Datasheet

1
R
3
=
+
-
1
1
R
1
G
T
R
2
1
1
R
1
V
LT
V
REF
(G
T
)
R
2
-
=
V
REF
(G
T
)
V
UT1
R
1
=
1
11
R
3
R
2
R
1
G
T
=
++
V
UT
= R
1
(V
REF
) G
T
V
LT
= R
1
// R
2
(V
REF
) G
T
+
-
V
B
LM385A-1.2V
LMC7215
R1
R3
R2
R4
V
REF
2.74M
180k
768k
301k
2.2PF
V
B
4 Cells
NiMH
SHDN
GND
IN
OUT
LP3982
FAULT
100k
2.2PF
CERAMIC
LP3982
SNVS185D –FEBRUARY 2002–REVISED APRIL 2013 www.ti.com
Figure 21. Minimum Battery Detector that Disconnects the Load Via the SHDN Pin of the LP3982
Resistor value for V
UT
and V
LT
are determined as follows:
(8)
(The application of Figure 21 used a G
T
of 5μ mho)
(9)
(10)
(11)
The above procedure assumes a rail-to-rail output comparator. Essentially, R
2
is in parallel with R
1
prior to
reaching the lower threshold, then R
2
becomes parallel with R
3
for the upper threshold. Note that the application
requires rail-to-rail input as well.
The resistor values shown in Figure 21 are the closest practical to calculated values.
Fast Start-Up
The LP3982 provides fast start-up time for better system efficiency. The start-up speed is maintained when using
the optional noise bypass capacitor. An internal 500 μA current source charges the capacitor until it reaches
about 90% of its final value.
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