LP3876-ADJ Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – 3A Fast Ultra Low Dropout Linear Regulator

Application Hints
V_IN RESTRICTIONS FOR PROPER START-UP
To prevent misoperation, ensure that V_IN is below 50mV
before start-up is initiated. This scenario can occur in sys-
tems with a backup battery using reverse-biased "blocking"
diodes which may allow enough leakage current to flow into
the V_IN node to raise it’s voltage slightly above ground when
the main power is removed. Using low leakage diodes or a
resistive pull down can prevent the voltage at V_IN from rising
above 50mV. Large bulk capacitors connected to V_IN may
also cause a start-up problem if they do not discharge fully
before re-start is initiated (but only if V_IN is allowed to fall
below 1V). A resistor connected across the capacitor will
allow it to discharge more quickly. It should be noted that the
probability of a "false start" caused by incorrect logic states
is extremely low.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the resistors R1 and R2 (see
Typical Application Circuit). The output is also dependent on
the reference voltage (typically 1.216V) which is measured
at the ADJ pin. The output voltage is given by the equation:
V_OUT = V_ADJ x ( 1 + R1 / R2)
This equation does not include errors due to the bias current
flowing in the ADJ pin which is typically about 10 nA. This
error term is negligible for most applications. If R1 is >
100kΩ , a small error may be introduced by the ADJ bias
current.
The tolerance of the external resistors used contributes a
significant error to the output voltage accuracy, with 1%
resistors typically adding a total error of approximately 1.4%
to the output voltage (this error is in addition to the tolerance
of the reference voltage at V_ADJ).
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: An input capacitor of at least 1µF is
required. Ceramic or Tantalum may be used, and capaci-
tance may be increased without limit
OUTPUT CAPACITOR: An output capacitor is required for
loop stability. It must be located less than 1 cm from the
device and connected directly to the output and ground pins
using traces which have no other currents flowing through
them (see PCB Layout section).
The minimum value of the output capacitance that can be
used for stable full-load operation is 10 µF, but it may be
increased without limit. The output capacitor must have an
ESR value as shown in the stable region of the curve (be-
low).
ESR Curve
20074470
C_FF (Feed Forward Capacitor)
The capacitor C_FF is required to add phase lead and help
improve loop compensation. The correct amount of capaci-
tance depends on the value selected for R1 (see Typical
Application Circuit). The capacitor should be selected such
that the zero frequency as given by the equation shown
below is approximately 45 kHz:
Fz = 45,000 = 1 / ( 2 x π x R1 x C_FF )
A good quality ceramic with X5R or X7R dielectric should be
used for this capacitor.
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 mΩ). 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
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