LP2981-N_15_15 Datasheet, PDF (16/29 Pages) Texas Instruments – LP2981-N Micropower 100-mA Ultra-Low Dropout Regulator in SOT-23 Package

LP2981-N
SNOS773M – MARCH 2000 – REVISED SEPTEMBER 2015
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8.2.2.1.1 Input Capacitor
An input capacitor with a value ≥ 1 μF is required with the LP2981-N (amount of capacitance can be increased
without limit).
This capacitor must be located a distance of not more than 0.5 inches from the input pin of the LP2981-N and
returned to a clean analog ground. Any good quality ceramic or tantalum can be used for this capacitor.
8.2.2.1.2 Output Capacitor
The output capacitor must meet both the requirement for minimum amount of capacitance and equivalent series
resistance (ESR) value. Curves are provided which show the allowable ESR range as a function of load current
for various output voltages and capacitor values (refer to Figure 36, Figure 37, Figure 38, and Figure 39).
NOTE
Important: The output capacitor must maintain its ESR in the stable region over the full
operating temperature range to ensure stability. Also, capacitor tolerance and variation
with temperature must be considered to ensure the minimum amount of capacitance is
provided at all times.
This capacitor should be located not more than 0.5 inches from the OUT pin of the LP2981-N and returned to a
clean analog ground.
8.2.2.2 Capacitor Characteristics
8.2.2.2.1 Tantalum
Tantalum capacitors are the best choice for use with the LP2981-N. Most good quality tantalums can be used
with the LP2981-N, but check the manufacturer's data sheet to be sure the ESR is in range.
It is important to remember that ESR increases at lower temperatures and a capacitor that is near the upper limit
for stability at room temperature can cause instability when it gets cold.
In applications which must operate at very low temperatures, it may be necessary to parallel the output tantalum
capacitor with a ceramic capacitor to prevent the ESR from going up too high (see below for important
information on ceramic capacitors).
8.2.2.2.2 Ceramic
Ceramic capacitors are not recommended for use at the output of the LP2981-N. This is because the ESR of a
ceramic can be low enough to go below the minimum stable value for the LP2981-N. A 2.2-μF ceramic was
measured and found to have an ESR of about 15 mΩ, which is low enough to cause oscillations.
If a ceramic capacitor is used on the output, a 1-Ω resistor should be placed in series with the capacitor.
8.2.2.2.3 Aluminum
Because of large physical size, aluminum electrolytics are not typically used with the LP2981-N. They must meet
the same ESR requirements over the operating temperature range, more difficult because of their steep increase
at cold temperature.
An aluminum electrolytic can exhibit an ESR increase of as much as 50x when going from 20°C to −40°C. Also,
some aluminum electrolytics are not operational below −25°C because the electrolyte can freeze.
8.2.2.3 Reverse Current Path
The internal PNP power transistor used as the pass element in the LP2981-N has an inherent diode connected
between the regulator output and input. During normal operation (where the input voltage is higher than the
output) this diode is reverse biased (See Figure 34).
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