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

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 m

Ω 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

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

terminated by tying them to V

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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