LP3871_15 Datasheet, PDF (12/26 Pages) Texas Instruments – LP387x 0.8-A Fast Ultra-Low-Dropout Linear Regulators

LP3871, LP3874
SNVS225H – FEBRUARY 2003 – REVISED JUNE 2015
Typical Applications (continued)
8.2.1 Design Requirements
For LP3871 and LP3874 typical applications, use the parameters listed in Table 1.
DESIGN PARAMETER
Input voltage
Output voltage
Output current
Output capacitor
Input capacitor
Output capacitor ESR range
Table 1. Design Parameters
EXAMPLE VALUE
2.5 V to 7 V
2.5 V
0.8 A
10 µF
10 µF
100 mΩ to 4 Ω
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8.2.2 Detailed Design Procedure
8.2.2.1 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 10 μF 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 Layout Guidelines).
The minimum value of 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 equivalent series resistance (ESR) value as shown in
the stable region of the curve (Figure 13). Tantalum capacitors are recommended for the output capacitor.
10
1.0
COUT > 10PF
STABLE REGION
0.1
.01
.001
0
0.2 0.4 0.6 0.8 1
LOAD CURRENT (A)
Figure 13. ESR Curve
8.2.2.2 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).
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