LP2987 Datasheet, PDF (20/24 Pages) National Semiconductor (TI) – Micropower, 200 mA Ultra Low-Dropout Voltage Regulator with Programmable Power-On Reset Delay; Low Noise Version Available (LP2988)

Application Hints (Continued)
DETAILS OF ERR/RESET CIRCUIT OPERATION: (Refer
to LP2987/8 Equivalent Circuit).
LP2987/8 Equivalent Circuit
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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_A
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
C_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_H
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 V_L.
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
V_IN to V_OUT (cathode on V_IN, anode on V_OUT), 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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