Supply device of circuit branches with LED diodes

1. Field of the Invention

The present invention refers to a supply device of circuit branches with LED diodes.

2. Description of the Related Art

Liquid crystal displays are widely used in mobile telephones; said displays use a large number of LED diodes to permit the phenomenon of backlighting. The LED diodes are distributed in the displays uniformly and use the same bias current; to obtain this they are connected in series.

To feed serially connected chains of LED diodes that emit white light, devices suitable for increasing the supply voltage above the value of the supply voltage at their-input are employed.

The most adopted circuit solutions provide for the use of a boost converter which, supplying many branches connected in parallel and each one made up of a series of LED diodes, permit the setting of the current or the voltage on each one.

To regulate the current that passes through one or more branches of LED diodes there are two different modes: a current one and a voltage one.

In the first mode only the current of the main branch can be set. The output current is read and compared with a reference to generate a control in pulse width modulation (PWM) mode; the circuit branches that are not controlled directly can even have a current very different from that of the main branch.

The disadvantage lies in the parallel connection of the circuit branches. Even if the current that flows in the main branch with the highest number of diodes is controlled directly, the secondary circuit branches can have an additional voltage and a different current. Adding a series of resistances in the secondary branches the current set on the main branch can be reached seeing that the resistances compensate the voltage jump error between the main branch and the secondaries that is due to the connection in parallel. In any case even if the object is reached a consistent quantity of power dissipation (on the compensation resistances) causes the decrease in the efficiency of the control.

This disadvantage can be present not only when supplying the circuit branches with a different number of diodes, but also if the number of LED diodes is equal in all the branches. In fact the voltage jump between the LED diodes could be different even if the same current flows. As a consequence it is necessary to impose a different voltage jump for each branch, but this is not possible by connecting all the branches in parallel. Only by regulating the current that flows through the circuit branches with a maximum value of voltage jump and inserting variable resistances in the other circuit branches the parallel connection can be maintained.

The voltage mode provides for the setting of the output voltage for each circuit branch by means of a boost converter and a voltage divider. To control the current that flows through each circuit branch a resistance, connected in series to the LED diodes, is added to each circuit branch; said resistance enable the current required to be adjusted. Nevertheless the value of the current cannot be known in advance given that it depends on the voltage at the terminals of the circuit branches, on the number of LED diodes present in each branch and on the fall in voltage on each LED diode; the latter depends on the flow of current and on the process technology. Therefore the correct resistance value must be assessed in the different cases and must be set so as to compensate the variation of voltage due to the process technology.

One embodiment of the present invention provides a supply device of circuit branches with LED diodes that overcomes the inconveniences of the known devices.

In one embodiment of the present invention, a supply device supplies at least two circuit branches, each of the at least two circuit branches comprising at least one LED diode. The device includes a supply circuit that provides the electric supply of the at least two circuit branches, the supply circuit comprising at least one variable resistance. The device also includes a controller coupled to the at least two circuit branches and suitable for varying the resistance in reply to a variation of the current that flows in one of the at least two circuit branches to change the electric supply of the at least two circuit branches.

Thanks to the present invention it is possible to produce a supply device of circuit branches with LED diodes that ensures the electric supply of each circuit branch preventing some circuit branch from turning off because of insufficient supply.

In a preferred embodiment said supply device guarantees the regulation of the current of each circuit branch.

The characteristics and advantages of the present invention will appear evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which:

FIG. 1 is a circuit diagram of the supply device of circuit branches with LED diodes in accordance with one embodiment of the invention;

FIG. 2 is a circuit diagram of the supply device of circuit branches with LED diodes in accordance with a first embodiment of the invention;

FIG. 3 is a circuit diagram of the supply device according to a second embodiment of the invention;

FIG. 4 shows more in detail a part of the circuit of FIG. 3 ;

FIG. 5 shows the time diagram of the voltage Vout of the device of FIG. 3 in the initial period of supply time;

FIG. 6 shows time diagrams at the voltage and current regime in question in the device of FIG. 3 .

With reference to FIG. 1 a circuit diagram of a lighting circuit according to one embodiment of the present invention is shown. The lighting circuit includes a supply device 1 and at least two circuit branches 10 , 20 with LED diodes that are powered by the supply device 1 . Each of said at least two circuit branches 10 , 20 comprises at least one LED diode 30 ; in particular in FIG. 1 each of the two circuit branches 10 , 20 comprises four LED diodes 30 . The device 1 comprises a supply circuit 2 suitable for supplying the electric supply of said at least two circuit branches 10 , 20 ; said supply circuit 2 provides the supply voltage Vout of the circuit branches 10 , 20 . Said supply circuit 2 includes at least one resistance R 2 . Preferably said supply circuit 2 includes a resistive divider with a resistance R 1 and the resistance R 2 connected in series; the resistive divider is positioned in parallel to said at least two circuit branches 10 , 20 . The resistance R 2 is a variable resistance and said supply device 1 comprises a control circuit 3 coupled to said at least two circuit branches 10 , 20 and suitable for varying the resistance R 2 in reply to a variation of the current of one of said at least two circuit branches 10 , 20 ; in this manner the control circuit 3 changes the electric supply of said at least two circuit branches. The supply circuit 2 preferably comprises a boost converter (not visible in FIG. 1 ) and the voltage at the terminals of the said variable resistance R 2 is used to vary the output voltage Vout to said boost converter.

FIG. 2 shows a supply device 1 A of the at least two circuit branches 10 , 20 with LED diodes in accordance with a first embodiment of the present invention. Each of the two circuit branches 10 , 20 comprises at least one LED diode 30 ; in particular in FIG. 2 each of the two circuit branches 10 , 20 comprises four LED diodes 30 . The device comprises a supply circuit 2 A suitable for providing the electric supply of said at least two circuit branches 10 , 20 . Said supply circuit 2 A comprises, for example, a boost converter 100 of the traditional type; it comprises the series of an inductor L and a resistance RL connected between a voltage Vbat and a terminal of a switch S 1 , preferably made up of a MOS transistor. Said terminal of the switch S 1 is connected to the anode of a Schottky diode Dz 1 whose cathode is connected to a series of a capacitor C 1 and a resistance Rc 1 connected to ground and to the two circuit branches 10 and 20 ; the cathode of the diode Dz 1 is also connected to the series of two resistances R 1 and R 2 connected to ground. The boost converter 100 comprises an operational error amplifier 11 having in input at the inverting terminal the voltage Vr at the terminals of the resistance R 2 and at the non-inverting terminal the reference voltage Vref and a comparator 12 suitable for comparing the voltage in output from the error amplifier 11 with a sawtooth voltage SW; the output of the comparator 12 drives the switch S 1 .

The resistance R 2 is a variable resistance and said supply device 1 comprises a control circuit 3 A coupled to said at least two circuit branches 10 , 20 and suitable for varying the resistance R 2 in reply to a variation of the current of one of said at least two circuit branches 10 , 20 .

The two circuit branches comprise resistances R 10 and R 20 positioned between the final LED diode 30 and ground; said control circuit 3 A is coupled at the terminals of said two resistances R 10 , R 20 .

The control circuit 3 A includes a first comparator 51 and a second comparator 52 having the non-inverting terminals connected with a terminal of said resistances R 10 and R 20 while on the inverting terminal the reference voltages Vref 10 and Vref 20 are present. The signals in output from the two comparators are sent to an OR gate 53 and the signal in output from the OR gate is sent to a counter 54 which by means of a signal Drive drives the variable resistance R 2 . If the voltage at the terminals of the resistance R 10 is lower than the voltage Vref 10 or if the voltage at the terminals of the resistance R 20 is lower than the voltage Vref 20 the counter 54 will increase the value of the resistance R 2 so that the current generator 100 sends a current with a higher value to the circuit branches 10 and 20 . In this manner the ratio of division of the resistances R 1 and R 2 is not chosen in advance but is dynamically adjusted to obtain the correct supply voltage of the circuit branches 10 and 20 . In fact, in this case account is taken of the process technology of the LEDs to reduce to a minimum the consumption of power, if a higher supply voltage than that required is regulated, or to prevent a circuit branch from being turned off because the supply voltage is not sufficient.

FIG. 3 shows a circuit diagram of a supply device 1 B that supplies the circuit branches 10 , 20 with LED diodes 30 in accordance with a second embodiment of the invention. The device 1 B of FIG. 3 differs from the device of FIG. 2 in the different circuit typology of the control circuit 3 B. The latter comprise switches S 10 and S 20 , preferably transistors, positioned in the circuit branches 10 and 20 and connected between the final LED diode 30 of the series of four LED diodes 30 and the resistances R 10 and R 20 . Each transistor S 10 , S 20 is driven by a respective circuit block 61 , 62 to obtain a pulse width modulation (PWM) regulation. The blocks 61 and 62 are capable of regulating the current that flows in the branches 10 and 20 with good precision. The blocks 61 and 62 regulate the duty-cycle D, that is they regulate the period of turn-on time Ton and the period of turn-off time Toff of the transistors S 10 and S 20 in a given period of time T; the duty-cycle D=(1−Toff)/Ton. In the starting conditions the resistance R 2 is set at the lowest value; in this manner the value of the supply voltage Vout of the circuit branches 10 and 20 will also be at the lowest value. Each block 61 , 62 will establish whether said voltage is sufficient for the supply of the respective circuit branch 10 , 20 . If the duty-cycle becomes unitary, that is the maximum period of turn-on time Ton is reached, the blocks 61 , 62 will send signals to the other logic blocks 63 and 64 . The latter will send said information to the counter device 54 that will increase the value of the resistance R 2 to increase the value of the voltage Vout; the same blocks 63 and 64 will see to zeroing the duty-cycle relating to each switch S 10 , S 11 . More precisely, in the case of only two circuit branches 10 and 20 , the signals in output from the logic blocks 63 and 64 are sent to a port OR 53 that sends its output signal to the counter device 54 . Said procedure will be repeated until the value of the voltage Vout is such that it feeds all the circuit branches correctly, preventing them from turning off.

The circuit block 61 is shown in more detail in FIG. 4 . The circuit block 61 comprises an operational error amplifier 67 having the inverting terminal connected with the terminal that is not grounded of the resistance R 10 and the non-inverting terminal connected to a reference voltage V 61 . The signal in output from the operational error amplifier is sent to the non-inverting terminal of a comparator 68 having the inverting terminal connected to a sawtooth voltage SW 61 . The output signal of said comparator 68 drives the switch S 10 . When the switch S 10 is closed we obtain

I ⁢ ⁢ 10 = V ⁢ ⁢ out - 4 ⁢ V ⁢ ⁢ 30 R ⁢ ⁢ 10 + Rs

where V 30 is the voltage at the terminals of each LED diode 30 and Rs is the resistance of the switch S 10 . The current is regulated at a value corrected by the feedback that forces the switch to turn on. In fact, with the sawtooth signal SW 61 , a pulsed signal with period T is generated and a pulse current I 10 flows in the circuit branch 10 . To regulate a correct average branch current Icorr it is necessary to impose V 61 =R 10 *Icorr so that the block 61 will regulate an average current Im=I 10 *D=Icorr.

FIG. 5 shows a time diagram of the course of the voltage Vout in the initial period of time, that is in initial transitory conditions, of the supplying of the circuit branches 10 and 20 for the device of FIG. 3 . FIG. 6 shows the time courses of the currents I 10 , I 20 and of the voltage Vout when the regime condition is reached again for the device of FIG. 3 .

The supply device according to the invention is applicable to more than two circuit branches containing LED diodes and in which the same circuit branches can contain a different number of LED diodes.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.