LTC1998
10
1998fb
APPLICATIONS INFORMATION
The calculation of the resistor values R1, R2 and R3 is
quite straightforward and similar to the procedure outlined
in the previous paragraph.
Choose a value for the sum of R1 + R2 + R3 as well as the
values for low battery threshold and hysteresis.
Solve for resistor R1:
RRRR
V
V
TH
1123
5
1
2
=++
⎛
⎝
⎜
⎞
⎠
⎟
()–
Solve for the sum of
()( ) –
.
RR RR R
V
V
BATT Th
12 12 3
5
1+=++
⎛
⎝
⎜
⎞
⎠
⎟
then solve for R2 and R3.
Example: A system needs to detect a low battery
voltage of 3V (V
BATT.Th
= 3V) with 250mV hysteresis
(V
TH2
= 3.25V). Set the value of the resistor divider
(R1 + R2 + R3) = 1M.
R1 = 539k, R1 + R2 = 667k, R2 = 128k, R3 = 333k.
Choose the closest 1% values, that is 536k, 332k, 127k.
Figure 4 graphically shows the function of the LTC1998 as
described above.
Figure 4. LTC1998 Function Plot
VERSATILE OUTPUT DRIVER
V
LOGIC
, BATTLO (Pins 5,6)
The LTC1998 uses a CMOS push-pull output stage to drive
the low battery output signal. This output pin (BATTLO)
has a separate supply pin, (V
LOGIC
) that can be used to
provide an output voltage rail matching the V
DD
logic of
microprocessors. The V
LOGIC
pin may be tied to a volt-
age lower than the voltage at the BATT pin. The V
LOGIC
pin may also be tied to a voltage higher than V
BATT
via a
series resistor greater than 10k. The output will then act
as an open-drain device.
In a given application, if it is possible for BATTLO to be
shorted to GND or a supply, a series resistor should be
added to limit the short-circuit current to 5mA.
1998 F04
PROGRAMMABLE
THRESHOLD RANGE
HYSTERESIS
PROGRAMMED
HYSTERESIS
THRESHOLD
PROGRAMMED
LOW BATT
THRESHOLD
V
LOGIC
BATTLO
BATTERY VOLTAGE
RECOVERS UNDER
REDUCED LOAD
BATTERY
VOLTAGE
3.25V
2.50V
