SHORT CIRCUIT PROTECTION

• US 20140139143A1
• Filed: 07/11/2013
• Published: 05/22/2014
• Est. Priority Date: 11/16/2012
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Navabi-Shirazi, Mohammad J., Hussain, Asif, Chen, Jingdong, Pandya, Manisha P.
• Granted Patent: US 9,185,768 B2
• Time in Patent Office: Days
• US Class Current: 315/310
• CPC Class Codes: G06F 1/26 Power supply means, e.g. regulation thereof for memories G11C
• CPC Class Codes: H02M 1/0025 Arrangements for modifying reference values, feedback values or error values in the control loop of a converter
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators
• CPC Class Codes: H05B 45/38 using boost topology
• CPC Class Codes: H05B 45/50 responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits
• CPC Class Codes: H05B 47/25 Circuit arrangements for protecting against overcurrent
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

A measured voltage drop across a power-line transistor is used as a sensing element to measure the current and detect an over-current condition for an LED backlight system. An over-current or short condition is detected when the measured voltage drop exceeds a threshold. Accurate detection of the over-current condition is achieved by calibrating the R_DS-ON (i.e., internal resistance between drain and source, when transistor is on) of the power-line transistor. In one embodiment, the calibration of R_DS-ON is performed by ramping down the threshold from an initial value and using the tripped threshold to determine the actual value for R_DS-ON. In another embodiment, the calibration of R_DS-ON is performed by using two thresholds, a first threshold to calibrate R_DS-ON and a second threshold to detect the over-current condition.

Claims

1. 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 a known 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 known 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.

2. The method of claim 1, 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.

3. The method of claim 2, wherein the driver is a boost converter for LED (light emitting diode) backlighting.

4. The method of claim 2, wherein the known 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 at maximum current, and

a boost efficiency.

5. The method of claim 2, 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.

6. The method of claim 2, wherein the calibrated threshold voltage level is stored in a non-volatile memory.

7. The method of claim 2, 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.

8. The method of claim 7 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.

9. The method of claim 8, 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.

10. The method of claim 2, wherein the calibrated threshold voltage level is determined using a pre-set minimum drain to source “

on”

resistance for the power-line transistor, instead of the actual drain to source “

on”

resistance, when the actual drain to source “

on”

resistance is lower than the pre-set minimum drain to source “

on”

resistance.

11. The method of claim 2, wherein calibrating the driver under normal operating conditions with the known input current comprises:

ramping a current up to a maximum value, and

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

12. A device configured for accurate detection and prevention of 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 accurately determined by applying a known 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 over-current condition for the device at the output of the device.

13. The device of claim 12,

wherein the actual drain to source “

on”

resistance for the power-line transistor is used to determine a calibrated threshold voltage level,

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.

14. The device of claim 13 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.

15. The device of claim 14 wherein the power converter is a boost converter for LED (light emitting diode) backlighting.

16. A method to use two threshold voltage levels and a measured voltage drop across a power-line transistor for a driver to accurately detect and prevent 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 a known input current;

using a comparator and the first threshold voltage level to accurately 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;

using the actual drain to source “

on”

resistance for the power-line transistor to determine the second threshold voltage level for detecting and preventing 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 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 LED (light emitting diode) backlighting.