LP2985IM5-2.0 datasheet(12/17 Pages) NSC

12 / 17 page

Application Hints (Continued)

One disadvantage of ceramic capacitors is that their capaci-

tance can vary with temperature. Most large value ceramic

capacitors (

≥ 2.2 µF) are manufactured 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

(which

could

cause

the

LP2985LV to oscillate). If Z5U or Y5V capacitors are used on

the output, a minimum capacitance value of 4.7 µF must be

observed.

A better choice for temperature coefficient in ceramic capaci-

tors is X7R, which holds the capacitance within ±15%. Un-

fortunately, the larger values of capacitance are not offered

by all manufacturers in the X7R dielectric.

Tantalum

Tantalum capacitors are less desirable than ceramics for use

as output capacitors because they are more expensive when

comparing equivalent capacitance and voltage ratings in the

1 µF to 4.7 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.

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.

On/off Input Operation

The LP2985LV is shut off by driving the ON/OFF input low,

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

used, the ON/OFF 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 ON/OFF 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. To prevent

mis-operation, the turn-on (and turn-off) voltage signals ap-

plied to the ON/OFF input must have a slew rate which is

≥ 40 mV/µs.

Caution: the regulator output voltage can not be guaranteed

if a slow-moving AC (or DC) signal is applied that is in the

range between the specified turn-on and turn-off voltages

listed under the electrical specification V

ON/OFF (see Electri-

cal Characteristics).

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2985LV 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

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

reverse voltage across the LP2985LV to 0.3V (see Absolute

Maximum Ratings).

MICRO SMD MOUNTING

The micro SMD package requires specific mounting tech-

niques which are detailed in National Semiconductor Appli-

cation Note # 1112. Referring to the section Surface Mount

Technology (SMT) Assembly Considerations, it should be

noted that the pad style which must be used with the 5-pin

package is the NSMD (non-solder mask defined) type.

For best results during assembly, alignment ordinals on the

PC board may be used to facilitate placement of the micro

SMD device.

MICRO SMD LIGHT SENSITIVITY

Exposing the micro SMD device to direct sunlight will cause

misoperation of the device. Light sources such as Halogen

lamps can also affect electrical performance if brought near

to the device.

The wavelenghts which have the most detrimental effect are

reds and infra-reds, which means that the fluorescent light-

ing used inside most buildings has very little effect on per-

formance. A micro SMD test board was brought to within 1

cm of a fluorescent desk lamp and the effect on the regu-

lated output voltage was negligible, showing a deviation of

less than 0.1% from nominal.

LP2985IM5-2.0 Datasheet(PDF) 12 Page - National Semiconductor (TI)

LP2985IM5-2.0 Datasheet(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL