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

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% 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

(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

V

R3 = 49.9k

Solving for R1:

Solving for R2:

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