LTC1998
8
1998fb
APPLICATIONS INFORMATION
LOW BATTERY THRESHOLD VOLTAGE AND
HYSTERESIS ADJUST
Low Battery Threshold Voltage Adjustment, Pin 3
The low battery threshold voltage is the battery voltage
which will trip the (BATTLO) pin high to low. It should be
adjusted via the threshold adjust pin (V
TH.A
). This is a high
input impedance pin that senses an externally applied
voltage and programs the low battery threshold voltage
(V
BATT.Th
). The V
TH.A
pin is designed to accommodate
voltages from 0V to 1.5V with respect to ground. This
allows the low battery threshold voltage to be set to any
voltage between 2.5V and 3.25V, that is:
VV
V
BATT Th
TH A
.
.
.
()
=+25
2
For instance, if the applied voltage at pin 3, V
TH.A
, is 1V
the LTC1998 will indicate a low battery condition when the
battery voltage pin (BATT) falls below 3V.
The voltage at the threshold adjust pin (V
TH.A
) can be set
with any voltage source. This pin allows a continuous time
adjustment, that is, the low battery threshold voltage may
be changed at any time. The high input impedance of the
V
TH.A
pin allows the use of a high valued resistive divider
(to minimize current drain) from the battery to set the
battery low threshold voltage, Figure 2.
The simple calculations of resistor values R1 and R2 are
illustrated below. Set a value for R1 + R2. This value will
affect the max amount of current drawn from the battery
when fully charged. For instance if R1 + R2 = 1M the
resistive divider will draw 4.1μA when the battery voltage
is 4.1V. Set the desired value of V
BATT.Th
(this value should
be between 2.5V and 3.25V) that is the value of the battery
voltage that will trip the internal circuitry of the LTC1998
and change the state of the battery low pin (BATTLO).
Solve for R R R
V
V
BATT Th
112
5
1=+
⎛
⎝
⎜
⎞
⎠
⎟
() –
.
Example: A Lithium-Ion battery is monitored and a bat-
tery low signal should be issued when it discharges to
2.85V, that is, V
BATT.Th
= 2.85V; if (R1 + R2) = 1M, then
R1 = 754.38k and R2 = 245.62k. Choose the closest 1%
value of R1 = 750k and R2 = 243k. Calculate the practical
value for V
BATT.Th
as it will be slightly different from 2.85V,
due to the use of standard 1% resistor values.
VV
RR
RRR
V
BATT Th.
()
.=
+
++
=5
12
112
2 849
The above low battery threshold of 2.849V is guaranteed to
within 1% even though 1% resistors are used to program
the V
TH.A
voltage applied to Pin 3.
For sake of completeness, the voltage at Pin 3 (V
TH.A
) can
be easily calculated by V
TH.A
= V
BATT.Th
(R2/(R1 + R2) =
0.6972V (when V
BATTERY
= V
BATT.Th
).
Figure 2. Resistor Divider Sets Threshold
1998 F02
LTC1998
R1
R2
V
TH.A
BATT
+
Li-Ion
1
2
3
