System and method of trickle charging a battery in a narrow rail architecture

1. An apparatus for charging a battery of an electronic device using a connected a/c power adapter, comprising:

a voltage regulator for regulating an input voltage provided by the a/c power adapter and providing a regulated output voltage;

a transistor connected between the regulated output voltage and the battery;

charging control circuitry responsive to the regulated output voltage applied thereto for controlling charging of the battery connected to the charging control circuitry;

wherein the charging control circuitry prevents the regulated voltage applied to the charging control circuitry from falling below a settable voltage level and switches a charging current between a quick charge level and a trickle charge level responsive to the state of the transistor;

wherein the charging control circuitry further drives the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if a battery voltage is approximately less than the settable voltage level.

2. The apparatus of claim 1 , wherein the charging control circuitry further includes:

an amplifier connected to drive the transistor; and

wherein the amplifier drives the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if the battery voltage is approximately less than the settable voltage level.

3. The apparatus of claim 2 , wherein the amplifier turns off the transistor to disconnect the battery from the charging control circuitry responsive to a first control input indicating the battery is fully charged.

4. The apparatus of claim 2 , wherein the amplifier turns on the transistor to connect the battery to the charging control circuitry responsive to a second control input indicating the a/c adapter has been disconnected from the voltage regulator.

5. The apparatus of claim 2 , wherein the charging control circuitry further includes:

a second comparator for generating a first output if the transistor is operating in a linear region of operation and for generating a second output if the transistor is not operating in a linear region of operation;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output; and

wherein the reference voltage at the first level causes the charging circuitry to provide a charging current at the trickle charge level and the reference voltage at the second level causes the charging circuitry to provide a changing current at the quick charge level.

6. The apparatus of claim 2 , wherein the charging control circuitry further includes:

a second comparator for generating a first output if a drain to source voltage of the transistor is greater than a predetermined voltage level and for generating a second output if the drain to source voltage of the transistor is not greater than the predetermined voltage level;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output; and

wherein the reference voltage at the first level causes the charging control circuitry to provide a charging current at the trickle charge level and the reference voltage at the second level causes the charging circuitry to provide a changing current at the quick charge level.

7. The apparatus of claim 2 , wherein a power dissipation within the transistor is less than 1 watt.

8. The apparatus of claim 2 , wherein the charging control circuitry further includes a back-up comparator for turning off the transistor if the amplifier does not respond quickly enough and when the regulated voltage applied to the charging circuitry drops below the settable voltage level.

9. An apparatus for charging a battery of an electronic device using a connected a/c power adapter, comprising:

a voltage regulator for regulating an input voltage provided by the a/c power adapter and providing a regulated output voltage;

charging control circuitry responsive to the regulated output voltage applied thereto for controlling charging of the battery connected to the charging circuitry, the charging circuitry including:

a transistor connected between the regulated output voltage and the battery;

an amplifier connected to drive the transistor;

a comparator for generating a first output if the transistor is operating in a linear region of operation and for generating a second output if the transistor is not operating in a linear region of operation;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output;

wherein the amplifier drives the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if the battery voltage is approximately less than the settable voltage level; and

wherein the reference voltage at the first level causes the charging control circuitry to provide a charging current at a trickle charge level and the reference voltage at the second level causes the charging circuitry to provide a changing current at a quick charge level.

10. The apparatus of claim 9 , wherein the amplifier turns off the transistor to disconnect the battery from the charging control circuitry responsive to a first control input indicating the battery is fully charged.

11. The apparatus of claim 9 , wherein the amplifier turns on the transistor to connect the battery to the charging control circuitry responsive to a second control input indicating the a/c adapter has been disconnected from the voltage regulator.

12. The apparatus of claim 9 , wherein a power dissipation within the transistor is less than 1 watt.

13. The apparatus of claim 9 , wherein the charging control circuitry further includes a back-up comparator for turning off the transistor if the amplifier does not respond quickly enough and when the regulated voltage applied to the charging circuitry drops below the settable voltage level.

14. Charging circuitry for charging a battery of an electronic device using a connected a/c power adapter, comprising:

a transistor connected between an applied regulator voltage and the battery;

circuitry responsive to the applied regulated voltage for charging the battery connected to the charging circuitry;

wherein the circuitry prevents the regulated voltage applied to the circuitry from falling below a settable voltage level and switches a charging current between a quick charge level and a trickle charge level responsive to a state of the transistor; and

wherein the control circuitry further drives the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if a battery voltage is approximately less than the settable voltage level.

15. The charging circuitry of claim 14 , wherein the circuitry further includes:

an amplifier connected to drive the transistor; and

wherein the amplifier drives the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if the battery voltage is approximately less than the settable voltage level.

16. The charging circuitry of claim 15 , wherein the amplifier turns off the transistor to disconnect the battery from the circuitry responsive to a first control input indicating the battery is fully charged.

17. The charging circuitry of claim 15 , wherein the amplifier turns on the transistor to connect the battery to the circuitry responsive to a second control input indicating the a/c adapter has been disconnected from the voltage regulator.

18. The charging circuitry of claim 15 , wherein the circuitry further includes:

a comparator for generating a first output if the transistor is operating in a linear region of operation and for generating a second output if the transistor is not operating in a linear region of operation;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output; and

wherein the reference voltage at the first level causes the circuitry to provide a charging current at the trickle charge level and the reference voltage at the second level causes the circuitry to provide a changing current at the quick charge level.

19. The charging circuitry of claim 15 , wherein the circuitry further includes:

a comparator for generating a first output if a drain to source voltage of the transistor is greater than a predetermined voltage level and for generating a second output if the drain to source voltage of the transistor is not greater than the predetermined voltage level;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output; and

wherein the reference voltage at the first level causes the circuitry to provide a charging current at the trickle charge level and the reference voltage at the second level causes the charging control circuitry to provide a changing current at the quick charge level.

20. The charging circuitry of claim 15 , wherein the circuitry further includes a back-up comparator for turning off the transistor if the amplifier does not respond quickly enough and when the regulated voltage applied to the charging circuitry drops below the settable voltage level.

21. The charging circuitry of claim 15 , wherein the circuitry further includes:

a comparator for generating a first output if a drain to source voltage of the transistor is greater than a predetermined voltage level and for generating a second output if the drain to source voltage of the transistor is not greater than the predetermined voltage level;

a reference voltage block for generating a reference voltage at a first level responsive to the first output and for generating the reference voltage at a second level responsive to the second output; and

wherein the reference voltage at the first level causes the circuitry to provide a charging current at the trickle charge level and the reference voltage at the second level causes the circuitry to provide a changing current at the quick charge level.

22. A method for charging a battery of an electronic device using a connected a/c power adapter, comprising the steps of:

regulating an input voltage provided by the a/c power adapter;

providing a regulated output voltage;

controlling charging of the battery responsive to the regulated output voltage, the step of controlling further comprising the steps of:

monitoring a state of a transistor connected between the regulated output voltage and the battery

preventing the regulated voltage from falling below a settable voltage level and switching a charging current between a quick charge level and a trickle charge level responsive to the state of the transistor; and

driving the transistor in a source follower configuration to maintain the regulated output voltage at the settable voltage level if the battery voltage is approximately less than the settable voltage level.

23. The method of claim 22 , wherein the step of controlling further comprises the steps of turning off the transistor to disconnect the battery responsive to a first control input indicating the battery is fully charged.

24. The method of claim 22 , wherein the step of controlling further comprises the step of turning on the transistor to connect the battery responsive to a second control input indicating the a/c adapter has been disconnected.

25. The apparatus of claim 22 , wherein the step of controlling further includes the steps of:

generating a first output if the transistor is operating in a linear region of operation;

generating a second output if the transistor is not operating in a linear region of operation;

generating a reference voltage at a first level responsive to the first output;

generating the reference voltage at a second level responsive to the second output; and

providing a charging current at the trickle charge level responsive to the reference voltage at the first level;

providing the changing current at the quick charge level responsive to the reference voltage at the second level.

26. The method of claim 22 , wherein the step of controlling further includes the steps of:

generating a first output if a drain to source voltage of the transistor is greater than a predetermined voltage level;

generating a second output if the drain to source voltage of the transistor is not greater than the predetermined voltage level;

generating a reference voltage at a first level responsive to the first output;

generating the reference voltage at a second level responsive to the second output;

providing a charging current at the trickle charge level responsive to the reference voltage at the first level; and

providing the changing current at the quick charge level responsive to the reference voltage at the second level.

27. The method of claim 22 , wherein the step of controlling further includes the step of turning off the transistor responsive to a response time of an amplifier driving the transistor and the regulated voltage dropping below the settable voltage level.