Input source detection for a battery charger

A system and method for determining if a USB port can source sufficient current to charge a rechargeable battery at a predetermined peak current level. Detection circuitry is disposed between the USB port and the battery. The detection circuitry includes a current source that is controlled to provide to the battery an increasing current that is sourced by the USB port. As the source current is increased from an initial value to a predetermined peak current source value, the output voltage of the USB port is monitored. If the USB port output voltage drops below a specified threshold voltage before the current source has ramped to the peak current source value, the load current is removed from the battery and an indication is provided that the USB port cannot provide the specified current. The detection process is then repeated after a specified delay interval. If the current source ramps up to the peak source current value and the USB port voltage has not decreased below the specified threshold voltage, charging of the rechargeable battery from the USB port continues at the peak source current.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates generally to battery chargers and more specifically to a system and method for verifying that a USB port has sufficient power to charge a rechargeable battery at a predetermined source current level while maintaining a minimum specified output voltage.

USB ports are provided in most presently manufactured desktop and laptop computers and are commonly used to interface peripheral devices to such computers. Due to the ubiquitous nature of USB ports in a computing environment, it is desirable to utilize such ports as a power source for the charging of rechargeable batteries. However, prior to the coupling of the USB port to a rechargeable battery it is desirable to verify that the specific USB port being used has sufficient power that to charge the rechargeable battery while avoiding possible avoid damage that to USB port circuitry.

Other solutions to this problem rely on the USB controller to identify the USB type by a complex data evaluation process. It would therefore be desirable to have a simple, effective and reliable technique for verifying that the USB port to be utilized in the battery charging process has sufficient power prior to the initiation of charging.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention a system and method is disclosed for verifying that a power source, such as a USB port, has sufficient power to charge a rechargeable battery. A controller is coupled to the USB port and to the rechargeable battery. The controller controls the application of source current from the USB port to the rechargeable battery by ramping the source current from the USB port in a controlled manner until the source current reaches a predetermined peak source current. While ramping the source current, the controller monitors the USB port output voltage. In the event the USB port output voltage decreases to a specified threshold voltage VIN(MIN)before the source current reaches the predetermined peak source current, the source current is disconnected from the rechargeable battery so as to discontinue the charging process since the decrease of the USB port output voltage indicates that the port is in a suspended state or has insufficient power to charge the battery. After removal of the source current from the rechargeable battery and after a predetermined delay interval, the controller repeats the above-described detection process. If the USB port output voltage does not decrease to the minimum specified threshold voltage Vin(min) by the time the source current reaches the predetermined peak source current, the controller permits the peak source current to continue to be provided from the power source to the rechargeable battery.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a method and system are disclosed for verifying that a USB port can source a predetermined peak source current without causing the USB output port voltage to drop below a predetermined minimum threshold voltage. The presently disclosed method and system are described below with reference toFIGS. 1-5.

Referring toFIG. 1, a Universal Serial Bus (USB) port100includes a voltage output VBUS102and a ground connection104and produces a USB port output voltage therebetween. The signals VBUSand GND are coupled to a controller that includes input power source detection circuitry110which is disposed between the USB port100and a rechargeable battery120. The detection circuitry110controls the application of charging current to the rechargeable battery120as described in greater detail below.

In response to a power on reset (POR) signal, the detection circuitry110loads a register (not shown) with a value that corresponds to a peak charging current ILOAd(PEAK). The peak charging current is the maximum current that is to be utilized for the charging of the rechargeable battery120. The signal VBUSis coupled to a current source130within the detection circuitry110. The current source130is controlled so as to ramp the charging current provided by the output port and applied to the rechargeable battery110during an interval TRISEfrom an initial charging current, such as 0 milliamps, to a final charging current which corresponds to the peak charging current ILOAd(PEAK)as illustrated in the lower portion ofFIG. 3. The charging current may be controlled by current source control circuitry132so as to increase linearly at a specified slew rate or in a non-linear manner from the initial charging current to the peak charging current. While the charging current is increasing, the USB port output voltage (VINto the detection circuitry110) is monitored using a comparator134. In the event the charging current has increased from the initial charging current to the peak charging current ILOAD(PEAK)and the comparator134indicates that the USB port output voltage VINto the detection circuitry110has not decreased below a predetermined minimum value VIN(MIN), the detection circuitry110permits the USB output port to continue to source the peak charging current to the rechargeable battery120to continue to charge the battery. In the event the comparator134determines that the USB port output voltage VINhas dropped below the predetermined minimum threshold voltage VIN(MIN)during the interval when the charging current is increasing from the initial charging current toward the peak charging current as depicted inFIG. 4, the detection circuitry110controls the current source130so as to cease the charging of the rechargeable battery120. More specifically, the comparator134, in response to a determination that the USB port output voltage VINhas dropped below the predetermined minimum threshold voltage VIN(MIN), generates a signal VIN—POORwhich is coupled to current source control circuitry132, which in turn causes the current source to cease sourcing current to the rechargeable battery120.

The comparator134output is also coupled to delay circuitry136. In the event the comparator134generates the signal VIN—POORindicating that the USB port output voltage VINhas dropped below the minimum threshold voltage VIN(MIN), the delay circuitry136generates a delay of length TINT, following which the delay circuitry136generates a Start signal which is coupled to the current source control circuitry132. In response to the Start signal, the current source control circuitry132controls the current source130to restart charging of the battery120by ramping the charge current, as previously discussed, from the initial charging current toward the peak charging current while monitoring the USB port output voltage VINto verify that such voltage does not drop below the predetermined minimum threshold voltage VIN(MIN)as illustrated inFIG. 4.

The current source130employed in the detection circuitry110may be implemented using a pulse width modulator (PWM) in which case the current control circuitry132includes the circuitry to provide appropriate pulse width modulation to generate the proper current as the source current is ramped from the initial charging current to the peak charging current. In this regard, a current sensing resistor138may be provided in series between the current source and the rechargeable battery to provide an indication to the current control circuitry132of the current supplied by the current source.

A more detailed block diagram illustrating input power source detection circuitry110athat employs pulse width modulation for control of the source current is depicted inFIG. 2a. The circuitry110aincludes a PWM controller132athat generates control signals for switching transistors Q1and Q3. The detection circuitry110includes a comparator134aand delay circuitry136athat functions as discussed above with respect toFIG. 1. The detection circuitry110aalso includes a transistor Q1that is controlled so as to provide a very low drain source resistance when forward biased and a very high drain source resistance with back biased so as to avoid damage to the USB port in a back biased state in which the rechargeable battery120voltage exceeds the USB port output voltage. The POR signal is coupled to control circuitry138awhich activates the loading of the register with the value corresponding to the peak source current and initiates the application of source current to the battery120.

The output from the PWM switching transistors Q2and Q3represents a signal that is generally a pulse width modulated waveform. This signal is coupled to an inductor-capacitor filter L1-C1to provide DC smoothing. The output of the inductor-capacitor filter is coupled to one end of a sense resistor R! and the other end of the sense resistor R1is coupled to the rechargeable battery120. Connections on either end of the sense resistor R! are fed back to the controller132ato provide an indication of the charging current to the PWM control circuitry132a.

In another embodiment illustrated inFIG. 2b, a voltage or current controlled linear current source is employed to generate the charging current is conveyed to the rechargeable battery120. More specifically, a linear current source employing a bipolar of field effect transistor130bprovides the charging current to the rechargeable battery120. The transistor130bis controlled by current control circuitry132bgenerally as discussed hereinabove with respect toFIG. 1to accomplish the timing and objectives discussed in connection withFIG. 1and depicted inFIGS. 3 and 4. The operation of the comparator134b, the delay circuitry136, transistor Q1and control circuitry138bare generally as discussed above in connection withFIGS. 1 and 2a.

A method of operation in accordance with the presently disclosed invention is depicted inFIG. 5. Referring toFIG. 5, in response to a power on reset (POR) signal, a default value for the peak charging current is loaded into a register as illustrated at step400. As shown at step402, detection circuitry110(FIG. 1) enables charging of the rechargeable battery120by applying a charging current that is increasing at a controlled rate. The charging current may be controlled to increase linearly or non-linearly. During the application of the increasing charging current to the rechargeable battery120, the USB port output voltage is monitored as illustrated at step404. If the USB port output voltage decreases below a predetermined threshold voltage VIN(MIN), as depicted in step406, the detection circuitry110disables charging of the rechargeable battery120and provides an indication that the USB output port cannot adequately source the required charging current. Following the disabling of charging of the rechargeable battery120, the detection circuitry110delays for a time interval TINTand then re-initiates charging of the rechargeable battery120as discussed in connection with step402and continues monitoring of the USB port output voltage as discussed in connection with decision step404to determine if the USB port output voltage VINdrops below the specified minimum threshold voltage VIN(MIN).

If the USB port output voltage does not decrease below the minimum threshold voltage VIN(MIN), the charging current supplied by the USB port continues to increase until the charging current reaches of the current ILOAD(PEAK)which corresponds to the maximum charging current specified by the stored value within the register. Once the charging current reaches the peak charging current ILOAD(PEAK), the detection circuitry110continues to source charging current from the USB port at the peak charging current. More specifically, if the charging current applied to the rechargeable battery120by the detection circuitry110reaches the peak charging current and the USB output port voltage has not decreased below a predetermined threshold voltage VIN(MIN) charging of the rechargeable battery continues at the peak charging current as illustrated in step410.

While the above-described system and method are discussed in terms of a Universal Serial Bus (USB) output port, it should be understood that the presently described detection circuitry and system may be employed with any power source that is employed to charge a rechargeable battery. It will be further apparent to those of ordinary skill in the art that modifications to and variations of the above-described system and method may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.