LP2956IMX/NOPB [NSC]
IC VREG DUAL OUTPUT, FIXED/ADJUSTABLE POSITIVE LDO REGULATOR, PDSO16, SMT-16, Fixed/Adjustable Positive Multiple Output LDO Regulator;
| 型号： | LP2956IMX/NOPB |
| 厂家： | 
National Semiconductor |
| 描述： | IC VREG DUAL OUTPUT, FIXED/ADJUSTABLE POSITIVE LDO REGULATOR, PDSO16, SMT-16, Fixed/Adjustable Positive Multiple Output LDO Regulator 光电二极管 输出元件 调节器 |
| 文件： | 总19页 (文件大小：977K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
March 2005
LP2956/LP2956A
Dual Micropower Low-Dropout Voltage Regulators
General Description
Features
n Output voltage adjusts from 1.23V to 29V
n Guaranteed 250 mA current (main output)
n Auxiliary LDO (75 mA) adjustable output
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).
n Auxiliary comparator with open-collector output
n Shutdown pin for main output
n Extremely low quiescent current
n Low dropout voltage
n Extremely tight line and load regulation
n Very low temperature coefficient
n Current and thermal limiting
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.
n Reverse battery protection
The error flag goes low if the main output voltage drops out
of regulation.
Applications
n High-efficiency linear regulator
An open-collector auxiliary comparator is included, whose
inverting input is tied to the 1.23V reference.
n Low dropout battery-powered regulator
n µP system regulator with switchable high-current VCC
Reverse battery protection is provided.
The parts are available in DIP and surface mount packages.
Block Diagram
LP2956
01133901
© 2005 National Semiconductor Corporation
DS011339
www.national.com
Connection Diagrams
16–Pin DIP
01133902
Order Number LP2956IN or LP2956AIN
See NS Package Number N16A
Order Number LP2956AMJ-QML or 5962-9554701QEA
See NS Package Number J16A
16-Pin Surface Mount
01133903
Order Number LP2956IM or LP2956AIM
See NS Package Number M16A
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2
Absolute Maximum Ratings (Note 1)
Input Supply Voltage
Feedback Input Voltage (Note 3)
Aux. Feedback Input Voltage (Note
3)
−20V to +30V
−0.3V to +5V
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
−0.3V to +5V
−0.3V to +30V
Storage Temperature Range
Operating Junction
−65˚C to +150˚C
Shutdown Input Voltage (Note 3)
Comparator Input Voltage (Notes 3,
4)
Temperature Range
−40˚C to +125˚C
−0.3V to +30V
Lead Temperature
Comparator Output Voltage (Notes
3, 4)
−0.3V to +30V
2 kV
(Soldering, 5 seconds)
Power Dissipation (Note 2)
260˚C
ESD Rating (Note 16)
Internally Limited
Electrical Characteristics
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Lim-
its are guaranteed 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 regu-
lator output has a 1 mA load, the auxiliary regulator output has a 100 µA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
Min Max
LP2956I
Min Max
Units
MAIN OUTPUT
VO
Output Voltage
5.0
4.975 5.025 4.950 5.050
4.940 5.060 4.900 5.100
4.930 5.070 4.880 5.120
V
1 mA ≤ IL ≤ 250 mA
5.0
Temperature Coefficient
Line Regulation
(Note 5)
20
100
150
ppm/˚C
VIN = 6V to 30V
0.03
0.04
0.1
0.2
0.2
0.4
%
%
Load Regulation
IL = 1 mA to 250 mA
IL = 0.1 mA to 1 mA (Note 6)
IL = 1 mA
0.16
0.20
0.20
0.30
V
IN–VO
Dropout Voltage
(Note 7)
60
100
150
300
420
400
520
600
800
500
530
0.2
100
150
300
420
400
520
600
800
500
530
0.2
IL = 50 mA
IL = 100 mA
IL = 250 mA
RL = 1Ω
240
310
470
380
0.05
mV
ILIMIT
Current Limit
mA
Thermal Regulation
(Note 8)
%/W
en
Output Noise Voltage
(10 Hz to 100 KHz)
IL = 100 mA
CL = 2.2 µF
400
260
80
CL = 33 µF
µV RMS
CL = 33 µF (Note 9)
VFB
IFB
Feedback Pin Voltage
Feedback Pin Bias
Current
1.23
20
1.215 1.245 1.205 1.255
V
40
60
10
20
40
60
10
20
nA
IO
Output Leakage
In Shutdown
I(SD IN) ≥ 1 µA
3
µA
(OFF)
VIN = 30V, VOUT = 0V
AUXILIARY OUTPUT
VFB Feedback Pin Voltage
1.23
1.22
1.25
1.21
1.26
V
3
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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. Lim-
its are guaranteed 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 regu-
lator output has a 1 mA load, the auxiliary regulator output has a 100 µA load.
Symbol
AUXILIARY OUTPUT
Feedback Voltage
Parameter
Conditions
Typical
LP2956AI
LP2956I
Units
Min
Max
Min
Max
1.21
1.26
1.20
1.27
20
ppm/˚C
Temperature Coefficient
IFB
Feedback Pin Bias
Current
10
20
30
20
30
nA
%
Line Regulation
6V ≤ VIN ≤ 30V
0.07
0.3
0.4
0.5
0.3
0.6
0.6
0.4
1.0
Load Regulation
Dropout Voltage
IL = 0.1 mA to 1 mA
IL = 1 mA to 75 mA (Note 10)
IL = 1 mA
0.1
%
V
IN–VO
100
400
500
200
300
600
700
700
850
200
300
600
700
700
850
mV
mV
mV
IL = 50 mA
IL = 75 mA
en
Output Noise
CL = 10 µF
300
100
(10 Hz–100 KHz)
CL = 33 µF (Note 9)
IL = 10 mA
µV RMS
mA
ILIM
Current Limit
VOUT = 0V (Note 13)
80
200
250
0.5
200
250
0.5
Thermal Regulation
(Note 8)
0.2
%/W
DROPOUT DETECTION COMPARATOR
IOH
Output “HIGH” Leakage
VOH = 30V
0.01
150
1
1
µA
mV
mV
mV
mV
µA
2
2
VOL
Output “LOW” Voltage
VIN = 4V
250
400
−150
−100
−230
−160
250
400
−150
−100
−230
−160
IO (COMP) = 400 µA
VTHR
(max)
VTHR
(min)
HYST
Upper Threshold Voltage (Note 11)
−240
−350
110
−320
−380
−450
−640
−320
−380
−450
−640
Lower Threshold Voltage (Note 11)
Hysteresis
(Note 11)
SHUTDOWN INPUT
IIN
Input Current to Disable
(Note 12)
0.03
0.5
0.5
Output
VIH
VIL
Shutdown Input High
Threshold
I(SD IN) ≥ 1 µA
VO ≥ 4.5V
900
900
mV
mV
1200
1200
Shutdown Input Low
Threshold
400
400
200
200
AUXILIARY COMPARATOR
VT(high)
Upper Trip Point
(Note 14)
1.236
1.20
1.28
1.20
1.28
V
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4
Electrical Characteristics (Continued)
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Lim-
its are guaranteed 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 regu-
lator output has a 1 mA load, the auxiliary regulator output has a 100 µA load.
Symbol
Parameter
Conditions
Typical
LP2956AI
LP2956I
Units
Min
Max
Min
Max
AUXILIARY COMPARATOR
1.19
1.19
1.18
1.29
1.27
1.28
1.19
1.19
1.18
1.29
1.27
1.28
VT(low)
Lower Trip Point
(Note 14)
1.230
V
HYST
IOH
Hysteresis
6
mV
µA
Output “HIGH” Leakage
VOH = 30V
0.01
1
2
1
2
VIN (COMP) = 1.3V
VIN (COMP) = 1.1V
IO(COMP) = 400 µA
0 ≤ VIN (COMP) ≤ 5V
VOL
IB
Output “LOW” Voltage
Input Bias Current
150
10
250
400
30
50
250
400
30
50
mV
nA
−30
−30
−50
−50
GROUND PIN CURRENT
IGND
Ground Pin Current
IL (Main Out) = 1 mA
IL (Aux. Out) = 0.1 mA
IL (Main Out) = 50 mA
IL (Aux. Out) = 1 mA
IL (Main Out) = 100 mA
IL (Aux. Out) = 1 mA
IL (Main Out) = 250 mA
IL (Aux. Out) = 1 mA
IL (Main Out) = 1 mA
IL (Aux. Out) = 50 mA
IL (Main Out) = 1 mA
IL (Aux. Out) = 75 mA
VIN = 4.5V
170
1.1
3
250
280
2
250
280
2
µA
(Note 15)
2.5
6
2.5
6
8
8
16
3
28
33
6
28
33
6
mA
8
8
6
8
8
10
325
350
10
325
350
IGND
Ground Pin Current
at Dropout (Note 15)
IL (Main Out) = 0.1 mA
IL (Aux. Out) = 0.1 mA
No Load on Either Output
270
120
µA
IGND
Ground Pin Current
180
180
at Shutdown (Note 15)
I
(SD IN) ≥ 1 µA
200
200
Note 1: 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.
Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, T (max), the junction-to-ambient thermal resistance, θ
,
J-A
J
and the ambient temperature, T . The maximum allowable power dissipation at any ambient temperature is calculated using: P(max) =
A
.
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.
Note 3: 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.
Note 4: May exceed the input supply voltage.
Note 5: Output or reference voltage temperature coefficient is defined as the worst case voltage change divided by the total temperature range.
Note 6: 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.
Note 7: 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.
5
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Electrical Characteristics (Continued)
Note 8: 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 V = 20V (3W pulse) for T = 10 ms on the Main regulator output. For the Auxiliary regulator output, specifications are for a 66 mA
IN
load pulse at V = 20V (1W pulse) for T = 10 ms.
IN
Note 9: Connect a 0.1 µF capacitor from the output to the feedback pin.
Note 10: 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.
Note 11: 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 = V
/V
.
OUT
REF
Note 12: 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.
Note 13: The auxiliary regulator output has foldback limiting, which means the output current reduces with output voltage. The tested limit is for V
= 0V, so the
OUT
output current will be higher at higher output voltages.
Note 14: 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.
Note 15: 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).
Note 16: 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).
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 auxil-
iary output has a 100 µA load.
Ground Pin Current
Ground Pin Current
01133918
01133919
Ground Pin Current
Ground Pin Current
01133920
01133921
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6
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. (Continued)
Ground Pin Current
Ground Pin Current
01133922
01133923
Ground Pin Current
vs Main Load
Dropout Characteristics
(Main Regulator)
01133925
01133924
Dropout Voltage vs
Temperature (Main Regulator)
Current Limit vs Regulator
(Main Regulator)
01133926
01133927
7
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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. (Continued)
Enable Transient
(Main Regulator)
Enable Transient
(Main Regulator)
01133929
01133928
Load Transient Response
(Main Regulator)
Load Transient Response
(Main Regulator)
01133931
01133930
Line Transient Response
(Main Regulator)
Line Transient Response
(Main Regulator)
01133932
01133933
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8
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. (Continued)
Ripple Rejection
(Main Regulator)
Ripple Rejection
(Main Regulator)
01133934
01133935
Ripple Rejection
(Main Regulator)
Thermal Regulation
(Main Regulator)
01133937
01133936
Output Impedance
(Main Regulator)
Output Noise Voltage
(Main Regulator)
01133938
01133939
9
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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. (Continued)
Feedback Bias Current
Divider Resistance
01133940
01133941
01133943
01133945
Dropout Characteristics
(Auxiliary Regulator)
Dropout vs Temperature
(Auxiliary Regulator)
01133942
Current Limit vs Temperature
(Auxiliary Regulator)
Line Transient Response
(Auxiliary Regulator)
01133944
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10
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. (Continued)
Load Transient Response
(Auxiliary Regulator)
Load Transient Response
(Auxiliary Regulator)
01133946
01133947
Ripple Rejection
Output Impedance
(Auxiliary Regulator)
(Auxiliary Regulator)
01133948
01133949
Output Noise Voltage
(Auxiliary Regulator)
Auxiliary Comparator
Sink Current
01133951
01133950
11
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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. (Continued)
Dropout Detection Comparator
Error Output Voltage
Threshold Voltages
01133952
01133953
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12
Application Hints
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2956 depending on
the maximum power dissipation and maximum ambient tem-
perature 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 pro-
duces the maximum DC input voltage to the regulator).
Figure 1 shows the voltages and currents which are present
in the circuit. The formula for calculating the power dissi-
pated in the regulator is also shown in Figure 1 (the currents
and power due to external resistive dividers are not included,
and are typically negligible).
01133910
*For best results, use L = 2H
FIGURE 2. Copper Heatsink Patterns
01133909
Table 2 shows some typical values of junction-to-ambient
thermal resistance (θ J-A) for values of L and W (1 oz.
copper).
FIGURE 1. Current/Voltage Diagram
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, TR(max). This is calculated
by using the formula:
TABLE 2.
Package
16-Pin
Plastic
DIP
L (In.)
H (In.)
θJ-A (˚C/W)
TR(max) = TJ(max) − T A(max)
1
0.5
70
where: TJ(max) is the maximum allowable junction
temperature
2
3
4
6
1
2
3
6
4
2
1
60
58
66
66
83
70
67
69
71
73
TA(max) is the maximum ambient temperature
1.5
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:
0.19
0.19
0.5
θ(J-A) = TR(max)/P(max)
16-Pin
Surface
Mount
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.
1
1.5
0.19
0.19
0.19
TABLE 1.
Part
Package
Pins
EXTERNAL CAPACITORS
LP2956IN
LP2956AIN
LP2956IM
LP2956AIM
16-Pin Plastic DIP
16-Pin Plastic DIP
16-Pin Surface Mt.
16-Pin Surface Mt.
4, 5, 12, 13
4, 5, 12, 13
1, 8, 9, 16
1, 8, 9, 16
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 electro-
lytics 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 tan-
talums below −25˚C. The important characteristic of the
capacitors is an ESR of 5Ω (or less) on the main regulator
Figure 2 shows copper patterns which may be used to
dissipate heat from the LP2956:
13
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Application Hints (Continued)
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.
The main output requires less capacitance at lighter load
currents. This capacitor can be reduced to 0.68 µF for cur-
rents 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 worst-case
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 insta-
bility. 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.
01133911
*See Application Hints
**Drive with high to shut down
MINIMUM LOAD ON MAIN OUTPUT
FIGURE 3. Adjustable Regulator
DROPOUT VOLTAGE
When setting the main output voltage using an external
resistive divider, a minimum current of 10 µA is recom-
mended through the resistors to provide a minimum load.
The dropout voltage of the regulator is defined as the mini-
mum input-to-output voltage differential required for the out-
put voltage to stay within 100 mV of the output voltage
measured with a 1V differential. The dropout voltage is in-
dependent of the programmed output voltage.
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.
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 pro-
grammed output voltage. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal
limiting.
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 3 ). The complete equa-
tion for the output voltage is:
Figure 4 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.
where VREF is the 1.23V reference and IFB is the Feedback
pin bias current (−20 nA typical). The minimum recom-
mended 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).
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 quies-
cent current is 130 µA, this added current is negligible.
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.
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14
Also, noise is no longer proportional to the output voltage, so
improvements are more pronounced at higher output volt-
ages.
Application Hints (Continued)
01133912
*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
01133913
**Exact value depends on dropout voltage. (See Application Hints)
FIGURE 4. ERROR Output Timing
where: V
= 1.23V and I = −10 nA (typical)
FB
REF
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.
FIGURE 5. Auxiliary Adjustable Regulator
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.
AUXILIARY LDO OUTPUT
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).
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.
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.
The output voltage is set by an external resistive divider, as
shown in Figure 5. 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
OUTPUT ISOLATION
The shutdown input equivalent circuit is shown in Figure 6.
The main regulator output is shut down when the NPN
transitor is turned ON.
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 con-
nected 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.
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 capaci-
tance. This is relatively inefficient, since large increases in
capacitance are required to get significant improvement.
01133914
FIGURE 6. Shutdown Circuitry
Noise can be reduced more effectively by a bypass capacitor
placed across R1 (refer to Figure 3 ). The formula for select-
ing the capacitor to be used is:
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.
IMPORTANT:
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.
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.
15
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16
Typical Applications
01133916
17
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Physical Dimensions inches (millimeters)
unless otherwise noted
16-Pin Surface Mount
Order Number LP2956IM or LP2956AIM
NS Package Number M16A
16-Pin Plastic Dual-In-Line Package
Order Number LP2956IN or LP2956AIN
NS Package Number N16A
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18
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
16-Pin Ceramic Dual-In-Line Package
Order Number LP2956AMJ-QML or 5962-9554701QEA
NS Package Number J16A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
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