LTC1998

9

1998fa

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

paragraph. Pin 4 effectively adjusts the threshold voltage

at which the low battery pin (BATTLO) changes state from

VV

V

TH

HA

2

25

2

=+

.

()

.

The actual hysteresis voltage is:

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

Figure 3. Combined Resistor Divider

1998 F03

LTC1998

R1

R2

R3

BATT

+

1

4

3