LTC1998 APPLICATIONS INFORMATIONHysteresis Adjustment, Pin 4. It is imperative that the hysteresis threshold adjust voltage at Pin 4 be set to a higher voltage than the low The LTC1998 has an adjustable hysteresis ranging from battery threshold adjust voltage at Pin 3, at all times, to 10mV to 0.75V. A large hysteresis is useful in the event that avoid oscillation at the BATTLO output pin. The hysteresis a low battery signal at the LTC1998’s BATTLO pin causes threshold adjust pin may be set with a voltage source or the system to shed some battery load, thus inducing with a resistor divider, just as with the low battery threshold system confusion as the partially loaded battery recov- adjust pin. ers and changes the status of Pin 6 (BATTLO). The 2.5V to 3.25V programming window of low battery threshold Combined Control of Threshold and Hysteresis voltage includes the hysteresis. If, for instance, the low battery threshold voltage is set to 2.5V, 750mV hyster- If a resistor divider is desired, then both threshold adjust esis can be added on top of the 2.5V. If the low battery dividers can be combined in order to save current. This threshold voltage is set to 3.15V, only 100mV hysteresis simple technique also guarantees that the hysteresis can be applied. threshold adjust voltage at Pin 4 is higher than the voltage at the V The programming of the hysteresis threshold adjust pin TH.A pin, Figure 3. (VH.A) is similar to the programming of the voltage thresh- old adjust pin (VTH.A) already described in the previous R1 + 1 paragraph. Pin 4 effectively adjusts the threshold voltage 4 BATT VH.A at which the low battery pin (BATTLO) changes state from R2 LTC1998 low to high. This threshold (VTH2) is defi ned as: Li-Ion 3 VTH.A V ( ) V = . V H.A R3 TH2 2 5 + 2 1998 F03 The actual hysteresis voltage is: Figure 3. Combined Resistor Divider VHYST = VTH2 – VBATT.Th 1998fb 9
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