LP2957_15 Datasheet, PDF (15/24 Pages) Texas Instruments – 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
www.ti.com
SNVS102C – JUNE 1998 – REVISED APRIL 2013
DESIGN EXAMPLE #2:
A 5V regulated output is to be powered from a battery made up of six NiCad cells. The cell data is:
cell voltage (full charged): 1.4V
cell voltage (90% discharged): 1.0V
The internal impedance of a typical battery is low enough that source loading during regulator turn-on is not
usually a problem.
In a battery-powered application, the turn-off voltage V_OFF should be selected so that the regulator is shut down
when the batteries are about 90% discharged (over discharge can damage rechargeable batteries).
In this case, the battery voltage will be 6.0V at the 90% discharge point (since there are six cells at 1.0V each).
That means for this application, V_OFF will be set to 6.0V.
Selecting the optimum voltage for V_ON requires understanding battery behavior. If a Ni-Cad battery is nearly
discharged (cell voltage 1.0V) and the load is removed, the cell voltage will drift back up. The voltage where the
regulator turns on must be set high enough to keep the regulator from re-starting during this time, or an on-off
pulsing mode can occur.
If the regulator restarts when the discharged cell voltage drifts up, the load on the battery will cause the cell
voltage to fall below the turn-off level, which causes the regulator to shut down. The cell voltage will again float
up and the on-off cycling will continue.
For NiCad batteries, a good cell voltage to use to calculate V_ON is about 1.2V per cell. In this application, this will
yield a value for V_ON of 7.2V.
We can now find R1, R2 and R3 assuming:
V_OFF = 6.0V V _ON = 7.2V R3 = 49.9k
Solving for R1:
(10)
Solving for R2:
(11)
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