LP3875EMP-18 Datasheet, PDF (8/25 Pages) Texas Instruments – LP3872/LP3875 1.5A Fast Ultra Low Dropout Linear Regulators

LP3872, LP3875
SNVS227F – FEBRUARY 2003 – REVISED APRIL 2013
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EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are required to assure stability. These capacitors must be
correctly selected for proper performance.
INPUT CAPACITOR: An input capacitor of at least 10µF is required. Ceramic, Tantalum, or Electrolytic
capacitors may be used, and capacitance 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 output capacitance that can be used for stable full-load operation is 10µF, but it may be
increased without limit. The output capacitor must have an ESR value as shown in the stable region of the curve
below. Tantalum capacitors are recommended for the output capacitor.
10
1.0
COUT > 10 PF
STABLE REGION
0.1
.01
.001
0
1
2
LOAD CURRENT (A)
Figure 18. ESR Curve
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and variation 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 temperature range. In general, a good Tantalum capacitor will show very little capacitance variation
with temperature, but a ceramic 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 Tantalum and aluminum electrolytic capacitors.
Both show increasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so
severe they may not be feasible for some applications (see Capacitor Characteristics 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 nominal capacitance at high and low limits of
the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically
maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of
course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
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