LP2989AIMX-1.8 datasheet(9/11 Pages) NSC

Part #	LP2989AIMX-1.8

Description	Micropower 500 mA Low Noise Low Dropout Regulator for Applications with Output Voltages < 2V Designed for Use with Very Low ESR Output Capacitors

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Manufacturer	NSC [National Semiconductor (TI)]

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Application Hints (Continued)

This could cause problems if a 10 µF Y5V capacitor were

used on the output since it will drop down to approximately

5 µF at high ambient temperatures (which could cause the

LP2989LV to oscillate).

Another significant problem with Z5U and Y5V dielectric

devices is that the capacitance drops severely with applied

voltage. A typical Z5U or Y5V capacitor can lose 60% of its

rated capacitance with half of the rated voltage applied to it.

For these reasons, X7R and X5R type ceramic capaci-

tors must be used on the input and output of the

LP2989LV.

TANTALUM: Tantalum output capacitors are not recom-

mended for use with the LP2989LV because:

Tantalum capacitors are less desirable than ceramics for use

as output capacitors because they are typically more expen-

sive when comparing equivalent capacitance and voltage

ratings in the 1 µF to 10 µF range.

Another important consideration is that Tantalum capacitors

have higher ESR values than equivalent size ceramics. This

means that while it may be possible to find a Tantalum

capacitor with an ESR value within the stable range, it would

have to be larger in capacitance (which means bigger and

more costly) than a ceramic capacitor with the same ESR

value.

Most 10 µF Tantalum capacitors have ESR values higher

than the 0.5

Ω maximum limit required to make the

LP2989LV stable.

It should also be noted that the ESR of a typical Tantalum will

increase about 2:1 as the temperature goes from 25˚C down

to −40˚C, so some guard band must be allowed.

FILM: Polycarbonate and polypropelene film capacitors

have excellent electrical performance: their ESR is the low-

est of the three types listed, their capacitance is very stable

with temperature, and DC leakage currrent is extremely low.

One disadvantage is that film capacitors are larger in physi-

cal size than ceramic or tantalum which makes film a poor

choice for either input or output capacitors.

However, their low leakage makes them a good choice for

the noise bypass capacitor. Since the required amount of

capacitance is only .01 µF, small surface-mount film capaci-

tors are avalable in this size.

SHUTDOWN INPUT OPERATION

The LP2989LV is shut off by driving the Shutdown input low,

and turned on by pulling it high. If this feature is not to be

used, the Shutdown input should be tied to V

IN to keep the

regulator output on at all times.

To assure proper operation, the signal source used to drive

the Shutdown input must be able to swing above and below

the specified turn-on/turn-off voltage thresholds listed in the

Electrical Characteristics section under V

ON/OFF.

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2989LV 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.

However, if the output is pulled above the input, this diode

will turn ON and current will flow into the regulator output.

In such cases, a parasitic SCR can latch which will allow a

high current to flow into V

IN (and out the ground pin), which

can damage the part.

In any application where the output may be pulled above the

input, an external Schottky diode must be connected from

V

IN to VOUT (cathode on VIN, anode on VOUT), to limit the

reverse voltage across the LP2989LV to 0.3V (see Absolute

Maximum Ratings).

www.national.com

9

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