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LP2956/LP2956A Dual Micropower Low-Dropout Voltage Regulators
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FEATURES
1
•
•
•
•
2
•
•
•
•
•
•
•
Output Voltage Adjusts From 1.23V to 29V
Ensured 250 mA Current (Main Output)
Auxiliary LDO (75 mA) Adjustable Output
Auxiliary Comparator With Open-Collector
Output
Shutdown Pin for Main Output
Extremely Low Quiescent Current
Low Dropout Voltage
Extremely Tight Line and Load Regulation
Very Low Temperature Coefficient
Current and Thermal Limiting
Reverse Battery Protection
APPLICATIONS
•
•
•
High-Efficiency Linear Regulator
Low Dropout Battery-Powered Regulator
μP System Regulator With Switchable HighCurrent VCC
DESCRIPTION
The LP2956 is a micropower voltage regulator with
very low quiescent current (170 μA typical at light
loads) and very low dropout voltage (typically 60 mV
at 1 mA load current and 470 mV at 250 mA load
current on the main output).
The LP2956 retains all the desirable characteristics of
the LP2951, but offers increased output current (main
output), an auxiliary LDO adjustable regulated output
(75 mA), and additional features.
The auxiliary output is always on (regardless of main
output status), so it can be used to power memory
circuits.
Quiescent current increases only slightly at dropout,
which prolongs battery life.
The error flag goes low if the main output voltage
drops out of regulation.
An open-collector auxiliary comparator is included,
whose inverting input is tied to the 1.23V reference.
Reverse battery protection is provided.
The parts are available in DIP and surface mount
packages.
BLOCK DIAGRAM
Figure 1. LP2956
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 1999–2013, Texas Instruments Incorporated
OBSOLETE
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SNVS101E – MAY 1999 – REVISED APRIL 2013
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CONNECTION DIAGRAM
16-Pin PDIP and CDIP
Figure 2. See Package Number N16A
See Package Number NFE
16-Pin Surface Mount SOIC
Figure 3. See Package Number D
2
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
ABSOLUTE MAXIMUM RATINGS (1) (2)
Storage Temperature Range
−65°C to +150°C
Operating Junction Temperature Range
−40°C to +125°C
Lead Temperature (Soldering, 5 seconds)
Power Dissipation
260°C
(3)
Internally Limited
−20V to +30V
Input Supply Voltage
Feedback Input Voltage
(4)
−0.3V to +5V
Aux. Feedback Input Voltage
Shutdown Input Voltage
−0.3V to +30V
(4) (5)
Comparator Output Voltage
(1)
−0.3V to +5V
(4)
Comparator Input Voltage
ESD Rating
(4)
−0.3V to +30V
(4) (5)
−0.3V to +30V
(6)
2 kV
Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply
when operating the device outside of its rated operating conditions.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
The maximum allowable power dissipation is a function of the maximum junction temperature, T J(max), the junction-to-ambient thermal
resistance, θ J-A, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated
(2)
(3)
using: P(max) =
. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the
regulator will go into thermal shutdown. See Application Hints for additional information on heat sinking and thermal resistance.
When used in dual-supply systems where the regulator load is returned to a negative supply, the output voltage must be diode-clamped
to ground.
May exceed the input supply voltage.
All pins are rated for 2 kV, except for the auxiliary feedback pin which is rated for 1.2 kV (human body model, 100 pF discharged
through 1.5 kΩ).
(4)
(5)
(6)
ELECTRICAL CHARACTERISTICS
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Limits are specified by production testing or correlation techniques using standard Statistical Quality Control (SQC) methods.
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback pin is tied to 5V Tap
pin, CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main
regulator output has a 1 mA load, the auxiliary regulator output has a 100 μA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
LP2956I
Min
Max
Min
Max
4.975
5.025
4.950
5.050
4.940
5.060
4.900
5.100
4.930
5.070
4.880
5.120
Units
MAIN OUTPUT
VO
Output Voltage
5.0
1 mA ≤ IL ≤ 250 mA
ΔVO/ΔT
ΔVO/VO
ΔVO/VO
(1)
(2)
Temperature Coefficient
Line Regulation
Load Regulation
5.0
(1)
VIN = 6V to 30V
IL = 1 mA to 250 mA
IL = 0.1 mA to 1 mA
V
20
100
150
ppm/°C
0.03
0.1
0.2
%
0.2
0.4
0.16
0.20
0.20
0.30
0.04
(2)
%
Output or reference voltage temperature coefficient is defined as the worst case voltage change divided by the total temperature range.
Load regulation is measured at constant junction temperature using low duty cycle pulse testing. Two separate tests are performed, one
for the range of 100 μA to 1 mA and one for the 1 mA to 250 mA range. Changes in output voltage due to heating effects are covered
by the thermal regulation specification.
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ELECTRICAL CHARACTERISTICS (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Limits are specified by production testing or correlation techniques using standard Statistical Quality Control (SQC) methods.
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback pin is tied to 5V Tap
pin, CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main
regulator output has a 1 mA load, the auxiliary regulator output has a 100 μA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
Min
VIN–VO
Dropout Voltage
(3)
IL = 1 mA
60
IL = 50 mA
240
IL = 100 mA
310
IL = 250 mA
ILIMIT
Current Limit
ΔVO/ΔPD
Thermal Regulation
en
Output Noise Voltage
(10 Hz to 100 KHz)
IL = 100 mA
VFB
Feedback Pin Voltage
IFB
Feedback Pin Bias Current
IO (OFF)
470
RL = 1Ω
380
(4)
0.05
CL = 2.2 μF
Min
Units
Max
100
100
150
150
300
300
420
420
400
400
520
520
600
600
800
800
500
500
530
530
0.2
0.2
mV
mA
%/W
μV RMS
400
CL = 33 μF
CL = 33 μF
Max
LP2956I
260
(5)
80
1.23
1.215
20
Output Leakage In Shutdown I(SD IN) ≥ 1 μA
VIN = 30V, VOUT = 0V
3
1.245
1.205
1.255
40
40
60
60
10
10
20
20
V
nA
μA
AUXILIARY OUTPUT
VFB
Feedback Pin Voltage
1.23
ΔVFB/ΔT
Feedback Voltage
Temperature Coefficient
20
IFB
Feedback Pin Bias Current
10
ΔVO/VO
ΔVO/VO
(3)
(4)
(5)
(6)
4
Line Regulation
Load Regulation
6V ≤ VIN ≤ 30V
0.07
IL = 0.1 mA to 1 mA
IL = 1 mA to 75 mA
0.1
(6)
1.22
1.25
1.21
1.26
1.21
1.26
1.20
1.27
V
ppm/°C
20
20
30
30
0.3
0.4
0.5
0.6
0.3
0.4
0.6
1.0
nA
%
%
Dropout voltage is defined as the input to output differential at which the output voltage drops 100 mV below the value measured with a
1V differential. At very low values of programmed output voltage, the input voltage minimum of 2V (2.3V over temperature) must be
observed.
Thermal regulation is the change in output voltage at a time T after a change in power dissipation, excluding load or line regulation
effects. Specifications are for a 200 mA load pulse at VIN = 20V (3W pulse) for T = 10 ms on the Main regulator output. For the Auxiliary
regulator output, specifications are for a 66 mA load pulse at VIN = 20V (1W pulse) for T = 10 ms.
Connect a 0.1 μF capacitor from the output to the feedback pin.
Load regulation is measured at constant junction temperature using low duty cycle pulse testing. Two separate tests are performed, one
for the range of 100 μA to 1 mA and one for the 1 mA to 75 mA range. Changes in output voltage due to heating effects are covered by
the thermal regulation specification.
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ELECTRICAL CHARACTERISTICS (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Limits are specified by production testing or correlation techniques using standard Statistical Quality Control (SQC) methods.
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback pin is tied to 5V Tap
pin, CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main
regulator output has a 1 mA load, the auxiliary regulator output has a 100 μA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
Min
VIN–VO
Dropout Voltage
IL = 1 mA
IL = 50 mA
400
IL = 75 mA
en
ILIM
ΔVO/ΔPD
Output Noise
(10 Hz–100 KHz)
CL = 10 μF
Current Limit
VOUT = 0V
Thermal Regulation
CL = 33 μF
IL = 10 mA
Max
100
500
LP2956I
Min
Max
200
200
300
300
600
600
700
700
700
700
850
850
(8)
(9)
mV
mV
mV
μV RMS
300
(7)
Units
100
80
200
200
250
250
0.2
0.5
0.5
%/W
0.01
1
1
μA
2
2
250
250
400
400
mA
DROPOUT DETECTION COMPARATOR
IOH
VOL
Output “HIGH” Leakage
Output “LOW” Voltage
VOH = 30V
VIN = 4V
150
IO (COMP) = 400 μA
VTHR (max)
VTHR (min)
HYST
Upper Threshold Voltage
Lower Threshold Voltage
Hysteresis
(10)
−240
(10)
−350
(10)
110
(11)
0.03
−320
−150
−320
−150
−380
−100
−380
−100
−450
−230
−450
−230
−640
−160
−640
−160
mV
mV
mV
mV
SHUTDOWN INPUT
IIN
Input Current to Disable
Output
VIH
Shutdown Input High
Threshold
I(SD IN) ≥ 1 μA
Shutdown Input Low
Threshold
VO ≥ 4.5V
VIL
0.5
0.5
900
900
1200
1200
μA
mV
400
400
200
200
mV
AUXILIARY COMPARATOR
VT(high)
VT(low)
Upper Trip Point
Lower Trip Point
(12)
(12)
1.236
1.230
1.20
1.28
1.20
1.28
1.19
1.29
1.19
1.29
1.19
1.27
1.19
1.27
1.18
1.28
1.18
1.28
V
V
Connect a 0.1 μF capacitor from the output to the feedback pin.
The auxiliary regulator output has foldback limiting, which means the output current reduces with output voltage. The tested limit is for
VOUT = 0V, so the output current will be higher at higher output voltages.
(9) Thermal regulation is the change in output voltage at a time T after a change in power dissipation, excluding load or line regulation
effects. Specifications are for a 200 mA load pulse at VIN = 20V (3W pulse) for T = 10 ms on the Main regulator output. For the Auxiliary
regulator output, specifications are for a 66 mA load pulse at VIN = 20V (1W pulse) for T = 10 ms.
(10) Dropout dectection comparator thresholds are expressed as changes in a 5V output. To express the threshold voltages in terms of a
differential at the Feedback terminal, divide by the error amplifier gain = VOUT/V REF.
(11) The shutdown input equivalent circuit is the base of a grounded-emitter NPN transistor in series with a current-limiting resistor. Pulling
the shutdown input high turns off the main regulator. For more details, see Application Hints.
(12) This test is performed with the auxiliary comparator output sinking 400 μA of current. At the upper trip point, the comparator output must
be ≥2.4V. At the low trip point, the comparator output must be ≤ 0.4V.
(7)
(8)
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ELECTRICAL CHARACTERISTICS (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Limits are specified by production testing or correlation techniques using standard Statistical Quality Control (SQC) methods.
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback pin is tied to 5V Tap
pin, CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main
regulator output has a 1 mA load, the auxiliary regulator output has a 100 μA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
Min
HYST
Hysteresis
IOH
Output “HIGH” Leakage
VOL
IB
Max
LP2956I
Min
Max
6
Output “LOW” Voltage
Input Bias Current
VOH = 30V
VIN (COMP) = 1.3V
0.01
VIN (COMP) = 1.1V
IO(COMP) = 400 μA
150
0 ≤ VIN (COMP) ≤ 5V
10
Units
mV
1
1
2
2
250
250
400
400
−30
30
−30
30
−50
50
−50
50
μA
mV
nA
GROUND PIN CURRENT
IGND
Ground Pin Current
(13)
IL (Main Out) = 1 mA
170
IL (Aux. Out) = 0.1 mA
IL (Main Out) = 50 mA
1.1
2
3
6
6
8
8
16
28
28
33
33
6
6
8
8
8
8
3
IL (Aux. Out) = 50 mA
IL (Main Out) = 1 mA
IGND
IGND
280
2.5
IL (Aux. Out) = 1 mA
IL (Main Out) = 1 mA
280
2
IL (Aux. Out) = 1 mA
IL (Main Out) = 250 mA
250
2.5
IL (Aux. Out) = 1 mA
IL (Main Out) = 100 mA
250
6
IL (Aux. Out) = 75 mA
10
10
Ground Pin Current at
Dropout (13)
VIN = 4.5V
325
325
IL (Main Out) = 0.1 mA
IL (Aux. Out) = 0.1 mA
270
350
350
Ground Pin Current at
Shutdown (13)
No Load on Either Output
I(SD IN) ≥ 1 μA
120
180
180
200
200
μA
mA
μA
(13) Ground pin current is the regulator quiescent current. The total current drawn from the source is the sum of the ground pin current,
output load current, and current through the external resistive dividers (if used).
6
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Typical Performance Characteristics
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
Ground Pin Current
Ground Pin Current
Figure 4.
Figure 5.
Ground Pin Current
Ground Pin Current
Figure 6.
Figure 7.
Ground Pin Current
Ground Pin Current
Figure 8.
Figure 9.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
8
Ground Pin Current
vs Main Load
Dropout Characteristics
(Main Regulator)
Figure 10.
Figure 11.
Dropout Voltage vs Temperature (Main Regulator)
Current Limit vs Regulator
(Main Regulator)
Figure 12.
Figure 13.
Enable Transient
(Main Regulator)
Enable Transient
(Main Regulator)
Figure 14.
Figure 15.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
Load Transient Response
(Main Regulator)
Load Transient Response
(Main Regulator)
Figure 16.
Figure 17.
Line Transient Response
(Main Regulator)
Line Transient Response
(Main Regulator)
Figure 18.
Figure 19.
Ripple Rejection
(Main Regulator)
Ripple Rejection
(Main Regulator)
Figure 20.
Figure 21.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
10
Ripple Rejection
(Main Regulator)
Thermal Regulation
(Main Regulator)
Figure 22.
Figure 23.
Output Impedance
(Main Regulator)
Output Noise Voltage
(Main Regulator)
Figure 24.
Figure 25.
Feedback Bias Current
Divider Resistance
Figure 26.
Figure 27.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
Dropout Characteristics
(Auxiliary Regulator)
Dropout vs Temperature
(Auxiliary Regulator)
Figure 28.
Figure 29.
Current Limit vs Temperature
(Auxiliary Regulator)
Line Transient Response
(Auxiliary Regulator)
Figure 30.
Figure 31.
Load Transient Response
(Auxiliary Regulator)
Load Transient Response
(Auxiliary Regulator)
Figure 32.
Figure 33.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 6V, CL = 2.2 μF (Main Output) and 10 μF (Auxiliary Output), Feedback is tied to 5V Tap pin,
CIN = 1 μF, VSD = 0V, Main Output pin is tied to Output Sense pin, Auxiliary Output is programmed for 5V. The main regulator
output has a 1 mA load, the auxiliary output has a 100 μA load.
12
Ripple Rejection
(Auxiliary Regulator)
Output Impedance
(Auxiliary Regulator)
Figure 34.
Figure 35.
Output Noise Voltage
(Auxiliary Regulator)
Auxiliary Comparator
Sink Current
Figure 36.
Figure 37.
Error Output Voltage
Dropout Detection Comparator
Threshold Voltages
Figure 38.
Figure 39.
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APPLICATION HINTS
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2956 depending on the maximum power dissipation and maximum
ambient temperature of the application. Under all expected operating conditions, the junction temperature must
be within the range specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the maximum power dissipated by the regulator, P(max), must be
calculated. It is important to remember that if the regulator is powered from a transformer connected to the AC
line, the maximum specified AC input voltage must be used (since this produces the maximum DC input
voltage to the regulator). Figure 40 shows the voltages and currents which are present in the circuit. The formula
for calculating the power dissipated in the regulator is also shown in Figure 40 (the currents and power due to
external resistive dividers are not included, and are typically negligible).
Figure 40. Current/Voltage Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, TR(max). This is
calculated by using the formula:
TR(max) = TJ(max) − T A(max)
(1)
where: TJ(max) is the maximum allowable junction temperature
TA(max) is the maximum ambient temperature
Using the calculated values for TR(max) and P(max), the required value for junction-to-ambient thermal
resistance, θ (J-A), can now be found:
θ(J-A) = TR(max)/P(max)
(2)
The heatsink for the LP2956 is made using the PC board copper. The heat is conducted from the die, through
the lead frame (inside the part), and out the pins which are soldered to the PC board. The pins used for heat
conduction are shown in Table 1.
Table 1.
Part
Package
Pins
LP2956IN
16-Pin Plastic DIP
4, 5, 12, 13
LP2956AIN
16-Pin Plastic DIP
4, 5, 12, 13
LP2956IM
16-Pin Surface Mt.
1, 8, 9, 16
LP2956AIM
16-Pin Surface Mt.
1, 8, 9, 16
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Figure 41 shows copper patterns which may be used to dissipate heat from the LP2956:
*For best results, use L = 2H
Figure 41. Copper Heatsink Patterns
Table 2 shows some typical values of junction-to-ambient thermal resistance (θ
copper).
J-A)
for values of L and W (1 oz.
Table 2.
Package
16-Pin Plastic
DIP
16-Pin Surface
Mount
L (In.)
H (In.)
θJ-A (°C/W)
1
0.5
70
2
1
60
3
1.5
58
4
0.19
66
6
0.19
66
1
0.5
83
2
1
70
3
1.5
67
6
0.19
69
4
0.19
71
2
0.19
73
EXTERNAL CAPACITORS
A 2.2 μF (or greater) capacitor is required between the main output pin and ground to assure stability. The
auxiliary output requires 10 μF to ground. Without these capacitors, the part may oscillate. Most types of
tantalum or aluminum electrolytics will work here. Film types will work, but are more expensive. Many aluminum
electrolytics contain electrolytes which freeze at −30°C, which requires the use of solid tantalums below −25°C.
The important characteristic of the capacitors is an ESR of 5Ω (or less) on the main regulator output and an ESR
of 1Ω (or less) on the auxiliary regulator output (the ESR may increase by a factor of 20 or 30 as the temperature
is reduced from +25°C to −30°C). The value of these capacitors may be increased without limit.
14
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The main output requires less capacitance at lighter load currents. This capacitor can be reduced to 0.68 μF for
currents below 10 mA or 0.22 μF for currents below 1 mA.
Programming the main output for voltages below 5V requires more output capacitance for stability. For the worstcase condition of 1.23V output and 250 mA of load current, a 6.8 μF (or larger) capacitor should be used.
A 1 μF capacitor should be placed from the 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 input is used.
Stray capacitance to the Feedback terminal can cause instability. This problem is most likely to appear when
using high value external resistors to set the output voltage. Adding a 100 pF capacitor between the Output and
Feedback pins and increasing the output capacitance to 6.8 μF (or greater) will cure the problem.
MINIMUM LOAD ON MAIN OUTPUT
When setting the main output voltage using an external resistive divider, a minimum current of 10 μA is
recommended through the resistors to provide a minimum load.
It should be noted that a minimum load current is specified in several of the electrical characteristic test
conditions, so the specified value must be used to obtain test limit correlation.
PROGRAMMING THE MAIN OUTPUT VOLTAGE
The main output may be pin-strapped for 5V operation using its internal resistive divider by tying the Output and
Sense pins together and also tying the Feedback and 5V Tap pins together.
Alternatively, it may be programmed for any voltage between the 1.23V reference and the 29V maximum rating
using an external pair of resistors (see Figure 42 ). The complete equation for the output voltage is:
(3)
where VREF is the 1.23V reference and IFB is the Feedback pin bias current (−20 nA typical). The minimum
recommended load current of 1 μA sets an upper limit of 1.2 MΩ on the value of R2 in cases where the regulator
must work with no load (see MINIMUM LOAD ON MAIN OUTPUT).
If IFB is ignored in the calculation of the output voltage, it will produce a small error in VMAIN OUT. Choosing R2 =
100 kΩ will reduce this error to 0.16% (typical) while increasing the resistor program current to 12 μA. Since the
typical quiescent current is 130 μA, this added current is negligible.
*See Application Hints
**Drive with high to shut down
Figure 42. Adjustable Regulator
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DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the minimum input-to-output voltage differential required for the
output voltage to stay within 100 mV of the output voltage measured with a 1V differential. The dropout voltage is
independent of the programmed output voltage.
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the main output falls out of regulation by more than about
5%. This figure results from the comparator's built-in offset of 60 mV divided by the 1.23V reference (refer to
block diagram). The 5% low trip level remains constant regardless of the programmed output voltage. An out-ofregulation condition can result from low input voltage, current limiting, or thermal limiting.
Figure 43 gives a timing diagram showing the relationship between the main output voltage, the ERROR output,
and input voltage as the input voltage is ramped up and down to a regulator whose main output is programmed
for 5V. The ERROR signal becomes low at about 1.3V input. It goes high at about 5V input, where the main
output equals 4.75V. Since the dropout voltage is load dependent, the input voltage trip points will vary with load
current. The main output voltage trip point does not vary.
The comparator has an open-collector output which requires an external pull-up resistor. This resistor may be
connected to the regulator main output or some other supply voltage. Using the main output prevents an invalid
“HIGH” on the comparator output which occurs if it is pulled up to an external voltage while the regulator input
voltage is reduced below 1.3V. In selecting a value for the pull-up resistor, note that while the output can sink
400 μA, this current adds to battery drain. Suggested values range from 100 kΩ to 1 MΩ. The resistor is not
required if the output is unused.
*In shutdown mode, ERROR will go high if it has been pulled up to an external supply. To avoid this invalid response,
pull up to regulator output
**Exact value depends on dropout voltage. (See Application Hints)
Figure 43. ERROR Output Timing
If a single pull-up resistor is used to the regulator output, the error flag may briefly rise up to about 1.3V as the
input voltage ramps up or down through the 0V to 1.3V region.
In some cases, this 1.3V signal may be mis-interpreted as a false high by a μP which is still “alive” with 1.3V
applied to it.
To prevent this, the user may elect to use two resistors which are equal in value on the error output (one
connected to ground and the other connected to the regulator output).
If this two-resistor divider is used, the error output will only be pulled up to about 0.6V (not 1.3V) during power-up
or power-down, so it can not be interpreted as a high signal. When the regulator output is at 5V, the error output
will be 2.5V, which is still clearly a high signal.
OUTPUT ISOLATION
The regulator outputs can be left connected to an active voltage source (such as a battery) with the regulator
input power shut off, as long as the regulator ground pin is connected to ground. If the ground pin is left floating,
damage to the regulator can occur if the output is pulled up by an external voltage source.
16
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REDUCING MAIN OUTPUT NOISE
In reference applications it may be advantageous to reduce the AC noise present on the main output. One
method is to reduce regulator bandwidth by increasing output capacitance. This is relatively inefficient, since
large increases in capacitance are required to get significant improvement.
Noise can be reduced more effectively by a bypass capacitor placed across R1 (refer to Figure 42 ). The formula
for selecting the capacitor to be used is:
(4)
This gives a value of about 0.1μF. When this is used, the output capacitor must be 6.8 μF (or greater) to
maintain stability. The 0.1 μF capacitor reduces the high frequency noise gain of the circuit to unity, lowering the
output noise from 260 μV to 80 μV using a 10 Hz to 100 kHz bandwidth. Also, noise is no longer proportional to
the output voltage, so improvements are more pronounced at higher output voltages.
where: VREF = 1.23V and IFB = −10 nA (typical)
Figure 44. Auxiliary Adjustable Regulator
AUXILIARY LDO OUTPUT
The LP2956 has an auxiliary LDO regulator output (which can source up to 75 mA) that is adjustable for voltages
from 1.23V to 29V.
The output voltage is set by an external resistive divider, as shown in Figure 44. The maximum output current is
75 mA, and the output requires 10 μF from the output to ground for stability, regardless of load current.
SHUTDOWN INPUT
The shutdown input equivalent circuit is shown in Figure 45. The main regulator output is shut down when the
NPN transitor is turned ON.
Figure 45. Shutdown Circuitry
The current into the input should be at least 0.5 μA to assure the output shutdown function. A resistor may be
placed in series with the input to minimize current draw in shutdown mode, provided this minimum input current
requirement is met.
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IMPORTANT:
The shutdown input must not be left floating: a pull-down resistor (10 kΩ to 50 kΩ recommended) must be
connected between the shutdown input and ground in cases where the input is not actively pulled low.
SCHEMATIC DIAGRAM
18
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TYPICAL APPLICATIONS
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REVISION HISTORY
Changes from Revision D (April 2013) to Revision E
•
20
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