LP2986IM-3.3 National Semiconductor, LP2986IM-3.3 Datasheet - Page 13

• Manufacturer Part Number: LP2986IM-3.3
• Description: IC, LDO, 3.3V, 200MA, 8SOIC
• Manufacturer: National Semiconductor

Specifications of LP2986IM-3.3

• Primary Input Voltage: 16V
• Output Voltage: 3.3V
• Dropout Voltage Vdo: 180mV
• No. Of Pins: 8
• Output Current: 200mA
• Voltage Regulator Case Style: SOIC
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Output Voltage Fixed: 3.3V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Hints
IMPORTANT: The output capacitor must maintain its ESR in
the stable region over the full operating temperature range of
the application to assure stability.
The minimum required amount of output capacitance is
4.7 µF. Output capacitor size can be increased without limit.
It is important to remember that capacitor tolerance and
variation with temperature must be taken into consideration
when selecting an output capacitor so that the minimum
required amount of output capacitance is provided over the
full operating temperature range. A good Tantalum capacitor
will show very little variation with temperature, but a ceramic
may not be as good (see next section).
CAPACITOR CHARACTERISTICS
TANTALUM: The best choice for size, cost, and perfor-
mance are solid tantalum capacitors. Available from many
sources, their typical ESR is very close to the ideal value
required on the output of many LDO regulators.
Tantalums also have good temperature stability: a 4.7 µF
was tested and showed only a 10% decline in capacitance
as the temperature was decreased from +125˚C to −40˚C.
The ESR increased only about 2:1 over the same range of
temperature.
However, it should be noted that the increasing ESR at lower
temperatures present in all tantalums can cause oscillations
when marginal quality capacitors are used (where the ESR
of the capacitor is near the upper limit of the stability range at
room temperature).
CERAMIC: For a given amount of a capacitance, ceramics
are usually larger and more costly than tantalums.
Be warned that the ESR of a ceramic capacitor can be low
enough to cause instability: a 2.2 µF ceramic was measured
and found to have an ESR of about 15 mΩ.
If a ceramic capacitor is to be used on the LP2986 output, a
1Ω resistor should be placed in series with the capacitor to
provide a minimum ESR for the regulator.
Another disadvantage of ceramic capacitors is that their
capacitance varies a lot with temperature:
Large ceramic capacitors are typically manufactured with the
Z5U temperature characteristic, which results in the capaci-
tance dropping by a 50% as the temperature goes from 25˚C
to 80˚C.
This means you have to buy a capacitor with twice the
minimum C
OUT
ESR Curves For 2.5V Output
to assure stable operation up to 80˚C.
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13
ALUMINUM: The large physical size of aluminum electrolyt-
ics makes them unattractive for use with the LP2986. Their
ESR characteristics are also not well suited to the require-
ments of LDO regulators.
The ESR of an aluminum electrolytic is higher than a tanta-
lum, and it also varies greatly with temperature.
A typical aluminum electrolytic can exhibit an ESR increase
of 50X when going from 20˚C to −40˚C. Also, some alumi-
num electrolytics can not be used below −25˚C because the
electrolyte will freeze.
USING AN EXTERNAL RESISTIVE DIVIDER
The LP2986 output voltage can be programmed using an
external resistive divider (see Basic Application Circuits).
The resistor connected between the Feedback pin and
ground should be 51.1k. The value for the other resistor (R1)
connected between the Feedback pin and the regulated
output is found using the formula:
It should be noted that the 25 µA of current flowing through
the external divider is approximately equal to the current
saved by not connecting the internal divider, which means
the quiescent current is not increased by using external
resistors.
A lead compensation capacitor (C
place a zero in the loop response at about 50 kHz. The value
for C
A good quality capacitor must be used for C
the value is accurate and does not change significantly over
temperature. Mica or ceramic capacitors can be used, as-
suming a tolerance of
If a ceramic is used, select one with a temperature coeffi-
cient of NPO, COG, Y5P, or X7R. Capacitor types Z5U, Y5V,
and Z4V can not be used because their value varies more
that 50% over the −25˚C to +85˚C temperature range.
SHUTDOWN INPUT OPERATION
The LP2986 is shut off by driving the Shutdown input low,
and turned on by pulling it high. If this feature is not to be
used, the Shutdown input should be tied to V
regulator output on at all times.
To assure proper operation, the signal source used to drive
the Shutdown input must be able to swing above and below
the specified turn-on/turn-off voltage thresholds listed as V
and V
It is also important that the turn-on (and turn-off) voltage
signals applied to the Shutdown input have a slew rate which
is not less than 40 mV/µs.
CAUTION: the regulator output state can not be guaranteed
if a slow-moving AC (or DC) signal is applied that is in the
range between V
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2986 has an inherent diode connected between the regu-
lator output and input.
During normal operation (where the input voltage is higher
than the output) this diode is reverse-biased.
However, if the output is pulled above the input, this diode
will turn ON and current will flow into the regulator output.
F
L
, respectively (see Electrical Characteristics).
can be found using:
V
OUT
H
C
and V
F
= 1.23 x (1 + R1/51.1k)
= 1/(2π x R1 x 50k)
±
20% or better is selected.
L
.
F
) must also be used to
F
to ensure that
IN
to keep the
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H