LTC1998_1 Datasheet, PDF (9/12 Pages) Linear Technology – 2.5μA, 1% Accurate SOT-23 Comparator and Voltage Reference for Battery Monitoring

LTC1998
APPLICATIONS INFORMATION
Hysteresis Adjustment, Pin 4.
The LTC1998 has an adjustable hysteresis ranging from
10mV to 0.75V. A large hysteresis is useful in the event that
a low battery signal at the LTC1998’s BATTLO pin causes
the system to shed some battery load, thus inducing
system confusion as the partially loaded battery recov-
ers and changes the status of Pin 6 (BATTLO). The 2.5V
to 3.25V programming window of low battery threshold
voltage includes the hysteresis. If, for instance, the low
battery threshold voltage is set to 2.5V, 750mV hyster-
esis can be added on top of the 2.5V. If the low battery
threshold voltage is set to 3.15V, only 100mV hysteresis
can be applied.
The programming of the hysteresis threshold adjust pin
(V_H.A) is similar to the programming of the voltage thresh-
old adjust pin (V_TH.A) already described in the previous
paragraph. Pin 4 effectively adjusts the threshold voltage
at which the low battery pin (BATTLO) changes state from
low to high. This threshold (V_TH2) is defined as:
V_TH2
=
2.5V
+
(V_H.A )
2
The actual hysteresis voltage is:
V_HYST = V_TH2 – V_BATT.Th
It is imperative that the hysteresis threshold adjust
voltage at Pin 4 be set to a higher voltage than the low
battery threshold adjust voltage at Pin 3, at all times, to
avoid oscillation at the BATTLO output pin. The hysteresis
threshold adjust pin may be set with a voltage source or
with a resistor divider, just as with the low battery threshold
adjust pin.
Combined Control of Threshold and Hysteresis
If a resistor divider is desired, then both threshold adjust
dividers can be combined in order to save current. This
simple technique also guarantees that the hysteresis
threshold adjust voltage at Pin 4 is higher than the voltage
at the V_TH.A pin, Figure 3.
+
R1
1
4
BATT
V_H.A
R2
LTC1998
3 _VTH.A
R3
1998 F03
Figure 3. Combined Resistor Divider
1998fa
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