Abstract:
This invention includes a charging circuit employing an off-the-shelf battery protection circuit. The invention offers an improvement to the battery protection circuit by providing means to accommodate trickle charging during undervoltage conditions. A pass transistor is provided with a trickle resistor coupled in parallel. The pass transistor is driven by a drooping voltage circuit that causes the pass transistor to open in undervoltage conditions, thus forcing charging current through the trickle resistor. Indicating means are also included to tell the user whether the circuit is in trickle mode or normal charging mode.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates generally to battery charging and protection circuits, and more specifically to battery charging circuits incorporating a trickle charge during battery undervoltage conditions. 
     2. Background Art 
     Battery protection circuits are well known in the art. Protection circuits for rechargeable cells are prolific in battery packs employing lithium-ion and lithium polymer cells. When rechargeable cells are charged, the voltage of the cell rises. Lithium-based rechargeable cells typically have a maximum termination voltage of 4.1 or 4.2 volts. This means that if a lithium-based battery is charged beyond the termination voltage—known as an “overvoltage condition” the reliability of the cell may be compromised. Most lithium battery protection circuits known in the art sense the voltage of the cell and terminate charging by opening a switch when the cell reaches the proper termination voltage. 
     There is another condition, known as an “undervoltage condition”, when the cell voltage drops below it&#39;s recommended operating point. For a typical lithium-ion cell, this voltage is about 2.5 volts. When the voltage drops below this level, possibly due to over discharge, cell manufacturers suggest that rapid charging may damage the cell. Consequently, the battery must be slowly charged until it reaches the minimum operational threshold. The slow charging current, often called a “trickle current” is on the order of a few hundred milliamps. Once the minimum threshold is reached, a full charging current, like 1 amp for example, may be applied until the maximum termination voltage is reached. 
     There is thus a need for a compact, low-cost charging circuit that accommodates trickle charging for undervoltage conditions, as well as terminating charging prior to an overvoltage condition occurring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a prior art protection circuit  100 . 
     FIG. 2 is an illustration of one preferred embodiment of a charging circuit that facilitates trickle charging during undervoltage conditions in accordance with the invention. 
     FIG. 3 shows an internal schematic diagram of the NCP802 protection integrated circuit. 
     FIG. 4 is an illustration of an indication charging circuit that provides charging status notice to the user in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     Cell protection circuits available on the market today provide termination of charging when the voltage of a rechargeable cell reaches a predetermined threshold. One such protection circuit includes the NCP802 manufactured by OnSemiconductor. The data sheet and application notes for this part may be found at www.onsemi.com/pub/Collateral/NCP802-D.PDF. This invention incorporates such a protection device and further provides a circuit that facilitates trickle charging during undervoltage conditions. While the NCP802 integrated circuit (IC) will be used herein for exemplary purposes, it will be clear to those of ordinary skill in the art that other protection circuits having similar characteristics are interchangeable with the NCP802. 
     Referring now to FIG. 1, illustrated therein is a prior art protection circuit  100 . The circuit  100  includes a protection control IC  101  like the NCP802. Protection circuits of this type typically have at least 5 terminals: A voltage sensing terminal  102  and corresponding ground terminal  103  to sense the voltage of the cell; a charge control pin  105  and a discharge control pin  104 ; and a negative input pin  6 . Terminals B+  112  and B−  113  are typically coupled to either a power source or load for charging and discharging the cell  111 . 
     The charge control pin  105  and the discharge control pin  104  typically control a charge transistor  107  and a discharge transistor  108 . Metal oxide semiconductor field effect transistors (MOSFET) are typically used due to their low impedance in the saturation mode. Two transistors are required because each MOSFET has a corresponding parasitic body diode  109 , 110  due to its manufacture. Consequently, each MOSFET  107 ,  108  can only prevent current from flowing in a single direction. The body diodes  109 , 110  allow the current to flow in the other direction. To stop all current flow, therefore, both MOSFETs  107 , 108  must be turned off. Thus the need for a charge control pin  105  and a discharge control pin  104 . The negative input pin  106  is provided to connect to excess current detectors. It also serves as a common node for the charge control pin  105  when the MOSFETs  107 , 108  are open. 
     The operation of the circuit  100  is as follows: When the voltage across the voltage sensing terminal  102  and corresponding ground  103  increases above a predetermined threshold, the charge control pin  105  is actuated, thereby opening the charge transistor  109 . Discharge of the cell  111  is still allowed through the parasitic diode  109 , but no current can flow in the charging direction, which corresponds to current flowing from + to − in the cell  111 . When an undervoltage situation is sensed across the voltage sense pin  102  and corresponding ground  103 , the discharge control pin  104  is actuated, thereby opening the discharge transistor  108 . Charging current may still flow through the parasitic diode  110 , but no discharge current may flow. The transistors  107 , 108  return to their closed positions when the cell voltage reenters it&#39;s operational voltage range. The IC  101  may include hysteresis across these transitions. 
     Referring now to FIG. 2, illustrated therein is one preferred embodiment of a charging circuit  200  that facilitates trickle charging during undervoltage conditions in accordance with the invention. As stated above, the protection IC  101  itself turns the charge and discharge transistors  107 , 108  either completely on or completely off. As also stated, manufacturers of cells suggest that when the cell  111  is in an undervoltage condition, it is desirable to reduce the charging current to a trickle level. The invention accommodates this by including a pass transistor  201  and a trickle resistor  202  in series with the cell  111 . The pass transistor  201  is driven by a “drooping voltage” source  203 . 
     In its simplest embodiment, the circuit senses a voltage across parasitic diode  110  when the discharge transistor  108  is open. This may be accomplished in a number of ways, including using operational amplifiers to sense the voltage, single transistors and the like. The voltage droop circuit  203  has advantages in that it is capable of more accurately sensing voltage changes in the parasitic diode  110 . Additionally, it is capable of sensing smaller voltages that other circuits. 
     When the cell  111  is operating within it&#39;s operational voltage range, the pass transistor  201  is in the fully saturated mode and is conducting current. If the cell  111  is in an undervoltage condition, however, the discharge transistor  108  opens. While current may still flow through the parasitic diode  110  for charging, the impedance of the charge path  204  increases. This causes current to begin flowing through resistors  205  and  206 , thereby reducing the voltage at node  207 , causing transistor  208  to turn off. 
     When transistor  208  turns off, the voltage at node  209  increases, causing transistor  210  to turn on, thereby causing transistor  211  to turn on. This pulls the voltage of node  212  to be pulled down, causing the pass transistor  201  to turn off. Once the pass transistor  201  is turned off, the only path for current to flow to the cell is through the trickle resistor. By increasing the resistance of the trickle resistor, the charging current may be reduced to the trickle level. 
     The circuit thus opens the pass transistor  201  during undervoltage conditions, reducing the charging current to a trickle level through trickle resistor  202 . Once the cell  111  reaches it&#39;s minimum threshold level, the pass transistor  201  turns back on to allow charging at the normal rate. 
     Referring now to FIG. 3, illustrated therein is an internal schematic diagram of the NCP802 protection IC  101 . The negative input pin  106  is internally coupled to the negative input an operational amplifier (op-amp)  301 . The positive input  301  is coupled to a voltage reference  302  from the ground pin  103  and the chassis reference  303 . When an external short between the negative input pin  106  and the ground pin  103  is removed—by opening the discharge transistor for example—the virtual short of the op-amp will cause the negative input pin  106  to float above the chassis reference  303  to the value of the voltage reference  302 . 
     Referring now to FIG. 4, illustrated therein is an indication charging circuit  400  that takes advantage of the virtual short of the internal op-amp. When the circuit  400  is in normal operation, e.g. when the cell  111  is within it&#39;s normal operation range, the transistors  107 , 108  are closed and the negative input pin  106  is pulled to ground. In this circuit  400 , the negative input pin  106  is employed as an output by coupling the negative input pin to a MOSFET transistor. As the negative input pin  106  is effectively tied low, MOSFET  401  is off, and transistor  402  is on, causing current to flow through a first light emitting diode (LED)  403 , thereby indicating normal charge. 
     When either the discharge transistor  108  opens, however, the negative input  106  is pulled up to the internal reference voltage, thereby causing transistor  401  to turn on. This actuation causes transistor  402  to turn off. Current in thereby directed through a second LED  404 , thereby indicating trickle charge mode. 
     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims. For example, while the NCP802 has been used as an example, any other protection device with similar characteristics may be used in its stead.