Light emitting diode control circuit with hysteretic control and low-side output current sensing

An LED control circuit controls a switching operation of a switch by hysteretic control. The LED control circuit includes a controller integrated circuit (IC) that senses a current sense voltage from a current sense resistor that is on a low-side of the switch. The LED control circuit senses the current sense voltage during on-time of the switch to determine when to turn off the switch. During off-time of the switch, the controller IC determines when to turn on the switch by comparing a sawtooth voltage to a turn-on threshold that is generated from the on-time of the switch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrical circuits, and more particularly but not exclusively to light emitting diode control circuits.

2. Description of the Background Art

A light emitting diode (LED) may be used in various lighting applications. For example, one or more LEDs may provide illumination by driving the LEDs using a transistor. An LED control circuit may receive an input voltage to generate a regulated output current that is provided to the LEDs. The LED control circuit may include a controller integrated circuit (IC) to control the switching operation of the transistor by pulse width modulation (PWM) or hysteretic control. When employed in a continuous conduction mode (CCM) buck topology, hysteretic control provides the benefits of no or minimum flicker and output current overshoot. However, in conventional CCM buck converters with hysteretic control, the output current is delivered during the on-time and the off-time of the transistor. Therefore, the output current needs to be continuously sensed during the switching cycle for regulation. This requires output current sensing, which leads to power loss on the sense resistor, during both the on-time and the off-time.

SUMMARY

In one embodiment, an LED control circuit controls a switching operation of a switch by hysteretic control. The LED control circuit includes a controller integrated circuit (IC) that senses a current sense voltage from a current sense resistor that is on a low-side of the switch. The LED control circuit senses the current sense voltage during on-time of the switch to determine when to turn off the switch. During off-time of the switch, the controller IC determines when to turn on the switch by comparing a sawtooth voltage to a turn-on threshold that is generated from the on-time of the switch.

DETAILED DESCRIPTION

For ease of reading, subscripts and superscripts that appear in the drawings are formatted below as normal fonts . For example, a signal that is labeled in the drawings as VEXAMPLEis simply written below as VEXAMPLE.

FIG. 1shows a schematic diagram of an LED control circuit100in accordance with an embodiment of the present invention. In the example ofFIG. 1, the LED control circuit100has a continuous conduction mode (CCM) buck converter topology with hysteretic control. In the example ofFIG. 1, the LED control circuit100comprises an inductor110, a diode string112,a switch in the form of a transistor114, an LED circuit113, a sense resistor RS, and a controller integrated circuit (IC)140. The diode string112may comprise a single diode or a plurality of diodes that are connected in series. Similarly, the LED circuit113may comprise a single LED or a plurality of LEDs that are connected in series. The LED control circuit100receives an input voltage VIN, which is filtered by an input capacitor115. In one embodiment, the input voltage VIN is a DC (i.e., direct current) voltage.

In the example ofFIG. 1, the transistor114is a metal oxide semiconductor field effect transistor (MOSFET) with a drain that is connected to a cathode of the diode string112, a gate that is connected to a gate pin151of the controller IC140, and a source that is connected to an end of the sense resistor RS. The other end of the sense resistor RS is connected to ground. Because the sense resistor RS is disconnected from the input voltage VIN when the transistor114is off, the sense resistor RS is referred to as being on the low side of the transistor114. Components on other side of the transistor114towards the input voltage VIN, e.g., diode string112, is referred to as being on the high side of the transistor114.

Briefly, when the transistor114is on, the input voltage VIN is connected to ground through the transistor114. The resulting output current ILED flows through the inductor110, the diode string112, the transistor114, and the sense resistor RS. Accordingly, a current sense voltage VCS that is developed by the output current ILED on the sense resistor RS is indicative of the of the output current ILED. When the transistor114is off, the input voltage VIN is disconnected from ground, and the output current ILED flows through the inductor110, the diode string112, and the LED circuit113. The controller IC140controls the switching operation of the transistor114to regulate the output current ILED, and thus the illumination provided by the LED circuit113.

In one embodiment, the controller IC140comprises a turn off circuit160, a sawtooth generator170, and a turn on circuit180. Circuits of the controller IC140that are not necessary to the understanding of the invention, such as soft-start circuits, protection circuits, internal bias circuits, etc., are not shown for clarity of illustration.

In the example ofFIG. 1, the controller IC140senses the output current ILED by low-side current sensing. More particularly, the controller IC140includes a current sense (CS) pin152for receiving the current sense voltage VCS, which is indicative of the output current ILED. The turn off circuit160, which comprises a comparator161, is configured to turn off the transistor114based on the current sense voltage VCS. The comparator161compares the current sense voltage VCS to a threshold voltage162, which serves as a turn-off threshold. When the current sense voltage VCS is higher than the threshold voltage162, the comparator161generates a comparator output voltage VCOM2that resets an SR flip-flop141, thereby generating a gate drive signal GATE that turns off the transistor114. A gate driver142provides suitable drive current to drive the gate of the transistor114.

The sawtooth generator170is configured to generate the sawtooth voltage VSAW, which serves as an increasing control signal for determining when to turn on the transistor114. In the example ofFIG. 1, the sawtooth generator170comprises a switch171, a capacitor172, a constant current source173, and a switch174. When the switch174is closed, the current source173charges the capacitor172to generate the sawtooth voltage VSAW. Opening the switch174stops the charging of the capacitor172. In the example ofFIG. 1, the state of the switch174is dictated by the gate drive signal GATE. More particularly, the switch174is closed when the Q output of the SR flip-flop141is at a logic low (i.e., when the transistor114is turned off), and the switch174is open when the Q output of the SR flip-flop141is at a logic high (i.e., when the transistor114is turned on). In the example ofFIG. 1, closing the switch171shorts the capacitor172to reset the sawtooth voltage VSAW. In one embodiment, the state of the switch171is dictated by a comparator output voltage VCOM1that is generated by a comparator184. The generation of the comparator output voltage VCOM1is further explained below.

In the example ofFIG. 1, the turn on circuit180comprises an on-time detector185, an operational transconductance amplifier (OTA)181, and the comparator184. In one embodiment, the OTA181provides error compensation. An RC circuit183at the output of the OTA181sets the phase and gain of the OTA181. The values of the resistor and capacitor of the RC circuit183may be set for loop compensation. In the example ofFIG. 1, the on-time detector185is configured to detect an on-time of the transistor114from the current sense voltage VCS to generate an on-time voltage VCS-TON that is indicative of the on-time of the transistor114. The on-time detector185may be implemented by a timer circuit or other suitable circuit for measuring on-time. In the example ofFIG. 1, the longer the on-time of the transistor114, the higher the level of the of on-time voltage VCS-TON; the shorter the on-time of the transistor114, the lower the level of the on-time voltage VCS-TON. The OTA181compares the on-time voltage VCS-TON to a reference voltage182to generate a comparator output voltage VCOM, which serves as a turn-on threshold voltage. The comparator184compares the comparator output voltage VCOM to the sawtooth voltage VSAW to generate the comparator output voltage VCOM1. When the sawtooth voltage VSAW increases to the level of the comparator output voltage VCOM, the comparator output voltage VCOM1is asserted to set the SR flip-flop141and thereby turn on the transistor114. Asserting the comparator output voltage VCOM1also closes the switch171to reset the sawtooth voltage VSAW.

In the example ofFIG. 1, the transistor114is turned off based on the threshold voltage162and the current sense voltage VCS. The transistor114is turned on based on the level of the sawtooth voltage VSAW relative to the comparator output voltage VCOM, which is generated from the on-time voltage VCS-TON. The off-time of the transistor114is controlled by sensing the on-time of the transistor114to generate the on-time voltage VCS-TON and setting the value of the comparator output voltage VCOM based on the value of the on-time voltage VCS-TON. In the example ofFIG. 1, when the on-time voltage VCS-TON is greater than the reference voltage182, the comparator output voltage VCOM increases, thereby increasing the off-time of the transistor114. When the on-time voltage VCS-TON is less than the reference voltage182, the comparator output voltage VCOM decreases, thereby decreasing the off-time of the transistor114.

The controller IC140controls the transistor114in accordance with hysteretic control because both the turn on and the turn off of the transistor114are actively controlled based on the output current ILED. Energy efficiency is improved because the current sense voltage VCS is sensed only during the on-time of the transistor114to determine when to turn the transistor114off. The current sense voltage VCS is not sensed during the off-time of the transistor114. Instead, during the off-time of the transistor114, the instance of when to turn on the transistor114is determined based on the internally generated sawtooth voltage VSAW and the on-time voltage VCS-TON.

FIG. 2shows waveforms of signals of the LED control circuit100in accordance with an embodiment of the present invention.FIG. 2shows, from top to bottom, the current sense voltage VCS, the comparator output voltage VCOM2, the sawtooth voltage VSAW, the comparator output voltage VCOM1, and the gate drive signal GATE.FIG. 2also shows the levels of the threshold voltage162, an onset voltage VCS-ON (FIG. 2, 211), and the comparator output voltage VCOM (FIG. 2, 215).

In the example ofFIG. 2, the onset voltage VCS-ON (FIG. 2, 211) is the level of the current sense voltage VCS at the beginning of the on-time (FIG. 2, 212) of the transistor114. The comparator output voltage VCOM (FIG. 2, 215) is generated at the beginning of the on-time of the transistor114(FIG. 2, 212) when the current sense voltage VCS reaches the onset voltage VCS-ON (FIG. 2, 210). More particularly, the on-time detector185measures the on-time of the transistor114, reads the value of the current sense voltage VCS, and generates the on-time VCS-TON when the sense voltage VCS reaches the onset voltage VCS-ON.

The sawtooth voltage VSAW increases (FIG. 2, 213) from the onset voltage VCS-ON to the threshold voltage162during the on-time of the transistor114(FIG. 2, 214). The on-time of the transistor114ends when the current sense voltage VCS reaches the threshold voltage162. The on-time detector185senses the time it took for the current sense voltage VCS to increase from the onset voltage VCS-ON to the threshold voltage162to generate the on-time voltage VCS-TON, which is used to generate the comparator output voltage VCOM (FIG. 2, 215).

When the current sense voltage VCS reaches the threshold voltage162, the comparator output voltage VCOM2is asserted (FIG. 2, 216), which turns off the transistor114(FIG. 2, 217) and initiates its off-time (FIG. 2, 218). The sawtooth voltage VSAW increases during the off-time of the transistor114(FIG. 2, 219). When the sawtooth voltage VSAW reaches the comparator output voltage VCOM, the comparator output voltage VCOM1is asserted (FIG. 2, 220) to turn on the transistor114and begin the next switching cycle.

FIG. 3shows a flow diagram of a method of operating an LED control circuit in accordance with an embodiment of the present invention. The method ofFIG. 3may be performed by the LED control circuit100ofFIG. 1.

In the example ofFIG. 3, a turn-on threshold (e.g., comparator output voltage VCOM) is generated based on a detected on-time of the switch (e.g., on-time voltage VCS-TON) (step401). A current sense voltage (e.g., current sense voltage VCS) is sense during the on-time of the switch (step402). The switch is turned off when the current sense voltage reaches a turn-off threshold (e.g., threshold voltage162) (step403). An increasing control signal (e.g., sawtooth voltage VSAW) is generated during the off-time of the switch (step404). The control signal is compared to the turn-on threshold to determine when to turn on the switch (step405). The switch is turned on when the control signal reaches the turn-on threshold (step406).

FIG. 4shows a flow diagram of a method of operating the LED control circuit100ofFIG. 1in accordance with an embodiment of the present invention. In the example ofFIG. 4, the steps501-504may be performed at startup of the LED control circuit100, and the steps505-509may be performed at steady-state during normal operation.

At startup, the transistor114is turned on until the current sense voltage VCS reaches the threshold voltage162(step501). The transistor114is turned off when the current sense voltage VCS reaches the threshold voltage162(step502), and then turned back on after some (e.g., random, temporary, predetermined) time (step503). The comparator output voltage VCOM is generated at the beginning of the on-time of the transistor114(step504), which occurs when the on-time detector185detects that the current sense voltage VCS reaches the onset voltage VCS-ON. In the example ofFIG. 1, the onset voltage VCS-ON is a reference voltage that is internal to the on-time detector185.

Continuing the example ofFIG. 4, the transistor114is kept on until the current sense voltage VCS reaches the threshold voltage162(step505). The transistor114is turned off when the current sense voltage VCS reaches the threshold voltage162(step506). The transistor114is turned on when the sawtooth voltage VSAW reaches the comparator output voltage VCOM (step507). The comparator output voltage VCOM is updated at the beginning of the on-time of the transistor114(step508). The transistor114is turned off based on the comparator output voltage VCOM2of the comparator161(step509). More specifically, the transistor114is turned off the when the current sense voltage VCS reaches the threshold voltage162. The cycle comprising the steps505-509is thereafter repeated during normal operation.

LED control circuits with low-side current sensing and hysteretic control have been disclosed. While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.