LP2957AIS National Semiconductor, LP2957AIS Datasheet - Page 11

• Manufacturer Part Number: LP2957AIS
• Description: 5V Low-Dropout Regulator for P Applications
• Manufacturer: National Semiconductor

Application Hints
Solving for R2:
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
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% dis-
charge point (since there are six cells at 1.0V each). That
means for this application, V
Selecting the optimum voltage for V
ing battery behavior. If a Ni-Cad battery is nearly discharged
OFF
(Continued)
will be set to 6.0V.
ON
requires understand-
OFF
11
(cell voltage 1.0V) and the load is removed , the cell volt-
age 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 oc-
cur.
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
value for V
We can now find R1, R2 and R3 assuming:
V
Solving for R1:
Solving for R2:
ON
OFF
is about 1.2V per cell. In this application, this will yield a
= 6.0V V
ON
of 7.2V.
ON
= 7.2V R3 = 49.9k
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