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:

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

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

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 ).

This high peak current causes V

equal to the source impedance multiplied by the current. If V

IN

drops below V

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.

DS011340-14

external supply. To avoid this invalid response, pull up to regulator output.

FIGURE 2. ERROR Output Timing

DS011340-8

*Minimum value (increase as required for smooth turn-on characteristic).

FIGURE 3. Snap-On/Snap-Off Output

DS011340-9

FIGURE 4. Snap-On/Snap-Off Input

and Output Voltage Diagram

DS011340-10

FIGURE 5. Transformer Powered Input Supply

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