Circuit for controlling load switch

A circuit for controlling load switch is disclosed in the present invention. The circuit is applied to a load circuit which includes a load switch and a working element. The circuit includes a first comparator, a second comparator, and a third comparator. When an inductance current achieves the average current, the first comparator triggered to conduct a first switch. When the inductance current achieves a default ripple current, the second comparator is triggered to conduct a second switch. When either one of the first switch and the second switch is conducted, the load switch is turned off to make a variable voltage be increased. When the first switch is conducted, a capacitance voltage of a controlled capacitance is increased to increase a turned-off time. When the second switch is conducted, the capacitance voltage is decreased to decrease the turned-off time to control the load switch.

This application claims the benefit of Taiwan Patent Application Serial No. 104114036, filed May 1, 2015, the subject matter of which is incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention is related to a circuit for controlling load switch, and more particularly related to a circuit for controlling load switch by using three comparators and two switches to adjust the average current.

2. Description of the Prior Art

FIG. 1is a circuit diagram showing a conventional integrated light emitted diode (LED) circuit. As shown, the conventional integrated LED circuit PA1includes a load circuit PA11and a driver circuit PA12. The load circuit PA11is a LED circuit, and further includes a voltage source PA111, a full bridge rectifier circuit PA112, a resistor PA113, a diode PA114, a capacitor PA115, at least a LED PA116, an inductor PA117, a first switch PA118, a resistor PA119, a capacitor PA120, and a capacitor PA121.

The full bridge rectifier PA112is coupled to the voltage source PA111and coupled to the resistor PA113, the diode PA114, the capacitor PA115, and the LED PA116. The inductor PA117has one end coupled to the diode PA114and the drain of the first switch PA118and another end coupled to the capacitor PA115and the LED PA116. The resistor PA119is coupled to the source of the first switch PA118and also to CS pin of the driver circuit PA12. The capacitor PA120is coupled to the resistor PA113and also to VCC pin of the driver circuit PA12. The capacitor PA121is coupled to COMP pin of the driver circuit PA12. The first switch PA118is coupled to OUT pin of the driver circuit PA12.

As the first switch PA118is conducted, an average circuit Ia is generated flowing through the LED PA116and an inductor current Ib is generated flowing through the inductor PA117. In general, operation mode of the load circuit PA11is decided by on time of the first switch PA118, which controls the increasing and decreasing of the inductor current Ib. The average of the peak value and the valley value of the inductor current Ib is the average current Ia. The above mentioned operation modes mainly include continuous conduction mode (CCM), discontinuous conduction mode (DCM), and boundary conduction mode (BCM), which are decided by the charging/discharging action of the inductor current Ib.

However, each of the operation modes has both advantage and disadvantage. Take the CCM mode for example, CCM mode has the advantage of small input and output ripple, small total harmonic distortion (THD) and Electra Magnetic Interference (EMI), and easier to executing filtering task, however, due to the restriction of circuit design of the load circuit PA11in present, it is necessary to set the switching frequency or the switching time of the load switch under CCM mode. Once the setting is completed, as the inductance of the inductor PA117has a significant change, a large ripple current would be generated even though the average current Ia is still stable. Thus, the LED with higher withstand current is needed such that the cost of the LED PA116would be increased and thus the need to improve the technology in present exists.

SUMMARY OF THE INVENTION

In view of the restriction of the circuit design in present, it is a general problem of large ripple current due to the significant change of inductance which may increase the circuit cost. Accordingly, it is a main object of the present invention to provide a circuit for controlling load switch under CCM by using three comparators and two switches to control on and off of the load switch so as to such that the generation of ripple current can be controlled in responsive to different inductance values effectively and thus the above mentioned problem can be resolved.

In accordance with the above mentioned object, a circuit for controlling load switch is provided in the present invention. The circuit is applied to a load circuit. The load circuit includes a load inductor, a load switch, and at least a working element, wherein the load inductor is electrically connected to the load switch and the working element and generates an inductor current as the load switch is conducted. The circuit for controlling load switch is utilized for controlling the load switch under a continuous conduction mode (CCM) and comprises a first comparator, a second comparator, a first switch, a second switch, a first controlled capacitor, a third comparator, and a processing module. The first comparator has a first comparing input, a second comparing input, and a first comparing output. The first comparing input is electrically connected to the load inductor for receiving the inductor current. The second comparing input is utilized for receiving a default average current. The first comparing output transmits a first conduction signal as the inductor current reaches the default average current. The second comparator has a third comparing input, a fourth comparing input, and a second comparing output. The third comparing input is electrically connected to the load inductor for receiving the inductor current. The fourth comparing input is utilized for receiving a default ripple current, which is greater than the default average current. The second comparing output is utilized for transmitting a second conduction signal as the inductor current reaches the default ripple current. The first switch has a gate electrically connected to the first comparing output and a drain electrically connected to a first constant current source. The first switch is conducted when receiving the first conduction signal. The second switch has a gate electrically connected to the second comparing output, a source electrically connected to a second constant current source, and a drain electrically connected to a source of the first switch. The second switch is conducted when receiving the second conduction signal. The first controlled capacitor has one end electrically connected to the source of the first switch and the drain of the second switch and another end grounded to store a capacitor voltage. The third comparator has a fifth comparing input, a sixth comparing input, and a third comparing output. The fifth comparing input is utilized for receiving a variable voltage, which gradually increases as the load switch is turned off. The sixth comparing input is electrically connected to the source of the first switch and the drain of the second switch for receiving the capacitor voltage. The third comparing output transmits a re-conduction signal as the variable voltage reaches the capacitor voltage.

Wherein, as the first switch is conducted, the first constant current source charges the first controlled capacitor, and as the second switch is conducted, the second constant current source discharges the first controlled capacitor. As the load switch is conducted, the inductor current gradually increases, after the inductor current reaches the default ripple current, the second switch is conducted and the load switch is turned off by the processing module to trigger the variable voltage to increase, and as the variable voltage reaches the capacitor voltage, the processing module receives the re-conduction signal to re-conduct the load switch so as to adjust the off time of the load switch adequately.

In accordance with a preferred embodiment of the present invention, the processing module comprises a NOR gate, a first inverter, a SR latch, and a second inverter. The NOR gate has a first NOR input, a second NOR input, and a NOR output. The first NOR input is electrically connected to the first comparing output. The second NOR input is electrically connected to the second comparing output. The NOR output is utilized for transmitting an OFF signal as the first NOR input receives the first conduction signal or the second NOR input receives the second conduction signal. The first inverter is electrically connected to the third comparing output for inverting the re-conduction signal to generate an inverted re-conduction signal. The SR latch is electrically connected to the NOR output and the first inverter for transmitting a latch OFF signal after receiving the OFF signal and for transmitting a latch ON signal after receiving the inverted re-conduction signal. The second inverter is electrically connected to the SR latch and the load switch for inverting the latch OFF signal and the latch ON signal so as to generate and transmit an inverted latch OFF signal and an inverted latch ON signal respectively. The load switch is turned off when receiving the inverted latch OFF signal and the load switch is conducted when receiving the inverted latch ON signal.

In accordance with a preferred embodiment of the present invention, the circuit for controlling load switch further comprises a second controlled capacitor, a controlled switch, and a third constant current source. The second controlled capacitor is electrically connected to the fifth comparing input and has the variable voltage. The controlled switch is electrically connected to the fifth comparing input and is triggered to conduct as the load switch is conducted. The third constant current source is electrically connected to the second controlled capacitor for charging the second controlled capacitor to increase the variable voltage as the controlled switch is conducted. In addition, the load circuit is a light emitted diode (LED) circuit and the working element is an LED.

By using the circuit for controlling load switch provided in the present invention, off time of the load switch can be adequately controlled such that the frequency corresponding to the stabilized ripple current can be automatically adjusted to make sure that the circuit can be adopted to the circuit with different inductance. Thus, the cost of working elements can be reduced such that the problem of the conventional art can be effectively resolved.

By using the circuit for controlling load switch provided in the present invention, because the off time of the load switch can be adequately controlled, the problem of the conventional art can be effectively resolved.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There are various embodiments of the circuit for controlling load switch in accordance with the present invention, which are not repeated hereby. Only one preferred embodiment is mentioned in the following paragraph as an example.

Please refer toFIGS. 2 and 3, whereinFIG. 2is a circuit diagram of a circuit for controlling load switch in accordance with a preferred embodiment of the present invention andFIG. 3is a schematic diagram showing the waveforms corresponding to the inductor current, the default average current, the default ripple current, and the load switch in accordance with a preferred embodiment of the present invention.

As shown, the circuit1for controlling load switch is applied to a load circuit2. The load circuit2includes a load inductor21, a load switch22, and at least a working element23(only one of them is labelled). The load inductor21is electrically connected to the load switch22and the working element23and generates an inductor current IL as the load switch22is conducted. The circuit1for controlling load switch is utilized for controlling on/off time of the load switch22under a continuous conduction mode (CCM) so as to generate an adjusted average current Ia flowing through the working element23according to the inductor current IL. As a preferred embodiment, the load circuit2is an LED circuit, and the working element23is an LED. However, the present invention is not so restricted. In addition, the average current Ia is the average of the highest and lowest values of the inductor current IL. The load circuit2may also include the elements shown in the conventional art such as the voltage source, the full-bridge rectifier, the resistor, the diode, and the capacitor.

The circuit1for controlling load switch includes a first comparator11, a second comparator12, a first switch13, a second switch14, a first controlled capacitor15, a third comparator16, a processing module17, and a voltage control module18.

The first comparator11has a first comparing input111, a second comparing input112, and a first comparing output113. The first comparing input111is electrically connected to the load inductor21for receiving the inductor current IL. The second comparing input112is utilized for receiving a default average current Ir1. Concretely speaking, the default average current Ir1is a default value decided based on the circuit design in practice. As a preferred embodiment, the default average current Ir1can be set as 10 mA.

The second comparator12has a third comparing input121, a fourth comparing input122, and a second comparing output123. The third comparing input121is electrically connected to the load inductor21for receiving the inductor current IL. The fourth comparing input122is utilized for receiving a default ripple current Ir2. The default ripple current Ir2is greater than the default average current Ir1. In general, the present ripple current Ir2can be set as 20% greater than the default average current Ir1. For example, as the default average current Ir1is 10 mA, the default ripple current Ir2in accordance with a preferred embodiment would be 12 mA.

The first switch13has a gate electrically connected to the first comparing output113and a drain electrically connected to a first constant current source3. The second switch14has a gate electrically connected to the second comparing output123, a source electrically connected to a second constant current source4, and a drain electrically connected to a source of the first switch13.

The first controlled capacitor15has one end electrically connected to the source of the first switch13and the drain of the second switch14and has another end grounded for storing a capacitor voltage Vc.

The third comparator16has a fifth comparing input161, a sixth comparing input162, and a third comparing output163. The fifth comparing input161is utilized for receiving a variable voltage Vr. The sixth comparing input162is electrically connected to the source of the first switch13and the drain of the second switch14for receiving the capacitor voltage Vc.

The processing module17is electrically connected to the first comparing output113and the second comparing output123. Concretely speaking, the processing module17includes a NOR gate171, a first inverter172, a SR latch173, and a second inverter174.

The NOR gate171has a first NOR input1711, a second NOR input1712, and a NOR output1713. The first NOR input1711is electrically connected to the first comparing output113and the second NOR input1712is electrically connected to the second comparing output123.

The first inverter172is electrically connected to the third comparing output163. The SR latch173is electrically connected to the NOR output1713and the first inverter172. Concretely speaking, the SR latch173has a SB pin electrically connected to the NOR gate, a RB pin electrically connected to the first inverter172. The second inverter174is electrically connected to the SR latch173and the load switch22.

The voltage control module18includes a second controlled capacitor181, a controlled switch182, and a third constant current source183. The second controlled capacitor181is electrically connected to the fifth comparing input161and has the variable voltage Vr. That is, the variable voltage Vr is the capacitor voltage of the second controlled capacitor181. The controlled switch182is electrically connected to the fifth comparing input161. Concretely speaking, the gate of the controlled switch182is also electrically connected to the second inverter174, and the third constant current source183is electrically connected to the second controlled capacitor181.

The first comparing output113transmits a first conduction signal S1(such as [1] of the digital signal) as the inductor current IL reaches the default average current Ir1. The second comparing output123transmits a second conduction signal S2(such as [1] of the digital signal) as the inductor current IL reaches the default ripple current Ir2. The first switch13is conducted when receiving the first conduction signal S1, and the second switch14is conducted when receiving the second conduction signal S2. The variable voltage Vr received by the fifth comparing input161gradually increases as the load switch22is turned off, and the third comparing output163transmits a re-conduction signal S3as the variable voltage Vr reaches the capacitor voltage Vc.

The NOR input1713transmits an OFF signal S4as the first NOR input1711receives the first conduction signal S1or the second NOR input1712receives the second conduction signal S21. The first inverter172is utilized for inverting the re-conduction signal S3to generate and transmit an inverted re-conduction signal S5. The SR latch173is utilized for transmitting a latch OFF signal S6when receiving the OFF signal S4and transmits a latch ON signal S7when receiving the inverted re-conduction signal S5. The second inverter174is utilized for inverting the latch OFF signal S6and the latch ON signal S7to generate an inverted latch OFF signal S8and an inverted latch ON signal S9respectively, such that the load switch22would be turned off when receiving the inverted latch OFF signal S8but would be conducted when receiving the inverted latch ON signal S9.

The inverted latch ON signal S9would be also received by the controlled switch182such that the controlled switch182would be turned on as the load switch22is conducted. The third constant current source183charges the second controlled capacitor181as the controlled switch182is conducted to enhance the variable voltage Vr.

Please refer toFIGS. 2 to 4, whereinFIG. 4is a schematic diagram showing the waveforms corresponding to the variable voltage and the capacitor voltage in accordance with a preferred embodiment of the present invention. As shown, in the beginning, the load switch22is triggered to be conducted to enter the time zone T1. Then, the inductor current IL gradually increases. As the inductor current IL reaches the default ripple current Ir2, the second comparing output123is triggered to transmit the second conduction signals S2to conduct the second switch14. Meanwhile, the NOR output1713is triggered to transmit the OFF signal S4such that the inverted latch OFF signal S8would be generated to turn off the load switch22and the time zone Ta begins. At this time, the third constant current source183charges the second controlled capacitor181to enhance the variable voltage Vr gradually. As the variable voltage Vr reaches the capacitor voltage Vc (such as the time zone Te ofFIG. 4, which is merely for demonstration rather than being corresponding to time zones Ta, Tb, Tc, and Td), the third comparing output163transmits the re-conduction signal S3such that the load switch22would be conducted again after receiving the inverted latch ON signal S8and the time zone T2begins. The following time zone T3is similar to time zone T1, the following time zones Tb, Tc, and Td are similar to time zone Ta, and thus are not repeated here.

It is noted that the default average current Ir1of the preferred embodiment of the present invention is the average of the highest value and the lowest value of the inductor current IL. Take the time zone T1as an example, as the default average current Ir1is 10 mA, the inductor current IL increases from 0 mA should be enhanced to 20 mA to have the average current reaches the default average current Ir1. However, the load switch22would be turned off as the inductor current IL reaches the default ripple current Ir2to gradually reduce the inductor current IL such that the default average current Ir1cannot be achieved.

In addition, as the inductor current IL gradually increases and reaches the default ripple current Ir2to conduct the second switch14, the second constant current source4would discharge the first controlled capacitor15to reduce the capacitor voltage Vc (as shown inFIG. 4, the waveform100indicates the capacitor voltage Vc which decreases after time zone Tf such that the off time of the load switch22becomes shorter than the previous time zone).

It is also noted that, in time zone T4, the inductor current IL is enhanced from point A to point B, and the default average current Ir1is the average value of the current at point A and the current at point B. Then, the first comparing output113is triggered to transmit the first conduction signal S1to conduct the first switch13so as to have the first constant current source3charges the first controlled capacitor15to enhance the capacitor voltage Vc (as shown inFIG. 4, the waveform100indicates that the capacitor voltage Vc, which is enhanced after time zone Te such that the needed charging time becomes longer and thus the off time of the load switch22becomes longer than the previous time zone, i.e. time zone Tf>time zone Te).

In addition, the NOR output1713transmits the OFF signal S4after the first NOR input1711receives the first conduction signal S1such that the inverted latch OFF signal S8would be generated to turn off the load switch22. The other portions of the time zone are similar to the time zone T1and thus are not repeated here.

In addition, as shown inFIG. 3, in which time zone T4is shorter than time zone T3, time zone T3is shorter than time zone T2, and time zone T2is shorter than time zone T1, the spirit of the present invention is that after the load switch22is conducted, the system may decide which condition (the inductor current IL reaches the default average current Ir1or reaches the default ripple current Ir2) should be used to turn off the load switch22for adjusting the off time of the load switch22. That is, the system can adjust the off time of the load switch22automatically. For example, as shown inFIGS. 3 and 4, in which time zone Ta is shorter than time zone Tb, time zone Tb is shorter than time zone Tc, time zone Tc is shorter than time zone Td, but time zone Te is greater than time zone Td and time zone Tf is greater than time zone Te, off time of the load switch22can be automatically adjusted so as to access the adequate off time to generate the adjusted average current Ia to stabilize the output of the working element23.

In accordance with the present invention, the width of the above mentioned time zones is adjusted by using the charging/discharging operation of the first controlled capacitor15. For a greater capacitor voltage Vc, the time needed for enhancing the variable voltage Vr to reach the capacitor voltage Vc would be longer such that the off time of the load switch22becomes longer. On the other hand, for a smaller capacitor voltage Vc, the time needed for enhancing the variable voltage Vr to reach the capacitor voltage Vc would be shorter such that the off time of the load switch22would be shorter (as shown inFIG. 4, the time zone Te is smaller than the time zone Tf). Thereby, the width of the time zones can be adjusted.

In conclusion, by using the circuit for controlling load switch provided in the present invention, off time of the load switch can be adequately controlled such that the ripple current can be steadily controlled to make sure that the present invention can be adopted to the circuit with different inductance. Thus, the cost of the working element can be reduced and the ripple current can be steadily controlled such that the problem of the conventional art can be effectively resolved.

The detail description of the aforementioned preferred embodiments is for clarifying the feature and the spirit of the present invention. The present invention should not be limited by any of the exemplary embodiments described herein, but should be defined only in accordance with the following claims and their equivalents. Specifically, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the scope of the invention as defined by the appended claims.