LP2957AIS National Semiconductor, LP2957AIS Datasheet - Page 9

• Manufacturer Part Number: LP2957AIS
• Description: 5V Low-Dropout Regulator for P Applications
• Manufacturer: National Semiconductor

Application Hints
*
external supply. To avoid this invalid response, pull up to regulator output.
*
If a single pull-up resistor is connected to the regulator out-
put, the error flag may briefly rise up to about 1.3V as the in-
put voltage ramps up or down through the 0V to 1.3V region.
In some cases, this 1.3V signal may be mis-interpreted as a
false high by a µP which is still “alive” with 1.3V applied to it.
To prevent this, the user may elect to use two resistors
which are equal in value on the error output (one connected
to ground and the other connected to the regulator output).
If this two-resistor divider is used, the error output will only be
pulled up to about 0.6V (not 1.3V) during power-up or
power-down, so it can not be interpreted as a high signal.
When the regulator output is in regulation (4.8V to 5V), the
error output voltage will be 2.4V to 2.5V, which is clearly a
high signal.
OUTPUT ISOLATION
The regulator output can be connected to an active voltage
source (such as a battery) with the regulator input turned off,
as long as the regulator ground pin is connected to
ground . If the ground pin is left floating, damage to the
regulator can occur if the output is pulled up by an external
voltage source.
SNAP-ON/SNAP-OFF OPERATION
The LP2957 output can be wired for snap-on/snap-off opera-
tion using three external resistors:
*Minimum value (increase as required for smooth turn-on characteristic).
When connected as shown, the shutdown input holds the
regulator off until the input voltage rises up to the turn-on
threshold (V
When the input power is shut off (and the input voltage starts
to decay) the output voltage will snap off when the input volt-
age reaches the turn-off threshold, V
*
In shutdown mode, ERROR will go high if it has been pulled up to an
Exact value depends on dropout voltage, which varies with load current.
FIGURE 3. Snap-On/Snap-Off Output
FIGURE 2. ERROR Output Timing
ON
), at which point the output “snaps on”.
(Continued)
OFF
.
DS011340-14
DS011340-8
9
It is important to note that the voltage V
lower than V
called the hysteresis).
Hysteresis is required when using snap-on/snap-off output,
with the minimum amount of hysteresis required for a spe-
cific application being dependent on the source impedance
of whatever is supplying V
Caution: A type of low-frequency oscillation can occur if
This high peak current causes V
equal to the source impedance multiplied by the current. If V
IN
ing current from the source. This will allow V
above V
stay in this oscillating mode and never come into regulation.
HYSTERESIS IN TRANSFORMER-POWERED
APPLICATIONS:
If the unregulated DC input voltage to the regulator comes
from a transformer, the required hysteresis is easily mea-
sured by loading the source with a resistive load.
If the regulator is powered from a battery, the source imped-
ance will probably be low enough that other considerations
will determine the optimum values for hysteresis (see Design
Example # 2).
For best results, the load resistance used to test the trans-
former should be selected to draw about 600 mA for the
maximum load current test, since this is the maximum peak
current the LP2957 could be expected to draw from the
source.
drops below V
FIGURE 5. Transformer Powered Input Supply
ON
V
hysteresis ). When the output snaps on, the regu-
lator must draw sufficient current to power the
load and charge up the output capacitor (in most
cases, the regulator will briefly draw the maximum
current allowed by its internal limiter).
For this reason, it is best to assume the LP2957
may pull a peak current of about 600 mA from the
source (which is the listed maximum short-circuit
load current of 530 mA plus the ground pin current
of 70 mA ).
FIGURE 4. Snap-On/Snap-Off Input
, and the cycle will start over. The regulator will
ON
ON
and Output Voltage Diagram
and V
OFF
(the difference in these voltage levels is
, the regulator will turn off and stop draw-
OFF
are too close together (insufficient
IN
.
IN
to drop by an amount
OFF
IN
DS011340-9
must always be
DS011340-10
to rise back up
www.national.com