LTC1998 APPLICATIONS INFORMATION The calculation of the resistor values R1, R2 and R3 is VERSATILE OUTPUT DRIVER quite straightforward and similar to the procedure outlined in the previous paragraph. VLOGIC, BATTLO (Pins 5,6) Choose a value for the sum of R1 + R2 + R3 as well as the The LTC1998 uses a CMOS push-pull output stage to drive values for low battery threshold and hysteresis. the low battery output signal. This output pin (BATTLO) has a separate supply pin, (V Solve for resistor R1: LOGIC) that can be used to provide an output voltage rail matching the VDD logic of ⎛ 5V ⎞ microprocessors. The VLOGIC pin may be tied to a volt- R1= R ( 1+R2+R3) – 1 ⎝⎜ V ⎠⎟ age lower than the voltage at the BATT pin. The VLOGIC TH2 pin may also be tied to a voltage higher than VBATT via a Solve for the sum of series resistor greater than 10k. The output will then act as an open-drain device. ⎛ 5V ⎞ (R1+R ) 2 = (R1+R2+R ) 3 – 1 In a given application, if it is possible for BATTLO to be ⎝⎜ VBAT .TTh ⎠⎟ shorted to GND or a supply, a series resistor should be added to limit the short-circuit current to 5mA. then solve for R2 and R3. Example: A system needs to detect a low battery 3.25V voltage of 3V (VBATT.Th = 3V) with 250mV hysteresis PROGRAMMED HYSTERESIS (V HYSTERESIS TH2 = 3.25V). Set the value of the resistor divider THRESHOLD PROGRAMMABLE (R1 + R2 + R3) = 1M. THRESHOLD RANGE PROGRAMMED BATTERY VOLTAGE LOW BATT R1 = 539k, R1 + R2 = 667k, R2 = 128k, R3 = 333k. RECOVERS UNDER THRESHOLD REDUCED LOAD Choose the closest 1% values, that is 536k, 332k, 127k. 2.50V BATTERY Figure 4 graphically shows the function of the LTC1998 as VOLTAGE described above. VLOGIC BATTLO 1998 F04 Figure 4. LTC1998 Function Plot 1998fb 10
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