LP3852 Datasheet, PDF (12/19 Pages) National Semiconductor (TI) – 1.5A Fast Response Ultra Low Dropout Linear Regulators

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.
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 10µF 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 amount of output capacitance that can be
used for stable operation is 10µF. For general usage across
all load currents and operating conditions, the part was
characterized using a 10µF Tantalum input capacitor. The
minimum and maximum stable ESR range for the output
capacitor was then measured which kept the device stable,
assuming any output capacitor whose value is greater than
10µF (see Figure 1 below).
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FIGURE 1. ESR Curve for C_OUT (with 10µF Tantalum
Input Capacitor)
It should be noted that it is possible to operate the part with
an output capacitor whose ESR is below these limits, as-
suming that sufficient ceramic input capacitance is provided.
This will allow stable operation using ceramic output capaci-
tors (see next section).
OPERATION WITH CERAMIC OUTPUT CAPACITORS
LP385X voltage regulators can operate with ceramic output
capacitors if the values of the input and output capacitors are
selected appropriately. The total ceramic output capacitance
must be equal to or less than a specified maximum value in
order for the regulator to remain stable over all operating
conditions. This maximum amount of ceramic output capaci-
tance is dependent upon the amount of ceramic input ca-
pacitance used as well as the load current of the application.
This relationship is shown in Figure 2, which graphs the
maximum stable value of ceramic output capacitance as a
function of ceramic input capacitance for load currents of
1.5A.
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FIGURE 2. Maximum Ceramic Output Capacitance vs
Ceramic Input Capacitance
If the maximum load current is 1.5A and a 10µF ceramic
input capacitor is used, the regulator will be stable with
ceramic output capacitor values from 10µF up to about
150µF. When calculating the total ceramic output capaci-
tance present in an application, it is necessary to include any
ceramic bypass capacitors connected to the regulator out-
put.
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).
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