LP2987 datasheet(20/24 Pages) NSC

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Application Hints (Continued)

DETAILS OF ERR/RESET CIRCUIT OPERATION: (Refer

to LP2987/8 Equivalent Circuit).

10001710

LP2987/8 Equivalent Circuit

The output of comparator U2 is the ERR/RESET flag. Since

it is an open-collector output, it requires the use of a pull-up

resistor (R

P). The 1.23V reference is tied to the inverting

input of U2, which means that its output is controlled by the

voltage applied to the non-inverting input.

The output of U1 (also an open-collector) will force the

non-inverting input of U2 to go low whenever the LP2987/8

regulated output drops about 5% below nominal.

U1’s inverting input is also held at 1.23V. The other input

samples the regulated output through a resistive divider (R

and R

B). When the regulated output is at nominal voltage,

the voltage at the divider tap point will be 1.23V. If this

voltage drops about 60 mV below 1.23V, the output of U1 will

go low forcing the output of U2 low (which is the ERROR

state).

Power-ON reset delay occurs when a capacitor (shown as

DLY) is connected to the Delay pin. At turn-ON, this capaci-

tor is initially fully discharged (which means the voltage at

the Delay pin is 0V). The output of U1 keeps C

DLY fully

discharged (by sinking the 2.2 µA from the current source)

until the regulator output voltage comes up to within about

5% of nominal. At this point, U1’s output stops sinking cur-

rent and the 2.2 µA starts charging up C

DLY.

When the voltage across C

DLY reaches 1.23V, the output of

U2 will go high (note that D1 limits the maximum voltage to

about 2V).

SELECTING C

DLY: The maximum recommended value for

this capacitor is 1 µF. The capacitor must not have exces-

sively high leakage current, since it is being charged from a

2.2 µA current source.

Aluminum electrolytics can not be used, but good-quality

tantalum, ceremic, mica, or film types will work.

SHUTDOWN INPUT OPERATION

The LP2987/8 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 as V

and V

L, respectively (see Electrical Characteristics).

It is also important that the turn-on (and turn-off) voltage

signals applied to the Shutdown input have a slew rate which

is not less than 40 mV/µs.

CAUTION: the regulator output state can not be guaranteed

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

range between V

H and VL.

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2987/8 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 LP2987/8 to 0.3V (see Absolute

Maximum Ratings).

BYPASS CAPACITOR (LP2988)

The capacitor connected to the Bypass pin must have very

low leakage. The current flowing out of the Bypass pin

comes from the Bandgap reference, which is used to set the

output voltage. Since the Bandgap circuit has only a few

microamps flowing in it, loading effects due to leakage cur-

rent will cause a change in the regulated output voltage.

Curves are provided which show the effect of loading the

Bypass pin on the regulated output voltage.

Care must be taken to ensure that the capacitor selected for

bypass will not have significant leakage current over the

operating temperature range of the application.

A high quality ceramic capacitor which uses either NPO or

COG type dielectiric material will typically have very low

leakage. Small surface-mount polypropolene or polycarbon-

ate film capacitors also have extremely low leakage, but are

slightly larger in size than ceramics.

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