LED driver
An LED driver is provided. The LED driver comprises a plurality of driving units and a fold circuit. The driving units are coupled to an LED string including a plurality of LEDs for driving the LEDs. The fold circuit is coupled to the LED string and divides the LED string into a plurality of LED segments when an input voltage is lower than a threshold voltage. The LED segments are coupled to each other in parallel, and the input voltage is supplied to the LED segments.
1. An LED driver, comprising:
a plurality of driving units, configured to be coupled to an LED string including a plurality of LEDs, the plurality of driving units for driving the plurality of LEDs;
a fold circuit, configured to be coupled to the LED string, for dividing the LED string into a plurality of LED segments in response to an input voltage being lower than a threshold voltage and for coupling the LED segments to each other in parallel, and the input voltage is supplied to the LED segments; and
a first driving unit of one of the plurality of LED segments having a selectable reference voltage that is selectable between a first reference voltage and a second reference voltage wherein the first reference voltage is less than the second reference voltage, the fold circuit configured to select the first reference voltage to the first driving unit in response to the input voltage being less than the threshold voltage and to select the second reference voltage to the first driving unit in response to the input voltage being no less than the threshold voltage.
2. The LED driver as claimed in claim 1 , further comprising a detection circuit detecting the input voltage, and generating a fold signal when the input voltage is lower than the threshold voltage, wherein the fold circuit divides the LED string into the LED segments in response to the fold signal.
3. The LED driver as claimed in claim 2 , wherein the detection circuit is coupled to the LED string to detect a voltage at the LED string for detecting the input voltage.
4. The LED driver as claimed in claim 3 , wherein the detection circuit is coupled to the middle of the LED string for detecting the input voltage.
5. The LED driver as claimed in claim 2 , wherein the driving units are coupled to the LEDs of the LED string respectively, the detection circuit is coupled to one of the driving units to detect the operation status of this driving unit for detecting the input voltage.
6. The LED driver as claimed in claim 1 , wherein the driving units are coupled to the LEDs of the LED string respectively, the driving units are current regulators, each of the driving units has a reference signal for controlling an LED current, and the reference signal of any driving unit is higher than or equal to the reference signal of the previous driving unit when the input voltage is higher than the threshold voltage.
7. The LED driver as claimed in claim 1 , wherein the driving units are coupled to the LEDs of the LED string respectively, the driving units are current regulators, each of the driving units has a reference signal for controlling an LED current; when the LED string is divided into the LED segments, the reference signal of any driving unit of each LED segment is higher than or equal to the reference signal of the previous driving unit of the same LED segment.
8. The LED driver as claimed in claim 1 , wherein the driving units are coupled to the LEDs of the LED string respectively, the driving units are current regulators, each of the driving units has a reference signal for controlling an LED current; when the LED string is divided into the LED segments, the LED segments comprise a first LED segment upstream to the dividing point of the LED string and a second LED segment downstream to the dividing point; the reference signal of the driving unit corresponding to the Nth LED of the second LED segment is lower than the reference signal of the driving unit corresponding to the Nth LED of the first LED segment.
9. The LED driver as claimed in claim 1 , wherein the LED segments comprise a first LED segment upstream to the dividing point of the LED string and a second LED segment downstream to the dividing point when the LED string is divided into the LED segments, in which the LED driver comprises a first current-sense element and a second current-sense element; the second current-sense element senses a first LED segment current flowing through the first LED segment, and the first current-sense element senses a total LED current of the first LED segment current and a second LED segment current flowing through the second LED segment.
10. The LED driver as claimed in claim 1 , wherein a terminal of the LED string is coupled to an input power, and the fold circuit comprises:
a switch, configured to be coupled between the dividing point of the LED string and the input power; and
a control circuit, coupled to the switch, and turning on the switch for dividing the LED string into the LED segments when the input voltage is lower than the threshold voltage.
11. The LED driver as claimed in claim 10 , wherein the control circuit comprises a current sink circuit coupled to the switch and for coupling to receive the input power, the current sink circuit turns on the switch and sinks a bleeding current from the input power when the input voltage is lower than the threshold voltage.
12. The LED driver as claimed in claim 1 , wherein each driving unit comprises:
a transistor, coupled between the LED and a ground; and
an operational amplifier, controlling the transistor for driving the LED in response to a reference signal.
13. The LED driver as claimed in claim 1 , further comprising a bleeder circuit coupled to an input power and sinking a bleeding current from the input power in accordance with a current-sense signal correlated to a total LED current for increasing a current flowing through a dimmer.
14. The LED driver as claimed in claim 13 , wherein the bleeding current is increased when the total LED current is decreased.
15. The LED driver as claimed in claim 13 , wherein the bleeder circuit comprises:
a buffer, receiving the current-sense signal; and
a current sink circuit, coupled to the input power and an output of the buffer for sinking the bleeding current from the input power in accordance with the current-sense signal and a bleeding reference signal.
16. The LED driver as claimed in claim 13 , wherein the bleeder circuit comprises:
an operation unit, generating an output signal in response to the current-sense signal and a bleeding reference signal; and
a current sink circuit, coupled to the input power and the operation unit for sinking the bleeding current from the input power in accordance with the output signal of the operation unit.
17. The LED driver as claimed in claim 13 , further comprising a current-sense element sensing the total LED current for generating the current-sense signal.
18. The LED driver as claimed in claim 1 , wherein the input voltage is a rectified AC voltage.