LP3879 Datasheet, PDF (8/11 Pages) National Semiconductor (TI) – Micropower 800mA Low Noise Ceramic Stable Voltage Regulator for Low Voltage Applications

Block Diagram
Application Information
PACKAGE INFORMATION
The LP3879 is offered in the 8 lead PSOP or LLP surface
mount packages to allow for increased power dissipation
compared to the SO-8 and Mini SO-8.
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP3879 requires external
capacitors for regulator stability. These capacitors must be
correctly selected for good performance.
INPUT CAPACITOR: A capacitor whose value is at least 4.7
µF (±20%) is required between the LP3879 input and
ground. A good quality X5R / X7R ceramic capacitor should
be used.
Capacitor tolerance and temperature variation must be con-
sidered when selecting a capacitor (see Capacitor Charac-
teristics section) to assure the minimum requirement of
input capacitance is met over all operating conditions.
The input capacitor must be located not more than 0.5" from
the input pin and returned to a clean analog ground. Any
good quality ceramic or tantalum capacitor may be used,
assuming the minimum input capacitance requirement is
met.
OUTPUT CAPACITOR: The LP3879 requires a ceramic
output capacitor whose size is at least 10 µF (±20%). A good
quality X5R / X7R ceramic capacitor should be used. Ca-
pacitance tolerance and temperature characteristics must be
considered when selecting an output capacitor.
The LP3879 is designed specifically to work with ceramic
output capacitors, utilizing circuitry which allows the regula-
tor to be stable across the entire range of output current with
an ultra low ESR output capacitor.
The output capacitor selected must meet the requirement for
minimum amount of capacitance and also have an ESR
(equivalent series resistance) value which is within the
stable range. A curve is provided which shows the stable
ESR range as a function of load current (see Figure 1).
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FIGURE 1. Stable Region For Output Capacitor ESR
Important: The output capacitor must maintain its ESR
within the stable region over the full operating temperature
range of the application to assure stability.
The output capacitor ESR forms a zero which is required to
add phase lead near the loop gain crossover frequency,
typically in the range of 50kHz to 200 kHz. The ESR at lower
frequencies is of no importance. Some capacitor manufac-
turers list ESR at low frequencies only, and some give a
formula for Dissipation Factor which can be used to calculate
a value for a term referred to as ESR. However, since the DF
formula is usually at a much lower frequency than the range
listed above, it will give an unrealistically high value. If good
quality X5R or X7R ceramic capacitors are used, the actual
ESR in the 50 kHz to 200 kHz range will not exceed 25 milli
Ohms. If these are used as output capacitors for the LP3879,
the regulator stability requirements are satisfied.
It is important to remember that capacitor tolerance and
variation with temperature must be taken into consideration
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