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參數(shù)資料
| 型號： | LP2957IT |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 5V Low-Dropout Regulator for P Applications |
| 中文描述： | 5 V FIXED POSITIVE LDO REGULATOR, 0.8 V DROPOUT, PSFM5 |
| 封裝： | BENT STAGGERED, TO-220, 5 PIN |
| 文件頁數(shù)： | 11/14頁 |
| 文件大?。?/td> | 378K |
| 代理商： | LP2957IT |
Application Hints
(Continued)
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
OFF
will be set to 6.0V.
Selecting the optimum voltage for V
requires understand-
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
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:
Solving for R2:
www.national.com
11
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