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
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 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.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Input Voltage
ESD Susceptibility (Note 2)
Power Dissipation (Note 3)
Junction Temperature (T
Jmax
)
−26V to +0.3V
2 kV
Internally limited
125˚C
Storage Temperature Range
Lead Temperature (Soldering, 10 sec.)
−65˚C to +150˚C
230˚C
Operating Ratings
(Note 1)
Junction Temperature Range (T
J
)
Maximum Input Voltage (Operational)
−40˚C to +125˚C
−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
Reference Voltage
Conditions
5 mA
≤
I
O
≤
1A
5 mA
≤
I
O
≤
1A,
V
O
− 1V
≥
V
IN
≥
−26V
Output Voltage
Range
Line Regulation
Load Regulation
Dropout Voltage
V
IN
= −26V
I
O
= 5 mA, V
O
− 1V
≥
V
IN
≥
−26V
50 mA
≤
I
O
≤
1A
I
O
= 0.1A,
∆V
O
≤
100 mV
I
O
= 1A,
∆V
O
≤
100 mV
Quiescent Current
Dropout Quiescent
Current
Ripple Rejection
Output Noise
ON /OFF Input
Voltage
ON /OFF Input
Current
Output Leakage
Current
Current Limit
V
ripple
= 1 Vrms,
f
I
O
= 5 mA
10 Hz − 100 kHz, I
O
= 5 mA
(V
OUT
: ON)
(V
OUT
: OFF)
V
ON/OFF
= 0.8V (V
OUT
: ON)
V
ON/OFF
= 2.4V (V
OUT
: OFF)
V
IN
= −26V, V
ON/OFF
= 2.4V
V
OUT
= 0V
V
OUT
= 0V
2
1.5
A
200
1.2
1.3
0.1
40
60
2.4
10
100
250
µA
µA
450
0.8
µV
V
ripple
Typical
(Note 4)
−1.210
Min
−1.234
−1.27
Max
−1.186
−1.15
−3
Units
V
V
V
V
%/V
%
V
V
mA
mA
dB
−2
−25
0.004
0.04
0.1
0.6
0.7
16
= 1 kHz,
60
50
−24
0.04
0.4
0.2
0.3
0.8
1
5
50
I
O
≤
1A
V
IN
= V
O
, I
O
≤
1A
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 kΩ resistor.
Note 3:
The maximum power dissipation is a function of T
Jmax
,
θ
JA
and T
A
. The maximum allowable power dissipation at any ambient temperature is P
D
= (T
Jmax
− T
A
)/θ
JA
. 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,
θ
JA
is 50˚C/W; with 1 square inch of copper area,
θ
JA
is 37˚C/W; and with 1.6 or more square inches of copper area,
θ
JA
is 32˚C/W.
Note 4:
Typicals are at T
J
= 25˚C and represent the most likely parametric norm.
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