Single inductor serial-parallel LED driver

Abstract

An LED driver circuit is disclosed that can drive a plurality of LED strings that are arranged in parallel, each LED string having a plurality of component LEDs that are series-connected. The LED strings can be the same type of LEDs in each string, or have different types of LEDs from one string to another. The LED driver includes a voltage control loop that dynamically regulates the output voltage across the parallel arrangement of LED strings. The output voltage is dynamically adjusted to accommodate the LED string with the largest operational voltage drop. This enables LED displays to constructed using different types of LEDs strings, but still supply the LED strings in a power efficient manner. Further, each LED string also includes its own individual current regulation loop so that the current, and therefore brightness, of each LED string can be individually adjusted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements and in which:

FIG. 1 illustrates a serial LED driver with a single string of LEDs connected in series.

FIG. 2 illustrates an LED driver to drive a set of parallel LED strings, where the individual strings have LEDs connected in series.

FIG. 3 illustrates an LED circuit having a set of parallel LED strings that are driven according to one embodiment of the present invention.

FIG. 4 illustrates main components of a current regulation loop used to control current variations in each of the parallel LED strings.

FIG. 5 further illustrates an exemplary implementation of the LED circuit according one embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An LED driver circuit is disclosed for an LED display system that includes the ability to control the current in each LED string and the voltage output. The present invention also provides improved power efficiency and scalability. In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail, as they are well known to those skilled in the art.

FIG. 3 illustrates a LED display system 300 having an LED driver 301 driving a plurality LED strings 310₁ to 310_n according to embodiments of the invention, where each LED string 310 includes a plurality of LEDs 309. As shown, the LED strings 310₁ to 310_n are connected in parallel with each other, but the LEDs 309 in each LED string are series connected. The total number of LEDs 309 in each individual string 310 may vary depending on the particular type of display system 300 that is intended. Further, the type of LEDs 309 may vary from string to string. For example, a first string 310₁ may contain one or more flash LEDs, and a second string 310₂ may contain backlighting LEDs.

The LED driver 301 includes both a voltage regulation loop 324 and current regulation loops 314₁ to 314_n, according to one embodiment of the present invention. As such, the voltage Vout 308 is regulated to the minimum voltage necessary to supply all of the LED strings 310. In other words, the Vout 308 is determined to meet the voltage requirement of the LED string 310 that requires the most voltage drop to be operational. Further, the current in each LED string 310 can be individually regulated by the corresponding current regulators 314₁ to 314_n. The LED driver 301 further includes an output voltage capacitor 315, one terminal of which is connected to a ground 330 and the other to a common node connecting the output of a voltage regulation loop 324 and anodes of the LED strings 310₁ to 310_n.

The input to the voltage regulation loop 324 includes an input voltage source VIN 326 via an inductor 325, a reference voltage input VREF 322, and a connection to a feedback signal 328. The feedback signal 328 originates from an output line 320 of a minimum voltage selector 312. The output voltages 316₁ to 316_n from the cathodes of the last LEDs of the serial LED strings 310₁-310_n are fed into the input of the minimum voltage selector 312. The number of input lines to the minimum voltages selector 312 corresponds to the LED strings 310₁-310_n. As will be discussed further below, the minimum voltage selector 312 selects the lowest of the input voltages 316₁-316_n from the ends of the LED strings 310, and then the voltage control loop 324 drives Vout 308 so that these the lowest input voltage is set approximately equal to Vref 322. This ensures that Vout 308 is sufficient to drive all of the LED strings 310 regardless of any differing voltage requirements among the LED strings 310. The current regulation loop 314₁ is connected to the cathode of the last LED in the LED string 310₁ to set and maintain the current in the LED string 310₁. Similarly, the other serial LED strings 310₂-310_n have their individual current regulation loops 314₂-314_n. The internal circuitry of the current regulation loops 314₁-314_n will be described in more detail with respect to FIG. 4.

The operation of the voltage control loop 324 and the minimum voltage selector 312 will now be described in more detail by means of an example. Consider two exemplary serial LED strings 310₁ and 310_n. Assume that the total voltage drop across serial LED string 310₁ is 6 Volts and that across serial LED string 310_n is 8 Volts, due to differing LED characteristics. The minimum voltage selector 312 receives the two voltage values 316₁ and 316_n corresponding to the two voltage drops. If Vout=8 volts, then voltage 316₁=2 v and voltage 316_n=0 v. However, the current regulation loops 314 require some minimum voltage drop to be operational. Therefore, at Vout=8 v, LED string 310_n may not be fully turned-on at Vout=8 v, if there is 0 volts at 316_n. Therefore, the minimum voltage selector 312 selects the lowest voltage values from nodes 316₁ to 316_n and outputs the minimum voltage to the feedback input 328. The control loop 332 then compares the minimum voltage to Vref 322, and drives the FET 330 so that the minimum voltage 316 is equal to Vref 322. Specifically, the control loop 332 increases or decreases the on-off duty-cycle of FET 330 so that Vout 308 adjusted as necessary in order for the minimum voltage 316 to be equal to Vref 322. In doing so, a minimum voltage at each of the nodes 316 is guaranteed so that the current regulation loops 314 are all operational. Further, each of the LEDs string 310₁-310_n is also guaranteed to have enough voltage drop to remain operational. In this specific example herein, Vref 322 may be set to say 0.4 v, which requires Vout=8.4 volts, so as accommodate the 8 v drop across the LED string 310_n.

In summary, minimum voltage selector 312 and the voltage regulation loop 324 operate so that the Vout 308 to accommodate the LED string 310 with the highest voltage drop, in order to achieve dynamic voltage regulation. But Vout 308 is not set unnecessarily high, so as to minimize power requirements. Using this technique, all the parallel serial LED strings 310₁-310_n will have the sufficient voltage for the individual LEDs, which are a part of a particular string.

FIG. 4 illustrates one embodiment of the current regulation loops 314. Referring to FIG. 4, the current regulation loop 314 includes an operational amplifier 416 (hereinafter, referred to as “OPAMP 416”) having a positive input terminal 402, a negative input terminal 404 and an output terminal 418 connected to the gate of a FET 408. Other types of transistors could be used besides FETs, including BJTs. The positive terminal 402 receives a reference voltage VREF1404, which is determined based on the desired current that is to flow through the serial LED string 310. The negative terminal 404 is connected to the source of the FET 408 at node 410, which is connected to one terminal of a resistor 412. The second terminal of the resistor 412 is connected to a ground 414. Resistor 412 is preferably a highly accurate, stable resistor so that a voltage measurement at node 410 will be used to accurately determine the current through the LED string 310. The drain of the FET 408 is connected to the cathode of the last LED 309 of the corresponding serial LED string 310 at a node 316, as shown.

Still referring to FIG. 4, during operation, OPAMP 416 detects the voltage drop across resistor 412 by measuring the voltage at node 410 and comparing it to VREF1404. The OPAMP 416 generates an output voltage 418 that controls the gate voltage of the FET 408, and therefore the conductivity of FET 408 based on the difference between the voltage at node 410 and the reference voltage 404. More specifically, the OPAMP 416 measures the voltage across the resistor 412 and drives the FET 408 so that the voltage across the resistor 412 substantially matches the reference voltage 404. As such, the conductivity of FET 416, and therefore the current flow through the corresponding LED string 310, can be adjusted higher or lower (i.e. regulated) by adjusting the reference voltage 404. The reference voltage VREF1404 can be different for each of the serial LED strings 310₁-310_n so as to individually tailor the current flow through each LED string 310. In sum, the current regulation loops 314 individually regulate the current in each LED string 310, according to adjustments made to the corresponding voltage reference 404. Since the current flow controls the brightness of an LED, then adjusting the reference voltage in a particular current regulation loop also controls the brightness of the LED string 310.

FIG. 5 further shows the LED driver 301 with the current regulation loops 314 illustrated in FIG. 4. As discussed above, the voltage regulation loop 324 provides dynamic voltage regulation by setting the output voltage Vout 308 so as to satisfy the LED string 310 with the highest voltage drop requirements. Further, the current regulation loops 314 also provide individual current regulation for each of the LED strings 310, based on the corresponding reference voltages Vref 404.

Let the currents flowing through each of the serial LED string 500₁, 500₂ . . . be denoted be i₁, i₂, . . . . The current i₁ has the value:

$\begin{array}{cc}{i}_1=\frac{\mathrm{VREF}1}{\mathrm{RB}1}& \left(1\right)\end{array}$

where VREF1=reference voltage VREF1404 at the positive terminal 402 of the current regulation loop 400 and RB1=resistor 412 shown in FIG. 4. Since RB1 is a precision resistor, it is almost of a constant value. Therefore, as can be seen from equation (1), reference voltage VREF1404 can be used to vary the value of the current i₁ in the first serial LED string 310₁. The same holds true for the other serial LED strings 310₂ to 310_n, as was discussed in reference to FIG. 4 above.

If reference voltages in the current regulation loop are equal (i.e. Vref₁=Vref_n), then the voltage drop between Vout 308 and node 410₁ is equal to the voltage drop between Vout 308 and node 410_n. However, the voltage differences between the node Vout 308 and the cathode of the last LEDs 309 (node 316) of each of the serial LED strings 310 can vary depending upon the brightness requirements for each serial LED string 310. The extra or differing voltage drop between LED strings 310 is accounted for by the FETs 408 in the current loops. In other words, if the one LED string 310 requires a higher voltage drop than another LED string, the extra voltage in the LED string with the lower voltage drop is dropped across the corresponding FET 408, assuming the current loop reference voltages 404 are equal. Since different regions of the display may need different optical outputs, the flexibility in varying the output voltage VOUT 308, if needed, adds to the design features of the LED driver 301. Therefore, a stable output voltage VOUT 308 across the terminals of the output capacitor 315 is maintained while attaining different brightness levels for different LED strings. Meanwhile, the ability to adjust the current draw of each LED string through the current loop adds addition brightness adjusting, and power efficiency savings.

As is mentioned elsewhere, FIG. 5 is an exemplary embodiment of the present invention. Depending upon whether a constant (or a static) display is required or a varying (or a dynamic) display is required, different features of the claimed invention can be implemented, thereby resulting in more embodiments. Such embodiments will be apparent to those skilled in the art and can be learnt by the practice of the invention.

CONCLUSION

Example embodiments of the methods, circuits, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. A light emitting diode (LED) circuit, comprising: a plurality of LED strings connected in parallel with each other, each LED string including a plurality of component LEDs connected in series;a minimum voltage selector circuit to select the LED string with the largest voltage drop across its terminals, and output a minimum voltage associated with said selected LED string with said largest voltage drop;a voltage control loop receiving said minimum voltage from said minimum voltage selector, said voltage control loop generating an output voltage sufficient to drive said plurality of LED strings based on said minimum voltage; anda plurality of current regulation loops, each current regulation loop determining a regulated current for a corresponding one of said plurality of LED strings.

2. The LED circuit of claim 1, wherein said current regulation loops are independently adjustable by adjusting a corresponding reference voltage for each current regulation loop.

3. The LED circuit of claim 1, wherein said plurality of LED strings contain one or more strings of differing LEDs.

4. The LED circuit of claim 3, wherein said plurality of current regulation loops includes: a first current regulation loop that causes a first current to flow through a first LED string of said plurality of LED strings; anda second current regulation loop that causes a second current to flow through a second LED string of said plurality of LED strings, said second current different from said first current.

5. The LED circuit of claim 3, wherein said plurality of LED strings contain a string of flash LEDs, and a string of backlighting LEDs.

6. The LED circuit of claim 5, wherein said plurality of current regulation loops includes a first current loop to cause a first current to flow through said string of flash LEDs, and a second current loop to cause a second current to flow through said string of backlighting LEDs.

7. The LED circuit of claim 1, wherein said voltage control loop adjusts said output voltage based on comparing said minimum voltage to a first reference voltage.

8. The LED circuit of claim 7, wherein said voltage control loop includes: a control loop circuit that compares said minimum voltage with a first reference voltage, and generates an output control voltage; anda field effect transistor (FET) having a drain that is series connected to a voltage supply through an inductor, a source coupled to ground, and a gate controlled by said output control voltage.

9. The LED circuit of claim 8, wherein said output control voltage drives said FET so that said output voltage that drives said LED strings is sufficient to cause said minimum voltage to be approximately equal to said first reference voltage.

10. The LED circuit of claim 8, wherein said output voltage for said plurality of LED strings is tapped from said drain of said FET.

11. The LED circuit of claim 1, wherein each current regulation circuit further comprises of: a field effect transistor (FET) having a drain coupled to a corresponding LED string of said plurality of LED strings, and a source coupled to ground through a resistor, said FET controlling a current through said LED string, and therefore the brightness of said LED string; andan operational amplifier having a first input coupled to a reference voltage and a second input coupled to said source of said FET, and an output voltage coupled to a gate of said FET.

12. The LED circuit of claim 11, wherein said operational amplifier drives a gate of said FET so that a source voltage of said FET substantially matches said reference voltage.

13. The LED circuit of claim 11, wherein said current through said LED string, and thereby an optical brightness of said LED string, is regulated by adjusting said reference voltage.

14. A light emitting diode (LED) circuit, comprising: a plurality of LED strings connected in parallel with each other, each LED string including a plurality of component LEDs connected in series;means for providing an output voltage for said plurality of LED strings, said output voltage determined to accommodate a LED string of said plurality of LED strings having the largest voltage drop of said plurality of LED strings; andmeans for individually providing a regulated current for each of said LED strings, said regulated current for each LED string adjustable based on a desired brightness said LED string.

15. The LED circuit of claim 14, wherein said means for individually providing a regulated current for each of said LED strings, includes a plurality of current regulation loops corresponding to said plurality of LED strings, each current regulation loop providing a corresponding regulated current based on a voltage reference input.

16. A driver circuit for a plurality of light emitting diode (LED) strings connected in parallel with each other, each LED string including a plurality of component LEDs connected in series, comprising: a minimum voltage selector circuit to select the LED string with the largest voltage drop across its terminals, and output a minimum voltage associated with said selected LED string with said largest voltage drop;a voltage control loop receiving said minimum voltage from said minimum voltage selector, said voltage control loop generating an output voltage sufficient to drive said plurality of LED strings based on said minimum voltage; anda plurality of current regulation loops, each current regulation loop determining a regulated current for a corresponding one of said plurality of LED strings.