Short circuit protection

1. A device configured for accurate detection and prevention of an over-current condition at an output of the device, the device comprising:

a power converter configured for converting an input power from an external supply to an output power at the output of the device;

a power-line transistor configured for switching on and off the input power from the external supply; and

a comparator configured for comparing a measured voltage drop across the power-line transistor with a threshold voltage level,

wherein an actual drain to source “on” resistance for the power-line transistor is determined by applying an input current through the power-line transistor and ramping down the threshold voltage level from an initial value until the measured voltage drop across the power-line transistor equals the threshold voltage level,

wherein the actual drain to source “on” resistance for the power-line transistor is used to detect and prevent the over-current condition for the device at the output of the device.

2. The device of claim 1 ,

wherein the actual drain to source “on” resistance for the power-line transistor is used to determine a calibrated threshold voltage level, and

wherein the over-current condition for the device occurs when the measured voltage drop across the power-line transistor exceeds the calibrated threshold voltage level.

3. The device of claim 2 further comprising:

a logic configured for switching on and off the power-line transistor,

wherein the power-line transistor is switched off when the over-current condition for the device occurs.

4. The device of claim 3 wherein the power converter is a boost converter for a light emitting diode backlight.

5. A method to accurately detect and prevent an over-current condition for a driver using a measured voltage drop across a power-line transistor for the driver, the method comprising:

setting a threshold voltage level to an initial value;

calibrating the driver with an input current;

comparing, with a comparator, the measured voltage drop across the power-line transistor with the threshold voltage level while the threshold voltage level is ramping down from the initial value;

tripping the comparator to generate a tripped threshold voltage level when the measured voltage drop across the power-line transistor equals the threshold voltage level;

using the tripped threshold voltage level and the input current to accurately determine an actual drain to source “on” resistance for the power-line transistor; and

using the actual drain to source “on” resistance for the power-line transistor to detect and prevent the over-current condition for the driver.

6. The method of claim 5 , wherein using the actual drain to source “on” resistance for the power-line transistor to detect and prevent the over-current condition for the driver comprises:

using the actual drain to source “on” resistance for the power-line transistor to determine a calibrated threshold voltage level,

wherein the over-current condition for the driver occurs when the measured voltage drop across the power-line transistor exceeds the calibrated threshold voltage level;

switching off the power-line transistor when the over-current condition for the driver occurs.

7. The method of claim 6 , wherein the driver is a boost converter for a light emitting diode backlight.

8. The method of claim 6 , wherein the input current is based on a system configuration, the system configuration comprising:

an input supply voltage,

a number of LEDs,

a voltage drop across each LED of the number of LEDs at a maximum current, and

a boost efficiency.

9. The method of claim 6 , wherein the calibrated threshold voltage level is set to be equal to a maximum expected input current times the actual drain to source “on” resistance for the power-line transistor times m,

wherein m is a scale factor allowing for margin.

10. The method of claim 6 , wherein the calibrated threshold voltage level is stored in a non-volatile memory.

11. The method of claim 6 , wherein the initial value of the threshold voltage level is set to be equal to a maximum expected input current times a minimum expected drain to source “on” resistance for the power-line transistor times k,

wherein k is a scale factor allowing for margin,

wherein the initial value protects the driver from entering the over-current condition during calibration.

12. The method of claim 11 further comprising:

before starting the step of calibrating the driver under normal operating conditions, testing the driver for the over-current condition with an LED backlighting off.

13. The method of claim 12 , wherein testing the driver for the over-current condition with the LED backlighting off comprises:

setting the threshold voltage level to the initial value;

operating the driver with the LED backlighting off;

comparing the measured voltage drop across the power-line transistor with the threshold voltage level while the threshold voltage level is ramping down from the initial value;

tripping the driver when the measured voltage drop across the power-line transistor exceeds the threshold voltage level;

determining that there are no existing short or over-current conditions when the driver does not trip or the driver trips at a very low voltage; and

determining that there are existing short or over-current conditions when the driver trips at a high voltage.

14. The method of claim 6 , wherein the calibrated threshold voltage level is determined using a pre-set minimum drain to source “on” resistance for the power-line transistor when the actual drain to source “on” resistance is lower than the pre-set minimum drain to source “on” resistance.

15. The method of claim 6 , wherein calibrating the driver under normal operating conditions with the input current comprises:

ramping a current up to a maximum value, and

allowing enough time for the driver to reach a stable condition.

16. A method to use at least two threshold voltage levels and a measured voltage drop across a power-line transistor for a driver to accurately detect and prevent an over-current condition for the driver, the method comprising:

setting a first threshold voltage level to a first initial value that protects the driver from entering the over-current condition during calibration;

setting a second threshold voltage level to a second initial value that is greater than the first initial value for the first threshold voltage level;

calibrating the driver under normal operating conditions with an input current;

using a comparator and the first threshold voltage level to determine an actual drain to source “on” resistance for the power-line transistor when the measured voltage drop across the power-line transistor equals the first threshold voltage level while the first threshold voltage level is ramping down from the first initial value;

using the actual drain to source “on” resistance for the power-line transistor to determine the second threshold voltage level for detecting and preventing the over-current condition for the driver.

17. The method of claim 16 , wherein using the actual drain to source “on” resistance for the power-line transistor to determine the second threshold voltage level for detecting and preventing the over-current condition comprises:

using the actual drain to source “on” resistance for the power-line transistor to determine and set a calibrated second threshold voltage level,

wherein the over-current condition for the driver occurs when the measured voltage drop across the power-line transistor exceeds the calibrated second threshold voltage level;

switching off the power-line transistor when the over-current condition for the driver occurs.

18. The method of claim 17 , wherein the calibrated second threshold voltage level is set to be equal to a maximum expected input current times the actual drain to source “on” resistance for the power-line transistor times m, wherein m is a scale factor allowing for margin.

19. The method of claim 17 , wherein the calibrated second threshold voltage level is dynamically adjusted and set based on an actual load current flowing through the driver.

20. The method of claim 17 , wherein the driver is a boost converter for a light emitting diode backlight.