LP2957_15 Datasheet, PDF (12/24 Pages) Texas Instruments – 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
SNVS102C – JUNE 1998 – REVISED APRIL 2013
SNAP-ON/SNAP-OFF OPERATION
The LP2957 output can be wired for snap-on/snap-off operation using three external resistors:
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*Minimum value (increase as required for smooth turn-on characteristic).
Figure 32. Snap-On/Snap-Off Output
When connected as shown, the shutdown input holds the regulator off until the input voltage rises up to the turn-
on threshold (V _ON), at which point the output “snaps on”.
When the input power is shut off (and the input voltage starts to decay) the output voltage will snap off when the
input voltage reaches the turn-off threshold, V_OFF.
Figure 33. Snap-On/Snap-Off Input and Output Voltage Diagram
It is important to note that the voltage V_OFF must always be lower than V_ON (the difference in these voltage levels
is called the hysteresis).
Hysteresis is required when using snap-on/snap-off output, with the minimum amount of hysteresis required for
a specific application being dependent on the source impedance of whatever is supplying V_IN.
Caution: A type of low-frequency oscillation can occur if V_ON and V_OFFare too close together (insufficient
hysteresis). When the output snaps on, the regulator 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_IN to drop by an amount equal to the source impedance multiplied by the current.
If V _IN drops below V_OFF, the regulator will turn off and stop drawing current from the source. This will allow V_IN to
rise back up above V_ON, and the cycle will start over. The regulator will 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
measured by loading the source with a resistive load.
Figure 34. Transformer Powered Input Supply
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