LED driver gate clamp systems and methods

Aspects of the subject technology relate to display circuitry such as backlight control circuitry for operating light-emitting diodes (LEDs). The backlight control circuitry may include a pulse-width-modulation (PWM) transistor and a current regulation transistor coupled in series with at least one LED. The current regulation transistor may have a gate terminal that receives a feedback-controlled gate voltage. The backlight control circuitry may include a gate clamp circuit coupled to the gate terminal of the current regulation transistor that clamps the gate voltage during a portion of a PWM on pulse.

TECHNICAL FIELD

The present description relates generally to electronic devices with light-emitting-diodes, and more particularly, but not exclusively, to electronic devices with light-emitting-diodes with pulse-width-modulation.

BACKGROUND

Electronic devices such as computers, media players, cellular telephones, set-top boxes, and other electronic equipment are often provided with light-emitting-diodes (LEDs) for illuminating portions of the device and/or providing visual indicators of device status.

In some devices, LEDs are included in displays such as organic light-emitting diode (OLED) displays and liquid crystal displays (LCDs) typically include an array of display pixels arranged in pixel rows and pixel columns. Liquid crystal displays commonly include a backlight unit and a liquid crystal display unit with individually controllable liquid crystal display pixels. The backlight unit commonly includes one or more light-emitting diodes (LEDs) that generate light that exits the backlight toward the liquid crystal display unit. The liquid crystal display pixels are individually operable to control passage of light from the backlight unit through that pixel to display content such as text, images, video, or other content on the display.

DETAILED DESCRIPTION

The subject disclosure provides electronic devices such as cellular telephones, media players, computers, laptops, tablets, set-top boxes, wireless access points, and other electronic equipment that may include light-emitting-diodes (LEDs). For example, electronic devices may include LEDs in displays that may be used to present visual information and status data and/or may be used to gather user input data, keyboards, flash LEDs, and/or other components. The brightness of the LEDs may be controlled by a pulse-width-modulation (PWM) signal.

Various examples are described herein in the context of LEDs and associated LED control circuitry implemented in display backlights. However, it should be appreciated that these examples are merely illustrative and the disclosed LED control systems and methods described herein may be implemented in other contexts in which PWM control of LEDs is desired (e.g., for illumination of keyboards, flash components, etc.).

A display may include an array of display pixels. Each display pixel may include one or more colored subpixels for displaying color images. Each display pixel may include a layer of liquid crystals disposed between a pair of electrodes operable to control the orientation of the liquid crystals. Controlling the orientation of the liquid crystals controls the polarization of backlight generated by a backlight unit of the display. This polarization control, in combination with polarizers on opposing sides of the liquid crystal layer, allows light passing into the pixel to be manipulated to selectively block the light or allow the light to pass through the pixel.

The backlight unit may include one or more strings of light-emitting diodes and associated backlight control circuitry that generate the backlight for the display. The strings of light-emitting diodes may be arranged along one or more edges of a light guide plate that distributes backlight generated by the strings to the LCD unit, or may be arranged to form a two-dimensional grid of LEDs.

The backlight unit includes backlight control circuitry for operating the strings of LEDs. The backlight control circuitry may include a pulse-width-modulation (PWM) transistor and a current regulation transistor coupled in series with each string of LEDs. The PWM transistor is operated as a switch such that the current through the LEDs is pulsed and a duty cycle of the LED current is used to control dimming of the LEDs. The current regulation transistor forms a portion of a current regulation loop that includes an operational amplifier coupled to the gate of the current regulation transistor. The operational amplifier uses a feedback loop to control the current through the current regulation transistor during the on pulses of the PWM cycle of the PWM transistor.

However, during the off pulses of the PWM transistor, there is no current flowing in the LEDs or the current regulation transistor. In some scenarios, without current flow, the current regulation transistor, which is in the regulating feedback loop, goes out of regulation. The operational amplifier still tries to correct the loop and drives the gate (e.g., in case of a metal oxide semiconductor field effect transistor (MOSFET) implementation of the current regulation transistor) of the current regulation transistor up to the supply rail voltage. In these scenarios, when the PWM switch is turned back on, the gate voltage of the current regulation transistor, being at the supply rail voltage, transitions back down to the regulation value. However, because the operational amplifier takes some time to bring the gate voltage of the current regulation transistor down to the desired voltage, a current spike can occur in the LED PWM pulse. If care is not taken, this type of current spike can occur anytime the feedback loop of the current regulation transistor is opened and closed again.

Backlight control circuitry and associated methods of operation of the backlight circuitry disclosed herein may help reduce or eliminate this type of voltage spike that occurs at the rising edge of the PWM cycle. Reducing or eliminating voltage spikes at the rising edge of the PWM cycle may help improve the accuracy of the average current that is provided to the strings of LEDs, may help avoid electromagnetic interference (EMI) with other components of the device, and/or may help improve the reliability of the LED driver circuitry.

As described in further detail hereinafter, the backlight control circuitry and associated methods may include using a gate clamp circuit coupled to the gate of the current regulation transistor to clamp the gate voltage of the current regulation transistor to a voltage at or slightly above the regulating voltage for the on pulse of the PWM cycle. In this way, once the feedback loop of the current regulation transistor is broken, the gate voltage remains or rises slightly and, once the loop is reconnected, the gate voltage can be returned to the regulation voltage quickly. In backlight systems, the output current is often known, and it is therefore possible to create a replica of the gate voltage and, if desired, add a small margin to the replica voltage and use this replica voltage as a reference for clamping.

The gate clamp circuit may include a replica circuit that duplicates the regulated gate voltage of the current driver device or may include a sample-and-hold circuit that samples the gate voltage during an on pulse of the PWM cycle and holds the gate voltage during the off pulse of the PWM cycle.

An illustrative electronic device of the type that may be provided with one or more LEDs and associated LED control circuitry (e.g., in a display) is shown inFIG. 1. In the example ofFIG. 1, device100has been implemented using a housing that is sufficiently small to be portable and carried by a user (e.g., device100ofFIG. 1may be a handheld electronic device such as a tablet computer or a cellular telephone). As shown inFIG. 1, device100may include a display such as display110mounted on the front of housing106. Display110may be substantially filled with active display pixels or may have an active portion and an inactive portion. Display110may have openings (e.g., openings in the inactive or active portions of display110) such as an opening to accommodate button104and/or other openings such as an opening to accommodate a speaker, a light source, or a camera.

Display110may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch-sensitive. Display110may include display pixels formed from light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), plasma cells, electrophoretic display elements, electrowetting display elements, liquid crystal display (LCD) components, or other suitable display pixel structures. Arrangements in which display110is formed using LCD pixels and LED backlights are sometimes described herein as an example. This is, however, merely illustrative. In various implementations, any suitable type of display technology may be used in forming display110if desired.

Housing106, which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials.

The configuration of electronic device100ofFIG. 1is merely illustrative. In other implementations, electronic device100may be a computer such as a computer that is integrated into a display such as a computer monitor, a laptop computer, a somewhat smaller portable device such as a wrist-watch device, a pendant device, or other wearable or miniature device, a media player, a gaming device, a navigation device, a computer monitor, a television, or other electronic equipment.

For example, in some implementations, housing106may be formed using a unibody configuration in which some or all of housing106is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). Although housing106ofFIG. 1is shown as a single structure, housing106may have multiple parts. For example, housing106may have upper portion and lower portion coupled to the upper portion using a hinge that allows the upper portion to rotate about a rotational axis relative to the lower portion. A keyboard such as a QWERTY keyboard and a touch pad may be mounted in the lower housing portion, in some implementations.

In some implementations, electronic device100may be provided in the form of a computer integrated into a computer monitor. Display110may be mounted on a front surface of housing106and a stand may be provided to support housing (e.g., on a desktop).

FIG. 2is a schematic diagram of display110showing how the display may be provided with a liquid crystal display unit204and a backlight unit202. As shown inFIG. 2, backlight unit202generates backlight208and emits backlight208in the direction of liquid crystal display unit204. Liquid crystal display unit204selectively allows some or all of the backlight208to pass through the liquid crystal display pixels therein to generate display light210visible to a user. Backlight unit may include one or more subsections206. In some implementations, subsections206may be elongated subsections that extend horizontally or vertically across some or all of display110(e.g., in an edge-lit configuration for backlight unit202). In other implementations, subsections206may be square or nearly square subsections (e.g., in a two-dimensional array backlight configuration). Accordingly, subsections206may be defined one or more strings of LEDs disposed in that subsection. Subsections206may be controlled individually for local dimming of backlight208.

FIG. 3shows a schematic diagram of exemplary LED control circuitry300(e.g., backlight control circuitry that may be implemented in backlight unit202). In the example ofFIG. 3, circuitry300includes at least one string302of LEDs304. Strings302each include one or more LEDs304in series. LEDs304received a voltage Vo, at a first end of string302from, for example, DC/DC converter306. LEDs304are also coupled, at a second end of string302, in series with cascaded transistors308and322(e.g., field effect transistors such as metal oxide semiconductor field effect transistors) that control the current through LEDs304. Transistor322, in combination with a reference voltage supply that supplies a reference voltage VREF and operational amplifier330may form a current regulation controller for LEDs304that controls a peak current through string302. Transistor308may combine with a pulse-width-modulation signal provider that provides a PWM signal310, to control an average current through string302by reducing the peak current allowed by the current regulation controller according to a duty cycle of the PWM signal.

As shown inFIG. 3, transistor308is a pulse-width modulation transistor having a first source/drain terminal316coupled to LEDs304, a gate terminal320configured to receive pulse-width modulation signal310, and a second source/drain terminal318. Transistor322is a current regulation transistor having a first source/drain terminal324coupled to second source/drain terminal318of pulse-width modulation transistor308, a gate terminal328coupled to an output332of an operational amplifier330, and a second source/drain terminal326coupled to a ground voltage through a resistor R. In this way, backlight circuitry300includes a cascade of two active devices to control the current, one arranged for current regulation and the other implemented as a switch for PWM pulse-width current control.

PWM signal310includes alternating on pulses312and off pulses314. The width (e.g., the length of time) of pulses312and314can be controlled to control a duty cycle of the current through LEDs304to, for example, provide a dimming of LEDs304without changing the voltage Vo. During off pulses314, when transistor308is off, there is no current flowing in LEDs304or transistor322. As shown inFIG. 3, current regulation transistor322forms part of a current regulation feedback loop including amplifier330. In some situations, the impedance of operational amplifier330may be used help reduce or prevent voltage spikes at gate terminal328at or near the rising edge of PWM signal310. However, in some situations, it may be desirable to provide backlight circuitry300with a separate gate clamp circuit (e.g., in situations in which the impedance of amplifier330may not be enough to reduce the voltage spike at the rising edge of PWM signal310).

FIGS. 4 and 5show exemplary implementations of backlight circuitry300having a gate clamp circuit. As shown inFIG. 4, circuitry300may include a clamping circuit such as gate clamp circuit400. The clamping circuit clamps a voltage of the current regulation controller (e.g., including transistor322and amplifier330) during at least a portion of an on pulse of PWM signal310. In the example ofFIG. 4, gate clamp circuit400is coupled (e.g., by conductive trace402) to gate terminal328of current regulation transistor322(e.g., between gate terminal328and ground404). Gate clamp circuit400clamps the gate voltage of gate terminal328of current regulation transistor322to the desired voltage. For example, gate clamp circuit400obtains a gate voltage of the gate terminal of the current regulation transistor during an on pulse of a pulse-width modulation cycle of the pulse-width modulation transistor and clamps the gate voltage of the gate terminal of the current regulation transistor to the obtained gate voltage during a portion of a subsequent on pulse of the pulse-width modulation cycle of the pulse-width modulation transistor. In this way, breaking the regulation loop, or coupling from drain326to gate328, will not be able to raise the gate voltage above its previous voltage during regulation.

As described in further detail hereinafter, gate clamp circuit may be active for a short period during and shortly after the PWM-on rising edge, and can be disabled once voltage transients have subsided, thus allowing the control loop of transistor322and amplifier330to raise the gate voltage, if desired for regulation, to generate a higher LED current. Gate clamp circuit400may create a replica of the gate voltage during a PWM-on pulse312to use as a reference voltage for clamping. In some scenarios, the reference voltage may be offset slightly higher (e.g., to allow for minor shifts or inaccuracies in duplicating the original PWM on pulse gate voltage).

FIG. 5shows additional details of an exemplary implementation of gate clamp circuit400. In the example ofFIG. 5, gate clamp circuit400includes a reference generator circuit500, a clamp timing circuit504, and a clamping circuit506. Reference generator circuit500obtains a reference voltage that is substantially equal to the gate voltage of transistor322during an on pulse312of a PWM signal310for current regulation transistor322. Clamp circuit506pulls current when a current gate voltage of the gate terminal328of the current regulation transistor322exceeds the obtained reference gate voltage. Timing circuit504activates the gate clamp circuit (e.g., activates clamping circuit506) during a portion of a subsequent on pulse. The portion of the subsequent on pulse may be a relatively small fraction of the time of the on pulse (e.g., less than 10 percent, less than 5 percent, less than 3 percent, less than one percent, or less than a fraction of a percent of the time of the on pulse). For example, the portion of the subsequent on pulse may be as small as or less than 5 microseconds, 1 microsecond, 100 nanoseconds, 50 nanoseconds or 20 nanoseconds.

In the example ofFIG. 5, reference generator circuit500is implemented as a sample-and-hold circuit that samples the gate voltage during the PWM-on pulse312and holds the sampled gate voltage during the subsequent PWM-off pulse314. As shown, the sample-and-hold circuit500includes a power supply508that receives the gate voltage of gate terminal328, the power supply being coupled through a switch512to a capacitor510for holding the received gate voltage as a reference voltage. However, this is merely illustrative.

In other implementations, reference generator circuit500may be implemented as a scaled down replica of the output stage (e.g., transistor322and/or amplifier330) so that a gate voltage of the replica device can be used as the clamp reference voltage. Clamp timing circuit504and/or clamping circuit506may be provided with a reference generator circuit500that is implemented as a sample-and-hold circuit or a replica circuit (as examples).

In the example ofFIG. 5, clamping circuit506is implemented as a source follower circuit (e.g., a voltage follower stage such as a p-type metal-oxide-semiconductor (PMOS) output stage). As shown, source follower circuit506may include transistor516having a first source/drain terminal switchably coupled to the gate voltage of the gate terminal328of the current regulation transistor322, a gate terminal coupled to a first side of a current sink518, and a second source/drain terminal coupled to a second side of the current sink518. The first source/drain terminal of transistor516is switchably coupled to the gate voltage of the gate terminal328of the current regulation transistor322by a switch514of clamp timing circuit504. Switch514may be operated so that clamp circuit506is only active during a short period after the PWM on pulse312has started. In some implementations, source follower circuit506may include additional circuitry such as an additional transistor having a gate that is coupled to the gate of transistor516as shown.

FIG. 6depicts a flow diagram of an example process for headroom voltage reduction for electronic device displays in accordance with various aspects of the subject technology. For explanatory purposes, the example process ofFIG. 6is described herein with reference to the components ofFIGS. 1-5. Further for explanatory purposes, the blocks of the example process ofFIG. 6are described herein as occurring in series, or linearly. However, multiple blocks of the example process ofFIG. 6may occur in parallel. In addition, the blocks of the example process ofFIG. 6need not be performed in the order shown and/or one or more of the blocks of the example process ofFIG. 6need not be performed.

In the depicted example flow diagram, at block600, a current through a string such as string302of LEDs such as LEDs304may be controlled using a pulse-width-modulation transistor such as transistor308and a current regulation transistor such as a transistor322coupled in series with the string of LEDs. Controlling the current may include providing PWM signal310to gate terminal320of transistor308and regulating the current using operational amplifier330having output332switchably coupled to gate terminal328of transistor322.

At block602, during an on pulse312of the pulse-width-modulation transistor308, a gate voltage of a gate terminal328of the current regulation transistor322may be obtained. The gate voltage of the gate terminal328of the current regulation transistor322may be obtained by a reference generator circuit such as reference generator circuit500ofFIG. 5(e.g., a replica circuit or a sample-and-hold circuit) that is coupled to gate terminal328.

At block604, during an off pulse314of the pulse-width-modulation transistor308, the obtained gate voltage of the current regulation transistor322may be stored (e.g., by the reference generator circuit).

At block606, during a portion of a subsequent on pulse312of the pulse-width-modulation transistor308, a clamping circuit506coupled to the gate terminal328of the current regulation transistor322may be activated. A timing circuit such as clamp timing circuit504may be operated to activate the clamping circuit.

At block608, while the clamping circuit506is active during the subsequent on pulse312of the pulse-width-modulation transistor308, the gate terminal328of the current regulation transistor322may be clamped by the clamping circuit506to the stored gate voltage. Clamping gate terminal328may include activating the clamping circuit if the gate voltage of gate terminal328starts to exceed the stored clamp reference voltage and holding the gate voltage of gate terminal328by pulling current (e.g., using current sink518).

In accordance with various aspects of the subject disclosure, an electronic device with a display is provided, the display including at least one light-emitting diode and a pulse-width modulation transistor having a first source/drain terminal coupled to the at least one light-emitting diode, a gate terminal configured to receive a pulse-width modulation signal, and a second source/drain terminal. The display also includes a current regulation transistor having a first source/drain terminal coupled to the second source/drain terminal of the pulse-width modulation transistor, a gate terminal coupled to an output of an operational amplifier, and a second source drain terminal coupled to a ground voltage through a resistor. The display also includes a gate clamp circuit coupled to the gate terminal of the current regulation transistor.

In accordance with other aspects of the subject disclosure, a method is provided that includes controlling current through at least one light-emitting diode using a pulse-width-modulation transistor and a current regulation transistor coupled in series with the at least one light-emitting diode. The method also includes, during an on pulse of the pulse-width-modulation transistor, obtaining a gate voltage of a gate terminal of the current regulation transistor. The method also includes, during an off pulse of the pulse-width-modulation transistor, storing the gate voltage of the current regulation transistor. The method also includes, during a subsequent on pulse of the pulse-width-modulation transistor, clamping the gate terminal of the current regulation transistor to the stored gate voltage using a clamping circuit coupled to the gate terminal of the current regulation transistor.

In accordance with other aspects of the subject disclosure, an electronic device having a display with a backlight is provided, the backlight including a light-emitting diode and a pulse-width modulation controller configured to control an average current through the light-emitting diode. The backlight also includes a current regulation controller configured to control a peak current through the light-emitting diode. The backlight also includes a clamping circuit coupled to the current regulation controller and configured to clamp a voltage of the current regulation controller during at least a portion of an on pulse of the pulse-width modulation controller.