Method and apparatus for managing charging current

1. An apparatus, comprising:

first circuitry arranged to draw current from a charging source at multiple current levels using one or more current sinks coupled to an input line arranged to couple to the charging source and a voltage comparator having a first input coupled to the input line and a second input arranged to receive a reference voltage; and

second circuitry arranged to determine a charging current capacity of the charging source using the first circuitry by detecting a voltage drop of the current signal on the input line and determining a charging current capacity value of the charging source based upon a value of total sinking current drawn by the charging source when the voltage drop is detected.

2. The apparatus of claim 1 , the voltage comparator arranged to detect a voltage droop in the input line when current drawn by the one or more current sinks exceeds the charging current capacity.

3. The apparatus of claim 1 , the first circuitry coupled to the charging source through a USB connection.

4. The apparatus of claim 1 , the second circuitry comprising a hardware state machine arranged to:

apply a search routine that couples one or more of the current sinks to the charging source;

determine the charging current capacity of the charging source based upon the search routine; and

store the determined charging current capacity.

5. The apparatus of claim 1 , comprising circuitry arranged to:

detect the presence of a V BUS signal from a USB connector coupled to the charging source; and

couple the V BUS signal to the first circuitry.

6. The apparatus of claim 1 , comprising a battery charger coupled to the second circuitry, the battery charger comprising a current limiting transistor operable to adjust the charging current within the battery charger according to the determined charging current capacity of the charging source.

7. The apparatus of claim 1 , comprising a low dropout regulator arranged to receive power from the charging source and to supply power to the first and second circuitry.

8. A method, comprising:

connecting an input line to one or more current sources arranged to draw a respective one or more sinking currents;

drawing multiple current levels of a current signal received from a charging source;

determining a current level at which voltage of the current signal droop;

detecting a voltage drop of the current signal on the input line when the one or more current sources are connected to the input line; and

determining a charging current capacity value of the charging source based upon a value of total sinking current drawn by the one or more current sources when the voltage drop is detected.

9. The method of claim 8 , the determining the current level comprising performing a binary search routine that opens or closes the current signal to one or more current sinks of a multiplicity of current sinks arranged to couple to the charging source.

10. The method of claim 9 , comprising controlling a transistor to open or close the one or more current sinks.

11. The method of claim 9 , comprising storing a current capacity value based on a current level at which voltage of the current signal drops.

12. The method of claim 11 , comprising limiting current output of a current regulator coupled to a value that corresponds to the current level at which the voltage of the current signal droops.

13. The method of claim 9 , comprising:

performing a binary search that comprises:

monitoring voltage of the current signal when a level of sinking current is sunk from the charging source;

increasing the level of sinking current if no voltage drop is observed; and

decreasing the level of sinking current if a voltage drop is observed.

14. The method of claim 13 , comprising

coupling one or more current sinks to the charging source to adjust the level of sinking current; and

storing a current capacity value as a bit sequence in which each bit corresponds to a separate current sink and indicates whether that current sink is coupled or decoupled from the charging source when the voltage of the current signal droops.

15. A system, comprising:

a portable device comprising a set of platform components;

a battery arranged to provide power to the set of platform components;

an input line arranged to couple to a charging source;

one or more current sinks coupled to the input line;

a voltage comparator having a first input connected to the input line and a second input arranged to receive a reference voltage;

first circuitry arranged to draw current from the charging source at multiple current levels using one or more current sinks coupled to an input line arranged to couple to the charging source and a voltage comparator having a first input coupled to the input line and a second input arranged to receive a reference voltage;

second circuitry arranged to determine a charging current capacity of the charging source using the first circuitry by detecting a voltage drop of the current signal on the input line and determining a charging current capacity value of the charging source based upon a value of total sinking current drawn by the charging source when the voltage drop is detected.

16. The system of claim 15 , the system coupled to the charging source through a USB connection.

17. The system of claim 15 , the voltage comparator arranged to detect a voltage droop in the input line when current drawn by one or more sinks of the multiplicity of current sinks exceeds the charging current capacity.