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LTC1998

2.5µA, 1% Accurate SOT-23 Comparator and Voltage Reference for Battery Monitoring

Linear Technology 

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 (VTH.A). This is a high

input impedance pin that senses an externally applied

voltage and programs the low battery threshold voltage

(VBATT.Th). The VTH.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:

VBATT.Th

=

2.5V

+

(VTH.A

2

)

For instance, if the applied voltage at pin 3, VTH.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 (VTH.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

VTH.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.

+

R1

1

BATT

3 VTH.A

R2

LTC1998

2

1998 F02

Figure 2. Resistor Divider Sets Threshold

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 VBATT.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

R1= (R1+R2)

⎛

⎝⎜

5V

VBATT.Th

⎞

– 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, VBATT.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 VBATT.Th as it will be slightly different from 2.85V,

due to the use of standard 1% resistor values.

VBATT.Th

=

5V

R1+ R2

R1+ (R1+ R2)

=

2.849V

The above low battery threshold of 2.849V is guaranteed to

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

the VTH.A voltage applied to Pin 3.

For sake of completeness, the voltage at Pin 3 (VTH.A) can

be easily calculated by VTH.A = VBATT.Th (R2/(R1 + R2) =

0.6972V (when VBATTERY = VBATT.Th).

1998fa

8

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