LINEARLY REGULATED BATTERY CHARGER

• US 20090206794A1
• Filed: 04/27/2009
• Published: 08/20/2009
• Est. Priority Date: 01/21/2003
• Status: Active Grant

• Current Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Original Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Inventors: Ferguson, Bruce R.
• Granted Patent: US 7,759,906 B2
• Time in Patent Office: Days
• US Class Current: 320/134
• CPC Class Codes: H02J 2207/30 Charge provided using DC bus or data bus of a computer
• CPC Class Codes: H02J 7/0047 with monitoring or indicating devices or circuits
• CPC Class Codes: H02J 7/007182 in response to battery voltage
• CPC Class Codes: H02J 7/04 Regulation of charging current or voltage

Abstract

A battery charger for an electronic device receives current limited power from an external power source, such as a Universal Serial Bus power interface. The battery charger can linearly regulate a charging current to an internal battery and limit the charging current so as not to demand current in excess of what the external power source can provide. A bi-directional pass element coupled between a system power terminal and the internal battery controls the charging current and effectively isolates the internal battery from a system load during charging of the battery while providing a low impedance path from the internal battery to the system load during discharging of the battery.

Claims

1. A battery charger comprising:

a first input configured to accept DC power from an AC adapter;

a second input configured to accept DC power from a Universal Serial Bus power interface;

a first output configured to provide power to a system load;

a second output configured for connection to a battery;

a first isolation circuit coupled between the first input and the first output to prevent current flow in a reverse direction to the first input;

a second isolation circuit and a current sensing circuit coupled in series between the second input and the first output, wherein the second isolation circuit prevents current flow in a reverse direction to the second input, and the current sensing circuit measures the current flow from the Universal Serial Bus power interface;

a pass element coupled between the first output and the second output, wherein the pass element conducts a charging current in a first direction to the second output during charging of the battery; and

a charging controller configured to linearly regulate the charging current of the pass element based on the current flow from the Universal Serial Bus power interface.

2. The battery charger of claim 1, wherein the charging controller reduces the charging current in response to an increase in current to the system load when the current flow from the Universal Serial Bus power interface exceeds a predefined current threshold.

3. The battery charger of claim 1, further comprising a current limiting circuit coupled between the Universal Serial Bus power interface and the second input.

4. The battery charger of claim 1, further comprising:

a first bypass transistor coupled across the first isolation circuit; and

a second bypass transistor coupled across the second isolation circuit, wherein the first bypass transistor and the second bypass transistor are selectively enabled to couple an output of the AC adapter or an output of the Universal Serial Bus power interface to the first output.

5. The battery charger of claim 4, wherein the second bypass transistor is automatically disabled when power is detected at the first input.

6. The battery charger of claim 1, wherein the first isolation circuit comprises a first diode with an anode coupled to the first input and a cathode coupled to the first output while the second isolation circuit comprises a second diode with an anode coupled to the second input and a cathode coupled to the first output.

7. The battery charger of claim 1, wherein the pass element is a series-connected transistor configured to additionally conduct current in a second direction from the second output to the first output during discharging of the battery.

8. The battery charger of claim 7, wherein discharging of the battery occurs when voltage at the second output is greater than voltage at the first output.

9. The battery charger of claim 7, wherein discharging of the battery occurs in response to a discharge command.

10. A battery controller comprising:

a pass element coupled between a system power terminal and a battery terminal;

a battery control loop that senses a voltage difference between the system power terminal and the battery terminal, wherein the battery control loop outputs a feedback control signal based on the voltage difference; and

a pass element driver that accepts the feedback control signal, wherein the pass element driver linearly regulates current conducted by the pass element using the feedback control signal.

11. The battery controller of claim 10, wherein the pass element is a P-channel enhancement mode MOSFET with a source terminal coupled to the system power terminal and a drain terminal coupled to the battery terminal.

12. The battery controller of claim 10, wherein the pass element is a MOSFET with a configurable body contact.

13. The battery controller of claim 12, wherein the configurable body contact is coupled to the system power terminal during a charging mode and to the battery terminal during a discharging mode.

14. The battery controller of claim 12, wherein the configurable body contact is coupled to a transistor terminal with a relatively higher voltage during a shutdown mode to prevent current flow in a body diode and thereby fully disconnecting the battery terminal from the system power terminal.

15. The battery controller of claim 12, further comprising a comparator with inputs coupled across the MOSFET to sense a voltage polarity of the MOSFET to generate an output to control connections of the configurable body contact.

16. The battery controller of claim 10, wherein power is selectively provided to the system power terminal by an AC adapter or a USB power interface.

17. The battery controller of claim 16, further comprising:

a current sensor that outputs a current sense voltage indicative of the level of current supplied by the USB power interface to the system power terminal; and

an error amplifier that compares the current sense voltage to a reference level and overrides the feedback control signal to reduce the current conducted by the pass element when the current sense voltage exceeds the reference level.

18. The battery controller of claim 10, further comprising a switching diode coupled across the pass element to improve battery response during a discharging mode, wherein the switching diode is inactive during a charging mode.

19. A Universal Serial Bus battery charger comprising:

means for selectively coupling a Universal Serial Bus power interface to a system power terminal of a portable device;

means for coupling a battery to the system power terminal through a pass element;

means for sensing an input current at the Universal Serial Bus power interface; and

means for linearly controlling the current conducted by the pass element to ensure that the input current is less than a predetermined limit.

20. The Universal Serial Bus battery charger of claim 19, further comprising means for sensing a voltage difference between the system power terminal and the battery to determine an operating mode, wherein a charging mode occurs when voltage at the system power terminal is greater than voltage at the battery and a discharging mode occurs when voltage at the battery is greater than voltage at the system power terminal.