LP3962EP Datasheet, PDF (10/20 Pages) National Semiconductor (TI) – 1.5A Fast Ultra Low Dropout Linear Regulators

Applications Information
V_IN RESTRICTIONS FOR PROPER START-UP
Because the LP396XEP devices use on-chip CMOS logic for
analog trimming of the output voltage, care must be taken
not to apply an input voltage which can allow this logic to
shift into random undefined logic states, as this can ad-
versely affect the regulated output voltage. This will most
likely occur if an input voltage between about 50mV and
200mV is applied to V_IN for a significant amount of time
(more than several seconds). To prevent misoperation, en-
sure that V_IN is below 50mV before start-up is initiated. This
problem can occur in systems 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 pre-
vent the voltage at V_IN from rising above the sensitive
threshold. 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 probabil-
ity of a "false start" caused by incorrect logic states is ex-
tremely low .
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 LP3962EP/5EP requires a low
source impedance to maintain regulator stability because the
internal bias circuitry is connected directly to V_IN. The input
capacitor must be located less than 1 cm from the
LP3962EP/5EP 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 mΩ).
To determine the minimum input capacitance amount and
ESR value, an approximation which should be used is:
C_IN ESR (mΩ) / 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
LP3962EP/5EP 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 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:
0.2Ω ≤ C_OUT ESR ≤ 5Ω
The lower limit of 200 mΩ means that ceramic capacitors are
not suitable for use as LP3962EP/5EP 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
capacitor, approximately 3 µF of ceramic capacitance can be
applied 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 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
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.
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