Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
1999-05-21
2001-10-16
Tso, Edward H. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
Reexamination Certificate
active
06304088
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a voltage monitor circuit.
2. Background of the Invention
A voltage monitor circuit is included in an electrical device such as a cellular telephone or a personal digital assistant (PDA) for monitoring the voltage level of a battery used to operate the device. The voltage monitor circuit ensures that the electrical device is operative only when a desired minimum level of battery voltage is present. When the battery voltage falls below the desired minimum voltage level, the voltage monitor circuit signals the electrical device to shut down. The voltage monitor circuit inhibits further operation of the electrical device until the battery voltage is restored.
FIG. 1
illustrates the typical behavior of the battery voltage over time for a battery being operated in an electrical device. In
FIG. 1
, it is assumed that a fully charged battery is inserted into the electrical device at time T0. When a fully charged battery is inserted, the battery voltage as seen by the electrical device increases rapidly (but not immediately due to the charging of capacitors) to a maximum voltage value as shown by curve
12
a
. A voltage monitor circuit associated with the electrical device detects when the battery voltage increases to a value greater than a predefined low threshold voltage, V
T
(Low), and signals the electrical device to begin operation. The battery voltage decreases as the battery charge is consumed (curve
12
b
). When the battery voltage falls below the low threshold voltage, VT(Low), at time T2 in
FIG. 1
, the voltage monitor circuit detects the low voltage condition and signals the electrical device to shut down its circuits. Ideally, the electrical device should not operate again until the battery charge is fully restored or restored to a sufficiently high level. However, as described below, the electrical device is often caused to operate before the battery is restored sufficiently because the battery voltage exhibits the well-known phenomenon called “bounce back” as illustrated by curve
12
c
in FIG.
1
.
Because the battery voltage is load dependent, when the electrical device shuts down its circuits, the load on the battery is reduced and the battery voltage will recover to a value above the low threshold voltage, V
T
(Low). The electrical device sees what is apparently a valid operating voltage on the battery and turns on its circuit again. However, because only minimal battery charge actually remains, the battery voltage will almost immediately fall below the low threshold level V
T
(Low) again and the device will have to shut off its circuits almost as soon as it turns them on. Therefore, it is undesirable to operate the device under what is called the “bounce back” voltage.
To prevent device operation under the “bounce back” voltage, conventional voltage monitor circuits are typically designed with hysteresis. Two threshold voltages are used to control the proper operation of an electrical device: a high threshold voltage, V
T
(High), and a low threshold voltage, V
T
(Low). Referring again to
FIG. 1
, when a voltage monitor circuit employs hysteresis, an electrical device will not turn on unless the battery voltage has reached V
T
(High) which is at time T1 in FIG.
1
. After time T1, the electrical device will operate until the battery voltage falls below V
T
(Low) which occurs at time T2 in FIG.
1
. After time T2, even though the battery voltage recovers to above V
T
(Low), the voltage monitor circuit will prevent the electrical device from turning on because the battery voltage has not increased above V
T
(High). In this manner, the voltage monitor circuit precludes operation under the “bounce back” voltage.
The amount of hysteresis, i.e., the voltage values of V
T
(Low) and V
T
(High), of a voltage monitor circuit is established based on the type of battery and the load placed on the battery by the electrical device or electrical system. The threshold voltages V
T
(Low) and V
T
(High) are optimized individually for each electrical device or electrical system in which a voltage monitor circuit operates.
FIG. 2
is a circuit schematic of a conventional voltage monitor circuit
20
with adjustable hysteresis. Voltage monitor circuit
20
includes a resistor network including serially connected resistors
24
,
26
, and
28
, a first comparator
38
, a second comparator
40
, and a latch
48
. The resistor network is connected across the battery voltage (V
BAT
) to be monitored (node
22
) and a ground potential (node
30
). Resistors
24
,
26
, and
28
operate as a voltage divider to scale battery voltage V
BAT
down to provide two reference voltage levels for monitoring the battery voltage. A high battery voltage monitor level (V
High
) is provided at node
34
between resistors
26
and
28
. V
High
is a scaled voltage for monitoring a high level of the battery voltage. A low battery voltage monitor level (V
High
) is provided at node
32
between resistors
24
and
26
. V
Low
is a scaled voltage for monitoring a low level of the battery voltage.
First comparator
38
compares voltage V
Low
to a reference voltage V
Ref
(node
36
). Because V
Low
is connected to the inverting input terminal of first comparator
38
, first comparator
38
provides a low to high transition on output lead
42
when voltage V
Low
decreases from a value above V
Ref
to a value below V
Ref
. Second comparator
40
compares voltage V
High
also to reference voltage V
Ref
(node
36
). Because V
High
is connected to the non-inverting terminal of second comparator
40
, second comparator
40
provides a low to high transition on output lead
44
when voltage V
High
increases from a value below V
Ref
to a value above V
Ref
. Output lead
42
of first comparator
38
is connected to the reset terminal of latch
48
. Output lead
44
of second comparator
40
is connected to the set terminal of latch
48
. Latch
48
provides an output signal Power Good on output lead
50
. Signal Power Good indicates the status of the battery voltage to the electrical device in which voltage monitor circuit
20
operates. When signal Power Good is at a high logic level, the battery charge is within the desirable operative range. When signal Power Good is at a low logic level, the battery charge is below the desirable operative range.
In operation, second comparator
40
monitors the positive-going transition of battery voltage V
BAT
, that is, the increase in battery voltage illustrated by curve
12
a
in FIG.
1
. At time T0, signal Power Good is low and voltage monitor circuit
20
is monitoring voltage V
High
. Between time T0 and T1, the battery voltage is below V
T
(High), thus, voltage V
High
is less than V
Ref
and the output of comparator
40
is at a low logic level. Latch
48
is not set and signal Power Good remains at a low logic level. At time T1, the battery voltage reaches V
T
(High), voltage V
High
is now greater than V
Ref
and output lead
44
of second comparator
40
transitions to a high logic level, setting latch
48
. Signal Power Good transitions to a high logic level as a result. Voltage monitor circuit
20
then monitors voltage V
Low
. Between time T1 and T2, the battery voltage is above V
T
(Low). Thus, V
Low
is greater than V
Ref
and output lead
42
is at a low logic level. Signal Power Good remains set to a high logic level. At time T2, when the battery voltage drops below V
T
(Low), first comparator
38
provides a high logic level output on lead
42
causing latch
48
to reset. Signal Power Good thus transitions to a low logic level. Signal Power Good will remain at a low logic level, indicating inoperative battery charge, until second comparator
40
detects that the battery voltage has increased above V
T
(High) again. In this manner, voltage monitor circuit
20
ensures that no chattering occurs on signal Power Good and the electrical device will not be operated under the “bounce back” voltage.
The conventional voltage monitor circuit such as circuit
20
in
Cook Carmen C.
Micrel Incorporated
Ogonowsky Brian D.
Skjerven Morrill & MacPherson LLP
Tso Edward H.
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