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LTC1998CS6-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC1998CS6-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 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 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: VV V BATT Th TH A . . . () =+ 25 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. 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 R R R V VBATT Th 11 2 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, 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. 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 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). Figure 2. Resistor Divider Sets Threshold 1998 F02 LTC1998 R1 R2 VTH.A BATT + 1 2 3 |
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