5962-9650501QXA - NATIONAL SEMICONDUCTOR

LM2991

May 18, 2010

Negative Low Dropout Adjustable Regulator

General Description

The LM2991 is a low dropout adjustable negative regulator

with a output voltage range between −3V to −24V. The

LM2991 provides up to 1A of load current and features a

On /Off pin for remote shutdown capability.

The LM2991 uses new circuit design techniques to provide a

low dropout voltage, low quiescent current and low tempera-

ture coefficient precision reference. The dropout voltage at 1A

load current is typically 0.6V and a guaranteed worst-case

maximum of 1V over the entire operating temperature range.

The quiescent current is typically 1 mA with a 1A load current

and an input-output voltage differential greater than 3V. A

unique circuit design of the internal bias supply limits the qui-

escent current to only 9 mA (typical) when the regulator is in

the dropout mode (VOUT − VIN ≤ 3V).

The LM2991 is short-circuit proof, and thermal shutdown in-

cludes hysteresis to enhance the reliability of the device when

inadvertently overloaded for extended periods. The LM2991

is available in 5-lead TO-220 and TO-263 packages and is

rated for operation over the automotive temperature range of

−40°C to +125°C. Mil-Aero versions are also available.

Features

■Output voltage adjustable from −3V to −24V, typically −2V

to −25V

■Output current in excess of 1A

■Dropout voltage typically 0.6V at 1A load

■Low quiescent current

■Internal short circuit current limit

■Internal thermal shutdown with hysteresis

■TTL, CMOS compatible ON/OFF switch

■Functional complement to the LM2941 series

Applications

■Post switcher regulator

■Local, on-card, regulation

■Battery operated equipment

Typical Application

1126001

VOUT = VREF (1 + R2/R1)

*Required if the regulator is located further than 6 inches from the power supply filter capacitors. A 1 μF solid tantalum or a 10 μF aluminum electrolytic

capacitor is recommended.

**Required for stability. Must be at least a 10 μF aluminum electrolytic or a 1 μF solid tantalum to maintain stability. May be increased without bound to

maintain regulation during transients. Locate the capacitor as close as possible to the regulator. The equivalent series resistance (ESR) is critical, and should

be less than 10Ω over the same operating temperature range as the regulator.

LM2991 Negative Low Dropout Adjustable Regulator

Connection Diagrams

1126009

Front View

TO-220, 5-Lead, Straight

See NS Package Number T05A

1126002

Front View

TO-220 Flow LB03, 5-Lead, Staggered Bend

See NS Package Number T05D

1126011

Top View

TO263, 5-Lead, Surface-Mount

See NS Package TS5B

1126029

Top View

16-Lead, Ceramic, Dual-in-Line

See NS Package Number J16A

1126030

Top View

16-Lead, Ceramic, Surface-Mount

See NS Package Number WG16A

Ordering Information

Order Number Package Type NSC Package Package Marking Supplied As

LM2991S 5-Lead TO-263 TS5B LM2991S Rail of 45

LM2991SX 5-Lead TO-263 TS5B LM2991S Reel of 500

LM2991T 5-Pin TO-220 T05A LM2991T Rail of 45

LM2991T / LB03 5-Pin TO-220, Stagger T05D LM2991T Rail of 45

LM2991J-QML 16-Pin CERDIP J16A 5962-9650501QEA -

LM2991J-QMLV 16-Pin CERDIP J16A 5962-9650501VEA -

LM2991WG-QML 16-Lead CERPACK WG16A 5962-9650501QXA Tray of 42

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LM2991

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 Voltage −26V to +0.3V

ESD Susceptibility (Note 2) 2 kV

Power Dissipation (Note 3) Internally limited

Junction Temperature (TJmax)125°C

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

Lead Temperature (Soldering, 10 sec.) 230°C

Operating Ratings (Note 1)

Junction Temperature Range (TJ)−40°C to +125°C

Maximum Input Voltage (Operational) −26V

Electrical Characteristics

VIN = −10V, VO = −3V, IO = 1A, CO = 47 μF, R1 = 2.7 kΩ, TJ = 25°C, unless otherwise specified. Boldface limits apply over the

entire operating junction temperature range.

Parameter Conditions Typical Min Max Units

(Note 4)

Reference Voltage

5 mA ≤ IO ≤ 1A −1.210 −1.234 −1.186

5 mA ≤ IO ≤ 1A,

VO − 1V ≥ VIN ≥ −26V −1.27 −1.15

Output Voltage Range −2 −3 V

VIN = −26V −25 −24

Line Regulation IO = 5 mA, VO − 1V ≥ VIN ≥ −26V 0.004 0.04 %/V

Load Regulation 50 mA ≤ IO ≤ 1A 0.04 0.4 %

Dropout Voltage

IO = 0.1A, ΔVO ≤ 100 mV 0.1 0.2 V

0.3

IO = 1A, ΔVO ≤ 100 mV 0.6 0.8 V

Quiescent Current IO ≤ 1A 0.7 5mA

Dropout Quiescent Current VIN = VO, IO ≤ 1A 16 50 mA

Ripple Rejection Vripple = 1 Vrms, f ripple = 1 kHz, IO = 5 mA 60 50 dB

Output Noise 10 Hz − 100 kHz, IO = 5 mA 200 450 μV

ON /OFF Input Voltage (VOUT: ON) 1.2 0.8 V

(VOUT: OFF) 1.3 2.4

ON /OFF Input Current VON/OFF = 0.8V (VOUT: ON) 0.1 10 μA

VON/OFF = 2.4V (VOUT: OFF) 40 100

Output Leakage Current VIN = −26V, VON/OFF = 2.4V, VOUT = 0V 60 250 μA

Current Limit VOUT = 0V 2 1.5 A

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: Human body model, 100 pF discharged through a 1.5 kΩ resistor.

Note 3: The maximum power dissipation is a function of TJmax, θJA and TA. The maximum allowable power dissipation at any ambient temperature is PD =

(TJmax − TA)/θJA. If this dissipation is exceeded, the die temperature will rise above 125°C and the LM2991 will go into thermal shutdown. For the LM2991, the

junction-to-ambient thermal resistance is 53°C/W for the TO-220, 73°C/W for the TO-263, and junction-to-case thermal resistance is 3°C. If the TO-263 package

is used, the thermal resistance can be reduced by increasing the PC 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.

Note 4: Typicals are at TJ = 25°C and represent the most likely parametric norm.

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LM2991

Typical Performance Characteristics

Dropout Voltage

1126013

Normalized Output Voltage

1126014

Output Voltage

1126015

Output Noise Voltage

1126016

Quiescent Current

1126017

Maximum Output Current

1126018

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LM2991

Line Transient Response

1126019

Load Transient Response

1126020

Maximum Output Current

1126021

Ripple Rejection

1126022

Output Impedance

1126023

ON /OFF Control Voltage

1126024

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LM2991

Adjust Pin Current

1126025

Low Voltage Behavior

1126026

Maximum Power

Dissipation (TO-220)

1126027

Maximum Power Dissipation

(TO-263) (See Note 3)

1126028

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LM2991

Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, external capacitors are re-

quired to stabilize the control loop. These capacitors must be

correctly selected for proper performance.

INPUT CAPACITOR

An input capacitor is required if the regulator is located more

than 6 inches from the input power supply filter capacitor (or

if no other input capacitor is present).

A solid Tantalum or ceramic capacitor whose value is at least

1 µF is recommended, but an aluminum electrolytic (≥ 10 µF)

may be used. However, aluminum electrolytic types should

not be used in applications where the ambient temperature

can drop below 0°C because their internal impedance in-

creases significantly at cold temperatures.

OUTPUT CAPACITOR

The output capacitor must meet the ESR limits shown in Fig-

ure 1, which means it must have an ESR between about 25

mΩ and 10Ω.

1126005

FIGURE 1. Output Capacitor ESR Range

A solid Tantalum (value ≥ 1 µF) is the best choice for the

output capacitor. An aluminum electrolytic (≥ 10 µF) may be

used if the ESR is in the stable range.

It should be noted that the ESR of a typical aluminum elec-

trolytic will increase by as much as 50X as the temperature is

reduced from 25°C down to −40°C, while a Tantalum will ex-

hibit an ESR increase of about 2X over the same range. For

this and other reasons, aluminum electrolytics should not be

used in applications where low operating temperatures occur.

The lower stable ESR limit of 25 mΩ means that ceramic ca-

pacitors can not be used directly on the output of an LDO. A

ceramic (≥ 2.2 µF) can be used on the output if some external

resistance is placed in series with it (1Ω recommended). Di-

electric types X7R or X5R must be used if the temperature

range of the application varies more than ± 25° from ambient

to assure the amount of capacitance is sufficient.

CERAMIC BYPASS CAPACITORS

Many designers place distributed ceramic capacitors whose

value is in the range of 1000 pF to 0.1 µF at the power input

pins of the IC's across a circuit board. These can cause re-

duced phase margin or oscillations in LDO regulators.

The advent of multi-layer boards with dedicated power and

ground planes has removed the trace inductance that (previ-

ously) provided the necessary "de-coupling" to shield the

output of the LDO from the effects of bypass capacitors.

These capacitors should be avoided if possible, and kept as

far away from the LDO output as is practical.

MINIMUM LOAD

A minimum load current of 500 μA is required for proper op-

eration. The external resistor divider can provide the minimum

load, with the resistor from the adjust pin to ground set to 2.4

kΩ.

SETTING THE OUTPUT VOLTAGE

The output voltage of the LM2991 is set externally by a re-

sistor divider using the following equation:

VOUT = VREF x (1 + R2/R1) − (IADJ x R2)

where VREF = −1.21V. The output voltage can be programmed

within the range of −3V to −24V, typically an even greater

range of −2V to −25V. The adjust pin current is about 60 nA,

causing a slight error in the output voltage. However, using

resistors lower than 100 kΩ makes the error due to the adjust

pin current negligible. For example, neglecting the adjust pin

current, and setting R2 to 100 kΩ and VOUT to −5V, results in

an output voltage error of only 0.16%.

ON/OFF PIN

The LM2991 regulator can be turned off by applying a TTL or

CMOS level high signal to the ON/OFF pin. The impedance

of the voltage source driving the ON/OFF pin should be low

enough to source the ON/OFF pin input current to meet the

OFF threshold voltage level, 100 µA maximum at 2.4V.

If the ON/OFF function is not needed, the pin should be con-

nected to Ground. The ON/OFF pin should not be left floating,

as this is not a guaranteed operating condition.

See the Adjustable Current Sink Application, Figure 3

FORCING THE OUTPUT POSITIVE

Due to an internal clamp circuit, the LM2991 can withstand

positive voltages on its output. If the voltage source pulling

the output positive is DC, the current must be limited to 1.5A.

A current over 1.5A fed back into the LM2991 could damage

the device. The LM2991 output can also withstand fast posi-

tive voltage transients up to 26V, without any current limiting

of the source. However, if the transients have a duration of

over 1 ms, the output should be clamped with a Schottky

diode to ground.

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LM2991

Typical Applications

1126006

FIGURE 2. Fully Isolated Post-Switcher Regulator

1126010

FIGURE 3. Adjustable Current Sink

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LM2991

Equivalent Schematic

1126008

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LM2991

Physical Dimensions inches (millimeters) unless otherwise noted

5-Lead T0-220, Straight Leads (T)

Order Number LM2991T

NS Package Number T05A

5-Lead T0-220, Bent, Staggered Leads (T)

Order Number LM2991T Flow LBO3

NS Package Number T05D

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LM2991

TO-263 5-Lead Plastic Surface Mount Package

Ordering Number LM2991S

NS Package Number TS5B

16-Lead Ceramic Dual-in-Line Package

Order Number LM2991J-QMLV

5962-9650501VEA

Order Number LM2991J-QML

5962-9650501QEA

NS Package Number J16A

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LM2991

16-Lead Ceramic Surface-Mount Package

Order Number LM2991WG-QML

5962-9650501QXA

NS Package Number WG16A

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LM2991

Notes

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LM2991

Notes

LM2991 Negative Low Dropout Adjustable Regulator

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