LP2957_15 Datasheet, PDF (13/24 Pages) Texas Instruments – 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
www.ti.com
SNVS102C – JUNE 1998 – REVISED APRIL 2013
If the regulator is powered from a battery, the source impedance 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 transformer 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.
The difference in input voltage measured at no load and full load defines the amount of hysteresis
required for proper snap-on/snap-off operation (the programmed hysteresis must be greater than the
difference in voltages).
CALCULATING RESISTOR VALUES:
The values of R1, R2 and R3 can be calculated assuming the designer knows the hysteresis.
In most transformer-powered applications, it can be assumed that V_OFF (the input voltage at turn-off) should be
set for about 5.5V, since this allows about 500 mV across the LP2957 to keep the output in regulation until it
snaps off. V_ON (the input voltage at turn on) is found by adding the hysteresis voltage to V_OFF.
R1, R2 and R3 are found by solving the node equations for the currents entering the node nearest the shutdown
pin (written at the turn-on and turn-off thresholds).
The shutdown pin bias current (10 nA typical) is not included in the calculations:
Figure 35. Turn-ON Transition - Equivalent Circuit
Figure 36. Turn-OFF Transition - Equivalent
Circuits
(4)
Since these two equations contain three unknowns (R1, R2 and R3) one resistor value must be assumed and
then the remaining two values can be obtained by solving the equations.
The node equations will be simplified by solving both equations for R2, and then equating the two to generate an
expression in terms of R1 and R3.
(5)
Setting these equal to each other and solving for R1 yields:
(6)
The same equation solved for R3 is:
(7)
A value for R1 or R3 can be derived using either one of the above equations, if the designer assumes a value for
one of the resistors.
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