LM2991
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 tem-
perature 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 quiescent current to only 9 mA (typical)
when the regulator is in the dropout mode (V
OUT
−V
IN
≤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 avail-
able.
Features
nOutput voltage adjustable from −3V to −24V, typically
−2V to −25V
nOutput current in excess of 1A
nDropout voltage typically 0.6V at 1A load
nLow quiescent current
nInternal short circuit current limit
nInternal thermal shutdown with hysteresis
nTTL, CMOS compatible ON/OFF switch
nFunctional complement to the LM2941 series
Applications
nPost switcher regulator
nLocal, on-card, regulation
nBattery operated equipment
Typical Application
DS011260-1
VOUT =V
REF (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 10Ωover the same operating temperature range as the regulator.
September 2000
LM2991 Negative Low Dropout Adjustable Regulator
© 2000 National Semiconductor Corporation DS011260 www.national.com
Connection Diagrams and Ordering Information
5-Lead TO-220
Straight Leads
DS011260-9
Front View
Order Number LM2991T
See NS Package Number T05A
5-Lead TO-220
Bent, Staggered Leads
DS011260-2
Front View
Order Number LM2991T Flow LB03
See NS Package Number T05D
TO263
5-Lead Surface-Mount Package
DS011260-11
Top View
DS011260-12
Side View
Order Number LM2991S
See NS Package Number TS5B
16-Lead Ceramic Dual-in-Line Package
DS011260-29
Top View
Order Number LM2991J-QMLV
5962-9650501VEA
Order Number LM2991J-QML
5962-9650501QEA
See NS Package Number J16A
16-Lead Ceramic Surface-Mount Package
DS011260-30
Top View
Order Number LM2991WG-QML
5962-9650501QXA
See NS Package Number WG16A
LM2991
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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 (T
Jmax
) 125˚C
Storage Temperature Range −65˚C to +150˚C
Lead Temperature (Soldering, 10 sec.) 230˚C
Operating Ratings (Note 1)
Junction Temperature Range (T
J
) −40˚C to +125˚C
Maximum Input Voltage (Operational) −26V
Electrical Characteristics
V
IN
= −10V, V
O
= −3V, I
O
= 1A, C
O
= 47 µF, R1 = 2.7k, T
J
= 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 ≤I
O
≤1A −1.210 −1.234 −1.186 V
5mA≤I
O
≤1A, −1.27 −1.15 V
V
O
−1V≥V
IN
≥−26V
Output Voltage −2 −3 V
Range V
IN
= −26V −25 −24 V
Line Regulation I
O
= 5 mA, V
O
−1V≥V
IN
≥−26V 0.004 0.04 %/V
Load Regulation 50 mA ≤I
O
≤1A 0.04 0.4 %
Dropout Voltage I
O
= 0.1A, ∆V
O
≤100 mV 0.1 0.2 V
0.3
I
O
= 1A, ∆V
O
≤100 mV 0.6 0.8 V
1
Quiescent Current I
O
≤1A 0.7 5mA
Dropout Quiescent V
IN
=V
O
,I
O
≤1A 16 50 mA
Current
Ripple Rejection V
ripple
= 1 Vrms,
f
ripple
= 1 kHz, 60 50 dB
I
O
=5mA
Output Noise 10 Hz − 100 kHz, I
O
= 5 mA 200 450 µV
ON /OFF Input (V
OUT
: ON) 1.2 0.8 V
Voltage (V
OUT
: OFF) 1.3 2.4
ON /OFF Input V
ON/OFF
= 0.8V (V
OUT
: ON) 0.1 10 µA
Current V
ON/OFF
= 2.4V (V
OUT
: OFF) 40 100
Output Leakage V
IN
= −26V, V
ON/OFF
= 2.4V 60 250 µA
Current V
OUT
=0V
Current Limit V
OUT
= 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 deivce is in-
tended 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 kΩresistor.
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=(T
Jmax
−T
A
)/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.
LM2991
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Typical Performance Characteristics
Dropout Voltage
DS011260-13
Normalized Output Voltage
DS011260-14
Output Voltage
DS011260-15
Output Noise Voltage
DS011260-16
Quiescent Current
DS011260-17
Maximum Output Current
DS011260-18
Line Transient Response
DS011260-19
Load Transient Response
DS011260-20
Maximum Output Current
DS011260-21
Ripple Rejection
DS011260-22
Output Impedance
DS011260-23
ON /OFF Control Voltage
DS011260-24
LM2991
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Typical Performance Characteristics (Continued)
Application Hints
External Capacitors
Like any low-dropout regulator, external capacitors are re-
qired to stabilize the control loop. These capacitors must be
correctly selected for proper performance.
INPUT CAPACITOR: An input capacitor is reqiuired if the
regulator is located more than 6″from the input power supply
filter capacitor (or if no other input capaciotr is present).
Asolid Tantalum or ceramic capacitor whose value is at least
1 µF is recommended, but an aluminum electrolytic (≥10 µF)
may be used. However, aluminum electrolytics should not be
used in applications where the ambient temperature can
drop below 0˚C because their internal impedance increases
significantly at cold temperatures.
OUTPUT CAPACITOR: The output capacitor must meet the
ESR limits shown in the graph, which means it must have an
ESR between about 25 mΩand 10Ω.
Asolid Tantalum (value ≥1 µF) is the best choice for the out-
put 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
exhibit 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 tempera-
tures occur.
The lower stable ESR limit of 25 mΩmeans 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 exter-
nal resistance is placed in series with it (1Ωrecommended).
Dielectric 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 reduced 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 ″decoupling″to shield the out-
put of the LDO from the effects of bypass capacitors.
These capacitors should be avoided if possible, and kept as
far far away from the LDO output as is practical.
Adjust Pin Current
DS011260-25
Low Voltage Behavior
DS011260-26
Maximum Power
Dissipation (TO-220)
DS011260-27
Maximum Power Dissipation
(TO-263) (See Note 3)
DS011260-28
LM2991
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Application Hints (Continued)
MINIMUM LOAD
A minimum load current of 500 µA is required for proper op-
eration. The external resistor divider can provide the mini-
mum 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 resis-
tor divider using the following equation:
V
OUT
=V
REF
x(1+R
2
/R
1
)−(I
ADJ
xR
2
)
where V
REF
= −1.21V. The output voltage can be pro-
grammed 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 kΩmakes the adjust pin cur-
rent negligible. For example, neglecting the adjust pin cur-
rent, and setting R2 to 100 kΩand V
OUT
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 (see Adjustable
Current Sink Application).
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 di-
ode to ground.
Typical Applications
Output Capacitor ESR Range
DS011260-5
Fully Isolated Post-Switcher Regulator
DS011260-6
LM2991
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Typical Applications (Continued)
Adjustable Current Sink
DS011260-10
LM2991
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Equivalent Schematic
DS011260-8
LM2991
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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
LM2991
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
TO-263 5-Lead Plastic Surface Mount Package
Ordering Number LM2991S
NS Package Number TS5B
LM2991
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
16-Lead Ceramic Dual-in-Line Package
Order Number LM2991J-QMLV
5962-9650501VEA
Order Number LM2991J-QML
5962-9650501QEA
NS Package Number J16A
LM2991
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
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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
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can be reasonably expected to cause the failure of
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www.national.com
16-Lead Ceramic Surface-Mount Package
Order Number LM2991WG-QML
5962-9650501QXA
NS Package Number WG16A
LM2991 Negative Low Dropout Adjustable Regulator
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.
