LTC1998

8

1998fa

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

input impedance pin that senses an externally applied

voltage and programs the low battery threshold voltage

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

the LTC1998 will indicate a low battery condition when the

battery voltage pin (BATT) falls below 3V.

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

(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

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

11

2

5

1

=+

⎛

⎝⎜

⎞

⎠⎟

()

–

.

Example: A Lithium-Ion battery is monitored and a bat-

tery low signal should be issued when it discharges to

R1 = 754.38k and R2 = 245.62k. Choose the closest 1%

value of R1 = 750k and R2 = 243k. Calculate the practical

due to the use of standard 1% resistor values.

VV

RR

RR

R

V

BATT Th

.

()

.

=

+

++

=

5

12

11

2

2 849

The above low battery threshold of 2.849V is guaranteed to

within 1% even though 1% resistors are used to program

Figure 2. Resistor Divider Sets Threshold

1998 F02

LTC1998

R1

R2

BATT

+

1

2

3