lm2991qml National Semiconductor Corporation, lm2991qml Datasheet - Page 8

• Manufacturer Part Number: lm2991qml
• Description: Lm2991qml Negative Low Dropout Adjustable Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Typical Performance Characteristics
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 required if the regulator is located more
than 6" 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 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
mΩ and 10Ω.
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
electrolytic will increase by as much as 50X as the tempera-
ture 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
temperatures occur.
The lower stable ESR limit of 25 mΩ means that ceramic
capacitors 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 recom-
mended). Dielectric types X7R or X5R must be used if the
temperature range of the application varies more than
25˚C from ambient to assure the amount of capacitance is
sufficient.
Adjust Pin Current
20158725
±
8
(Continued)
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 (pre-
viously) provided the necessary "decoupling" 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
operation. 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
resistor divider using the following equation:
where V
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,
REF
V
OUT
= −1.21V. The output voltage can be pro-
Output Capacitor ESR Range
= V
Low Voltage Behavior
REF
x (1 + R
2
/R
1
) − (I
ADJ
20158726
x R
20158705
2
)