tape and Reel
LP2951ACMMX-3.3 3.5k Units Tape and Reel
LP2951CMM-3.3 L0CB 1k Units Tape and Reel
LP2951CMMX-3.3 3.5k Units Tape and Reel
5.0 LP2951ACMM L0DA 1k Units Tape and Reel
LP2951ACMMX 3.5k Units Tape and Reel
LP2951CMM L0DB 1k Units Tape and Reel
LP2951CMMX 3.5k Units Tape and Reel
J
(J08A)
−55 < TJ < 150 5.0 LP2951J/883 See MIL/AERO Datasheet 40 Units/Rail J08A
H
(H08C)
−55 < TJ < 150 5.0 LP2951H/883 See MIL/AERO Datasheet Tray H08C
WG
(WG10A)
−55 < TJ < 150 5.0 LP2951WG/883 See MIL/AERO Datasheet Tray WG10A
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LP2950/LP2951
Package Temperature
Range VOUT
Part
Number
Package
Marking
Transport
Media
NSC
Drawing
8-lead LLP −40 < TJ < 125 3.0 LP2951ACSD-3.0 51AC30 1k Units Tape and Reel SDC08A
LP2951ACSDX-3.0 4.5k Units Tape and Reel
LP2951CSD-3.0 51AC30B 1k Units Tape and Reel
LP2951CSDX-3.0 4.5k Units Tape and Reel
3.3 LP2951ACSD-3.3 51AC33 1k Units Tape and Reel
LP2951ACSDX-3.3 4.5k Units Tape and Reel
LP2951CSD-3.3 51AC33B 1k Units Tape and Reel
LP2951CSDX-3.3 4.5k Units Tape and Reel
5.0 LP2951ACSD 2951AC 1k Units Tape and Reel
LP2951ACSDX 4.5k Units Tape and Reel
LP2951CSD 2951ACB 1k Units Tape and Reel
LP2951CSDX 4.5k Units Tape and Reel
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LP2950/LP2951
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Supply Voltage −0.3 to +30V
SHUTDOWN Input Voltage,
Error Comparator Output
Voltage, (Note 9)
FEEDBACK Input Voltage −1.5 to +30V
(Note 9, Note 10)
Power Dissipation Internally Limited
Junction Temperature (TJ)+150°C
Ambient Storage Temperature −65° to +150°C
Soldering Dwell Time, Temperature
Wave
Infrared
Vapor Phase
4 seconds, 260°C
10 seconds, 240°C
75 seconds, 219°C
ESD Rating
Human Body Model(Note 18) 2500V
Operating Ratings (Note 1)
Maximum Input Supply Voltage 30V
Junction Temperature Range
(TJ) (Note 8)
LP2950AC-XX, LP2950C-XX −40° to +125°C
LP2951 −55° to +150°C
LP2951AC-XX, LP2951C-XX −40° to +125°C
Electrical Characteristics (Note 2)
Parameter Conditions
(Note 2)
LP2951 LP2950AC-XX LP2950C-XX
Units
LP2951AC-XX LP2951C-XX
Tested Tested Design Tested Design
Typ Limit Typ Limit Limit Typ Limit Limit
(Note 3)
(Note 16)
(Note 3) (Note 4) (Note 3) (Note 4)
3V Versions (Note 17)
Output Voltage TJ = 25°C 3.0 3.015 3.0 3.015 3.0 3.030 V max
2.985 2.985 2.970 V min
−25°C TJ 85°C 3.0 3.0 3.030 3.0 3.045 V max
2.970 2.955 V min
Full Operating 3.0 3.036 3.0 3.036 3.0 3.060 V max
Temperature Range 2.964 2.964 2.940 V min
Output Voltage 100μA IL 100mA 3.0 3.045 3.0 3.042 3.0 3.072 V max
TJ TJMAX 2.955 2.958 2.928 V min
3.3V Versions (Note 17)
Output Voltage TJ = 25°C 3.3 3.317 3.3 3.317 3.3 3.333 V max
3.284 3.284 3.267 V min
−25°C TJ 85°C 3.3 3.3 3.333 3.3 3.350 V max
3.267 3.251 V min
Full Operating 3.3 3.340 3.3 3.340 3.3 3.366 V max
Temperature Range 3.260 3.260 3.234 V min
Output Voltage 100μA IL 100mA 3.3 3.350 3.3 3.346 3.3 3.379 V max
TJ TJMAX 3.251 3.254 3.221 V min
5V Versions (Note 17)
Output Voltage TJ = 25°C 5.0 5.025 5.0 5.025 5.0 5.05 V max
4.975 4.975 4.95 V min
−25°C TJ 85°C 5.0 5.0 5.05 5.0 5.075 V max
4.95 4.925 V min
Full Operating 5.0 5.06 5.0 5.06 5.0 5.1 V max
Temperature Range 4.94 4.94 4.9 V min
Output Voltage 100μA IL 100mA 5.0 5.075 5.0 5.075 5.0 5.12 V max
TJ TJMAX 4.925 4.925 4.88 V min
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LP2950/LP2951
Parameter Conditions
(Note 2)
LP2951 LP2950AC-XX LP2950C-XX
Units
LP2951AC-XX LP2951C-XX
Tested Tested Design Tested Design
Typ Limit Typ Limit Limit Typ Limit Limit
(Note 3)
(Note 16)
(Note 3) (Note 4) (Note 3) (Note 4)
All Voltage Options
Output Voltage
Temperature
Coefficient
(Note 12)20 120 20 100 50 150 ppm/°C
Line Regulation
(Note 14)
(VONOM + 1)V Vin
30V (Note 15)
0.03 0.1 0.03 0.1 0.04 0.2 % max
0.5 0.2 0.4 % max
Load Regulation
(Note 14)
100μA IL 100mA 0.04 0.1 0.04 0.1 0.1 0.2 % max
0.3 0.2 0.3 % max
Dropout Voltage
(Note 5)
IL = 100μA 80 80 80 mV max
50 150 50 150 50 150 mV max
IL = 100mA 450 450 450 mV max
380 600 380 600 380 600 mV max
Ground IL = 100μA75 120 75 120 75 120 μA max
Current 140 140 140 μA max
IL = 100mA 8 12 8 12 8 12 mA max
14 14 14 mA max
Dropout Vin = (VONOM − 0.5)
V
110 170 110 170 110 170 μA max
Ground Current IL = 100μA 200 200 200 μA max
Current Limit Vout = 0 160 200 160 200 160 200 mA max
220 220 220 mA max
Thermal Regulation (Note 13) 0.05 0.2 0.05 0.2 0.05 0.2 %/W
max
Output Noise, CL = 1μF (5V Only) 430 430 430 μV rms
10 Hz to 100 kHz CL = 200μF160 160 160 μV rms
CL = 3.3μF
(Bypass = 0.01μF100 100 100 μV rms
Pins 7 to 1 (LP2951)
8-pin Versions Only LP2951 LP2951AC-XX LP2951C-XX
Reference 1.23
5
1.25 1.23
5
1.25 1.23
5
1.26 V max
Voltage 1.26 1.26 1.27 V max
1.22 1.22 1.21 V min
1.2 1.2 1.2 V min
Reference (Note 7) 1.27 1.27 1.285 V max
Voltage 1.19 1.19 1.185 V min
Feedback Pin 20 40 20 40 20 40 nA max
Bias Current 60 60 60 nA max
Reference Voltage (Note 12) 20 20 50 ppm/°C
Temperature
Coefficient
Feedback Pin Bias 0.1 0.1 0.1 nA/°C
Current Temperature
Coefficient
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LP2950/LP2951
Parameter Conditions
(Note 2)
LP2951 LP2950AC-XX LP2950C-XX
Units
LP2951AC-XX LP2951C-XX
Tested Tested Design Tested Design
Typ Limit Typ Limit Limit Typ Limit Limit
(Note 3)
(Note 16)
(Note 3) (Note 4) (Note 3) (Note 4)
Error Comparator
Output Leakage VOH = 30V 0.01 1 0.01 1 0.01 1 μA max
Current 2 2 2μA max
Output Low Vin = (VONOM − 0.5)
V
150 250 150 250 150 250 mV max
Voltage IOL = 400μA 400 400 400 mV max
Upper Threshold (Note 6) 60 40 60 40 60 40 mV min
Voltage 25 25 25 mV min
Lower Threshold (Note 6) 75 95 75 95 75 95 mV max
Voltage 140 140 140 mV max
Hysteresis (Note 6) 15 15 15 mV
Shutdown Input
Input 1.3 1.3 1.3 V
Logic Low (Regulator ON) 0.6 0.7 0.7 V max
Voltage High (Regulator
OFF)
2.0 2.0 2.0 V min
Shutdown Pin Input
Current
Vshutdown = 2.4V 30 50 30 50 30 50 μA max
100 100 100 μA max
Vshutdown = 30V 450 600 450 600 450 600 μA max
750 750 750 μA max
Regulator Output
Current in Shutdown
(Note 11) 3 10 3 10 3 10 μA max
20 20 20 μA max
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: Unless otherwise specified all limits guaranteed for VIN = (VONOM + 1)V, IL = 100μA and CL = 1μF for 5V versions and 2.2μF for 3V and 3.3V versions.
Limits appearing in boldface type apply over the entire junction temperature range for operation. Limits appearing in normal type apply for TA = TJ = 25°C.
Additional conditions for the 8-pin versions are FEEDBACK tied to VTAP, OUTPUT tied to SENSE, and VSHUTDOWN 0.8V.
Note 3: Guaranteed and 100% production tested.
Note 4: Guaranteed but not 100% production tested. These limits are not used to calculate outgoing AQL levels.
Note 5: Dropout Voltage is defined as the input to output differential at which the output voltage drops 100 mV below its nominal value measured at 1V differential.
At very low values of programmed output voltage, the minimum input supply voltage of 2V (2.3V over temperature) must be taken into account.
Note 6: Comparator thresholds are expressed in terms of a voltage differential at the Feedback terminal below the nominal reference voltage measured at Vin =
(VONOM + 1)V. 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 5V, the Error output is guaranteed to go low when the output drops by 95mV × 5V/1.235V = 384 mV. Thresholds remain constant
as a percent of Vout as Vout is varied, with the dropout warning occurring at typically 5% below nominal, 7.5% guaranteed.
Note 7: Vref Vout (Vin − 1V), 2.3V Vin 30V, 100μA IL 100mA, TJ TJMAX.
Note 8: The junction-to-ambient thermal resistances are as follows: 180°C/W and 160°C/W for the TO-92 package with 0.40 inch and 0.25 inch leads to the
printed circuit board (PCB) respectively, 105°C/W for the molded plastic DIP (N), 130°C/W for the ceramic DIP (J), 160°C/W for the molded plastic SOP (M), 200°
C/W for the molded plastic MSOP (MM), and 160°C/W for the metal can package (H). The above thermal resistances for the N, J, M, and MM packages apply
when the package is soldered directly to the PCB. Junction-to-case thermal resistance for the H package is 20°C/W. Junction-to-case thermal resistance for the
TO-252 package is 5.4°C/W. The value of θJA for the LLP package is typically 51°C/W but is dependent on the PCB trace area, trace material, and the number
of layers and thermal vias. For details of thermal resistance and power dissipation for the LLP package, refer to Application Note AN-1187.
Note 9: May exceed input supply voltage.
Note 10: When used in dual-supply systems where the output terminal sees loads returned to a negative supply, the output voltage should be diode-clamped to
ground.
Note 11: Vshutdown 2V, Vin 30V, Vout = 0, Feedback pin tied to VTAP.
Note 12: Output or reference voltage temperature coefficient is defined as the worst case voltage change divided by the total temperature range.
Note 13: 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 50mA load pulse at VIN = 30V (1.25W pulse) for T = 10ms.
Note 14: 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.
Note 15: Line regulation for the LP2951 is tested at 150°C for IL = 1mA. For IL = 100μA and TJ = 125°C, line regulation is guaranteed by design to 0.2%. See
Typical Performance Characteristics for line regulation versus temperature and load current.
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LP2950/LP2951
Note 16: A Military RETS specification is available on request. At time of printing, the LP2951 RETS specification complied with the boldface limits in this column.
The LP2951H, WG, or J may also be procured as Standard Military Drawing Spec #5962-3870501MGA, MXA, or MPA.
Note 17: All LP2950 devices have the nominal output voltage coded as the last two digits of the part number. In the LP2951 products, the 3.0V and 3.3V versions
are designated by the last two digits, but the 5V version is denoted with no code at this location of the part number (refer to ordering information table).
Note 18: Human Body Model (HBM) is 1.5kΩ in series with 100pF; LP2950 passes 2.5 kV(HBM) ESD; LP2951 passes 2.5 kV(HBM) except: Feedback pin passes
1kV(HBM) and Shutdown pin passes 2kV(HBM).
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LP2950/LP2951
Typical Performance Characteristics
Quiescent Current
854627
Dropout Characteristics
854628
Input Current
854629
Input Current
854630
Output Voltage vs. Temperature of 3
Representative Units
854631
Quiescent Current
854632
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LP2950/LP2951
Quiescent Current
854633
Quiescent Current
854634
Quiescent Current
854635
Short Circuit Current
854636
Dropout Voltage
854637
Dropout Voltage
854638
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LP2950/LP2951
LP2951 Minimum Operating Voltage
854639
LP2951 Feedback Bias Current
854640
LP2951 Feedback Pin Current
854641
LP2951 Error Comparator Output
854642
LP2951 Comparator Sink Current
854643
Line Transient Response
854644
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LP2950/LP2951
Load Transient Response
854645
Load Transient Response
854646
LP2951 Enable Transient
854647
Output Impedance
854648
Ripple Rejection
854649
Ripple Rejection
854650
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LP2950/LP2951
Ripple Rejection
854651
LP2951 Output Noise
854652
LP2951 Divider Resistance
854653
Shutdown Threshold Voltage
854654
Line Regulation
854655
LP2951 Maximum Rated Output Current
854656
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LP2950/LP2951
LP2950 Maximum Rated Output Current
854657
Thermal Response
854658
Output Capacitor ESR Range
854663
LP2951 Input Pin Current vs Input Voltage
0 5 10 15 20 25 30
0
20
40
60
80
100
120
INPUT PIN CURRENT, IIN (μA)
INPUT PIN VOLTAGE, VIN (V)
VSD = 2.0V
Output Load = Open
Ta= -50°C
Ta= -40°C
Ta= +25°C
Ta= +125°C
854672
LP2951 Input Pin Current vs Input Voltage
0 5 10 15 20 25 30
0
20
40
60
80
100
120
INPUT PIN CURRENT, IIN (μA)
INPUT PIN VOLTAGE, VIN (V)
VSD = 2.0V
Output Load = Short to Ground
Ta= -50°C
Ta= -40°C
Ta= +25°C
Ta= +125°C
854673
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LP2950/LP2951
Application Hints
OUTPUT CAPACITOR REQUIREMENTS
A 1.0 μF (or greater) capacitor is required between the output
and ground for stability at output voltages of 5V or higher. At
lower output voltages, more capacitance is required (2. F
or more is recommended for 3.0V and 3.3V versions). Without
this capacitor the part will oscillate. Most types of tantalum or
aluminum electrolytic work fine here; even film types work but
are not recommended for reasons of cost. Many aluminum
electrolytics have electrolytes that freeze at about −30°C, so
solid tantalums are recommended for operation below −25°
C. The important parameters of the capacitor are an ESR of
about 5Ω or less and a resonant frequency above 500 kHz.
The value of this capacitor may be increased without limit.
854663
FIGURE 1. Output Capacitor ESR Range
The reason for the lower ESR limit is that the loop compen-
sation of the feedback loop relies on the capacitance value
and the ESR value of the output capacitor to provide the zero
that gives added phase lead (See Figure 1).
fZ = (1 / (2 x π x COUT x ESR) )
Using the 2.2 µF value from the Output Capacitor ESR
Range curve (Figure 1), a useful range for fZ can be estimat-
ed:
fZ(MIN)= (1 / (2 x π x 2.2 µF x 5) ) = 14.5 kHz
fZ(MAX)= (1 / (2 x π x 2.2 µF x 0.05) ) = 318 kHz
For ceramic capacitors, the low ESR produces a zero at a
frequency that is too high to be useful, so meaningful phase
lead does not occur. A ceramic output capacitor can be used
if a series resistance is added (recommended value of resis-
tance about 0.1 to 2) to simulate the needed ESR. Only
X5R, X7R, or better, MLCC types should be used, and should
have a DC voltage rating at least twice the VOUT(NOM) value.
At lower values of output current, less output capacitance is
required for stability. The capacitor can be reduced to
0.33 μF for currents below 10 mA or 0.1 μF for currents below
1 mA. Using the adjustable versions at voltages below 5V
runs the error amplifier at lower gains so that more output
capacitance is needed. For the worst-case situation of a
100mA load at 1.23V output (Output shorted to Feedback) a
3.3 μF (or greater) capacitor should be used.
Unlike many other regulators, the LP2950 will remain stable
and in regulation with no load in addition to the internal voltage
divider. This is especially important in CMOS RAM keep-alive
applications. When setting the output voltage of the LP2951
versions with external resistors, a minimum load of 1 μA is
recommended.
Applications having conditions that may drive the LP2950/51
into nonlinear operation require special consideration. Non-
linear operation will occur when the output voltage is held low
enough to force the output stage into output current limiting
while trying to pull the output voltage up to the regulated value.
The internal loop response time will control how long it takes
for the device to regain linear operation when the output has
returned to the normal operating range. There are three sig-
nificant nonlinear conditions that need to be considered, all
can force the output stage into output current limiting mode,
all can cause the output voltage to over-shoot with low value
output capacitors when the condition is removed, and the
recommended generic solution is to set the output capacitor
to a value not less than 10 μF. Although the 10 μF value for
COUT may not eliminate the output voltage over-shoot in all
cases, it should lower it to acceptable levels (<10% of
VOUT(NOM)) in the majority of cases. In all three of these con-
ditions, applications with lighter load currents are more sus-
ceptible to output voltage over-shoot than applications with
higher load currents.
1) At power-up, with the input voltage rising faster than output
stage can charge the output capacitor.
VIN tRISE(MIN) > ((COUT / 100 mA) x ΔVIN)
Where ΔVIN = VOUT(NOM) + 1.0V
2) Recovery from an output short circuit to ground condition.
COUT(MIN) (160 mA - ILOAD(NOM))/((VOUT(NOM)/10)/25 μs))
3) Toggling the LP2951 SHUTDOWN pin from high (i.e. OFF)
to low (i.e. ON).
COUT(MIN) (160 mA - ILOAD(NOM))/((VOUT(NOM)/10)/25 μs))
854647
FIGURE 2. LP2951 Enable Transient
INPUT CAPACITOR REQUIREMENTS
A minimum 1 μF tantalum, ceramic or aluminum electrolytic
capacitor should be placed from the LP2950/LP2951 input pin
to ground if there is more than 10 inches of wire between the
input and the AC filter capacitor or if a battery is used as the
input.
ERROR DETECTION COMPARATOR OUTPUT
The comparator produces a logic low output whenever the
LP2951 output falls out of regulation by more than approxi-
mately 5%. This figure is the comparator's built-in offset of
about 60mV divided by the 1.235 reference voltage. (Refer to
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LP2950/LP2951
the block diagram in the front of the datasheet.) This trip level
remains “5% below normal” regardless of the programmed
output voltage of the 2951. For example, the error flag trip
level is typically 4.75V for a 5V output or 11.4V for a 12V out-
put. The out of regulation condition may be due either to low
input voltage, current limiting, or thermal limiting.
Figure 3 below gives a timing diagram depicting the ER-
ROR signal and the regulated output voltage as the LP2951
input is ramped up and down. For 5V versions, the ERROR
signal becomes valid (low) at about 1.3V input. It goes high
at about 5V input (the input voltage at which VOUT = 4.75V).
Since the LP2951's dropout voltage is load-dependent (see
curve in typical performance characteristics), the input volt-
age trip point (about 5V) will vary with the load current. The
output voltage trip point (approx. 4.75V) does not vary with
load.
The error comparator has an open-collector output which re-
quires an external pull up resistor. This resistor may be re-
turned to the output or some other supply voltage depending
on system requirements. In determining a value for this re-
sistor, note that while the output is rated to sink 400μA, this
sink current adds to battery drain in a low battery condition.
Suggested values range from 100k to 1 MΩ. The resistor is
not required if this output is unused.
854620
*When VIN 1.3V, the error flag pin becomes a high impedance, and the
error flag voltage rises to its pull-up voltage. Using VOUT as the pull-up volt-
age (see Figure 4), rather than an external 5V source, will keep the error flag
voltage under 1.2V (typ.) in this condition. The user may wish to divide down
the error flag voltage using equal-value resistors (10k suggested), to en-
sure a low-level logic signal during any fault condition, while still allowing a
valid high logic level during normal operation.
FIGURE 3. ERROR Output Timing
PROGRAMMING THE OUTPUT VOLTAGE (LP2951)
The LP2951 may be pin-strapped for the nominal fixed output
voltage using its internal voltage divider by tying the output
and sense pins together, and also tying the feedback and
VTAP pins together. Alternatively, it may be programmed for
any output voltage between its 1.235V reference and its 30V
maximum rating. As seen in Figure 4, an external pair of re-
sistors is required.
The complete equation for the output voltage is
where VREF is the nominal 1.235V reference voltage and IFB
is the feedback pin bias current, nominally -20nA. The mini-
mum recommended load current of 1 μA forces an upper limit
of 1.2 MΩ on the value of R2, if the regulator must work with
no load (a condition often found in CMOS in standby). IFB will
produce a 2% typical error in VOUT which may be eliminated
at room temperature by trimming R1. For better accuracy,
choosing R2 = 100 k reduces this error to 0.17% while in-
creasing the resistor program current to 12 μA. Since the
LP2951 typically draws 60 μA at no load with Pin 2 open-cir-
cuited, this is a small price to pay.
854607
*See Application Hints
**Drive with TTL-high to shut down. Ground or leave open if shutdown fea-
ture is not to be used.
Note: Pins 2 and 6 are left open.
FIGURE 4. Adjustable Regulator
Stray capacitance to the LP2951 Feedback terminal can
cause instability. This may especially be a problem when us-
ing high value external resistors to set the output voltage.
Adding a 100 pF capacitor between the Output pin and the
Feedback pin,and increasing the output capacitor to at least
3.3 μF, will fix this problem.
REDUCING OUTPUT NOISE
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor. This is the only way noise can be reduced on the
3 lead LP2950 but is relatively inefficient, as increasing the
capacitor from 1 μF to 220 μF only decreases the noise from
430 μV(RMS) to 160 μV(RMS) for a 100 kHz bandwidth at 5V
output.
Noise can be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity.
Pick
or about 0.01 μF. When doing this, the output capacitor must
be increased to 3.3 μF to maintain stability. These changes
reduce the output noise from 430μV to 100μV rms for a
100 kHz bandwidth at 5V output. With the bypass capacitor
added, noise no longer scales with output voltage so that im-
provements are more dramatic at higher output voltages.
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LP2950/LP2951
LLP MOUNTING<