Datasheet LTC1998 (Analog Devices) - 8

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