LP2957ISXSLASHNOPB datasheet(12/24 Pages) TI

12 / 24 page

LP2957, LP2957A

SNVS102C – JUNE 1998 – REVISED APRIL 2013

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SNAP-ON/SNAP-OFF OPERATION

The LP2957 output can be wired for snap-on/snap-off operation using three external resistors:

*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, VOFF.

Figure 33. Snap-On/Snap-Off Input and Output Voltage Diagram

It is important to note that the voltage VOFF must always be lower than VON (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 VIN.

Caution: A type of low-frequency oscillation can occur if VON and VOFFare 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 VIN to drop by an amount equal to the source impedance multiplied by the current.

If V IN drops below VOFF, the regulator will turn off and stop drawing current from the source. This will allow VIN to

rise back up above VON, 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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LP2957ISX/NOPB 数据表(PDF) 12 Page - Texas Instruments