TWI424787B - Voltage converter, backlight module control system and control method thereof - Google Patents

Description

Voltage converter, backlight module control system and control method thereof

The invention relates to a voltage converter and a backlight module control system, in particular to a DC-to-DC voltage converter and a backlight module control using the same and using a color sequential method to drive a liquid crystal display panel. system.

Due to the difference in process and material of different color LEDs, the conduction voltage of the LED junction is also different. Therefore, when different color LEDs are built on the same backlight module. Different operating diodes of different colors require different operating voltages, so the backlight module control system may require multiple voltage converters to provide the operating voltages required for the LEDs of different colors. Please refer to FIG. 1 , which is a schematic diagram of a conventional backlight module control system 100 . As shown in FIG. 1, the backlight module 100 includes a control system power supply 110, a plurality of voltage converters 120_1 ~ 120_N, a plurality of backlight subassembly 130_1 ~ 130_N and a plurality of resistors R ₁ ~ R _N, wherein Each of the backlight sub-modules 130_1~130_N includes a plurality of light-emitting diodes.

In the operation of the conventional backlight module control system 100, the voltage converters 120_1~120_N convert one input voltage provided by the power supply 110 into an operating voltage corresponding to the backlight sub-modules 130_1~130_N, The corresponding backlight sub-modules 130_1~130_N are driven. However, since a plurality of voltage converters are required, the manufacturing cost of the backlight module control system is increased.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a backlight module control system that requires only one voltage converter and a control method thereof to solve the above problems.

According to an embodiment of the invention, a voltage converter is disclosed. The voltage converter comprises a comparator, a sawtooth generator, a transconductance amplifier, a pulse width modulation signal generator, an inductor, a current limiting circuit and an output circuit. The comparator is configured to compare a reference voltage and a feedback voltage to generate a first current, the sawtooth generator is configured to generate a sawtooth current according to the reference voltage, and the transducing amplifier is used according to Inputting a voltage to generate a second current, the pulse width modulation signal generator is configured to generate a pulse width modulation signal according to the first current, the sawtooth current and the second current, and the current limiting circuit is used To limit the current on the inductor, and the output circuit is used to generate an output voltage and the feedback voltage based on the current from the inductor.

According to an embodiment of the invention, a backlight module control system is disclosed. The system comprises a plurality of backlight sub-modules, a control signal output unit, a voltage converter and a plurality of current control units. The control signal output unit is configured to provide a voltage control signal, a current control signal, and a plurality of pulse width modulation signals; the voltage converter is coupled to the control signal output unit and the plurality of backlight sub-modules, And controlling the signal to output an output voltage to the plurality of backlight sub-modules; the plurality of current control units are respectively coupled to the plurality of backlight sub-modules, and each current control unit is Determining a current of the corresponding backlight sub-module according to the current control signal, and determining whether the corresponding backlight sub-module is enabled according to a corresponding pulse width modulation signal, and further, the plurality of backlights The module will only have one backlight sub-module enabled at the same time.

According to an embodiment of the invention, a control method of a backlight module control system is disclosed. The method includes: providing a voltage control signal, a current control signal, and a plurality of pulse width modulation signals; controlling the signal according to the voltage to output an output voltage to the plurality of backlight submodules; determining according to the current control signal Corresponding to a current of the backlight sub-module; and determining, according to one of the plurality of pulse width modulation signals, a corresponding pulse width modulation signal to determine whether the corresponding backlight sub-module is enabled, wherein the plurality of backlights The source sub-module will only have one backlight sub-module enabled at the same time.

The backlight module control system and the control method thereof provided by the invention can provide the operations required for the plurality of backlight sub-modules in a case where only one voltage converter is used, by using the control signal outputted by the control signal output unit. The voltage drives the plurality of backlight sub-modules sequentially. Since the backlight module control system of the present invention requires only one voltage converter, the present invention can significantly reduce the manufacturing cost compared to the conventional backlight module control system requiring a plurality of voltage converters. In addition, since the plurality of backlight sub-modules in the present invention are sequentially driven, the voltage converter provided by the present invention can provide fast voltage conversion to output the correct voltage.

Please refer to FIG. 2, which is a schematic diagram of an embodiment of a backlight module control system 200 of the present invention. As shown in FIG. 2, the backlight module control system 200 includes a power supply 210, a voltage converter 220, a control signal output unit 270, and a plurality of backlight sub-modules (in this embodiment, three backlights are used). The module 230_1~230_3 is taken as an example and a plurality of current control units (the current embodiment is exemplified by three current control units 260_1~260_3), wherein the control signal output unit 270 includes a processor 240 and a first The digital analog converter 250_1 and the second digital analog converter 250_2, and the backlight sub-module 230_1 is a green backlight sub-module including a plurality of green LEDs, and the backlight sub-module 230_2 includes One of the plurality of red light-emitting diodes, the red backlight sub-module, and the backlight sub-module 230_3 are blue backlight sub-modules including a plurality of blue light-emitting diodes. In addition, the backlight module control system 200 is applied to a liquid crystal display panel driven by a color sequential method, that is, a plurality of backlight sub-modules 230_1 230 230_3 have only one backlight sub-module enabled at the same time.

Please refer to FIG. 2, FIG. 3 and FIG. 4 at the same time. FIG. 3 is a timing chart of the control signals of the backlight module control system 200 shown in FIG. 2, and FIG. 4 is the first and the first diagram shown in FIG. Schematic diagram of the voltage level of the input and output signals of the binary analog converters 250_1, 250_2 and the voltage converter 220. The backlight module on the operation control system 200, first, at time T _1, a first digital to analog converter receiving one of 250_ 240 outputs a first digital signal processor D _a0, to generate a voltage control signal _V ref_1, Thereafter, the voltage converter generates an output voltage V _out according to the voltage control signal V _{ref_1} and one of the input voltages V _in which the power supply 210 provides. At this time, the first digital signal D _a0 is at a low level (ie, The voltage level of the digital signal "0"), the voltage control signal V _{ref_1} is V _refH and the voltage level of the output voltage V _out is V ₀₁ , where V ₀₁ is one of the required operating voltages of the green backlight sub-module 230_1; In addition, the second digital analog converter 250_2 receives the two second digital signals D _r0 and D _r1 output by the processor 240 to generate a current control signal V _{ref_2} , and then the current control unit 260_1 corresponds to the current control signal V _{ref_2 .} The green backlight sub-module 230_1 has a current I _G . At this time, the second digital signals D _r0 and D _r1 are all low-level (that is, all are digital signals “0”), and the current control signal V _{ref_2} Voltage level To V _21; Further, current control unit 260_1 ~ 260_3 processor also receives a plurality of PWM (Pulse Width Modulation, PWM) signal (provided by 240 in the present embodiment is a three pulse width modulated signal _V dmG , V _dmR , V _dmB ) to determine whether the corresponding backlight sub-module is enabled. In time T ₁ , since only the green backlight sub-module 230_1 needs to be enabled, the pulse width modulation signal V _dmR , V _dmB is a low voltage level (ie, the backlight sub-modules 230_2, 230_3 are not enabled), and the pulse width modulation signal V _{dmG has dimming in} addition to the green backlight sub-module 230_1. The function.

In the time T ₂ , the first digital signal D _a0 is a high level (that is, the digital signal “1”), so the voltage level of the voltage control signal V _{ref_1} is V _refL , and the voltage level of the output voltage V _out is It is V ₀₂ , where V ₀₂ is one of the required operating voltages of the red backlight sub-module 230_2; in addition, the second digital signals D _r0 and D _r1 are respectively low and high levels (that is, the digital signal “0” respectively , "1"), and the voltage level of the current control signal V _{ref_2} is V ₂₂ , therefore, the current control unit _{260_2} makes the corresponding red backlight sub-module 230_2 have a current I _R according to the current control signal V _{ref_2} ; In time T ₂ , since only the red backlight sub-module 230_2 needs to be enabled, the pulse width modulation signals V _dmG and V _{dmB are} at a low voltage level (ie, the backlight sub-modules 230_1, 230_3 are not In addition to the enabling of the green backlight sub-module 230_2, the pulse width modulation signal V _dmR also has a dimming function.

Similarly, in time T ₃ , the first digital signal D _a0 is a low level (ie, a digital signal “0”), so the voltage level of the voltage control signal V _{ref_1} is V _refH , and the output voltage V _out is The voltage level is V ₀₁ , where V ₀₁ is one of the required operating voltages of the blue backlight sub-module 230_3; in addition, the second digital signals D _r0 and D _r1 are respectively high and low levels (ie, respectively, digitally The signal "1", "0"), and the voltage level of the current control signal V _{ref_2} is V ₂₃ , therefore, the current control unit _{260_3} causes the corresponding blue backlight sub-module 230_3 to have current according to the current control signal V _{ref_2} I _B; in addition, during the time T _3, as the blue backlight subassembly only needs 230_3 is enabled, so the pulse width modulation signal _{V dmG,} V dmR line is a low voltage level (i.e., the backlight-order mode Groups 230_1, 230_2 are not enabled).

In addition, the time T ₁ to T ₃ is a frame time, that is, the green backlight sub-module 230_1 , the red backlight sub-module 230_2 , and the blue backlight sub-module 230_3 are in a frame. Time is enabled in time.

In addition, the conversion relationship between the first digital signal D _a0 , the voltage control signal V _{ref_1 ,} and the output voltage V _out shown in FIG. 4 is only an example. In practice, as long as the output voltage V _out meets the required driving The operating voltage of the backlight sub-module, the digital signal D _a0 and the voltage control signal V _{ref_1} can be set according to the designer's consideration; similarly, as long as the current of the backlight sub-module 230_1~230_3 is I _G , I _R , I _B , the second digital signal D _r0 , D _r1 and the current control signal V _{re_2} can also be set according to the designer's consideration.

It should be noted that, in this embodiment, since the operating voltages of the green backlight sub-module 230_1 and the blue backlight sub-module 230_3 are the same, the processor 240 outputs only one first digit signal D _a0 , however, If the green backlight sub-module 230_1 and the blue backlight sub-module 230_3 require different operating voltages, the processor may also output two or more first digital signals such that the voltage control signal V _{ref_1} and the output voltage V _out There are at least three voltage levels to drive the backlight sub-modules 230_1~230_3, and these design variations are also within the scope of the present invention.

In addition, in the present invention, the voltage control signal V _{ref_1} and the current control signal V _{ref_2} are respectively generated by the first digital signal D _a0 and the second digital signal D _r0 , D _r1 output by the processor 240 via a digital analog converter. However, the processor 240 can also directly output the voltage control signal V _{ref_1} and the current control signal V _{ref_2} . In addition, the first digital analog converter 250_1 can also be integrated into the voltage converter 220, and these design changes are also included in this embodiment. The scope of the invention.

In addition, since the backlight module control system of the present invention is applied to a color sequential driving liquid crystal display panel, the voltage converter requires a fast conversion speed, however, the conventional DC/DC voltage converter is limited by the system. The low frequency is wide, so when the voltage control signal V _{ref_1} changes, the conventional voltage converter cannot immediately react to output the correct output voltage, so the present invention additionally proposes a voltage converter to be able to quickly switch to the desired output. Voltage.

Please refer to FIG. 5, which is a schematic diagram of the voltage converter 220 shown in FIG. As shown in FIG. 5, the voltage converter 220 includes a comparator 510, a sawtooth generator 520, a transconductance amplifier 530, a voltage dividing circuit 532, a pulse width modulation signal generator 540, and an inductor L. a current limiting circuit 550 and an output circuit 560, wherein the current limiting circuit 550 includes a comparator 552, a flip-flop 554, an AND gate 556, a driving circuit 558, a transistor MN ₁ and a resistor R _L1 , and the output circuit 560 includes a Schottky diode 562, a capacitor C _load and a voltage dividing circuit 564, wherein the voltage dividing circuit 564 includes two resistors R _F1 and R _F2 .

Please refer to FIG. 5 and FIG. 6 at the same time. FIG. 6 is a timing chart of the signals of the voltage converter 220 shown in FIG. 5 at each stage. In the first stage S ₁ , the output voltage V _{out of the} system is equal to V ₀₂ shown in FIG. 3 and has been maintained at a steady state, and the duty cycle of the pulse width modulation signal V _PWM is also in a steady state. At this time, the voltage control signal V _{ref_1} is V _refL , and the feedback voltage V _{fb is} also equal to (or very close to) V _refL ; in the second phase S ₂ , the voltage control signal V _{ref_1} instantaneously becomes a higher voltage value V _refH , at this time, the comparator 510 compares the voltage control signal V _{ref_1} and the feedback voltage V _fb to generate a first current I _c , and the sawtooth generator 520 also generates a large upper and lower boundary according to the voltage control signal V _{ref_1 .} The sawtooth current I _a , in addition, the transconductance amplifier 530 generates a second current I _ad according to the voltage b*V _in from the voltage dividing circuit 532 (in this example, b=R _F1 /(R _F1 +R _{F2 of));} then, the PWM signal generator 540 according to the first current I _c, and I _a sawtooth current I _ad a second current to produce a pulse width modulated signal V _{the PWM;} seen from the above when the voltage control signal V _{When ref_1} changes to a higher reference value, I _c +I _ad quickly becomes a smaller current and I _a also quickly forms a higher current, comparing I _c +I _ad with I _a , which can make the pulse width modulation signal V _PWM quickly convert and maintain at the highest level, and because the V _{PWM is} at the highest level, the transistor MN _{1 is} fully turned on.

When the transistor MN _{1 is} fully turned on, the voltage level of the node Node1 will drop to be close to the ground voltage, so that the inductor L has a large voltage difference on both sides of the inductor L, and the inductor current I _L rises rapidly, in order to prevent the inductor current I _L Too high to burn out the entire circuit, so when the inductor current I _{L is} close to a predetermined value (ie, the highest current), the current limiting circuit 550 forcibly turns off the transistor MN ₁ , when the transistor MN _{1 is} turned off, the inductor The energy of the current I _L is thus transferred to the external capacitor C _load by the Schottky diode 562, at which point the input positive terminal of the comparator 552 in the current limiting circuit 550 will return to zero, and in this example, The reference voltage V _refCL of one of the input negative terminals of the comparator 552 is set to 0.2V, so the compared voltage V _clo outputted by the comparator 552 passes through the flip-flop 554, the gate 556, and the driving circuit 558 to control the transistor MN ₁ Turning off or on, waiting for the next cycle causes the transistor MN _{1 to} turn back on, forming a current limiting circuit loop, and the entire voltage converter 220 is controlled by the current limiting circuit. At this time, the inductor current I _L is maintained at the highest output current (ie, the predetermined value), and the output voltage V _out can be quickly reached to the desired voltage level.

In the third stage S ₃ , when the output voltage V _{out is} close to the driving voltage value required by the backlight sub-module (in this example, V ₀₁ shown in FIG. 3), the output voltage V _out is divided by voltage. The feedback voltage V _fb generated by the circuit 564 gradually approaches the voltage control signal V _{ref_1} (V _refH ), and at this time, the first current I _c rapidly drops to 0, so that the pulse width modulation signal V _PWM can be quickly Switch back to the correct cycle.

In the fourth stage S ₄ , the voltage control signal V _{ref_1} instantaneously becomes a lower voltage value V _refL , at which time I _c +I _ad quickly becomes a larger current and I _a also rapidly forms a lower current, which will be I _c + Comparing I _ad with I _a allows the pulse width modulation signal V _{PWM to be} quickly converted and maintained at the lowest level, and since the pulse width modulation signal V _{PWM is} at the lowest level, the transistor MN _{1 is} turned off, and this The output voltage V _out will drop.

In the fifth stage S ₅ , when the output voltage V _{out is} close to the driving voltage value required by the backlight sub-module (in this example, V ₀₂ shown in FIG. 3), the output voltage V _out is divided by voltage. The feedback voltage V _fb generated by the circuit 564 gradually approaches the voltage control signal V _{ref_1} (V _refL ), and at this time, the first current I _c rapidly drops to 0, so that the pulse width modulation signal V _PWM can be quickly Switch back to the correct cycle.

In summary, the backlight module control system of the present invention is applied to a liquid crystal display panel driven by a color sequential method. Compared with the conventional backlight module control system, the present invention only needs a voltage converter, and The voltage converter has a relatively fast voltage conversion rate, so the manufacturing cost of the backlight module can be reliably reduced without degrading the display quality.

The above are only the preferred embodiments of the present invention, and all changes and modifications made to the scope of the present invention should be within the scope of the present invention.

100. . . Conventional backlight module control system

200. . . Backlight module control system

110, 210. . . Power Supplier

120_1~120_N, 220. . . Voltage converter

130_1~130_N, 230_1~230_3. . . Backlight sub-module

240. . . processor

250_1. . . First digital analog converter

250_2. . . Second digital analog converter

260_1~260_3. . . Current control unit

270. . . Control signal output unit

510, 552. . . Comparators

520. . . Sawtooth generator

530. . . Transduction amplifier

532, 564. . . Voltage dividing circuit

540. . . Pulse width modulation signal generator

550. . . Current limiting circuit

554. . . Positive and negative

556. . . Gate

558. . . Drive circuit

560. . . Output circuit

562. . . Xiao Ji diode

R ₁ ~ R _N , R _L1 , R _F1 , R _F2 . . . resistance

V _in . . . Input voltage

V _out . . . The output voltage

D _a0 . . . First digit signal

D _r0 , D _r1 . . . Second digit signal

V _{ref_1} . . . Voltage control signal

V _{ref_2} current control signal

V _dmG , V _dmR , V _dmB . . . Pulse width modulation signal

I _G , I _R , I _B , I _L , I _c , I _a , I _ad . . . Current

MN ₁ . . . Transistor

L. . . inductance

V _fb . . . Feedback voltage

Nodel. . . node

V _clo . . . Comparison voltage

V _PWM . . . Pulse width modulation signal

V _refCL . . . Reference voltage

Clk. . . Clock signal

V _refH , V _refL . . . Voltage level of voltage control signal V _{ref_1}

V ₀₁ , V ₀₂ . . . Voltage level of output voltage V _out

V ₂₁ , V ₂₂ , V ₂₃ . . . Voltage level of current control signal V _ref _2

T ₁ , T ₂ , T ₃ . . . time

Q. . . Positive and negative output terminal

R. . . Reversing device reset endpoint

S. . . Positive and negative set endpoint

S ₁ , S ₂ , S ₃ , S ₄ , S ₅ . . . stage

C _load . . . capacitance

Figure 1 is a schematic diagram of a conventional backlight module control system.

2 is a schematic view of an embodiment of a backlight module control system of the present invention.

Figure 3 is a timing diagram of the control signals of the backlight module control system shown in Figure 2.

Fig. 4 is a schematic diagram showing the voltage levels of the input and output signals of the first and second digital analog converters and the voltage converter shown in Fig. 2.

Figure 5 is a schematic diagram of the voltage converter shown in Figure 2.

Figure 6 is a timing diagram of the signals of the voltage converters shown in Figure 5 at various stages.

200. . . Backlight module control system

210. . . Power Supplier

220. . . Voltage converter

230_1~230_3. . . Backlight sub-module

240. . . processor

250_1. . . First digital analog converter

250_2. . . Second digital analog converter

260_1~260_3. . . Current control unit

270. . . Control signal output unit

V _in . . . Input voltage

V _out . . . The output voltage

D _a0 . . . First digit signal

D _r0 , D _r1 . . . Second digit signal

V _{ref_1} . . . Voltage control signal

V _{re_2} . . . Current control signal

V _dmG , V _dmR , V _dmB . . . Pulse width modulation signal

Claims (16)

A voltage converter includes: a comparator for comparing a reference voltage and a feedback voltage to generate a first current; and a sawtooth generator for generating a sawtooth current according to the reference voltage; a transconductance amplifier for generating a second current according to an input voltage; a Pulse Width Modulation (PWM) signal generator coupled to the comparator, the sawtooth generator, and the transconductance amplifier The first current, the sawtooth current, and the second current are used to generate a pulse width modulation signal; an inductor coupled to a voltage input terminal; and a current limiting circuit coupled to the pulse width a modulation signal generator and the inductor for limiting current on the inductor; and an output circuit coupled to the inductor, the comparator, and a voltage output terminal for determining a current from the inductor An output voltage and the feedback voltage are generated. The voltage converter of claim 1, further comprising: a voltage dividing circuit coupled to the voltage input terminal and the transducing amplifier for generating a voltage divider according to the input voltage The voltage is input, and the transconductance amplifier generates the second current according to the divided voltage input voltage. The voltage converter of claim 1, wherein the output circuit comprises: a Schottky diode; a capacitor coupled to the Schottky diode and the voltage output terminal And a voltage dividing circuit coupled to the voltage output terminal for generating the feedback voltage according to the output voltage. A backlight module control system includes: a plurality of backlight sub-modules, wherein each backlight sub-module includes at least one backlight; and a control signal output unit for providing a voltage control signal and a current a control signal and a plurality of first pulse width modulation (PWM) signals; a voltage converter coupled to the control signal output unit and the plurality of backlight sub-modules for controlling the signal according to the voltage And outputting an output voltage to the plurality of backlight sub-modules; and a plurality of current control units respectively coupled to the plurality of backlight sub-modules, each of the current control units being coupled to the control signal output unit And determining, according to the current control signal, a current corresponding to the backlight sub-module, and determining, according to a voltage level of one of the plurality of first pulse width modulation signals, corresponding to the first pulse width modulation signal Whether the corresponding backlight sub-module is enabled or not; wherein the plurality of backlight sub-modules are in a frame time The sequence is enabled and only one backlight sub-module is enabled at the same time. The backlight module control system of claim 4, wherein the plurality of backlight sub-modules are a red backlight sub-module, a green backlight sub-module, and a blue backlight sub-module And the red backlight sub-module includes at least one red LED, the green backlight sub-module includes at least one green LED, and the blue backlight sub-module includes at least one blue LED Polar body. The backlight module control system of claim 5, wherein the red backlight sub-module, the green backlight sub-module, and the blue backlight sub-module are sequentially ordered in the frame time. Enable. The backlight module control system of claim 4, further comprising: a power supply coupled to the voltage converter for providing an input voltage to the voltage converter. The backlight module control system of claim 4, wherein the control signal output unit comprises: a processor for providing at least a first digital signal and at least a second digital signal; a first digit An analog converter coupled to the processor and the voltage converter for outputting the voltage control signal according to the first digital signal; A second digital analog converter is coupled to the processor and the plurality of current control units for outputting the current control signal according to the second digital signal. The backlight module control system of claim 4, wherein the voltage converter comprises: a comparator for comparing the voltage control signal and a feedback voltage to generate a first current; a sawtooth wave a generator for controlling a signal according to the voltage to generate a sawtooth current; a transducing amplifier for generating a second current according to an input voltage; a Pulse Width Modulation (PWM) signal generator The inductor, the sawtooth generator, and the transconductance amplifier are configured to generate a pulse width modulation signal according to the first current, the sawtooth current, and the second current; an inductor coupled a current limiting circuit coupled to the pulse width modulation signal generator and the inductor for limiting current on the inductor; and an output circuit coupled to the inductor, the comparison And a voltage output terminal for generating an output voltage and the feedback voltage based on the current from the inductor. The backlight module control system of claim 9, further comprising: a voltage dividing circuit coupled to the voltage input terminal and the transducing amplifier for The input voltage is used to generate a divided voltage input voltage, and the transconductance amplifier generates the second current according to the divided voltage input voltage. The backlight module control system of claim 9, wherein the output circuit comprises: a Schottky diode; a capacitor coupled to the Schottky diode and the voltage An output terminal; and a voltage dividing circuit coupled to the voltage output terminal for generating the feedback voltage according to the output voltage. A control method for a backlight module control system includes: providing a voltage control signal, a current control signal, and a plurality of Pulse Width Modulation (PWM) signals; and controlling an output voltage according to the voltage control signal And a plurality of backlight sub-modules; determining a current corresponding to the backlight sub-module according to the current control signal; and determining a voltage level of the corresponding pulse width modulation signal according to the plurality of pulse width modulation signals The bit height determines whether the corresponding backlight sub-module is enabled or not; wherein the plurality of backlight sub-modules are sequentially enabled in a frame time, and only at the same time A backlight sub-module is enabled. The control method of claim 12, further comprising: setting a red backlight sub-module, a green backlight sub-module, and a blue backlight sub-module as the plurality of backlights a module, and the red backlight sub-module includes at least one red LED, the green backlight sub-module includes at least one green LED, and the blue backlight sub-module includes at least one blue illumination Diode. The control method of claim 13, further comprising: sequentially enabling the red backlight sub-module, the green backlight sub-module, and the blue backlight sub-module during the frame time . The control method of claim 12, further comprising: providing an input voltage. The control method of claim 12, further comprising: providing at least a first digital signal and at least a second digital signal; outputting the voltage control signal according to the first digital signal; and according to the second The digital signal outputs the current control signal.