LTC1998CS6#PBF Linear Technology, LTC1998CS6#PBF Datasheet - Page 8

• Manufacturer Part Number: LTC1998CS6#PBF
• Manufacturer: Linear Technology

Specifications of LTC1998CS6#PBF

• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 6
• Lead Free Status / RoHS Status: Compliant

LTC1998
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
input impedance pin that senses an externally applied
voltage and programs the low battery threshold voltage
(V
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:
For instance, if the applied voltage at pin 3, V
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
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
(to minimize current drain) from the battery to set the
battery low threshold voltage, Figure 2.
8
TH.A
BATT.Th
V
BATT Th
pin allows the use of a high valued resistive divider
). The V
.
Figure 2. Resistor Divider Sets Threshold
=
2 5
.
TH.A
V
+
+
(
pin is designed to accommodate
V
TH A
2
R1
R2
.
)
3
V
LTC1998
TH.A
BATT
2
1
TH.A
1998 F02
TH.A
). This is a high
) can be set
TH.A
, is 1V
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
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).
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
R1 = 754.38k and R2 = 245.62k. Choose the closest 1%
value of R1 = 750k and R2 = 243k. Calculate the practical
value for V
due to the use of standard 1% resistor values.
The above low battery threshold of 2.849V is guaranteed to
within 1% even though 1% resistors are used to program
the V
For sake of completeness, the voltage at Pin 3 (V
be easily calculated by V
0.6972V (when V
Solve for R
V
BATT Th
TH.A
.
voltage applied to Pin 3.
BATT.Th
=
1
5
V
=
BATT.Th
as it will be slightly different from 2.85V,
BATTERY
(
R
R
1
1
+
R
+
1
(
R
= 2.85V; if (R1 + R2) = 1M, then
R
+
1
2
TH.A
R
= V
)
+
2
⎛
⎝ ⎜
R
BATT.Th
V
2
= V
BATT Th
)
5
=
BATT.Th
BATT.Th
V
2 849
.
).
.
–
(this value should
1
(R2/(R1 + R2) =
V
⎞
⎠ ⎟
TH.A
) can
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