LP3855ES-2.5 National Semiconductor, LP3855ES-2.5 Datasheet - Page 12

• Manufacturer Part Number: LP3855ES-2.5
• Description: IC,VOLT REGULATOR,FIXED,+2.5V,CMOS,SIP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LP3855ES-2.5

• Rohs Compliant: NO

www.national.com
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: 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).
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.
FIGURE 1. ESR Curve for C
Input Capacitor)
OUT
(with 10µF Tantalum
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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).
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
FIGURE 2. Maximum Ceramic Output Capacitance vs
Ceramic Input Capacitance
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