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 -40C to +125C. Mil-Aero versions are also avail-
able.
Features
n
Output voltage adjustable from -3V to -24V, typically
-2V to -25V
n
Output current in excess of 1A
n
Dropout voltage typically 0.6V at 1A load
n
Low quiescent current
n
Internal short circuit current limit
n
Internal thermal shutdown with hysteresis
n
TTL, CMOS compatible ON/OFF switch
n
Functional complement to the LM2941 series
Applications
n
Post switcher regulator
n
Local, on-card, regulation
n
Battery operated equipment
Typical Application
DS011260-1
V
OUT
= V
REF
(1 + R2/R1)
*Required if the regulator is located further than 6 inches from the power supply filter capacitors. A 1 F solid tantalum or a 10 F aluminum electrolytic
capacitor is recommended.
*
*Required for stability. Must be at least a 10 F aluminum electrolytic or a 1 F 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 10
over the same operating temperature range as the regulator.
May 1999
LM2991
Negative
Low
Dropout
Adjustable
Regulator
1999 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
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2
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
)
125C
Storage Temperature Range
-65C to +150C
Lead Temperature (Soldering, 10 sec.)
230C
Operating Ratings
(Note 1)
Junction Temperature Range (T
J
)
-40C to +125C
Maximum Input Voltage (Operational)
-26V
Electrical Characteristics
V
IN
= -10V, V
O
= -3V, I
O
= 1A, C
O
= 47 F, R1 = 2.7k, T
J
= 25C, 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
5 mA
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
5
mA
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
= 5 mA
Output Noise
10 Hz - 100 kHz, I
O
= 5 mA
200
450
V
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
A
Current
V
ON/OFF
= 2.4V (V
OUT
: OFF)
40
100
Output Leakage
V
IN
= -26V, V
ON/OFF
= 2.4V
60
250
A
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 k
resistor.
Note 3: The maximum power dissipation is a function of T
Jmax
,
JA
and T
A
. The maximum allowable power dissipation at any ambient temperature is P
D
= (T
Jmax
- T
A
)/
JA
. If this dissipation is exceeded, the die temperature will rise above 125C and the LM2991 will go into thermal shutdown. For the LM2991, the
junction-to-ambient thermal resistance is 53C/W for the TO-220, 73C/W for the TO-263, and junction-to-case thermal resistance is 3C. 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,
JA
is 50C/W; with 1 square inch of copper area,
JA
is 37C/W; and with 1.6 or more square inches of copper area,
JA
is 32C/W.
Note 4: Typicals are at T
J
= 25C and represent the most likely parametric norm.
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3
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
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4
Typical Performance Characteristics
(Continued)
Application Hints
EXTERNAL CAPACITORS
The LM2991 regulator requires an output capacitor to main-
tain stability. The capacitor must be at least 10 F aluminum
electrolytic or 1 F solid tantalum. The output capacitor's
ESR must be less than 10
, or the zero added to the regu-
lator frequency response by the ESR could reduce the
phase margin, creating oscillations. The shaded area in the
Output Capacitor ESR graph indicates the recommended
ESR range. An input capacitor, of at least 1 F solid tantalum
or 10 F aluminum electrolytic, is also needed if the regulator
is situated more than 6 inches from the input power supply
filter.
MINIMUM LOAD
A minimum load current of 500 A 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 k
.
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
x R
2
)
where V
REF
= -1.21V. The output voltage can be pro-
grammed within the range of -3V to -24V, typically an even
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
Output Capacitor ESR Range
DS011260-5
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