Patent Description:
It will be appreciated that the scope of the invention is in accordance with the claims.

The accompanying drawings illustrate implementations of the present concepts. Features of the illustrated implementations can be more readily understood by reference to the following descriptions in conjunction with the accompanying drawings. Like reference numbers in the various drawings are used where feasible to indicate like elements. In some cases, parentheticals are utilized after a reference number to distinguish like elements. Use of the reference number without the associated parenthetical is generic to the element. The accompanying drawings are not necessarily drawn to scale. In the figures, the left-most digit of a reference number identifies the figure in which the reference number first appears. The use of similar reference numbers in different instances in the description and the figures may indicate similar or identical items.

The present concepts relate to a circuit for temporarily generating a negative voltage rail, on an as-needed basis (i.e., on demand), to increase the voltage difference across a given load to a level that meets the minimum voltage requirement of the load which may be greater than an available system supply voltage referenced to ground. The temporary negative voltage rail may be enabled and disabled in synchronization with (i.e., tracks) an enable and disable signal that controls the load. The difference in voltage across the load may be enlarged without increasing the positive voltage at the system power rail and without adding a switched-mode power supply (SMPS) or a charge pump to create a higher positive voltage rail or a negative voltage rail. The negative voltage rail, consistent with the present concepts, may be self-generated (i.e., generated using the existing system power supply) and may operate at the same frequency as the load.

Electronic circuit designers must often ensure sufficient voltage headroom for an electronic component or circuit (i.e., a load) to maintain proper operations. That is, the difference between the positive system voltage rail and the return voltage rail, which is often ground, should be above that required to enable the load. For example, a typical red-colored LED may have a forward voltage (Vf) between <NUM> volts (V) and <NUM> V, and a typical white-colored LED may have a forward voltage between <NUM> V and <NUM> V. To turn on such an LED, an LED driver may need to provide sufficient voltage and/or current with enough headroom to forward bias the LED, and any additional current control or current limiting circuitry in the conduction path.

However, the driver headroom may be too small to forward bias the LED and associated circuitry when operating from an insufficient voltage source. This may occur due to a low-voltage power rail on a board (e.g., a low-power daughterboard), or if the system is powered by a <NUM> lithium battery system (typically ranging from <NUM> V to <NUM> V) or from AA or AAA <NUM>. 5V batteries, for example. Or, even a <NUM> battery system (typically ranging from <NUM> V to <NUM> V) may provide insufficient headroom if driving multiple LEDs connected in series. When the source voltage is not great enough to overcome the forward voltage drop of an LED (or LEDs) and associated circuitry in the conduction path, then the LED may not conduct and may not emit light. Furthermore, where an LED is driven by a constant current driver (e.g., a two-transistor constant current source or similar), and the source voltage is insufficient to drive enough current, either the LED may not emit light or the light emitted by the LED may not meet target output levels.

To overcome such problems, the designer must increase the operating headroom of the LED driver, and thereby increase the voltage difference across the LED (i.e., the difference in voltage levels at the anode side of the LED and the cathode side of the LED) and other circuitry in the LED path. There are two conventional options for increasing the headroom, neither of which is desirable.

The first conventional option is to generate a system power rail with a higher positive voltage, and thereby increase the voltage level at the anode side of the LED. This can be achieved, for example, by introducing a boost converter, such as an SMPS or a charge pump. A boost converter can step up the input voltage to the LED.

The second conventional option is to create a negative power rail, and thereby decrease the voltage level at the cathode side of the LED. This can be achieved conventionally, for example, by introducing a negative supply SMPS or charge pump (e.g., in an inverted configuration).

Both of the conventional options would increase the voltage difference across the LED, either by increasing the positive voltage on the anode side of the LED or decreasing the negative voltage on the cathode side of the LED. Unfortunately, the addition of such integrated circuits (ICs) would add significant costs, take up considerable board area, increase design complexity, produce high-frequency electrical noise, and/or require ramp-up time and ramp-down time, all of which are undesirable. For example, these ICs may typically include <NUM> or more switches, multiple capacitors, oscillators, and gate drivers, and an SMPS may include an inductor, which is often the largest single component. Some of these ICs may operate at <NUM> kilohertz (kHz) to <NUM> range which may introduce unwanted noise to an otherwise clean system. Moreover, adding active ICs would require additional validation.

The present concepts can achieve the goal of increasing the headroom while avoiding the problems associated with the conventional options. The present concepts relate to a novel circuit that creates an on-demand, temporary, self-generated negative voltage power rail. In some implementations, this negative rail generator circuit may include only a few inexpensive off-the-shelf components, such as a capacitor, a diode, and a transistor. Furthermore, the negative rail generator circuit may be controlled by the same pulse width modulation (PWM) signal that controls the LED for dimming and enablement. Accordingly, a temporary negative rail may be generated on the cathode side of the LED anytime the LED is activated by the PWM signal. The negative rail generator circuit can increase the voltage difference in the LED path up to the referenced positive rail voltage minus certain voltage drops, which will be explained below. The present concepts may also include further improvements to reduce power consumption, which will be described in detail below.

The present concepts may provide a temporary negative rail as needed (e.g., when the LED is on) using only a few simple components. The negative rail generator circuit, consistent with the present concepts, may operate at low frequencies, e.g., the same operating frequency of the LED, which may be typically around <NUM> to <NUM>, for example, or even up to a couple of kilohertz at which some systems may drive their LEDs due to various system constraints. The present concepts may be utilized in any application where there is a desire to increase a voltage difference across a load beyond the existing voltage source level, provided that a switching signal (e.g., a clock signal) is available. For example, a speaker with an amplifier or a flexible circuit board with an LED can benefit from the present concepts.

<FIG> shows an example circuit diagram of a device <NUM> that includes a negative rail generator <NUM>, consistent with some implementations of the present concepts. The device <NUM> may include any electronic device, a system, or a component, such as a computer, a smart phone, a tablet, a personal digital assistant, an appliance, a wearable, an IoT, a television, a printer, a drone, a speaker, a router, etc., and any of a myriad of ever-evolving or yet-to-be-developed types of devices, systems, or components. The negative rail generator <NUM> may be on a printed circuit board (PCB) or in an integrated circuit (IC), inside the device <NUM>, among other configurations. For example, the negative rail generator <NUM> may be implemented as discrete components for simplicity and/or flexibility. Alternatively, the negative rail generator <NUM> may be implemented inside an IC to save board space and/or reduce costs.

In some implementations, the device <NUM> may include a power supply that provides power to various parts of the device <NUM>. Alternatively, the power supply may reside outside the device <NUM>. The power supply may be alternating current (AC), direct current (DC), regulated, unregulated, a generator, a converter, a transformer, a battery, an IC, or any other electrical circuit or component that can provide power to one or more loads in the device <NUM>. In the example circuit shown in <FIG>, the power supply in the device <NUM> may include a voltage source <NUM>. For example, the voltage source <NUM> may provide a certain positive DC supply voltage to a positive voltage rail <NUM>.

The device <NUM> may include one or more loads. A load may be any electrical circuit or component that consumes electrical power. For example, the device <NUM> may include an LED <NUM>. Although a single LED <NUM> is illustrated for simplicity, the LED <NUM> may include multiple LEDs in series and/or parallel. For example, the LED <NUM> may be a low power LED included in a computer, a mobile device, a peripheral, a display, a switch board, a control panel, etc., such as an indicator LED. Where the load includes the LED <NUM>, the supply voltage from the voltage source <NUM> can be called VLED. Also, the voltage source <NUM> may be included in an LED driver that controls the LED <NUM>, or the voltage source <NUM> may supply power to an LED driver that controls the LED <NUM>.

The LED <NUM> may include an anode side (also known as the high side or the positive side) and a cathode side (also known as the low side or the negative side). The LED <NUM> may glow and emit light when current flows through the LED <NUM> from the anode side to the cathode side. A characteristic of the LED <NUM> may include a forward voltage (Vf). If the difference between the voltage at the anode side and the voltage at the cathode side of the LED <NUM> (i.e., a voltage drop across the LED <NUM>) is larger than the minimum threshold forward voltage to forward bias the LED <NUM>, then the LED <NUM> may activate, i.e., current may flow through the LED <NUM> and the LED <NUM> may light up. If the difference in voltage levels across the LED <NUM> is not greater than the forward voltage threshold or the forward voltage for the target current level, then the LED <NUM> may not emit light (or it may emit light at insufficient output levels).

Typically, the forward voltage of the LED <NUM> may be between <NUM> V and <NUM> V (but other voltage values are possible), depending on the color of the light emitted by the LED <NUM> and the LED structure. For example, depending on the current, the LED <NUM> may emit red light and have a forward voltage of <NUM> V. Or, the LED <NUM> may emit blue light and have a forward voltage of <NUM> V. In some implementations, such as for low power LED applications, the LED <NUM> may operate from <NUM> mA to <NUM> mA of current.

The device <NUM> may include a current limiting resistor (not shown in <FIG>) or a constant current source <NUM>. For example, the current limiting resistor may be used where there is a stable positive supply voltage and/or where a very consistent light output is not required. Alternatively, the constant current source <NUM> may be used where the positive supply voltage can vary and/or where consistent light output is desired. In some implementations, the constant current source <NUM> may be part of an LED driver that controls the LED <NUM>. The constant current source <NUM> may provide a constant current at the cathode side of the LED <NUM> regardless of swinging fluctuations in the supply voltage at the positive voltage rail <NUM>. Furthermore, the constant current source <NUM> may be capable of providing a constant current for the LED <NUM> where voltage levels fluctuate at a negative rail in a dual-supply system. Since changes in the current through the LED <NUM> can change the luminous intensity of the LED <NUM>, the constant current source <NUM> may limit changes in the current through the LED <NUM> and thus maintain a consistent luminous intensity from the LED <NUM>. For example, in some implementations, the constant current source <NUM> may include a two-transistor circuit for driving the LED <NUM>, or it may include a more precise current-sensing topology using comparators and/or operational amplifiers. The present concepts may be implemented using various methods of constant current control.

In some implementations, the LED <NUM> may be controlled by a signal. For example, an enable signal to the constant current source <NUM> may cause sufficient current to flow through the LED <NUM> and thereby activate the LED <NUM>. Furthermore, in some implementations, the signal may be a pulse width modulation (PWM) signal <NUM> that turns on, when high, and turns off, when low, the LED <NUM> by controlling the constant current source <NUM>. Alternative control schemes are possible, for example, where a high PWM signal turns off the LED <NUM> and a low PWM signal turns on the LED <NUM>, depending on transistor arrangements. In the alternative implementation discussed above where the device <NUM> includes a current limiting resistor instead of the constant current source <NUM>, the PWM signal <NUM> to the negative rail generator <NUM> may enable and disable the LED <NUM>.

The PWM signal <NUM> may be generated by the LED driver. Alternatively, the PWM signal <NUM> may be a general-purpose input/output (GPIO) signal provided by a microcontroller unit (MCU), a system-on-chip (SoC), or any other controller with an oscillator capable of producing a PWM-type of signal. The duty cycle (i.e., the on time period Ton divided by the total time period Tperiod) of the PWM signal <NUM> may control the average intensity of the LED <NUM>. The frequency of the PWM signal <NUM> may be, for example, between <NUM> and <NUM>. Frequencies above about <NUM> may be sufficiently higher than the range at which human eyes can perceive blinking or flickering of the LED <NUM>. The designer may choose a higher frequency for the PWM signal <NUM>, for example, if the LED <NUM> were being recorded by a camera that can perceive blinking and flickering at higher frequencies than the human eye, or if there are interferences on the board with other frequencies. An excessively high frequency may unnecessarily waste power due to switching losses. In some implementations, the PWM signal <NUM> may be provided by a PWM driver or any IC for controlling the LED <NUM>.

The example in <FIG> illustrates a low-side control of the LED <NUM>, because the high side (the anode side) of the LED <NUM> may be connected to (i.e., electrically coupled to, either directly or indirectly with intervening components and/or circuits) VLED, while the low side (the cathode) of the LED <NUM> may be controlled (i.e., either opened or closed) to activate and deactivate the LED <NUM>. Therefore, a variable voltage drop across the constant current source <NUM> may be controlled to compensate for varying VLED levels to keep the same setpoint current flowing through the LED <NUM>. Alternative implementations of the present concepts using a high-side control of the LED <NUM> may also be possible.

<FIG> shows the negative rail generator <NUM> as a generic box for simplicity, but examples and details of the negative rail generator <NUM> will be explained below with reference to other figures. The negative rail generator <NUM> may be a component or a circuit that creates a negative voltage rail <NUM> on the cathode side of the LED <NUM> (e.g., the low side of the constant current source <NUM>). The negative rail generator <NUM> may be included in the LED driver or work in conjunction with the LED driver.

Conventionally, the constant current source <NUM> would be connected to ground (or switched to ground). Accordingly, the positive supply voltage provided by the voltage source <NUM> must be greater than the forward voltage of the LED <NUM> plus any voltage drop across the constant current source <NUM>. However, with the negative rail generator <NUM>, the voltage difference across the LED <NUM> can be enlarged beyond the positive reference supply voltage by creating the negative voltage rail <NUM> on the cathode side of the LED <NUM>. Accordingly, the negative rail generator <NUM> can be used in, for example, a device that includes a <NUM> V battery to drive a red-colored LED with a <NUM> Vf or a device that includes a <NUM> V battery (or two <NUM> V batteries in series) to drive a white-colored LED with a <NUM> Vf by self-generating the negative voltage rail <NUM>.

In some implementations, the negative rail generator <NUM> may be activated by the same PWM signal <NUM> that activates the LED <NUM>. Therefore, the negative rail generator <NUM> may create the negative voltage rail <NUM> temporarily (i.e., on demand) when additional headroom is desired to activate the LED <NUM>. That is, the negative rail generator <NUM> may not be activated when the LED <NUM> is not being activated by the PWM signal <NUM>.

Furthermore, the device <NUM> may include a recharge current source <NUM>. The recharge current source <NUM> will be explained in detail in reference to the later figures. In some implementations, the negative rail generator <NUM> and/or the recharge current source <NUM> may be included in an LED driver that controls the LED <NUM>.

<FIG> shows an example circuit diagram of the device <NUM> that includes an example configuration of negative rail generator <NUM>, consistent with some implementations of the present concepts. In this example, the constant current source <NUM> may include a limit transistor <NUM> (also called a pass transistor), a sense transistor <NUM> (also called a clamp transistor), and a sense resistor <NUM>. For example, the limit transistor <NUM> and the sense transistor <NUM> may be two bipolar junction transistors (BJTs). Conventionally, the sense resistor <NUM> would be connected to ground or the return. This two-transistor circuit may be capable of maintaining a constant current through the LED <NUM> even as the source voltage fluctuates. The constant current can ensure a constant light output from the LED <NUM>. In some implementations, the constant current source <NUM> may use metal-oxide-semiconductor field-effect transistors (MOSFETs) instead of BJTs, and could sense the current with an operational amplifier or other circuit configurations to improve accuracy.

As shown in <FIG>, in some implementations consistent with the present concepts, the negative rail generator <NUM> may include a capacitor <NUM>, a clamper, and a switch. These elements may be discrete components, or they may be included in an IC. In one example implementation, the clamper may include a diode clamp <NUM> connected to ground, and the switch may include a reset transistor <NUM> (also called a pulldown transistor) connected to ground. In some implementations, the clamper may include a clamping transistor (not pictured in <FIG>) instead of the diode clamp <NUM>. The diode clamp <NUM> may be, for example, a Schottky diode, a conventional diode, or a transistor controlled with appropriate timing. The reset transistor <NUM> may be, for example, an n-type BJT transistor that is controlled by the PWM signal <NUM>. Alternatively, the reset transistor may be a negative metal oxide semiconductor (NMOS) transistor or other types of transistor or switch. The capacitor <NUM> may include two pins. A pin <NUM> side <NUM> (also called the reset side) of the capacitor <NUM> may be connected to the reset transistor <NUM>. A pin <NUM> side <NUM> (also called the clamp side) of the capacitor <NUM> may be connected to the diode clamp <NUM> and the sense resistor <NUM>. Also, in <FIG>, the recharge current source <NUM> may include a recharge resistor <NUM> in one simple example implementation. These components shown in <FIG> are one example implementation of the present concepts, which have been provided for explanation and illustration purposes. Other types of components and circuits may be used to achieve the effects and benefits of the present concepts.

In some implementations, the negative rail generator <NUM> may temporarily generate the negative voltage rail <NUM> on the cathode side of the LED <NUM> in synchronization with the LED driver by using the same PWM signal <NUM> that drives the LED <NUM>. Alternatively, the negative rail generator <NUM> and the LED driver may be controlled by different signals but nonetheless operate sufficiently in synchronization with each other.

In some implementations, the amplitude of the PWM signal <NUM> may be high enough to enable the reset transistor <NUM> (e.g., the base-emitter voltage (VBE) if a BJT or typically between <NUM> V and <NUM> V if a MOSFET). In one example implementation, the PWM signal <NUM> may have an amplitude of <NUM> V. In some implementations, the MCU that generates the PWM signal <NUM> may run on the same positive voltage rail <NUM> as the LED <NUM>. In other implementations, the amplitude of the PWM signal <NUM> may be higher or lower, such as <NUM> V. Other voltage levels are also possible depending on, for example, the design of the device <NUM>.

When the PWM signal <NUM> is low and therefore the LED <NUM> is off and the reset transistor <NUM> is off, the capacitor <NUM> may be charged through the recharge path that includes the recharge current source <NUM>. For example, if the voltage source <NUM> provides <NUM> VLED and the diode clamp <NUM> has a forward voltage of <NUM> mV when it is on, the voltage at the pin <NUM> side <NUM> of the capacitor <NUM> may be <NUM> V and the voltage at the pin <NUM> side <NUM> of the capacitor <NUM> may be <NUM> mV (i.e., the Vf of the diode clamp <NUM> above ground).

When the PWM signal <NUM> goes high and turns on the LED <NUM> using the constant current source <NUM>, the PWM signal <NUM> may also turn on the reset transistor <NUM>, which switches on the capacitor-diode network in the negative rail generator <NUM>. That is, turning on the reset transistor <NUM> may form an electrical path from the positive voltage rail <NUM>, through the LED <NUM>, the limit transistor <NUM>, the sense resistor <NUM>, and into the capacitor <NUM>. Turning on the reset transistor <NUM> may also pull the voltage level on the pin <NUM> side <NUM> of the capacitor <NUM> to ground, and the voltage level on the pin <NUM> side <NUM> of the capacitor <NUM> may become negative, thereby creating the negative voltage rail <NUM>. The pin <NUM> side <NUM> of the capacitor <NUM> may be called a negative voltage node.

The present concepts may utilize the fact that the capacitor <NUM> may not be able to immediately change the voltage across itself. For example, where the voltage source <NUM> provides <NUM> VLED, when the PWM signal <NUM> turns on the reset transistor <NUM>, thereby pulling the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> to ground (<NUM> V), the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> (i.e., at the negative voltage rail <NUM>) may be clamped at negative VLED (<NUM> V) minus a voltage drop across the diode clamp <NUM> (e.g., <NUM> mV). Therefore, in this example, the negative voltage rail <NUM> may start out at about -<NUM> V (<NUM> V - <NUM> mV) when the PWM signal <NUM> goes high. Since the capacitor <NUM> cannot immediately change the voltage difference across itself, when the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> swings -<NUM> V (from <NUM> V to <NUM> V), the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> may also swing -<NUM> V (from <NUM> V to -<NUM> V). This preserves the voltage difference of <NUM> V across the capacitor <NUM>. Accordingly, the negative voltage rail <NUM> having a negative voltage level of -<NUM> V may have been generated temporarily on the pin <NUM> side <NUM> of the capacitor <NUM>. Moreover, the diode clamp <NUM> may block conduction from ground (<NUM> V) to the negative voltage node, thereby allowing it to become negative.

As time passes while the PWM signal <NUM> remains high, the negative voltage level of the negative voltage rail <NUM> may slowly become less negative, i.e., decay towards <NUM> V, as the LED current passes into the capacitor <NUM>. This decay will be explained in detail below. When the PWM signal <NUM> goes low again, the LED <NUM> may turn off, the reset transistor <NUM> may turn off, and the capacitor <NUM> may recharge through the recharge path so that the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> approaches <NUM> V again and the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> is approximately <NUM> V again to begin the next cycle.

Furthermore, to prevent the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> from rising in the positive voltage direction when the pin <NUM> side <NUM> of the capacitor <NUM> is driven high again, the diode clamp <NUM> may be placed in order to clamp the pin <NUM> side <NUM> to ground (or at least the forward voltage of the diode clamp <NUM>). For example, if the diode clamp <NUM> were a Schottky diode having a forward voltage (Vf) of <NUM> mV, then when the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> goes to, for example <NUM> V, then the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> may only rise to about <NUM> mV. When the PWM signal cycle repeats, the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> may go from about <NUM> V to about <NUM> V, while the voltage level at the pin <NUM> side <NUM> of the capacitor <NUM> may go from about <NUM> mV to about -<NUM> V.

A perfect <NUM> V swing may not occur, because there may be some voltage drops due to the diode clamp <NUM>, the reset transistor <NUM>, the equivalent series resistance (ESR) of the capacitor <NUM>, and/or the recharge resistor <NUM>. In some implementations, the maximum negative voltage level that the negative rail generator <NUM> can generate may be the source voltage level provided by the voltage source <NUM> (or some other source for the recharge) minus the forward voltage (Vf) of the diode clamp <NUM>. In the above example, the maximum negative voltage may be -<NUM> V (<NUM> V - <NUM> mV). A theoretical maximum voltage difference that the negative rail generator <NUM> can generate may be twice the VLED. However, the negative rail generator <NUM> and the device <NUM> may be designed and their components sized properly to achieve sufficient headroom to drive the LED <NUM>.

Consistent with the present concepts, the capacitor <NUM> may be sized large enough such that the negative voltage rail <NUM> does not decay too much during the time the LED <NUM> is on (Ton). If the voltage level of the negative voltage rail <NUM> is not negative enough to maintain the required bias between the pin <NUM> side <NUM> of the capacitor <NUM> and the positive voltage rail <NUM>, the LED <NUM> may no longer be forward biased and therefore turn off or its current may decrease below the target level and therefore dim.

For example, the following equations may reflect the above-described decay effect: <MAT> <MAT> where I is the LED current plus the emitter current of the sense transistor <NUM> (which can be significant in a BJT transistor circuit), C is the capacitance of the capacitor <NUM>, dv/dt is the change in voltage over time, and Ton is the on time period of one cycle, D is the duty cycle, and Fs is the frequency of the PWM signal <NUM>, respectively. In one example implementation, the current I may be about <NUM> mA (<NUM> mA LED current plus <NUM> mA emitter current of the sense transistor <NUM>).

By combining Equation <NUM> and Equation <NUM> above, the following equation may be derived. <MAT> Equation <NUM> for dv may represent the change in the voltage (i.e., the degree of decay) across the capacitor <NUM> for a given constant current driven through the LED <NUM> when it is on. This change in voltage level, starting from about -<NUM> V in the above example and heading towards ground (<NUM> V), should not be so great as to eliminate the headroom required to maintain the forward biased state of the LED <NUM>. As Equation <NUM> shows, the negative voltage rail <NUM> may become less negative linearly with time. And therefore, the capacitor <NUM> should be sized large enough in consideration of the duty cycle and the frequency of the PWM signal <NUM>, such that the capacitor <NUM> does not reduce the negative voltage of the negative voltage rail <NUM> so much that the LED <NUM> cannot be kept biased during the on time period. Even as the negative voltage rail <NUM> decays, the constant current source <NUM> may be capable of maintaining a constant current through the LED <NUM>.

Moreover, the frequency and the duty cycle of the PWM signal <NUM> may impact the selection of the capacitor <NUM> and the peak recharge current source requirements. That is, certain combinations of frequencies and duty cycles of the PWM signal <NUM> may be required to keep the LED <NUM> on. For example, the LED <NUM> may be driven by a <NUM> to <NUM> PWM signal <NUM> for dimmer/brightness control, and the LED <NUM> may reach peak currents of <NUM> mA to <NUM> mA. With these example PWM signal frequencies and LED current loads, the capacitor <NUM> may be around <NUM> microfarads (µF) to <NUM>µF for duty cycles up to around <NUM>%.

Furthermore, another consideration includes the peak current capability of the recharge current source <NUM> for the capacitor <NUM>. In one implementation (shown in <FIG>), the recharge current source <NUM> may include the recharge resistor <NUM>. The recharge resistor <NUM> may have a low enough resistance to quickly recharge the capacitor <NUM> in the off time periods (Toff) when the PWM signal <NUM> is low and the LED <NUM> is off. That time period Toff may be Tperiod × (<NUM> - D), where Tperiod is the total time of one cycle and D is the duty cycle. When the PWM signal <NUM> is high during the on time period, the recharge resistor <NUM> may conduct to ground.

Consistent with the present concepts, the negative rail generator <NUM> may require that the LED <NUM> be controlled by the PWM signal <NUM>, rather than a constantly on signal (such as a steady enable signal), in order to cycle the charging and discharging of the capacitor <NUM> in synchronization with the activation and deactivation of the LED <NUM>. In other words, the PWM signal <NUM> may not have a <NUM>% duty cycle as there would be no time to recharge the capacitor <NUM>. The off time period (i.e., when the PWM signal <NUM> is low) may be required to reset the negative rail generator <NUM> and to recharge the capacitor <NUM> for the next cycle. Therefore, a very high duty cycle (e.g., greater than <NUM>%) may not be feasible, depending on several factors. Because the capacitor <NUM> would have to replace (recharge) in a much shorter amount of time (Toff) the same amount of charge that it discharged during the period the PWM signal <NUM> is high (Ton), the capacitor <NUM> would receive a recharge pulse with a very high peak current compared to the average LED current. For example, if the frequency is very high and/or the duty cycle is very high (i.e., a low amount of off time periods), the recharge of the capacitor <NUM> may be insufficient and the negative rail generator <NUM> may fail to function properly.

The limit on the amount of off time required (i.e., how high the duty cycle can be) may be dependent on the recharge current source capabilities and acceptable losses of current. That is, the higher the duty cycle, the higher the peak recharge current requirement. For example, using a high duty cycle, if the peak current through the LED <NUM> is around <NUM> mA, the recharge current for the capacitor <NUM> may be around <NUM> mA or higher. The charge in and out of the capacitor <NUM> should be rebalanced through the ongoing cycles of the PWM signal <NUM>. That is, the capacitor <NUM> may sufficiently recharge during the off time periods (when the PWM signal <NUM> is low) to replace the discharge during the on time periods (when the PWM signal <NUM> is high). Accordingly, the recharge current source <NUM> may supply sufficient charge quickly enough to replace the charge in the capacitor <NUM> in the off time given the duty cycle of the PWM signal <NUM>. The recharge current level may increase as the voltage swing on the capacitor <NUM> increases. Additionally, the recharge current level may be limited by the recharge resistor <NUM> (i.e., the impedance of the recharge current source <NUM>).

By utilizing the same PWM signal <NUM> that activates the LED <NUM>, the negative voltage rail <NUM> may be temporarily generated only when needed (i.e., when the LED <NUM> is activated). The present concepts may generate the negative voltage rail <NUM> using only a minimal number of simple, small, inexpensive, off-the-shelf, readily available components to increase the voltage difference across the LED <NUM> by up to nearly twice the source voltage level. The present concepts can increase the LED driver headroom without adding any additional high frequency noise to the device <NUM>, such as an SMPS operating at <NUM> to <NUM> or a <NUM>-switch charge pump operating at <NUM> to <NUM> would. The negative rail generator <NUM>, consistent with the present concepts, may operate at low frequencies, e.g., the same frequency as the LED <NUM>, which may be around <NUM> to <NUM>, for example.

Accordingly, the present concepts may utilize the capacitor <NUM>, the diode clamp <NUM>, and the reset transistor <NUM>-placed where ground (or the return of the constant current source <NUM>) would normally be-to self-generate a temporary, on-demand negative voltage at the negative voltage rail <NUM> whenever the PWM signal <NUM> drives the LED <NUM>. This increases the bias across the LED path and drives the LED <NUM>, even where the voltage level at the positive voltage rail <NUM> referenced to ground would normally be insufficient to forward bias the LED <NUM>.

In one example implementation, the device <NUM> may include the voltage source <NUM> that provides <NUM> V of source voltage, the LED <NUM> that requires <NUM> V of forward voltage to turn on, the sense transistor <NUM> having <NUM> mV of base-emitter voltage (VBE), and the limit transistor <NUM> having <NUM> mV of collector-emitter voltage (VCE). The PWM signal <NUM> may be a <NUM> V GPIO signal from an MCU that controls the LED <NUM> and also provides a "clock" signal to create the negative voltage rail <NUM>. The <NUM> V GPIO signal may be generated from the <NUM> V voltage source <NUM>. Conventionally, a voltage source of at least <NUM> V (<NUM> V + <NUM> mV + <NUM> mV) would be needed to drive the constant current source <NUM> at the target current and thus activate the LED <NUM>. However, with the present concepts using the negative rail generator <NUM>, the LED <NUM> having a <NUM> Vf can be forward biased using the voltage source <NUM> that provides only <NUM> V by temporarily creating the negative voltage rail <NUM> that increases the voltage difference across the LED path above <NUM> V. Therefore, where the device <NUM> runs on a battery system, the present concepts may extend the runtime of the device <NUM> since the battery system may be more depleted and still forward bias the LED <NUM> compared to conventional devices.

There may be a voltage drop across the reset transistor <NUM>. When using a BJT as the reset transistor <NUM>, the voltage drop may be the collector-emitter voltage (VCE), which may be relative to the current. Accordingly, the voltage drop may be high (e.g., around <NUM> mV to <NUM> mV) since, at a high duty cycle, the recharge current peak may be much higher (e.g., more than <NUM> times higher) than the average LED current. So, in this example, the maximum negative voltage of the negative rail generator <NUM> may be about - <NUM> V (<NUM> V - <NUM> mV - <NUM> mV), assuming a high duty cycle and a high efficiency reset method (explained below in reference to <FIG>). The voltage drop across the reset transistor <NUM> may be reduced by using a MOSFET instead, such that the drain-source on resistance (RDS(on)) multiplied by the current could be a very low voltage drop.

With -<NUM> V on the negative voltage rail <NUM> (the cathode side) and <NUM> V on the positive voltage rail <NUM> (the anode side), the bias for the LED path may be as high as, for example, <NUM> V (<NUM> V + <NUM> V) instead of only <NUM> V without the negative voltage rail <NUM>. Accordingly, in this example, the capacitor <NUM> may discharge and the negative voltage rail <NUM> may decay as much as <NUM> mV (<NUM> V - <NUM> V) before the required headroom is lost and the current through the LED <NUM> drops below the setpoint (e.g., <NUM> mA) of the constant current source <NUM>.

Equation <NUM> may be rearranged to solve for any of the other variables. For example, the designer may want to solve for the minimum C in Equation <NUM> to select the size of the capacitor <NUM>. <MAT> The frequency Fs of the PWM signal <NUM> may typically range from <NUM> to <NUM>. The duty cycle D of the PWM signal <NUM> may range between <NUM>% and <NUM>%, exclusive of <NUM>%. The capacitance C may typically range from <NUM>µF to <NUM>µF, which are commonly available as <NUM> or <NUM> package sized multi-layer ceramic capacitors (MLCCs). For example, the current I may be <NUM> mA (<NUM> mA of the LED current plus <NUM> mA of the emitter current from the sense transistor <NUM>), the duty cycle D may be <NUM>%, the frequency Fs may be <NUM>, and the change in voltage dv may be <NUM> mV. In this example, the capacitance C may be calculated as <NUM>µF using Equation <NUM>. Therefore, the capacitor <NUM> required to support this example device <NUM> may be at least <NUM>µF, but with derating, a capacitor with an even larger capacitance may be recommended.

Equation <NUM> illustrates that a capacitor with less capacitance may be used if the current I is reduced, the duty cycle D is reduced, the frequency Fs is increased, and/or the acceptable change in voltage dv is increased. On the other hand, slower frequencies and higher duty cycles may require larger capacitors. For example, if the designer wishes to increase the duty cycle, perhaps to brighten the light output intensity of the LED <NUM>, then the size of the capacitor <NUM> may be increased, e.g., from <NUM>µF to <NUM>µF. But this change may increase power loss and increase the root mean square (RMS) current of the capacitor <NUM>. Sizing the capacitor <NUM> appropriately may be important to keeping power losses, costs, and peak current levels balanced.

Using a larger capacitor may reduce the peak currents, since the peak voltage swing will be reduced. However, the cost, size, and/or availability of larger capacitors may limit the designer to select a practical capacitance for the capacitor <NUM>. For example, if the voltage swing is <NUM> mV and the capacitance is <NUM>µF, then the amount of charge that needs to be replaced may be <NUM> microcoulombs (µC) (<NUM> mV × <NUM>µF). Furthermore, following this example, if the frequency is <NUM> (i.e., Tperiod = <NUM>) and the duty cycle is <NUM>%, then the average current replaced during the off time period may be <NUM> mA (<NUM>µC / {(<NUM> - <NUM>%) × <NUM>}). This value is very high-<NUM> times the LED current of <NUM> mA. Even if the recharge current source <NUM> can supply such a high recharge current, power losses may be significant, as the duty cycle increases and the off time period shrinks. Power losses through the recharge path may increase dramatically, because the peak current will increase and power losses (P = I<NUM> × R) will increase at the square of the current. If the recharge current source <NUM> cannot supply the required recharge current within the recharge time window, the negative rail generator <NUM> may fail after a number of cycles.

<FIG> shows an example circuit diagram of the device <NUM> that includes an example configuration of the negative rail generator <NUM>, consistent with some implementations of the present concepts. In this alternative example configuration, the recharge current source <NUM> may include a recharge transistor <NUM>.

As explained above in reference to <FIG>, the recharge resistor <NUM> (shown in <FIG>) should have low enough resistance to quickly recharge the capacitor <NUM> in the off time periods when the PWM signal <NUM> is low. However, when the PWM signal <NUM> is high, the low resistance of the recharge resistor <NUM> may allow significant power loss from the positive voltage rail <NUM> to ground through the recharge path. This power loss may be prevented by switching the recharge path open during the off time periods when the PWM signal <NUM> is high. This may be implemented using one or more transistors (e.g., BJTs or MOSFETs) that are controlled by either the same PWM signal <NUM> or an inverted version of the PWM signal <NUM>.

In one example configuration, the recharge transistor <NUM> may be a PNP-type BJT (shown in <FIG>) controlled by the same PWM signal <NUM> that drives the LED <NUM>. In an alternative example configuration, the recharge transistor <NUM> may be an NPN-type BJT controlled by an inverse PWM signal <NUM> (shown in <FIG>) that may be generated by inverting the PWM signal <NUM> that drives the LED <NUM> using an inverter circuit (not shown in <FIG>). The PWM signal <NUM> that controls the PNP-type transistor or the inverse PWM signal <NUM> that controls the NPN-type transistor may be provided by an MCU (e.g., the same MCU that provides the PWM signal <NUM> for the LED <NUM>).

With either configuration, when the reset transistor <NUM> is on and the LED <NUM> is emitting light, the recharge transistor <NUM> may not conduct, thereby disconnecting the recharge path and preventing power loss to ground. Conversely, when the reset transistor <NUM> is off and the LED <NUM> is off, the recharge transistor <NUM> may conduct in order to recharge the capacitor <NUM>. The PWM signal <NUM> (or the inverse PWM signal <NUM>) may have a high enough amplitude to turn on and off the recharge transistor <NUM>. In some implementations, a level shifter transistor may be added to increase the amplitude of the PWM signal <NUM> (or the inverse PWM signal <NUM>) that controls the recharge transistor <NUM>.

The recharge transistor <NUM>, consistent with the present concepts, may significantly reduce power consumption by the device <NUM> when the LED <NUM> is on. And, the recharge transistor <NUM> may be able to recharge the capacitor <NUM> when the LED <NUM> is off so that, at the next cycle, the negative voltage rail <NUM> can be generated to increase the headroom when the LED <NUM> is turned on. Using the recharge transistor <NUM> shown in <FIG> can drastically improve the power efficiency of the device <NUM> compared to using the recharge resistor <NUM> shown in <FIG>.

<FIG> show graphs of example electrical characteristics of the device <NUM> that includes the negative rail generator <NUM>, consistent with some implementations of the present concepts. The values of voltage, current, and time on the axes of the graphs are provided merely as examples. Other values are possible, consistent with the present concepts. Moreover, the curves in the graphs have been simplified (i.e., they are simulated waveforms) for the purposes of illustration and explanation of the present concepts. The curves may not exactly represent real-life measurement curves that may be observed with actual implementations of the present concepts. Many variations of these curves, especially with different implementations of the present concepts, may be possible. For example, <FIG> have a common x-axis, indicating time. Although the example curves have been drawn in perfect synchronization with one another with respect to time for illustration and for simplicity, real-life implementations of the present concepts may exhibit time delays in the curves with respect to one another (e.g., ramp-up periods).

<FIG> shows a graph of example voltage levels that may be supplied by the voltage source <NUM> at the positive voltage rail <NUM> (i.e., at the anode side of the LED <NUM>). <FIG> shows a graph of example voltage difference levels across the LED <NUM> (i.e., the voltage drop across the LED <NUM>). <FIG> shows a graph of example voltage levels at the collector side of the reset transistor <NUM> (i.e., the pin <NUM> side <NUM> of the capacitor <NUM>). <FIG> shows a graph of example voltage levels at the negative voltage rail <NUM>. <FIG> shows a graph of example voltage levels of the PWM signal <NUM>. <FIG> shows a graph of the example current levels flowing through the LED <NUM>.

<FIG> may relate to an example of the device <NUM> that may include the voltage source <NUM> supplying a constant <NUM> V to the positive voltage rail <NUM>, as shown in <FIG>, the constant current source <NUM> providing <NUM> mA of constant current, the capacitor <NUM> having <NUM>µF capacitance, and the LED <NUM> having a forward voltage of <NUM> V (e.g., a red-colored LED). Conventionally, the voltage source <NUM> may not be able to forward bias the LED <NUM> to turn it on without replacing the voltage source <NUM> with a higher voltage supply or adding a negative voltage source from an SMPS. But, consistent with the present concepts, the device <NUM> may use the PWM signal <NUM>, having <NUM> V amplitude, <NUM> frequency, and <NUM>% duty cycle, as shown in <FIG>, to generate the negative voltage rail <NUM> and sufficient headroom to forward bias the LED <NUM> to conduct, for example, at a constant <NUM> mA per the current setting target.

In this example, the PWM signal <NUM> shown in <FIG> cycles every <NUM>, it is high at <NUM> V for <NUM>, and it is low at <NUM> V for <NUM>. When the PWM signal <NUM> goes high from <NUM> V to <NUM> V at <NUM> (or at <NUM>, <NUM>, etc.), the reset transistor <NUM> may conduct, thereby pulling the voltage level at its collector side to ground (<NUM> V), as shown in <FIG>. This may cause the negative voltage rail <NUM> to charge to about -<NUM> V, as shown in <FIG>. Accordingly, about <NUM> V (<NUM> V + <NUM> V) of voltage difference is created across the LED path, which is more than sufficient to light up the LED <NUM> since <NUM> V is greater than the <NUM> V drop across the LED <NUM> plus <NUM> mV drop for the sense resistor <NUM> plus a small voltage drop across the reset transistor <NUM>.

As time passes from <NUM> to <NUM> while the PWM signal <NUM> remains high (i.e., the on time period), the capacitor <NUM> may discharge, and the voltage level at the negative voltage rail <NUM> may decay (i.e., becomes less negative) from about -<NUM> V at <NUM> to about -<NUM> V at <NUM>, as shown in <FIG>. Even at <NUM>, the voltage difference is about <NUM> V (<NUM> V + <NUM> V), which is still large enough to keep the LED <NUM> lit, since <NUM> V is greater than the sum of the voltage drops across the LED <NUM>, the sense resistor <NUM>, and the reset transistor <NUM>. Therefore, in this example, the negative voltage rail <NUM> did not decay so much that the LED <NUM> did not remain lit at the target current setting throughout the on time period.

At <NUM>, the PWM signal <NUM> may go low (i.e., fall from <NUM> V to <NUM> V), and the PWM signal <NUM> remains low for <NUM> from <NUM> until <NUM>, as shown in <FIG>. During this off time period, the constant current source <NUM> may stop driving the LED <NUM>, such that the voltage drop across the LED <NUM> is less than <NUM> V, as shown in <FIG>, thereby turning off the LED <NUM>. Furthermore, the reset transistor <NUM> may turn off, and the recharge transistor <NUM> turn on, thereby raising the voltage levels at the collector side of the reset transistor <NUM>, as shown in <FIG>. This may cause the negative voltage rail <NUM> to practically disappear (i.e., the voltage is no longer negative), as shown in <FIG>, while the capacitor <NUM> charges for the next cycle. And at <NUM>, when the PWM signal <NUM> goes high again, i.e., the beginning of the next cycle, the described process may start again.

<FIG> shows a flowchart illustrating a negative rail generating method <NUM> that can implement some of the present concepts. The negative rail generating method <NUM> may be performed, for example, using any one of the various implementations described above or other implementations that are consistent with the present concepts but not explicitly described above.

In act <NUM>, an LED may be controlled using a PWM signal. For example, in some implementations, the PWM signal may activate and deactivate an LED driver or switch having a constant current source on the cathode side (the low side) of the LED. The anode side (the high side) of the LED may be connected to a positive voltage rail having a positive voltage supplied by a power source.

In act <NUM>, the same PWM signal that controls the LED may also turn on and off a negative voltage generator that may be positioned between the constant current source and ground. Accordingly, by leveraging the same PWM signal, the LED and the negative voltage generator may be activated and deactivated in synchronization with each other (i.e., both are on or both are off at the same time periods). When the negative voltage generator is activated, it may temporarily create a negative voltage rail on the low side of the LED, thereby increasing the difference in voltage across the LED and circuitry in the conduction path to be greater than the reference positive voltage to ground provided by the power supply.

In some implementations, the negative voltage generator may include a reset transistor that is activated and deactivated by the PWM signal. The reset transistor may be a n-type BJT whose emitter is connected to ground and whose collector is connected to a capacitor or an NMOS transistor in the same functional configuration. The other side of the capacitor may be connected to the constant current source and also to a diode clamp that is connected to ground.

When the PWM signal goes high, the capacitor may swing the voltage at the cathode side of the LED to a negative voltage level to temporarily generate the negative voltage rail, thereby creating sufficient headroom to turn on the LED. As the capacitor discharges, the negative voltage rail may decay. When the PWM signal goes low, the negative voltage rail essentially disappears and the LED conduction path is opened, and the capacitor may recharge via a recharge current source. In one implementation, the recharge current source may include a resistor in a recharge path from the positive voltage rail to the low side of the capacitor.

Alternatively, the recharge current source may include a recharge transistor or switch that turns off while the LED is on to prevent loss of power through the recharge path to ground. Using this alternative implementation, in act <NUM>, the same PWM signal may be used to control the recharge current source, or a level shifted or inverted version of the PWM signal may be used. That is, the PWM signal may turn on the recharge transistor (e.g., a PNP-type) to recharge the capacitor while the PWM signal turns off the LED, and the PWM signal may turn off the recharge transistor to prevent power loss while the PWM signal turns on the LED.

Claim 1:
A circuit, comprising:
a load (<NUM>);
a capacitor (<NUM>) having a first pin (<NUM>) and a second pin (<NUM>), the second pin of the capacitor being electrically coupled to a low side of the load (<NUM>), the load having a high side electrically coupled to a positive rail (<NUM>) for supplying a positive voltage; a clamper (<NUM>) having a first pin and a second pin, the first pin of the clamper being electrically coupled to ground, the second pin of the clamper being electrically coupled to the second pin (<NUM>) of the capacitor (<NUM>);
a recharge current source (<NUM>) electrically coupled to the positive rail (<NUM>) and the first pin (<NUM>) of the capacitor (<NUM>);
and
a switch (<NUM>) having a first pin and a second pin, the first pin of the switch being electrically coupled to ground, the second pin of the switch being electrically coupled to the first pin (<NUM>) of the capacitor (<NUM>), the switch, when closed, creating a negative rail (<NUM>) having a negative voltage on the low side of the load (<NUM>).