LP2967 Datasheet, PDF (14/17 Pages) National Semiconductor (TI) – Dual Micropower 150 mA Low-Dropout Regulator in micro SMD Package

Application Hints (Continued)
LP2967-2.5V Region Of Stability with 2.2 µF C_OUT
10114265
No-Load Operation
If a 2.2 µF output capacitor is used, the minimum stable ESR
value rises to about 0.5 Ω at load currents below 1 mA. If
the minimum output load is < 1 mA (with C_OUT = 2.2 µF), a
Tantalum output capacitor should be used (the ESR of a
ceramic will be too low). It should be noted that if a 4.7 µF (or
larger) output capacitor is used, the part is fully stable with
either Tantalum or ceramic from no load to full load output
current.
Bypass Capacitor
Connecting a 10 nF capacitor to the Bypass pin significantly
reduces noise on the regulator output. It should be noted that
the capacitor is connected directly to a high impedance
circuit in the bandgap reference. Because this circuit has
only a few microamperes flowing into it, any significant load-
ing on this node will cause a change in the regulated output
voltage. For this reason, DC leakage current through the
noise bypass capacitor must never exceed 100 nA, and
should be kept as low as possible for best output voltage
accuracy. The types of capacitors best suited for the noise
bypass capacitor are ceramic and film capacitors. High qual-
ity ceramic capacitors with either NPO or COG dielectric
typically have very low leakage. 10 nF polypropylene and
polycarbonate film capacitors are available in small surface
mount packages and typically have extremely low leakage
current.
CAPACITOR CHARACTERISTICS
Ceramic
Ceramic capacitors have the lowest ESR values, which
make them best for eliminating high frequency noise. The
outputs of LP2967 require a minimum of 2.2 µF of capaci-
tance. The ESR of a typical 2.2 µF ceramic capacitor is in the
range of 4 mΩ to 20 mΩ, which easily meets the ESR limits
required for stability by the LP2967. One disadvantage of
ceramic capacitors is that their capacitance can vary with
temperature. Most large value ceramic capacitors are manu-
factured with the Z5U or Y5V temperature characteristic,
which results in the capacitance dropping by more than 50%
as the temperature goes from 25˚C to 85˚C. This could
cause problems if a 2.2 µF capacitor were used on the
output since it will drop down to approximately 1 µF at high
ambient temperatures. This could cause the LP2967 to os-
cillate. If Z5U or Y5V capacitors are used on the output, a
minimum capacitance value of 4.7 µF must be used.
A better choice for temperature coefficient in ceramic capaci-
tors is X7R or X5R which hold the capacitance to within
±15% over the full temperature range. Unfortunately, the
larger values of capacitance are not offered by all manufac-
turers in the X7R dielectric.
Tantalum
For the LP2967, tantalum capacitors are less desirable than
ceramic for use as output capacitors because they are typi-
cally more expensive when comparing equivalent capaci-
tance and voltage ratings in the 2.2 µF to 4.7 µF range of
capacitance. Tantalum capacitors have good temperature
stability: a 4.7µF was tested and showed a 10% decline in
capacitance as the temperature was decreased from +125˚C
to −40˚C while the ESR increased by about 2:1 over the
same range of temperatures. This increase in ESR at lower
temperatures can cause oscillations when marginal quality
capacitors are used and the upper limit for ESR value is
exceeded.
Aluminum
The large physical size of aluminum electrolytic capacitors
make them unattractive for use with the LP2967. Their ESR
characteristics are also not well suited to the requirements of
LDO regulators. The ESR of an aluminum electrolytic is
higher than that of a tantalum, and it also varies greatly with
temperature. A typical aluminum electrolytic can exhibit an
ESR increase of 50X when going from 20˚C to −40˚C. Also,
some aluminum electrolytic capacitors can not be used be-
low −25˚C because the electrolyte will freeze.
SHUTDOWN OPERATION
The two LDO regulators in the LP2967 have independent
shutdown pins. A low logic level signal at either of the shut-
down pins SD1 or SD2 will turn off the corresponding regu-
lator output V_OUT1 or V_OUT2. Pins SD1 and SD2 must be
terminated by tying them to V_IN for a proper operation when
the shutdown function is not required.
REVERSE CURRENT PATH
The internal power transistor in the LP2967 has an inherent
parasitic diode. During normal operation, the input voltage is
higher than the output voltage and the parasitic diode is
reverse biased. However, if the output is pulled above the
input in an application, then current flows from the output to
the input if the parasitic diode gets forward biased. The
output can be pulled above the input as long as the current
in the parasitic diode is limited to 150mA.
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