Datasheet

V
OUT
V
REF
R
1
R
2
V
IN
+
-

LP3982
SNVS185D –FEBRUARY 2002–REVISED APRIL 2013 www.ti.com
Figure 17. Regulator Topology Simplified
Similarity in the output capabilities exists between op amps and linear regulators. Just as rail-to-rail output op
amps allow their output voltage to approach the supply voltage, low dropout regulators (LDOs) allow their output
voltage to operate close to the input voltage. Both achieve this by the configuration of their output transistors.
Standard op amps and regulator outputs are at the source (or emitter) of the output transistor. Rail-to-rail op amp
and LDO regulator outputs are at the drain (or collector) of the output transistor. This replaces the threshold (or
diode drop) limitations on the output with the less restrictive source-to-drain (or V
SAT
) limitations. There is a trade-
off, of course. The output impedance become significantly higher, thus providing a critically lower pole when
combined with the capacitive load. That's why rail-to-rail op amps are usually poor at driving capacitive loads and
recommend a series output resistor when doing so. LDOs require the same series resistance except that the
internal resistance of the output capacitor will usually suffice. Refer to the Output Capacitance section for more
information.
Output Capacitance
The LP3982 is specifically designed to employ ceramic output capacitors as low as 2.2 μF. Ceramic capacitors
below 10μF offer significant cost and space savings, along with high frequency noise filtering. Higher values and
other types and of capacitor may be used, but their equivalent series resistance (ESR) should be maintained
below 0.5Ω
Ceramic capacitor of the value required by the LP3982 are available in the following dielectric types: Z5U, Y5V,
X5R and X7R. The Z5U and Y5V types exhibit a 50% or more drop in capacitance value as their temperature
increases from 25°C, an important consideration. The X5R generally maintain their capacitance value within
±20%. The X7R type are desirable for their tighter tolerance of 10% over temperature.
Ceramic capacitors pose a challenge because of their relatively low ESR. Like most other LDOs, the LP3982
relies on a zero in the frequency response to compensate against excessive phase shift in the regulator's
feedback loop. If the phase shift reaches 360° (i.e.; becomes positive), the regulator will oscillate. This
compensation usually resides in the zero generated by the combination of the output capacitor with its equivalent
series resistance (ESR). The zero is intended to cancel the effects of the pole generated by the load capacitance
(C
L
) combined with the parallel combination of the load resistance (R
L
) and the output resistance (R
O
) of the
regulator. The challenge posed by low ESR capacitors is that the zero it generates can be too high in frequency
for the pole that it's intended to compensate. The LP3982 overcomes this challenge by internally generating a
strategically placed zero.
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