LP2957ISXSLASHNOPB datasheet(15/24 Pages) TI

15 / 24 page

LP2957, LP2957A

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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 VOFF 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, VOFF will be set to 6.0V.

Selecting the optimum voltage for VON 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 VON is about 1.2V per cell. In this application, this will

yield a value for VON of 7.2V.

We can now find R1, R2 and R3 assuming:

VOFF = 6.0V V ON = 7.2V R3 = 49.9k

Solving for R1:

(10)

Solving for R2:

(11)

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LP2957ISX/NOPB датащи(PDF) 15 Page - Texas Instruments