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Capacitor Characteristics

Ceramics

Tantalum

ON/OFF Operation

Reverse Input-Output Voltage

LP2985

150-mA LOW-NOISE LOW-DROPOUT REGULATOR

WITH SHUTDOWN

SLVS522J – JULY 2004 – REVISED AUGUST 2006

APPLICATION INFORMATION (continued)

The LP2985 allows for low-noise performance with the use of a bypass capacitor that is connected to the

internal bandgap reference via the BYPASS pin. This high-impedance bandgap circuitry is biased in the

microampere range and, thus, cannot be loaded significantly, otherwise, its output – and, correspondingly, the

minimized as much as possible and never should exceed 100 nA.

Ceramic capacitors are ideal choices for use on the output of the LP2985 for several reasons. For capacitances

in the range of 2.2

µF to 4.7 µF, ceramic capacitors have the lowest cost and the lowest ESR, making them

choice candidates for filtering high-frequency noise. For instance, a typical 2.2-

µF ceramic capacitor has an ESR

in the range of 10 m

Ω to 20 mΩ and, thus, satisfies minimum ESR requirements of the regulator.

Ceramic capacitors have one major disadvantage that must be taken into account – a poor temperature

coefficient, where the capacitance can vary significantly with temperature. For instance, a large-value ceramic

capacitor (

≥2.2 µF) can lose more than half of its capacitance as the temperature rises from 25°C to 85°C. Thus,

a 2.2-

µF capacitor at 25°C drops well below the minimum C

rises. For this reason, select an output capacitor that maintains the minimum 2.2

µF required for stability over

the entire operating temperature range. Note that there are some ceramic capacitors that can maintain a

±15%

capacitance tolerance over temperature.

Tantalum capacitors can be used at the output of the LP2985, but there are significant disadvantages that could

prohibit their use:

•

In the 1-

µF to 4.7-µF range, tantalum capacitors are more expensive than ceramics of the equivalent

capacitance and voltage ratings.

•

Tantalum capacitors have higher ESRs than their equivalent-sized ceramic counterparts. Thus, to meet the

ESR requirements, a higher-capacitance tantalum may be required, at the expense of larger size and higher

cost.

•

The ESR of a tantalum capacitor increases as temperature drops, as much as double from 25

°C to –40°C.

Thus, ESR margins must be maintained over the temperature range to prevent regulator instability.

The LP2985 allows for a shutdown mode via the ON/OFF pin. Driving the pin LOW (

≤0.3 V) turns the device

OFF; conversely, a HIGH (

≥1.6 V) turns the device ON. If the shutdown feature is not used, ON/OFF should be

connected to the input to ensure that the regulator is on at all times. For proper operation, do not leave ON/OFF

unconnected, and apply a signal with a slew rate of

≥40 mV/µs.

There is an inherent diode present across the PNP pass element of the LP2985.

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