Sep 2001 Adjustable Low-Battery Threshold Detector Extends Battery Runtime

DESIGN FEATURES
Adjustable Low-Battery Threshold
Detector Extends Battery Runtime
by Brendan Whelan
The LTC1998 is the first LithiumIon low-battery detector that
guarantees 1% accuracy even when
using inexpensive 1% programming
resistors. This device facilitates more
reliable battery capacity management;
it extends battery runtime and
improves prediction of remaining battery charge. The LTC1998 achieves
this performance by combining an
accurate internal reference and a proprietary comparison circuit. Other
features include:
❏ Low supply current, 2.5µA
typical
❏ Adjustable low-battery threshold,
2.5V to 3.25V
❏ Adjustable hysteresis, 10mV to
750mV
❏ Rail-to-rail push-pull output
eliminates pullup resistor
❏ Dedicated output supply pin
ensures compatibility with
microprocessors
❏ Small ThinSOT™ 6-lead SOT-23
Package
The LTC1998, in the SOT-23 package, compares the voltage of a single
Lithium-Ion cell to an internal reference voltage. When the battery voltage
falls below a predetermined low-battery threshold, the output pin
(BATTLO) changes state to indicate a
low-battery condition. The low-bat-
cate a false recharged battery condition. The hysteresis is programmable
via the hysteresis-adjust pin.
tery threshold voltage is adjustable
from 2.5V to 3.25V via the threshold
adjust pin. The threshold voltage is
guaranteed to be within 1% of the
programmed voltage as long as the
threshold-adjust pin voltage is programmed with a resistor divider
composed of 1% or better resistors.
The BATTLO output of the LTC1998
can be used by a microprocessor or
microcontroller in a battery-powered
system to ensure that the battery has
sufficient remaining capacity to allow
proper operation or ensure that stored
settings are not lost. The high accuracy of the LTC1998 improves the
ability of the system to predict the
remaining battery capacity. The
LTC1998 can provide a longer runtime than a system using a less
accurate threshold detector since it
requires a smaller guardband for
detector error. The LTC1998 can also
be used to protect a battery from
damage from overdischarge. In addition, the 2.5µA typical supply current
is less than the effective self-discharge
current of most Li-Ion cells.
In many applications, the system
reduces the battery load current during a low-battery condition, thus
allowing the battery voltage to recover
over time. The large adjustable hysteresis of the LTC1998 allows it to
ignore this voltage change so that the
output does not change state to indi-
Programmable Thresholds
An accurate internal reference and
internal divider set the low-battery
threshold level at which the BATTLO
signal changes from high to low. The
threshold can be adjusted via the
threshold-adjust pin (VTH.A), as in Figure 1. A proprietary threshold
adjustment circuit maintains a highly
accurate threshold voltage even when
using 1% external resistors to set the
threshold voltage. This gives the
LTC1998 the accuracy of a trimmed
fixed-threshold device as well as the
flexibility of an adjustable threshold.
Hysteresis is programmable via the
hysteresis-adjust pin VH.A). This pin
works in exactly the same way as the
low-battery threshold adjust pin, except that it controls the threshold at
which the BATTLO signal changes
state from low to high, indicating a
charged battery condition. Both the
low-battery threshold voltage and the
hysteresis-threshold voltage may be
programmed to be between 2.5V and
3.25V; the hysteresis is the difference
between these two thresholds. The
hysteresis may be as large as 750mV.
This large hysteresis prevents false
state changes at the output due to
transients or battery recovery. The
2.5V–4.2V
+
+
LTC1998
R1
845k
4
R2
147k
2
3
VH.A
VTH.A
GND
BATT
VLOGIC
BATTLO
1
5
6
THRESHOLD = 2.7V
Figure 1. Threshold adjustment
with resistor divider
Linear Technology Magazine • September 2001
R1
787k
1%
R2
66.5k
1%
R3
147k
1%
0.1µF
VDD
LTC1998
4
3
2
VH.A
VTH.A
GND
BATT
VLOGIC
BATTLO
1
5
6
VTHRESHOLD = 2.7V
HYSTERESIS = 100mV
µP
BATTERY
OK
GND
Figure 2. Single resistor divider sets both thresholds
21
DESIGN FEATURES
2.5V–4.2V
+
2.5V–4.2V
R1
787k
1%
4
3
R4
147k
1%
2
VH.A
VTH.A
GND
BATT
VLOGIC
BATTLO
1
6
BATTERY
OK
R3
147k
1%
GND
SW1 OPEN:
VTHRESHOLD = 2.7V
HYSTERESIS = 100mV
SW1
R2
66.5k
1%
µP
5
R1
787k
1%
+
VDD
LTC1998
R2
66.5k
1%
R3
147k
1%
1.2V–3.25V
REGULATOR*
0.1µF
0.1µF
3
2
VH.A
VTH.A
GND
BATT
VLOGIC
BATTLO
1
5
6
*LT1762 FOR 150mA
LT1962 FOR 300mA
µP
BATTERY
OK
GND
VTHRESHOLD = 2.7V
HYSTERESIS = 100mV
SW1 CLOSED:
VTHRESHOLD = 2.6V
HYSTERESIS = 100mV
Figure 4. VLOGIC pin ensures compatibility with processor logic level
Figure 3. Dual low-battery threshold circuit
system can be designed with multiple
low-battery threshold levels. In such
an application, the load current can
be reduced when a low-battery signal
is produced. For example, the application can change the low-battery
threshold to a lower voltage with S1
and continue to run in a low power
mode, keeping system parameters
stored safely in memory. When the
battery voltage finally drops below
the new low-battery threshold, the
system can be shut down completely
in order to protect the battery from
damage due to overdischarging.
system may then be designed to automatically reduce the battery load
current when a battery-low signal is
received. If the battery voltage recovers
under this reduced load, the output
of the LTC1998 will maintain a low
output state. Both thresholds may be
programmed using just three external resistors, as shown in Figure 2,
minimizing component count. The
threshold-adjust and hysteresisadjust pins exhibit very high input
impedance, which allows the use of
large external program resistors to
reduce the battery drain current.
The threshold-adjust function is
continuous; threshold voltages can
be adjusted at any time and changes
take effect instantaneously, without
the use of a clock or reset. This allows
simple switched resistors to change
thresholds. Figure 3 illustrates how a
Versatile Output Stage
The LTC1998’s BATTLO output is a
rail-to-rail push-pull CMOS output
driver with a dedicated driver supply
pin, VLOGIC. A break-before-make
circuit prevents power supply cross-
conduction currents during switching to reduce battery load transients.
The VLOGIC pin may be wired to BATT
or driven by a lower supply voltage to
allow compatibility with low voltage
microprocessors (Figure 4). The
BATTLO pin will drive a high output
signal to the voltage set by the VLOGIC
pin. The VLOGIC pin can be any voltage
between 1V and VBATT. This feature
helps to protect a low voltage microprocessor or microcontroller from
overvoltage stress at its batterythreshold detect input pin. The VLOGIC
pin can also be driven from a separate
supply so that output drive current is
not taken from the battery, further
increasing battery life. The BATTLO
output can drive a voltage that is a
higher potential than the BATT pin,
provided that a resistor is connected
from the VLOGIC pin to the required
supply, as in Figure 5.
5V
2.5V–4.2V
+
VDD
LTC1998
4
Conclusion
R1
787k
1%
0.1µF
R2
66.5k
1%
R3
147k
1%
4
LTC1998
3
2
VH.A
VTH.A
GND
BATT
VLOGIC
BATTLO
1
5
6
R4
1M
VDD
µP
BATTERY
OK
GND
VTHRESHOLD = 2.7V
HYSTERESIS = 100mV
Figure 5. Simple pull-up resistor increases compatible logic range
The LTC1998 low-battery threshold
detector provides a small (SOT-23-6),
accurate, low power solution to
capacity management problems in
Lithium-Ion battery applications. It
can improve useful battery life and is
flexible enough to work in many different applications. Low cost 1%
resistors can be used to configure
thresholds while maintaining 1%
threshold accuracy. Its ultralow power
consumption minimizes battery drain
current.
For more information on parts featured in this issue, see
http://www.linear-tech.com/go/ltmag
22
Linear Technology Magazine • September 2001