LM2991QML datasheet(8/13 Pages) NSC

8 / 13 page

Typical Performance Characteristics (Continued)

Adjust Pin Current

Low Voltage Behavior

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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

Ω 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 m

Ω 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 (1

Ω 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.

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.

Output Capacitor ESR Range

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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 k

Ω.

SETTING THE OUTPUT VOLTAGE

The output voltage of the LM2991 is set externally by a

resistor divider using the following equation:

OUT =VREF x(1+R2/R1)−(IADJ xR2)

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,

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