Constant input current filter for power supplies and related system and method

1. A system comprising:

a first capacitor; and

a current source configured to draw a constant input current from a power source and to generate an output current, the current source comprising an n-type field effect transistor that is biased to operate as a constant current source;

wherein the current source is configured to provide the output current to the first capacitor and charge the first capacitor during a first time period associated with operation of a load; and

wherein the current source is configured to provide the output current to the load and the first capacitor is configured to provide an additional current to the load during a second time period associated with operation of the load.

2. The system of claim 1 , wherein the current source further comprises:

a voltage source coupled to a drain of the transistor;

a first resistor coupled to the drain of the transistor;

a second resistor coupled between the voltage source and the first resistor;

a second capacitor coupled to a source of the transistor; and

a third resistor coupled between the second capacitor and the first and second resistors.

3. The system of claim 1 , wherein the current source further comprises:

a Zener diode coupled across the transistor.

4. The system of claim 3 , wherein the Zener diode is configured to conduct current around the transistor when the first capacitor is charged to a specified voltage level.

5. The system of claim 1 , further comprising:

a DC-DC converter coupled to the current source and the first capacitor, the DC-DC converter configured to be coupled to the load.

6. The system of claim 1 , wherein the load comprises an electronic device having a time-varying output power characteristic.

7. The system of claim 1 , wherein:

the load comprises a wireless radio;

the current source is configured to provide the output current to the first capacitor and charge the first capacitor during time periods when the wireless radio is not transmitting; and

the current source is configured to provide the output current to the wireless radio and the first capacitor is configured to provide the additional current to the wireless radio during time periods when the wireless radio is transmitting.

8. A method comprising:

drawing a constant input current from a power source using a current source;

generating an output current using an n-type field effect transistor in the current source that is biased to operate as a constant current source;

during a first time period associated with operation of a load, charging a first capacitor using the output current; and

during a second time period associated with operation of the load, providing the output current to the load and discharging the first capacitor to provide an additional current to the load.

9. The method of claim 8 , further comprising biasing the transistor using:

a voltage source coupled to a drain of the transistor;

a first resistor coupled to the drain of the transistor;

a second resistor coupled between the voltage source and the first resistor;

a second capacitor coupled to a source of the transistor; and

a third resistor coupled between the second capacitor and the first and second resistors.

10. The method of claim 8 , further comprising:

conducting current around the transistor using a Zener diode when the first capacitor is charged to a specified voltage level.

11. The method of claim 8 , further comprising:

providing the output current and the additional current to a DC-DC converter that is coupled to the load.

12. The method of claim 8 , wherein the load comprises an electronic device having a time-varying output power characteristic.

13. The method of claim 8 , wherein the load and the current source collectively comply with a CE101 power lead requirement defined in a MIL-STD-461 standard for power leads operating in a range of 30Hz to 10kHz.

14. The method of claim 8 , wherein:

the load comprises a wireless radio;

the output current is provided to the first capacitor and charges the first capacitor during time periods when the wireless radio is not transmitting; and

the output current and the additional current are provided to the wireless radio during time periods when the wireless radio is transmitting.

15. An apparatus comprising:

a current source configured to draw a constant input current from a power source and to generate an output current, the current source comprising an n-type field effect transistor that is biased to operate as a constant current source;

wherein the current source is configured to provide the output current to a first capacitor and charge the first capacitor during a first time period associated with operation of a load; and

wherein the current source is configured to provide the output current to the load and the first capacitor is configured to provide an additional current to the load during a second time period associated with operation of the load.

16. The apparatus of claim 15 , wherein the current source further comprises:

a voltage source coupled to a drain of the transistor;

a first resistor coupled to the drain of the transistor;

a second resistor coupled between the voltage source and the first resistor;

a second capacitor coupled to a source of the transistor; and

a third resistor coupled between the second capacitor and the first and second resistors.

17. The apparatus of claim 16 , wherein a resistance of the first resistor is selected to properly bias the transistor.

18. The apparatus of claim 16 , wherein the voltage source comprises a battery.

19. The apparatus of claim 15 , wherein the current source further comprises:

a Zener diode coupled across the transistor.

20. The apparatus of claim 19 , wherein the Zener diode is configured to conduct current around the transistor when the first capacitor is charged to a specified voltage level.

21. The system of claim 2 , wherein a gate of the transistor is coupled between the second capacitor and the third resistor.