LP2957IS/NOPB - National Semiconductor

LP2957/LP2957A

5V Low-Dropout Regulator for µP Applications

General Description

The LP2957 is a 5V micropower voltage regulator with elec-

tronic shutdown, error flag, very low quiescent current

(150 µA typical at 1 mA load), and very low dropout voltage

(470 mV typical at 250 mA load current).

Output can be wired for snap-on/snap-off operation to elimi-

nate transition voltage states where µP operation may be un-

predictable.

Output crowbar (50 mA typical pull-down current) will bring

down the output quickly when the regulator snaps off or

when the shutdown function is activated.

The part has tight line and load regulation (0.04%typical)

and low output temperature coefficient (20 ppm/˚C typical).

The accuracy of the 5V output is guaranteed at room tem-

perature and over the full operating temperature range.

The LP2957 is available in the five-lead TO-220 and TO-263

packages.

Features

n5V output within 1.4%over temperature (A grade)

nEasily programmed for snap-on/snap-off output

nGuaranteed 250 mA output current

nExtremely low quiescent current

nLow Input-Output voltage required for regulation

nReverse battery protection

nExtremely tight line and load regulation

nVery low temperature coefficient

nCurrent and thermal limiting

nError flag signals when output is out of regulation

Applications

nHigh-efficiency linear regulator

nBattery-powered regulator

Package Outline

Bent, Staggered Leads

5-Lead TO-220 (T)

DS011340-16

Top View

Order Number LP2957AIT or LP2957IT

See NS Package Number T05D

Plastic Surface Mount Package

5-Lead TO-263 (S)

DS011340-17

Top View

DS011340-18

Side View

Order Number LP2957AIS or LP2957IS

See NS Package Number TS5B

June 1998

LP2957/LP2957A 5V Low-Dropout Regulator for µP Applications

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Operating Junction

Temperature Range −40˚C to +125˚C

Storage Temperature Range −65˚C to +150˚C

Lead Temperature

(Soldering, 5 Seconds) 260˚C

Power Dissipation (Note 2) Internally Limited

Input Supply Voltage −20V to +30V

Shutdown Input −0.3V to +30V

ESD Rating 2 kV

Electrical Characteristics

Limits in standard typeface are for T

=25˚C, and limits in boldface type apply over the full operating temperature range. Un-

less otherwise specified: V

=6V, I

=1 mA, C

=2.2 µF, V

=3V.

Symbol Parameter Conditions Typical LP2957AI LP2957I Units

Min Max Min Max

Output Voltage 5.0 4.975 5.025 4.950 5.050 V(Note 9) 4.940 5.060 4.900 5.100

1mA≤I

≤250 mA 5.0 4.930 5.070 4.880 5.120

Output Voltage

Temperature

Coefficient

(Note 3) 20 100 150 ppm/˚C

Line Regulation V

=6V to 30V 0.03 0.10 0.20 %

0.20 0.40

Load Regulation I

=1mAto250mA 0.04 0.16 0.20 %

=0.1 mA to 1 mA

(Note 4) 0.20 0.30

–V

Dropout Voltage I

=1 mA 60 100 100 mV

(Note 5) 150 150

=50 mA 240 300 300

420 420

=100 mA 310 400 400

520 520

=250 mA 470 600 600

800 800

GND

Ground Pin Current I

=1 mA 150 200 200 µA

(Note 6) 230 230

=50 mA 1.1 2 2 mA

2.5 2.5

=100 mA 3 6 6

250 mA 16 28 28

33 33

GND

Ground Pin Current I

=0 130 180 180 µA

in Shutdown (Note 6) V

=0.4V 200 200

GND

Ground Pin Current V

=4.5V 180 230 230 µA

at Dropout (Note 6) I

=0.1 mA 250 250

Off-State Output V

=5.3V 50 30 30 mA

(Sink) Pulldown Current V

=5V, V

=0.4V 20 20

Output Leakage I

(SD IN)

≥1 µA 3 10 10 µA

(Off) in Shutdown V

=30V, V

OUT

=0V 20 20

LIMIT

Current Limit R

=1Ω400 500 500 mA

530 530

Thermal Regulation (Note 7) 0.05 0.2 0.2 %/W

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Electrical Characteristics (Continued)

Limits in standard typeface are for T

=25˚C, and limits in boldface type apply over the full operating temperature range. Un-

less otherwise specified: V

=6V, I

=1 mA, C

=2.2 µF, V

=3V.

Symbol Parameter Conditions Typical LP2957AI LP2957I Units

Min Max Min Max

Output Noise Voltage C

=2.2 µF 500 µV

RMS

(10 Hz to 100 kHz)

=100 mA C

=33 µF 320

SHUTDOWN INPUT

(ON) Output Turn-On 1.155 1.305 1.155 1.305 V

Threshold Voltage 1.140 1.320 1.140 1.320

HYST Hysteresis 6 mV

Input Bias V

IN(SD)

=0V to 5V 10 −30 30 −30 30 nA

Current −50 50 −50 50

DROPOUT DETECTION COMPARATOR

Output “HIGH” V

=30V 0.01 1 1 µA

Leakage 22

Output “LOW” V

=4V 150 250 250 mV

Voltage I

(COMP) =400 µA 400 400

THR

Upper Threshold (Note 8) −240 −320 −150 −320 −150 mV

(Max) Voltage −380 −100 −380 −100

THR

Lower Threshold (Note 8) −350 −450 −230 −450 −230 mV

(Min) Voltage −640 −160 −640 −160

HYST Hysteresis (Note 8) 60 mV

Note 1: Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the de-

vice outside of its rated operating conditions.

Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, T J(MAX), the junction-to-ambient thermal resistance, θJA,

and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using:

Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. The junction-to-ambient

thermal resistance of the TO-220 (without heatsink) is 60˚C/W and 73˚C/W for the TO-263. If the TO-263 package is used, the thermal resistance can be reduced

by increasing the P.C. board copper area thermally connected to the package: Using 0.5 Square inches of copper area, θJA is 50˚C/W, with 1 square inch of copper

area, θJA is 37˚C/W; and with 1.6 or more square inches of copper area, θJA is 32˚C/W. The junction-to-case thermal resistance is 3˚C/W. If an external heatsink is

used, the effective junction-to-ambient thermal resistance is the sum of the junction-to-case resistance (3˚C/W), the specified thermal resistance of the heatsink se-

lected, and the thermal resistance of the interface between the heatsink and the LP2957 (see Application Hints).

Note 3: Output voltage temperature coefficient is defined as the worst case voltage change divided by the total temperature range.

Note 4: Regulation is measured at constant junction temperature using low duty cycle pulse testing. Parts are tested separately for load regulation in the load ranges

0.1 mA–1 mA and 1 mA–250 mA. Changes in output voltage due to heating effects are covered by the thermal regulation specification.

Note 5: Dropout voltage is defined as the input to output voltage differential at which the output voltage drops 100 mV below the value measured with a 1V input

to output differential.

Note 6: Ground pin current is the regulator quiescent current. The total current drawn from the source is the sum of the load current plus the ground pin current.

Note 7: 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 ef-

fects. Specifications are for a 200 mA load pulse at VIN =20V (3W pulse) for T =10 ms.

Note 8: Voltages are referenced to the nominal regulated output voltage.

Note 9: 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.

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Typical Performance Characteristics Unless otherwise specified: V

=6V, I

=1 mA, C

=2.2 µF, V

=3V, T

=25˚C

Ground Pin Current

DS011340-19

Ground Pin Current

DS011340-20

Ground Pin Current

vs Load

DS011340-21

Ground Pin Current

DS011340-22

Ground Pin Current

DS011340-23

Output Noise Voltage

DS011340-24

Ripple Rejection

DS011340-25

Ripple Rejection

DS011340-26

Ripple Rejection

DS011340-27

Line Transient Response

DS011340-28

Line Transient Response

DS011340-29

Output Impedance

DS011340-30

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Typical Performance Characteristics Unless otherwise specified: V

=6V, I

=1 mA, C

=2.2 µF,

VSD =3V, T

=25˚C (Continued)

Load Transient

Response

DS011340-31

Load Transient

Response

DS011340-32

Dropout

Characteristics

DS011340-33

Enable Transient

DS011340-34

Enable Transient

DS011340-35

Short-Circuit Output

Current and Maximum

Output Current

DS011340-36

Thermal Regulation

DS011340-37

Error Output

Sink Current

DS011340-38

Dropout Detection

Threshold Voltages

DS011340-39

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Typical Performance Characteristics Unless otherwise specified: V

=6V, I

=1 mA, C

=2.2 µF,

VSD =3V, T

=25˚C (Continued)

Block Diagram

Typical Application Circuits

Maximum Power Dissipation

(TO-263) (Note 2)

DS011340-40

Error Output Voltage

DS011340-41

Dropout Voltage

DS011340-42

DS011340-1

LP2957 Basic Application

DS011340-2

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Typical Application Circuits (Continued)

Application Hints

EXTERNAL CAPACITORS

A 2.2 µF (or greater) capacitor is required between the out-

put pin and ground to assure stability (refer to

Figure 1

Without this capacitor, the part may oscillate. Most type of

tantalum or aluminum electrolytics will work here. Film types

will work, but are more expensive. Many aluminum electro-

lytics contain electrolytes which freeze at −30˚C, which re-

quires the use of solid tantalums below −25˚C. The important

parameters of the capacitor are an ESR of about 5Ωor less

and a resonant frequency above 500 kHz (the ESR may in-

crease by a factor of 20 or 30 as the temperature is reduced

from 25˚C to −30˚C). The value of this capacitor may be in-

creased without limit. At lower values of output current, less

output capacitance is required for stability. The capacitor can

be reduced to 0.68 µF for currents below 10 mA or 0.22 µF

for currents below 1 mA.

A 1 µF capacitor should be placed from the input pin to

ground if there is more than 10 inches of wire between the in-

put and the AC filter capacitor or if a battery input is used.

This capacitor may have to be increased if the regulator

is wired for snap-on/snap-off output and the source im-

pedance is high (see

Snap-On/Snap-Off Operation

sec-

tion).

SHUTDOWN INPUT

A logic-level signal will shut off the regulator output when a

“LOW” (<1.2V) is applied to the Shutdown input.

To prevent possible mis-operation, the Shutdown input must

be actively terminated. If the input is driven from

open-collector logic, a pull-up resistor (20 kΩto 100 kΩrec-

ommended) must be connected from the Shutdown input to

the regulator input.

If the Shutdown input is driven from a source that actively

pulls high and low (like an op-amp), the pull-up resistor is not

required, but may be used.

If the shutdown function is not to be used, the cost of the

pull-up resistor can be saved by tying the Shutdown input di-

rectly to the regulator input.

IMPORTANT: Since the Absolute Maximum Ratings state

that the Shutdown input can not go more than 0.3V below

ground, the reverse-battery protection feature which protects

the regulator input is sacrificed if the Shutdown input is tied

directly to the regulator input.

If reverse-battery protection is required in an application, the

pull-up resistor between the Shutdown input and the regula-

tor input must be used.

MINIMUM LOAD

It should be noted that a minimum load current is specified in

several of the electrical characteristic test conditions, so the

value listed must be used to obtain correlation on these

tested limits. The part is parametrically tested down to

100 µA, but is functional with no load.

DROPOUT VOLTAGE

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 mea-

sured with a 1V differential. The dropout voltages for various

values of load current are listed under Electrical Characteris-

tics.

If the regulator is powered from a transformer connected to

the AC line, the minimum AC line voltage and maximum

load current must be used to measure the minimum voltage

at the input of the regulator. The minimum input voltage is

the lowest voltage level including ripple on the filter ca-

pacitor . It is also advisable to verify operation at minimum

operating ambient temperature , since the increasing ESR

of the filter capacitor makes this a worst-case test due to in-

creased ripple amplitude.

HEATSINK REQUIREMENTS

A heatsink may be required with the LP2957 depending on

the maximum power dissipation and maximum ambient tem-

perature of the application. Under all possible operating con-

ditions, 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), and

the maximum load current must also be used.

Figure 1

shows the voltages and currents which are present in the cir-

cuit. The formula for calculating the power dissipated in the

regulator is also shown in

Figure 1

LP2957 Application with Snap-On/Snap-Off Output

DS011340-4

*See Application Hints

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Application Hints (Continued)

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

(Max). This is calculated

by using the formula:

(Max) =T

(Max) − T

(Max)

where: T

(Max) is the maximum allowable junction tem-

perature

(Max) is the maximum ambient temperature

Using the calculated values for T

(Max) and P(Max), the re-

quired value for junction-to-ambient thermal resistance, θ

(JA)

, can now be found:

(JA)

(Max)/P(Max)

If the calculated value is 60˚C/W or higher , the regulator

may be operated without an external heatsink. If the calcu-

lated value is below 60˚C/W, an external heatsink is re-

quired. The required thermal resistance for this heatsink,

(HA)

, can be calculated using the formula:

(HA)

=θ

(JA)

−θ

(JC)

−θ

(CH)

where:

(JC)

is the junction-to-case thermal resistance, which is

specified as 3˚C/W for the LP2957.

(CH)

is the case-to-heatsink thermal resistance, which is de-

pendent on the interfacing material (see

Table 1

and

Table

Typical TO-220 Case-To-Heatsink

Thermal Resistance in ˚C/W

TABLE 1. (From AAVID)

Silicone Grease 1.0

Dry Interface 1.3

Mica with Grease 1.4

TABLE 2. (From Thermalloy)

Thermasil III 1.3

Thermasil II 1.5

Thermalfilm (0.002) 2.2

with Grease

(HA)

is the heatsink-to-ambient thermal resistance. It is this

specification (listed on the heatsink manufacturers data

sheet) which defines the effectiveness of the heatsink. The

heatsink selected must have a thermal resistance which is

equal to or lower than the value of θ

(HA)

calculated from the

above listed formula.

ERROR COMPARATOR

This comparator produces a logic “LOW” whenever the out-

put 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. An out-of-regulation condition can

result from low input voltage, current limiting, or thermal lim-

iting.

Figure 2

gives a timing diagram showing the relationship be-

tween the output voltage, the ERROR output, and input volt-

age as the input voltage is ramped up and down to the regu-

lator without snap-on/snap-off output. The ERROR signal

becomes low at about 1.3V input. It goes high at about 5V in-

put, where the output equals 4.75V. Since the dropout volt-

age is load dependent, the input voltage trip points will vary

with load current. The 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 output or some other supply voltage. Using

the regulator output prevents an invalid “HIGH” on the com-

parator 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 100k to 1 MΩ. The re-

sistor is not required if the output is unused.

DS011340-7

PTOTAL =(VIN − 5)I L+(V

IN)IG

*See EXTERNAL CAPACITORS

FIGURE 1. Basic 5V Regulator Circuit

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Application Hints (Continued)

If a single pull-up resistor is connected to the regulator out-

put, the error flag may briefly rise up to about 1.3V as the in-

put 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 in regulation (4.8V to 5V), the

error output voltage will be 2.4V to 2.5V, which is clearly a

high signal.

OUTPUT ISOLATION

The regulator output can be connected to an active voltage

source (such as a battery) with the regulator input turned 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.

SNAP-ON/SNAP-OFF OPERATION

The LP2957 output can be wired for snap-on/snap-off opera-

tion using three external resistors:

When connected as shown, the shutdown input holds the

regulator off until the input voltage rises up to the turn-on

threshold (V

), at which point the output “snaps on”.

When the input power is shut off (and the input voltage starts

to decay) the output voltage will snap off when the input volt-

age reaches the turn-off threshold, V

OFF

It is important to note that the voltage V

OFF

must always be

lower than V

(the difference in these voltage levels is

called the hysteresis).

Hysteresis is required when using snap-on/snap-off output,

with the minimum amount of hysteresis required for a spe-

cific application being dependent on the source impedance

of whatever is supplying V

Caution: A type of low-frequency oscillation can occur if

and V

OFF

are too close together (insufficient

hysteresis ). When the output snaps on, the regu-

lator must draw sufficient current to power the

load and charge up the output capacitor (in most

cases, the regulator will briefly draw the maximum

current allowed by its internal limiter).

For this reason, it is best to assume the LP2957

may pull a peak current of about 600 mA from the

source (which is the listed maximum short-circuit

load current of 530 mA plus the ground pin current

of 70 mA ).

This high peak current causes V

to drop by an amount

equal to the source impedance multiplied by the current. If V

drops below V

OFF

, the regulator will turn off and stop draw-

ing current from the source. This will allow V

to rise back up

above V

, and the cycle will start over. The regulator will

stay in this oscillating mode and never come into regulation.

HYSTERESIS IN TRANSFORMER-POWERED

APPLICATIONS:

If the unregulated DC input voltage to the regulator comes

from a transformer, the required hysteresis is easily mea-

sured by loading the source with a resistive load.

If the regulator is powered from a battery, the source imped-

ance will probably be low enough that other considerations

will determine the optimum values for hysteresis (see Design

Example #2).

For best results, the load resistance used to test the trans-

former should be selected to draw about 600 mA for the

maximum load current test, since this is the maximum peak

current the LP2957 could be expected to draw from the

source.

DS011340-14

*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, which varies with load current.

FIGURE 2. ERROR Output Timing

DS011340-8

*Minimum value (increase as required for smooth turn-on characteristic).

FIGURE 3. Snap-On/Snap-Off Output

DS011340-9

FIGURE 4. Snap-On/Snap-Off Input

and Output Voltage Diagram

DS011340-10

FIGURE 5. Transformer Powered Input Supply

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Application Hints (Continued)

The difference in input voltage measured at no load and

full load defines the amount of hysteresis required for

proper snap-on/snap-off operation (the programmed hys-

teresis must be greater than the difference in voltages).

CALCULATING RESISTOR VALUES:

The values of R1, R2 and R3 can be calculated assuming

the designer knows the hysteresis.

In most transformer-powered applications, it can be as-

sumed that V

OFF

(the input voltage at turn-off) should be set

for about 5.5V, since this allows about 500 mV across the

LP2957 to keep the output in regulation until it snaps off. V

(the input voltage at turn on) is found by adding the hyster-

esis voltage to V

OFF

R1, R2 and R3 are found by solving the node equations for

the currents entering the node nearest the shutdown pin

(written at the turn-on and turn-off thresholds).

The shutdown pin bias current (10 nA typical) is not included

in the calculations:

Since these two equations contain three unknowns (R1, R2

and R3) one resistor value must be assumed and then the

remaining two values can be obtained by solving the equa-

tions.

The node equations will be simplified by solving both equa-

tions for R2, and then equating the two to generate an ex-

pression in terms of R1 and R3.

Setting these equal to each other and solving for R1 yields:

The same equation solved for R3 is:

A value for R1 or R3 can be derived using either one of the

above equations, if the designer assumes a value for one of

the resistors.

The simplest approach is to assume a value for R3. Best re-

sults will typically be obtained using values between about

20 kΩand 100 kΩ(this keeps the current drain low, but also

generates realistic values for the other resistors).

There is no limit on the minimum value of R3, but current

should be minimized as it generates power that drains the

source and does not power the load.

SUMMARY: TO SOLVE FOR R1, R2 AND R3:

1. Assume a value for either R1 or R3.

2. Solve for the other variable using the equation for R1 or

R3.

3. Take the values for R1 and R3 and plug them back into

either equation for R2 and solve for this value.

DESIGN EXAMPLE #1:

A 5V regulated output is to be powered from a transformer

secondary which is rectified and filtered. The voltage V

measured at zero current and maximum current (600 mA) to

determine the minimum allowable hysteresis.

is measured using an oscilloscope (both traces are

shown on the same grid for clarity):

The full-load voltage waveform from a transformer-powered

supply will have ripple voltage as shown. The correct point to

measure is the lowest value of the waveform.

The 1.2V differential between no-load and full-load condi-

tions means that at least 1.2V of hysteresis is required for

proper snap-on/snap-off operation (for this example, we will

use 1.5V ).

As a starting point, we will assume:

OFF

=5.5V

OFF

+ HYST =5.5 + 1.5 =7V

R3 =49.9k

Solving for R1:

Turn-ON Transition

DS011340-11

Turn-OFF Transition

DS011340-12

FIGURE 6. Equivalent Circuits

DS011340-13

FIGURE 7. V

VOLTAGE WAVEFORMS

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Application Hints (Continued)

Solving for R2:

DESIGN EXAMPLE #2:

A 5V regulated output is to be powered from a battery made

up of six NiCad cells. The cell data is:

cell voltage (full charged): 1.4V

cell voltage (90%discharged): 1.0V

The internal impedance of a typical battery is low enough

that source loading during regulator turn-on is not usually a

problem.

In a battery-powered application, the turn-off voltage V

OFF

should be selected so that the regulator is shut down when

the batteries are about 90%discharged (over discharge can

damage rechargeable batteries).

In this case, the battery voltage will be 6.0V at the 90%dis-

charge point (since there are six cells at 1.0V each). That

means for this application, V

OFF

will be set to 6.0V.

Selecting the optimum voltage for V

requires understand-

ing battery behavior. If a Ni-Cad battery is nearly discharged

(cell voltage 1.0V) and the load is removed , the cell volt-

age will drift back up. The voltage where the regulator turns

on must be set high enough to keep the regulator from

re-starting during this time, or an on-off pulsing mode can oc-

cur.

If the regulator restarts when the discharged cell voltage

drifts up, the load on the battery will cause the cell voltage to

fall below the turn-off level, which causes the regulator to

shut down. The cell voltage will again float up and the on-off

cycling will continue.

For NiCad batteries, a good cell voltage to use to calculate

is about 1.2V per cell. In this application, this will yield a

value for V

of 7.2V.

We can now find R1, R2 and R3 assuming:

OFF

=6.0V V

=7.2V R3 =49.9k

Solving for R1:

Solving for R2:

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Schematic Diagram

DS011340-15

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Physical Dimensions inches (millimeters) unless otherwise noted

Bent, Staggered 5-Lead TO-220 (T)

Order Number LP2957AIT or LP2957IT

NS Package Number T05D

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

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TO-263 5-Lead Plastic Surface Mount Package

Order Number LP2957AIS or LP2957IS

NS Package Number TS5B

LP2957/LP2957A 5V Low-Dropout Regulator for µP Applications

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.