Embedded battery overtemperature protection and voltage regulator circuitry

A method and apparatus is provided for miniaturizing battery overtemperature protection and voltage regulation circuits in which such protection and regulation circuits are formed in an integrated circuit (200, 600) and embedded in the casings of individual batteries. In one aspect of the invention, a MOSFET switch (Q1), a temperature sensitive diode (D1) and a control circuit (604) may be formed in an integrated circuit (600) and embedded in the casing of a battery. The MOSFET switch may be placed in the charging path of the battery to provide an open circuit when the battery temperature exceeds a predetermined value. In another aspect of the invention, the operation of the MOSFET switch (Q1) may be controlled by a control circuit (204) to provide a constant output voltage from the battery, thereby improving the operation of the portable system using the battery.

TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to power sources for personal 
communications systems, and more particularly, to batteries with embedded 
overtemperature protection and/or voltage regulator circuitry. 
BACKGROUND OF THE INVENTION 
The rapid development of smaller, lighter and more efficient personal 
communication systems, such as personal computer notebooks, sub-notebooks, 
pocket cellular phones, miniature radios, stereos, and cassette players, 
has been fueled partly by the development of improved rechargeable 
batteries. For example, today's nickel cadmium batteries are more powerful 
than those in use just a few years ago. However, cadmium-based batteries 
are under increased scrutiny for both environmental and performance 
reasons. Consequently, the continuing demand for improved 
rechargeable-battery performance, combined with the need for a more 
environmentally acceptable battery, has driven the development of new 
rechargeable-battery technologies, such as, for example, nickel metal 
hydride batteries. Thus, the emerging generation of rechargeable batteries 
is not only intended to diminish the use of toxic heavy metals, but also 
to provide a significant improvement in energy density over existing 
cells. This improvement in energy density will allow designers of 
hand-held communications systems to significantly reduce the size of 
battery supplies and accordingly reduce the size of the hand-held systems. 
However, these reductions in battery size have not been accompanied by 
corresponding reductions in battery associated protection circuitry, which 
has hampered attempts to design smaller systems. 
Essentially, the operating life of a rechargeable battery will be reduced 
if it's temperature rises significantly during the recharging process. In 
fact, a rechargeable battery can explode at the higher temperatures. 
Typically, portable communications systems contain protection circuits 
that monitor battery temperature during the charging process, and open the 
charging path if a specific temperature threshold is exceeded. A P-N 
junction diode, which has temperature dependent, forward conduction 
characteristics, is located in the general vicinity of the battery or 
power pack (e.g., multiple batteries). The protection circuitry is 
designed to monitor current flow through the temperature dependent diode 
and open the charge path at a predetermined temperature. Consequently, 
both the battery supply and the portable system may be protected from 
potentially serious damage. Nevertheless, these protection circuits take 
up precious space and add unnecessary weight to existing portable systems. 
Furthermore, in the newer rechargeable batteries, such as the nickel metal 
hydride batteries, the charging reaction is exothermic rather than 
endothermic as in the nickel cadmium batteries. Consequently, the nickel 
metal hydride battery will warm as it charges, whereas the nickel cadmium 
cell temperature remains relatively constant until the cell moves into 
overcharge. Therefore, a temperature sensor in the protection circuit must 
be more sensitive to temperature surges in the nickel metal hydride 
batteries than for nickel cadmium batteries. However, sensitivity to these 
relatively small temperature surges may be limited unless the sensor is 
placed in direct, thermal contact with the battery. Yet, such physical 
contact is difficult to attain for multiple batteries. Also, space 
requirements may limit design flexibility with respect to optimum 
placement of the sensors. 
Some portable systems use throw-away power sources, such as alkaline 
batteries, instead of rechargeable batteries. Thus, overcharging is not a 
problem with such systems and thermal protection circuitry may not be 
needed. Nevertheless, the operation of portable systems using either 
throw-away or rechargeable batteries is affected significantly by 
variations in battery output voltage. Consequently, most portable systems 
employ voltage regulating circuits to maintain the battery supply voltage 
at a constant level. However, similar to the size and weight problems 
experienced with protection circuits, the relatively large size and weight 
of voltage regulating circuits have become a major design concern because 
of the trend toward miniaturization of the portable systems. 
SUMMARY OF THE INVENTION 
Accordingly, a need exists in the portable communications systems 
manufacturing industry for smaller, space-optimized battery 
overtemperature protection and voltage regulation circuits. In accordance 
with the present invention, a method and apparatus is provided for 
miniaturizing battery overtemperature protection and voltage regulation 
circuits in which such protection and regulation circuits are formed in an 
integrated circuit and embedded in the casings of individual batteries. In 
one aspect of the invention, a MOSFET switch, a temperature sensitive 
diode and a control circuit may be formed in an integrated circuit and 
embedded in the casing of a battery pack. The MOSFET switch may be placed 
in the charging path of the battery to provide an open circuit when the 
battery temperature exceeds a predetermined value. In another aspect of 
the invention, the operation of the MOSFET switch may be controlled to 
provide a constant output voltage from the battery, thereby improving the 
operation of the portable system. 
An important technical advantage of the present invention is that more 
sensitive and responsive overtemperature protection may be provided for 
the overall system by embedding the protection circuitry inside individual 
batteries. 
Another important technical advantage of the present invention is that 
miniaturization of the overall system may be accomplished more readily 
since the space formerly used outside the batteries is no longer needed 
for overtemperature protection and/or voltage regulator circuitry. 
An additional technical advantage of the present invention is that a single 
integrated circuit may be fabricated to provide an embedded 
overtemperature protection and voltage regulator circuit, which reduces 
manufacturing costs and the overall price of the system.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment of the present invention and its advantages are 
best understood by referring to FIGS. 1-7 of the drawings, like numerals 
being used for like and corresponding parts of the various drawings. 
FIGS. 1(a)-(c) illustrate a cross-sectional view of exemplary batteries for 
use in a portable communications system and alternative locations for 
placement of an embedded circuit according to the teachings of the present 
invention. As described below with respect to FIGS. 2-3 and 6-7, the 
present invention may include either one or both of an overtemperature 
protection circuit or a voltage regulator circuit formed on an integrated 
circuit chip. Consequently, the invention may be fabricated to be small 
enough to be embedded inside the case of either a rechargeable or 
throw-away battery. Consequently, for battery 100 in FIG. 1(a), which may 
represent, for example, any of rechargeable or throwaway batteries of AAA, 
AA, A, B, C or D sizes, the invention may be placed at locations 102 or 
106, or in any appropriate space between the cell material and the 
battery's casing to include a curved area at the side of the battery, such 
as location 104. However, since the integrated circuit of the present 
invention may be formed as a flat die, it may be preferable to locate the 
invention directly on a flat surface of the battery, such as, for example, 
a flat surface of the casing at respective top and bottom locations 102 
and 106, for maximum physical contact and maximum sensitivity to 
temperature variations, although such specific placement is not required 
to practice the invention. Essentially, a finite area will exist between 
the cell material and the battery's casing which is larger than the 
integrated circuit comprising the present invention. Consequently, the 
invention requires no additional space in the portable system and provides 
only an insignificant amount of additional weight. 
For battery 110 in FIG. 1(b), which may represent a typical flat, circular 
battery, the invention may be placed at respective bottom or side 
locations 108 or 112, which is within a space between the battery cell 
material and the casing. Similarly, for battery 114 in FIG. 1(c), which 
may represent a flat, elongated battery, such as one configuration used 
for a nickel metal hydride battery, the invention may be placed at 
respective top, bottom and side locations 118, 116 and 120, or again in 
spaces between the cell material and casing of the battery. Although it 
may be preferable to place the integrated circuit of the present invention 
between the cell material and the casing of a battery, it would be within 
the scope of the invention to place the present integrated circuit within 
the cell material itself, or at any other appropriate location within the 
battery. 
FIG. 2 illustrates an electrical schematic circuit diagram of a preferred 
embodiment of a voltage regulator circuit that may be embedded in a 
battery according to the teachings of the present invention. Voltage 
regulator circuit 200 shown in FIG. 2 may be fabricated as a single 
integrated circuit chip, or combined alternatively on a single integrated 
circuit chip with the overtemperature protection circuit described below 
with respect to FIGS. 6 and 7. Battery cell 202 may represent a cell in 
any of respective batteries 100, 110 or 114 shown in FIGS. 1(a)-(c), or 
any other appropriate rechargeable or throw-away battery used for portable 
communication systems. Transistor Q1, which may be a p-channel MOSFET 
switch, is connected in series with the positive pole of cell 202 and the 
positive, external connection (+) of the battery. Specifically, the 
negative pole of cell 202 is connected to the negative, external 
connection (-) of the battery. Typically, the cell's negative pole is 
connected to the battery's metallic casing. The positive pole of cell 202 
is connected to the source of p-channel MOSFET Q1 and node 206, which is 
connected to voltage input connection Vin and node 209 of control circuit 
204. The drain of transistor Q1 is connected to the positive, external 
connection (+) of the battery and node 207, which is connected to voltage 
output connection rout of control circuit 204. The specific wiring 
relationship between transistor Q1, cell 202, and the external connections 
of the battery is not explicitly shown in FIGS. 1(a)-(c), but the 
arrangement of such wiring may be readily understood by one of ordinary 
skill in the relevant art. The gate of transistor Q1 is connected to 
output connection 218 of control circuit 204. 
FIG. 3 illustrates an electrical circuit schematic diagram of the control 
circuit shown in FIG. 2. Control circuit 204 may be a low drop out 
regulator (LDO), such as, for example, Texas Instruments part number 
TL75LP05, which is produced by Texas Instruments Incorporated, or any 
appropriate integrated circuit designed to perform at least the function 
of regulating voltage, or used additionally in conjunction with a MOSFET 
switch. Control circuit 204 may be combined on a single integrated circuit 
chip with MOSFET switch Q1, or the two devices may be provided as separate 
integrated circuits. Referring to FIGS. 2 and 3, voltage input Vin is 
connected to a first pole of on/off switch 216. The switching pole of 
switch 216 is connected to node 214, and a third pole of the switch is 
connected to a switch standby control. In the position shown in FIG. 3, 
switch 216 is in the "off" or standby position. In the "on" position of 
switch 216, the switching pole is moved to a second position, whereby node 
214 is connected through the switch to voltage input connection Vin. Node 
214 is also connected to an output connection of thermal voltage reference 
212, which is used to provide temperature stability for the control 
circuit. Node 214 is also connected to a bias input of error amplifier 
210. A second output connection of thermal voltage reference 212 is 
connected to the negative signal input connection of error amplifier 210. 
An output of error amplifier 210 is connected to the gate of MOSFET switch 
Q1. Reference 212 and amplifier 210 are connected to circuit ground, which 
is wired to the negative external connection (-) of cell 202. Node 207 is 
connected to a terminal of resistor R1F. The opposite terminal of resistor 
R1F is connected to one terminal of resistor R2F and the positive signal 
input of error amplifier 210. The opposite terminal of resistor R2F is 
connected to circuit ground. 
In operation, switch 216 is moved from a standby or "off" position to an 
"on" position. FIG. 4 illustrates the decay of unregulated battery cell 
output voltages over time. These unregulated voltages could represent an 
output of voltage regulator circuit 200 with switch 216 in a standby 
position. Conversely, with switch 216 in an "on" position, thermal voltage 
reference 212 provides a thermally stable output reference voltage to the 
bias input of error amplifier 210, and a second, stable output reference 
voltage to the negative signal input of amplifier 210. Thus, a temperature 
independent reference level is set for amplifier 210. A thermally stable 
reference voltage output from reference 212 is also applied to node 206. 
Consequently, error amplifier 210 provides an output signal to the gate of 
MOSFET switch Q1, which is turned on to allow current flow from cell 202 
to the positive external connection (+) of the battery. Initially, as the 
voltage at output connection Vout is increased, the voltage at node 208 is 
decreased to provide a corresponding decreased output voltage from error 
amplifier 210 to the gate of Q1. As a result, the conduction of current 
through transistor switch Q1 is decreased, which decreases the output 
voltage Vout at the external positive connection (+) of the battery. 
Eventually, the steady state provides a stable output voltage Vout. Over 
time, as the output voltage Vin of cell 202 decays, the voltage at node 
208 is increased, and the output voltage from error amplifier 210 is 
increased, which in turn increases the conduction of transistor Q1. 
Consequently, the voltage Vout is maintained at a constant value over the 
useful life of cell 202. FIG. 5 illustrates regulation of battery cell 
output voltages over time according to the teachings of the present 
invention. Each of regulated voltages V1R and V2R may represent, for 
example, respective regulated voltages of voltage regulator circuit 200 
for a nickel metal hydride or nickel cadmium cell. However, a similarly 
stable output voltage trend over time, such as each of those shown in FIG. 
5, may be envisioned for any other appropriate battery cell (e.g., 
throw-away battery) used according to the teachings of the present 
invention. 
FIG. 6 illustrates an electrical schematic circuit diagram of a preferred 
embodiment of an overtemperature protection circuit that may be embedded 
in a battery according to the teachings of the present invention. In an 
aspect of the invention, similar to the voltage regulator embodiment, 
MOSFET switch Q1 of overtemperature protection circuit 600 may be 
connected in series with cell 202 and the two, external connections of the 
battery. Specifically, the negative external connection (-) of the battery 
is connected to the negative pole of cell 202. The positive pole of cell 
202 is connected to the source of MOSFET switch Q1 and voltage input 
connection Vin of overtemperature control circuit 604. The drain of switch 
Q1 is connected to the positive external connection (+) of the battery and 
node 607 of control circuit 604. The gate of Q1 is connected to node 622 
of control circuit 604. The cathode of diode D1, which may be a P-N 
junction diode, is connected to the negative external connection (-) of 
the battery, and the anode of D1 is connected to node 628 of control 
circuit 604. Control circuit 604 is generally shown as grounded to the 
negative external connection (-) of the battery. 
FIG. 7 illustrates an electrical schematic circuit diagram of control 
circuit 604 shown in FIG. 6. Referring to FIGS. 6 and 7, node 618 (Vin) is 
connected to one pole of on/off switch 616 and the source of switch Q1. 
Switch 616 is shown for illustrative purposes only, as a single-pole 
double-throw switch, but the invention is not intended to be limited to a 
particular type of switch. The function of on/off switch 616 is to switch 
to the "off" or standby position when a predetermined signal threshold 
(voltage or current) is detected at the switching contact, which is 
connected to node 624. Therefore, for example, switch 616 may be a MOSFET 
switch or other appropriate type of input signal-sensitive switching 
device. A MOSFET switch configuration may be selected whereby the 
transistor is cut off when a predetermined level of voltage (or current) 
is applied to the drain. On/off switch 616 is operated in the normally 
closed position, as shown by the solid line representing the operating 
position of the switching pole of switch 616. The dashed line represents 
the switching pole in the open or standby position. The switching pole of 
switch 616 is connected to node 614, and the control pole of the switch is 
connected to node 624 on the standby control line. For a MOSFET switch, 
the gate would be connected to node 624. In the position shown by the 
dashed line in FIG. 7, switch 616 is in the "off" or standby position. In 
the "on" or operating position of switch 616 shown by the solid line, the 
switching pole connects node 614 through the switch to voltage input 
connection Vin. Node 614 is also connected to an output connection of 
thermal voltage reference 612, which is used to provide temperature 
stability for the control circuit. Additionally, node 614 is connected to 
a bias input of error amplifier 610. A second output connection of thermal 
voltage reference 612 is connected to the negative signal input connection 
of error amplifier 610. A third output connection of thermal voltage 
reference 612 is connected to the negative signal input connection of 
error amplifier 626. Thus, a temperature independent reference level is 
set for amplifiers 610 and 626. An output of error amplifier 610 is 
connected to node 622 and the gate of transistor Q1. Reference 612, and 
amplifiers 610 and 626 are connected to circuit ground, which is wired to 
the negative external connection (-) of cell 202 and the battery. Node 607 
is connected to a terminal of resistor R1F and the drain of transistor Q1. 
The opposite terminal of resistor R1F is connected to one terminal of 
resistor R2F, node 608, and the positive signal input of error amplifier 
610. The opposite terminal of resistor R2F is connected to circuit ground. 
The anode of temperature sensitive diode D1 is connected to the positive 
signal input of error amplifier 626 at node 628. The signal output of 
error amplifier 626 is connected to node 624. 
In operation, control circuit 604 is designed generally to detect the 
amount of current flowing through diode D1, as a function of temperature. 
In a normal operating state, in which cell temperature is below a 
predetermined level, control circuit 604 may operate virtually identically 
to control circuit 204 of FIG. 3 to provide the above-described voltage 
regulation function of the invention. However, when the temperature of 
cell 602 rises, typically when cell 202 is being recharged, current flow 
through diode D1 is increased and the current signal input at node 628 is 
increased. Accordingly, the output voltage from error amplifier 626 is 
increased. When a predetermined level of voltage is sensed at node 624, 
switch 616 switches to the standby or "off" position (dashed line), which 
opens the circuit between node 614 and input voltage connection Vin. 
Therefore, the input voltage at Vin is removed from the bias input of 
error amplifier 610. The output voltage from error amplifier 610 at node 
622 is decreased below a predetermined level, which cuts off transistor 
Q1. Consequently, when the temperature sensed by diode D1 reaches a 
predetermined level (e.g., based on a threshold selected by the setting of 
switch 616), transistor Q1 may be cut off to present an effective open 
circuit to the recharge current flow path. Therefore, both the battery and 
portable system may be protected from potential damage due to 
overtemperature. 
In another aspect of the invention, for example, control circuit 604 may 
comprise a sub-miniaturized bimetallic switch. In such an arrangement, the 
bimetallic switch would be operated in its normally closed position. As 
long as current flow from diode D1 were to remain below a predetermined 
value, the switch would remain closed, and voltage from the positive pole 
of cell 202 would be applied to the base of transistor Q1 to maintain 
conduction. However, if the current through diode D1 (and through the 
bimetallic elements of the switch) were to exceed a predetermined value, 
the bimetallic switch would open and the voltage from cell 202 would be 
removed from the base of transistor Q1. Consequently, transistor Q1 would 
be turned off and the recharge path for the cell would be opened. 
It should be understood that the teachings of the present invention may 
also apply to rechargeable and throw-away batteries for use in 
applications other than portable communications systems, such as, for 
example, back-up power sources for digital clocks or thermostats, etc. In 
other words, the present invention may be used in conjunction with any 
appropriate embedded battery application. 
Although the present invention and its advantages have been described in 
detail, it should be understood that various changes, substitutions and 
alterations can be made herein without departing from the spirit and scope 
of the invention as defined by the appended claims.