Abstract:
A temperature stabilized, constant current source capable of charging a fully discharged battery of the present invention includes a feedback control stage that provides a substantially constant battery charging current at a particular temperature. A temperature stabilized current source stage coupled to a bias resistor includes a negative temperature coefficient current source that provides a countervailing control current to a positive temperature coefficient current source that is coupled from a sensing resistor. The temperature dependencies of the positive and negative temperature coefficient current sources tend to cancel each other out so as to provide a temperature stabilized current to the sensing resistor. The bias resistor provides a bias current to the temperature stabilized current source stage in such a way that the temperature stabilized current source stage provides a charging current to a fully discharged battery.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to analog integrated circuits, and more particularly to current sources implemented in analog integrated circuits. 
     Constant current sources and constant voltage sources are used for a variety of purposes in analog integrated circuits. As used herein, “constant” means that the output of the source remains at a relatively constant direct current (d.c.) level, although the output levels of such sources can typically be adjusted (“set”) with a control signal. Once set, the output of a constant current or voltage source may change with temperature (i.e. be “temperature dependent”) or may be stable with temperature. In many applications, it is desirable to have a constant current or voltage source that does not vary in output as the temperature changes. If the output of a constant, temperature stable, current source is coupled from an output resistor that is temperature stable, the result is a constant, temperature stable voltage source, as will be appreciated by those skilled in the art. These constant, temperature stable voltage sources are useful for many purposes, such as providing a reference voltage, for adjusting the threshold of a comparator, etc. that are useful power supplies for charging a battery 
     Unfortunately, however, conventional battery chargers are not capable of charging batteries that have been discharged below approximately 2.5 to 3.0 volts. One approach to charging batteries below 2.5 to 3.0 volts to a range of approximately 1.0 volt is described in U.S. Pat. 6,225,787 B1 issued to Chen et. al. entitled “TEMPERATURE STABILIZED CONSTANT CURRENT SOURCE SUITABLE FOR CHARGING A HIGHLY DISCHARGED BATTERY”. Unfortunately, the approach described in the &#39;787 patent is incapable of charging batteries that are fully depleted, i.e., in the range of 0.0 volts since the temperature stabilized constant current source described therein requires at least 1.0 volt battery voltage in order to generate an appropriate and sufficient battery charging current 
     Therefore, what is desired is a temperature stabilized, adjustable, yet constant current source suitably arranged to charge a battery that has been discharged to as low as 0.0 volts. 
     SUMMARY OF THE INVENTION 
     The invention is an electrical circuit that provides a temperature stabilized current source with a stable control voltage capable of charging a battery having been discharged to as low as approximately 0 volt. As used herein, “stable” means that the voltage remains essentially unchanged with changes in temperature, i.e. it is not temperature dependent. The circuit solves the problem of providing an adjustable temperature stabilized current source suitable for charging fully discharged (i.e. in the range of 0.0V) batteries. 
     The temperature stabilized, constant current source battery charger suitable for charging a fully discharged battery includes a current based feedback control circuit responsive to small signal changes in a battery charging current wherein the feedback control circuit maintains the battery charging current within a predetermined range of current values at a particular operating temperature. A temperature compensation circuit coupled from the feedback control circuit that is responsive to a temperature change having a sensing resistor of a given resistor technology coupled from at least one positive temperature coefficient voltage source and at least one negative temperature coefficient current source arranged to provide said sensing resistor with a temperature stabilized control current such that temperature dependencies of the negative temperature coefficient current source substantially countervails the at least one positive temperature coefficient voltage source such that a sense voltage developed by the sensing resistor is substantially constant over a predetermined operating range of temperatures. The current source further includes a bias resister coupled to the temperature compensation circuit arranged to provide a bias current to the temperature compensation circuit based upon a supply voltage such that the temperature compensation circuit is operative when the battery is fully discharged. 
     As a method for providing a temperature independent current suitable for charging a fully discharged battery, a battery charging current is maintained to the battery within a specified range of battery charging currents using a feedback control circuit and a bias resistor. The feedback controlled battery charging current is further temperature stabilized over a range of operating temperatures by a temperature stabilized current source having positive temperature coefficient current source and a countervailing negative temperature coefficient current source that maintains a temperature stabilized current through a sensing resistor. The bias resistor provides a bias current based upon a supply voltage to the temperature stabilized current source so as to provide a battery charging current to a fully discharged battery. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a temperature-stabilized, constant current source in accordance with the present invention; 
     FIGS. 2A-2D are graphical representations of an output current versus output voltage over a selected temperature range for a particular implementation of the invention. 
     FIG. 3 is a circuit diagram of a temperature-stabilized, constant current source of the present invention that can be used in the circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a temperature-stabilized, constant current source battery charger  100  in accordance with the present invention, includes a first stage  102  in the form of a feedback control circuit coupled from a second stage  104 . In the described embodiment, the second stage  104  takes the form of a temperature stabilized current controlled source circuit  104  that uses a voltage developed by a sensing resistor R sense    105  to compensate for any change (i.e., increase or decrease) in ambient temperature. A bias resistor R bias    106  passes a bias current I bias  to the temperature stabilized current controlled source circuit  104  that includes those situations where a battery  108  coupled thereto is fully charge depleted (also referred to as discharged) such that a battery voltage V Bat  is in the range of 0.0 volts. 
     In the described embodiment, at a particular ambient temperature, the output power stage  102  maintains a substantially constant battery charging current I bc  by producing a feedback current signal in proportion to any deviation (i.e., an increase or a decrease in I bc ) of a nominal battery charging current I bc  through the battery  108 . During operation, the output power stage  102  in combination with the bias resistor R bias    106  provides the current necessary to maintain the battery charging current I bc  at its nominal value even in those situations where the battery  108  is fully charge depleted. It should be noted that by fully charge depleted it is meant that the battery  108  is discharged such that the battery voltage V Bat  is in the range of substantially zero (0.0) volts. For example, in those situations where the V Bat  is substantially zero, the bias current I bias  is supplied to the control circuit  104  so as to be operative when the battery is fully depleted. In the described embodiment, a typical value of R bias  is approximately 8 K ohms with a 10 volt power supply which provides about 1 mA to the control circuit  104 . 
     In those situations where there is a change (either an increase or a decrease) in ambient temperature, the temperature stabilized current source circuit  104  maintains a substantially constant battery charging current I bc  by, in a preferred embodiment, maintaining a constant voltage (V sense ) across the sensing resistor R sense    105 . In this way, the temperature stabilized current source circuit  104  is able to maintain a constant battery charging current I bc  over a wide range of ambient temperatures. In a preferred embodiment, the temperature stabilized current source circuit  104  utilizes a negative temperature coefficient current source to compensate for any positive temperature coefficient current sources in order to maintain a constant sensing current I s  through the sensing resistor  105 . In this way, the battery charging current I bc  to the battery  108  is stable over any contemplated range of operating temperatures. 
     FIG. 3 illustrates a circuit diagram  300  of one embodiment of the battery charging circuit  100  in accordance with an embodiment of the invention. As shown, the output power stage  102  includes transistors labeled Q 1  and Q 2 . In this preferred embodiment, the transistors Q 1 -Q 2  are bipolar NPN transistors. The design and fabrication of bipolar transistors that are commercially available is well known to those skilled in the art. 
     The temperature stabilized current source circuit  104  also includes a number of current sources. More particularly, the temperature stabilized current source circuit  104  includes a matched dual current source  302 . The matched dual current source  302  includes a first current source I c1 , a second current source I c2 , and a biasing current source I 5  A gain stage I c3  provides a feedback current I fb  to the transistor Q 2  included in the output power stage  102 . In addition, a capacitor C 1  having a capacitance of approximately 400 nf is coupled between the collector and base of the transistor Q 2 . For example, at a particular operating temperature, if a nominal battery charging current I bc(nominal)  increases by an amount ΔI bc , such that the battery charging current I bc  is increased to (I bc(nominal) +ΔI bc ), a first feedback current I fb1  also increases. The increase in feedback current I fb ) increases the base drive of an NPN transistor Q 5  which increases the base drive of a PNP transistor Q 3 . The increase in the base drive of the PNP transistor Q 3  results in an increase in the base drive (I fb2 ) of the NPN transistor Q 2  which results in an increase in the transistor Q 2  collector current I c2  which pulls the collector of transistor Q 2  and the base of the transistor Q 1  low. In the described embodiment, the increased collector current I c2  has the effect of reducing the base drive of the NPN transistor Q 1 . In a preferred embodiment, the decrease in the base drive of the transistor Q 1  causes the increased battery charging current (I bc(nominal) +ΔI bc ) to be reduced by ΔI bc  thereby returning the battery charging current I bc  to its nominal value I bc(nominal) . It should be noted that when the battery charging current I bc  decreases, the feedback circuit  102  will increase the battery charging current I bc  to its nominal value I bc(nominal) . In a preferred embodiment, the emitter area of the transistor Q 8  is approximately 10 times that of the transistor Q 7  which gives a ΔV be  of approximately 60 mV. In this way, the active base emitter turnon voltage of the transistor Q 8  (i.e., V be,(Q8) ) is approximately 600 mV whereas the active base emitter turnon voltage of the transistor Q 7  (V be (Q7) ) is approximately 660 mV. 
     As is well understood in the art, when the temperature of an NPN bipolar transistor increases, the ΔV bc  of Q 7  and Q 8  will also increase. Therefore, as the temperature of the battery charging circuit  100  changes, the current through the sensing resistor R sense    105  will also change if it were not compensated with the present invention, resulting in a correspondingly undesireable change in the battery charging current I bc . 
     With a zero temperature coefficient sensing resistor R sense the combined positive temperature coefficient exhibited by the ΔV be  of the transistors Q 7  and Q 8  is substantially compensated for by a negative temperature coefficient current source  304 . In the described embodiment, the current source  304  includes a biasing circuit  306  (typically in the form of a IPTAT) coupled from a resistor R 1  and a transistor Q 9  having its collector tied to its base to form a V be  dependent diode. The emitters of the transistor Q 7  and Q 8  are coupled to both the resistor R 1  and the resistor R 2 . In the described embodiment, the base of the transistor Q 8  is coupled to the base of the transistor Q 7  having its emitter coupled to a resistor R 3  that is in turn coupled from the sensing resistor (R sense )  105 . It should be noted that the transistors Q 7 and Q 8  are preferably NPN transistors where the transistor Q 8  has a larger emitter area than the transistor Q 7 . Typically, the ratio of the emitter areas of the transistors Q 8  to Q 7 , is in the range of approximately 2.0 to approximately 16.0 (i.e., when the device size of the transistor Q 8  is twice the size of the transistor Q 7 , the ratio is 2.0). 
     During operation, in order to maintain a temperature stabilized battery charging current I bc , the sensing current I s  through the sensing resistor  105  must remain substantially temperature stabilized. In this way, a constant battery charging current I bc  is maintained through the battery  108  across any contemplated range of operating temperatures. 
     The operation of the constant current source circuit  104  of the present invention will now be discussed in greater detail. As shown, the current source  304  provides a current i 1  through the resistor R 1 . 
     The equations for the currents in transistors are: 
     
       
         ( i   2   +i   1 )* R   2   +V   be(Q7)   −V   be(Q8)   −i   3   *R   3 −( I   s   +i   3 ) R   s =0  (Equation 1) 
       
     
     by setting 
     
       
           R   2   =R   3   (Equation 2) 
       
     
     and 
     
       
           i   2   =i   3  and assuming  i   3   &lt;&lt;I   S , and  I   bQ7   &lt;&lt;I   e   (Equation 3) 
       
     
     Then: 
     
       
         Δ V   sense   =[V   be(Q7)   −V   be(Q8)   ]−i   1   *R   2   (Equation 4) 
       
     
     or, 
     
       
         Δ V   sense   =ΔV   be   +i   1   *R   2   (Equation 5) 
       
     
     Since i 1  (delivered by the current source  304 ) is set by resistors R 1  and R 2 , then by selecting appropriate resistor values R 1  and R 2  such that 
     
       
           i   1   *R   2  is approximately equal to Δ V   be   (Equation 6) 
       
     
     or more precisely 
     
       
           T.C.  of ( i   1   *R   2 )=− T.C.  of (Δ V   be )  (Equation 7) 
       
     
     then V sense  has zero temperature coefficient. In this way, for example, when ΔV be  increases by 10 millivolts, for example, then i 1 *R 2  will decrease by 10 millivolts thereby maintaining a constant current I s  through the sensing resistor R sense    105 . 
     The actual value for the various components in battery charging circuit  100  are dependent upon the application of the circuit, as will be appreciated by those skilled in the art. Typically, V supply  is in the range of approximately 7.0 to 10.0 volts and the current sources I c1  and I c2  can be, for example, 10 or 100 microampere current sources, whereas the sensing resistor  105  can be, for example, approximately 1 ohm whereas the bias resistor R bias  can be in the range of 8 K ohms. 
     It should be noted that it is contemplated that both R bias  and R sense  are ideal zero temperature coefficient type resistors. However, if the sense resistor R sense  has a positive or negative temperature coefficient, it can be compensated by changing the ratio of resistors R 1 /R 2 . For the proper operation of the present invention, the matched dual current sources I c1  and I c2  rely on good matching in ratio of resistors R 1  and R 2 . As will be appreciated to those skilled in the art, there are many types of resistor technologies (also referred to herein as resistor “types”) that can be provided on an integrated circuit. For example, in the book Analysis and Design of Analog Integrated Circuits, 2nd edition, P. Grey et al., John Wiley &amp; Sons, ©1977, 1978, a number of resistor technologies are described including, for example, base-diffused, emitter-diffused, pinched, epitaxial, pinched epitaxial, and thin film resistors. It is not important to the present invention which resistor technology is chosen as long as they have good matching. 
     The circuit and method of the present invention can, and typically do, form a part of a larger system and/or process. For example, the circuit of the present invention typically forms a part of a larger circuit that is integrated on a “chip” and packaged. The packaged integrated circuit is then made a part of a larger system by attaching it to a printed circuit (PC) board along with other electronic devices, connecting the resultant circuit to power supplies and to other devices and systems. It should therefore be understood for the product that results from the processes of the present invention include the circuit itself, integrated circuit chips including one or more circuits, larger systems (e.g. PC board level systems), products which include such larger systems, etc. It should also be noted that transistors Q 1  and Q 2  can be located off-chip for a particular application. 
     While this invention has been described in terms of several preferred embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent to those skilled in the art upon a reading of the specification and study of the drawings. It is therefore intended that the following appended claims include all such alternatives, modifications, permutations and equivalents as fall within the true spirit and scope of the present invention.