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LP3966ESX-5.0 Datasheet(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part # LP3966ESX-5.0
Description  3A Fast Ultra Low Dropout Linear Regulators
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LP3966ESX-5.0 Datasheet(HTML) 11 Page - National Semiconductor (TI)

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Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are re-
quired to assure stability. these capacitors must be correctly
selected for proper performance.
INPUT CAPACITOR: The LP3963/6 requires a low source
impedance to maintain regulator stability because the inter-
nal bias circuitry is connected directly to V
IN. The input
capacitor must be located less than 1 cm from the LP3963/6
device and connected directly to the input and ground pins
using traces which have no other currents flowing through
them (see PCB Layout section).
The minimum allowable input capacitance for a given appli-
cation depends on the type of the capacitor and ESR
(equivalent series resistance). A lower ESR capacitor allows
the use of less capacitance, while higher ESR types (like
aluminum electrolytics) require more capacitance.
The lowest value of input capacitance that can be used for
stable full-load operation is 68 µF (assuming it is a ceramic
or low-ESR Tantalum with ESR less than 100 m
Ω).
To determine the minimum input capacitance amount and
ESR value, an approximation which should be used is:
C
IN ESR (m
Ω)/C
IN (µF)
≤ 1.5
This shows that input capacitors with higher ESR values can
be used if sufficient total capacitance is provided. Capacitor
types (aluminum, ceramic, and tantalum) can be mixed in
parallel, but the total equivalent input capacitance/ESR must
be defined as above to assure stable operation.
IMPORTANT: The input capacitor must maintain its ESR and
capacitance in the "stable range" over the entire temperature
range of the application to assure stability (see Capacitor
Characteristics Section).
OUTPUT CAPACITOR: An output capacitor is also required
for loop stability. It must be located less than 1 cm from the
LP3963/6 device and connected directly to the output and
ground pins using traces which have no other currents flow-
ing through them (see PCB Layout section).
The minimum value of the output capacitance that can be
used for stable full-load operation is 33 µF, but it may be
increased without limit. The output capacitor’s ESR is critical
because it forms a zero to provide phase lead which is
required for loop stability. The ESR must fall within the
specified range:
0.2
Ω≤ C
OUT ESR
≤ 5Ω
The lower limit of 200 m
Ω means that ceramic capacitors are
not suitable for use as LP3963/6 output capacitors (but can
be used on the input). Some ceramic capacitance can be
used on the output if the total equivalent ESR is in the stable
range: when using a 100 µF Tantalum as the output capaci-
tor, approximately 3 µF of ceramic capacitance can be ap-
plied before stability becomes marginal.
IMPORTANT: The output capacitor must meet the require-
ments for minimum amount of capacitance and also have an
appropriate ESR value over the full temperature range of the
application to assure stability (see Capacitor Characteristics
Section).
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and varia-
tion with temperature must be taken into consideration when
selecting a capacitor so that the minimum required amount
of capacitance is provided over the full operating tempera-
ture range. In general, a good Tantalum capacitor will show
very little capacitance variation with temperature, but a ce-
ramic may not be as good (depending on dielectric type).
Aluminum electrolytics also typically have large temperature
variation of capacitance value.
Equally important to consider is a capacitor’s ESR change
with temperature: this is not an issue with ceramics, as their
ESR is extremely low. However, it is very important in Tan-
talum and aluminum electrolytic capacitors. Both show in-
creasing ESR at colder temperatures, but the increase in
aluminum electrolytic capacitors is so severe they may not
be feasible for some applications (see Capacitor Character-
istics Section).
CAPACITOR CHARACTERISTICS
CERAMIC: For values of capacitance in the 10 to 100 µF
range, ceramics are usually larger and more costly than
tantalums but give superior AC performance for bypassing
high frequency noise because of very low ESR (typically less
than 10 m
Ω). However, some dielectric types do not have
good capacitance characteristics as a function of voltage
and temperature.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. A typical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with half of
the rated voltage applied to it. The Z5U and Y5V also exhibit
a severe temperature effect, losing more than 50% of nomi-
nal capacitance at high and low limits of the temperature
range.
X7R and X5R dielectric ceramic capacitors are strongly rec-
ommended if ceramics are used, as they typically maintain a
capacitance range within ±20% of nominal over full operat-
ing ratings of temperature and voltage. Of course, they are
typically larger and more costly than Z5U/Y5U types for a
given voltage and capacitance.
TANTALUM: Solid Tantalum capacitors are recommended
for use on the output because their typical ESR is very close
to the ideal value required for loop compensation. They also
work well as input capacitors if selected to meet the ESR
requirements previously listed.
Tantalums also have good temperature stability: a good
quality Tantalum will typically show a capacitance value that
varies less than 10-15% across the full temperature range of
125˚C to −40˚C. ESR will vary only about 2X going from the
high to low temperature limits.
The increasing ESR at lower temperatures can cause oscil-
lations when marginal quality capacitors are used (if the ESR
of the capacitor is near the upper limit of the stability range at
room temperature).
ALUMINUM: This capacitor type offers the most capaci-
tance for the money. The disadvantages are that they are
larger in physical size, not widely available in surface mount,
and have poor AC performance (especially at higher fre-
quencies) due to higher ESR and ESL.
Compared by size, the ESR of an aluminum electrolytic is
higher than either Tantalum or ceramic, and it also varies
greatly with temperature. A typical aluminum electrolytic can
exhibit an ESR increase of as much as 50X when going from
25˚C down to −40˚C.
It should also be noted that many aluminum electrolytics only
specify impedance at a frequency of 120 Hz, which indicates
they have poor high frequency performance. Only aluminum
electrolytics that have an impedance specified at a higher
frequency (between 20 kHz and 100 kHz) should be used for
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11


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