Abstract:
An LED driver is disclosed that drives LEDs connected in parallel. Instead of applying current to all the parallel-connected LEDs at the same time, under control of a common PWM brightness control signal, the application of current to each parallel path is staggered by using staggered brightness control signals. The turning on of the LEDs in the different parallel paths will have the same duty cycle but will be out of phase. This reduces ripple in the power supply by reducing the magnitude of the instantaneous current sink. In one embodiment, a shift register contains a binary representation of the PWM duty cycle, and a clock shifts the bits along the shift register. The PWM brightness control signals for each parallel path of LEDs are tapped from different positions along the shift register so that the PWM brightness control signals are identical but staggered.

Description:
FIELD OF THE INVENTION 
     This invention relates to a light emitting diode (LED) driver that drives LEDs in parallel. 
     BACKGROUND 
     LEDs are rapidly replacing incandescent bulbs, fluorescent bulbs, and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. A typical forward voltage drop for a high power LED is about 3-4 volts. The brightness of an LED is controlled by the current through the LED, which ranges from a few milliamps to an amp or more, depending on the type of LED. For this reason, LED drivers typically include some means to control the LED current. 
     In applications where high brightness is needed, multiple LEDs are used. It is common to connect LEDs in series, since the current through all the LEDs in series will be the same. However, driving many LEDs in series requires relatively high voltages, since the driving voltage must be greater than the combined voltage drops of the series LEDs. Additionally, when different types of LEDs are driven, such as different colors of LEDs, it may be desired to drive each type of LED with a different current. Therefore, in some applications, LEDs are connected in parallel, with the current through each parallel path being separately controlled. There may be only one LED in each parallel path, a few LEDs connected in series in each parallel path, or another configuration of LEDs in each parallel path. In some cases, one parallel path may drive a single red LED, another parallel path may drive two or more green LEDs connected in series, and another parallel path may drive one or more blue LEDs, in order to achieve a target overall brightness and color. Parallel LEDs may also be used for redundancy in case one LED fails and becomes an open circuit. 
       FIG. 1A  is a typical prior art LED driver  10  that drives multiple LEDs  12  in parallel. Some parallel paths show multiple LEDs connected in series. Most components of the driver  10  are formed on an integrated circuit chip. The LEDs are typically connected to pins extending from the chip package. 
     A DC voltage regulator controller  14  up converts or down converts an input voltage (Vin), depending on the required voltage to drive the LEDs  12 . Typically, the voltage regulator is a switching regulator operating at a high frequency, such as 100 KHz-5 MHz, to keep component sizes small. The controller  14  switches a switching transistor at a certain pulse-width modulation (PWM) duty cycle to maintain an output voltage (Vout) at the desired level. The switching transistor and an output circuit are represented by block  16 . The output circuit comprises an inductor and diode (or synchronous rectifier) connected to the switching transistor to supply pulses of current to a smoothing capacitor  18 . The topology of the output circuit determines whether the voltage regulator is a step up or step down regulator. Such regulators are well known and need not be described in detail. 
     The anodes of the “top” LEDs in each parallel path are connected to Vout, as shown in  FIG. 1A , and the cathodes of the “bottom” LEDs in each path are connected to designated pins of the driver  10 . The current through each path is individually set, such as by the user connecting a current set resistor to corresponding pins of the driver  10 . Other types of current setting circuits may be used. A current setting circuit  20 - 22  for each parallel path is shown, where each circuit  20 - 22  may comprise a current set resistor or a current regulator. It is important to individually set the currents, since each parallel path may drive a different type (e.g., color) of LED that requires a different current to achieve the desired light output. 
     An LED&#39;s color is slightly dependent on the magnitude of the forward current. Once the current is fixed, it should be not changed in order to avoid color shift. Therefore, to adjust the perceived brightness of the LEDs  12 , a PWM brightness control unit  24  outputs a PWM signal at a relatively low frequency (e.g., 100 Hz-1000 Hz) and effectively turns all the LEDs on and off at the LF PWM duty cycle. An externally generated dimming control signal sets the LF PWM duty cycle. For example, if the duty cycle were 50%, the average current would be half of the peak current when the LEDs are on. Thus, the perceived brightness of the LEDs would be about half the brightness of the LEDs when fully on. 
       FIG. 1B  illustrates one type of brightness control unit  24 . An N-Bit counter  26  cyclically counts, such as a repeating 5-bit count from 0-31, at a rate set by a clock. The binary count is applied to preset count detectors  28  and  30 . Detector  28  detects a count that begins the high state of the PWM cycle, such as the count 00000. Upon count 00000 being detected, the detector  28  outputs a pulse. Detector  30  detects a count that begins the low state of the PWM cycle, such as 01010. Upon count 01010 being detected, the detector  30  outputs a pulse. The outputs of the detectors  28  and  30  are connected to the set and reset inputs, respectively, of an RS flip flop  32 . The output of the flip flop  32  is the PWM brightness control signal, where the duty cycle in the example is approximately 30%. 
     The frequency of the LF PWM signal must be high enough to prevent noticeable flicker of the LEDs. There may be thousands of HF PWM pulses generated by the voltage regulator during each pulse (on-time) of the LF PWM signal. In this way, the current through the LEDs is either a “fixed” peak current or zero current. This keeps the emitted color of the LEDs constant, but enables the perceived brightness to be adjusted by the LF PWM duty cycle. 
     Since there are multiple parallel sets of LEDs, the combined currents of the paths can be high. For example, some LEDs may be driven at 1 Amp. When the brightness control PWM signal goes high, the voltage regulator must instantaneously supply driving current to all the LEDs at the same time. The large instantaneous change in the current causes a voltage ripple in the battery or other power supply. Greatly increasing the value of the smoothing capacitor  18  to smooth out the current drain on the power supply is not desirable, since such high value capacitors are relatively large and expensive. The ripple in the power supply due to the high currents being switched on and off causes noise that affects other circuits connected to the power supply. Since the PWM brightness control signal is at audio frequencies, the noise may even be audible. 
     What is needed is an LED driver that produces less ripple and noise compared to the conventional LED drivers. 
     SUMMARY 
     An LED driver is disclosed that drives LEDs connected in parallel paths. Each parallel path may include one LED, or LEDs connected in series, or LEDs connected in parallel, or LEDs connected in a combination of series and parallel. 
     Instead of applying current to all the parallel-paths at the same time, under control of a PWM brightness control signal, the application of current to each parallel path is staggered. This reduces ripple in the power supply, since the current provided by the power supply when the PWM signal goes high is not the sum of all the LED peak currents. In fact, at medium and low duty cycles, the currents can be staggered such that all the LEDs are never on at the same time. 
     One way of doing this is to circulate a binary representation of the PWM duty cycle, such as 11111000 (duty cycle=62.5%), in an 8-bit shift register. Any number of bits (e.g., 128) can be used to more finely control the duty cycle. The bits (11111000) are initially loaded into the shift register, such as by a parallel loading process, and the bits are shifted through the shift register by a clock. The bit shifted out of the last position in the shift register is fed back into the first position, so the bit pattern recirculates through the shift register. The PWM brightness control signals for each parallel path of LEDs are tapped from different positions along the shift register so that the PWM brightness control signals are identical but staggered. The spacing of the taps may be equal, random or shaped, depending on the number of parallel paths and the current drawn by each path. The spacing should be that required to minimize ripple in the power supply. In one example, if the duty cycle is low enough, no high state PWM signal overlaps another high state PWM signal. 
     In another embodiment, the PWM brightness control signal is continuously and serially read into the shift register at one end, and the bits are shifted through the shift register. The PWM brightness control signals for each parallel path of LEDs are tapped from different positions along the shift register so that the PWM brightness control signals are identical but staggered. In this embodiment, there is no feeding back of any bits in the shift register, and the shift register acts as a clocked tapped delay line. 
     Other circuits may be used to create the staggered PWM signals. 
     Although embodiments of the invention have been summarized above, the scope of the invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a prior art LED driver, driving LEDs connected in different parallel paths. 
         FIG. 1B  illustrates a prior art PWM brightness control signal generator. 
         FIG. 2A  illustrates a tapped recirculating shift register used to stagger PWM brightness control signals to the LEDs in the different parallel paths, in accordance with one embodiment of the invention. 
         FIG. 2B  illustrates the staggering of the PWM brightness control signals. 
         FIG. 3  illustrates a tapped non-recirculating shift register used to stagger PWM brightness control signals to the LEDs in the different parallel paths, in accordance with another embodiment of the invention. 
         FIG. 4  illustrates an LED driver, driving LEDs connected in different parallel paths, using the staggered PWM brightness control signals, in accordance with one embodiment of the present invention. 
         FIG. 5  illustrates one type of current regulator for a single parallel path receiving a staggered PWM brightness control signal. 
         FIG. 6  illustrates one type of current set circuit for a single parallel path receiving a staggered PWM brightness control signal. 
     
    
    
     Elements that are the same or equivalent in the figures are identified with the same numeral. 
     DETAILED DESCRIPTION 
     The present invention is an LED driver that staggers PWM brightness control signals to different parallel paths. The LEDs in each path are switched on and off in accordance with the PWM duty cycle to create a certain perceived brightness level, but the paths are energized at different times. This results in the current sinks being spread out so that there is less ripple produced by the power supply. This reduces noise, eases the constraints on the power supply, improves the color output of the LEDs, reduces artifacts caused by lights flickering, and provides other benefits. 
     Fig,  2 A illustrates an N-Bit shift register  40 . The number of bits determines the fineness in selecting PWM brightness control signal duty cycles. In one embodiment, the duty cycle of the LEDs is set by a 7-bit word, which represents 128 possible duty cycles (approximately 0.8% change in brightness per step). This would require a 128 bit shift register. Any size bit word may be used in the invention, such as 2-12 bits. For simplicity, only a 3-bit duty cycle word will be used in the examples, corresponding to 8 possible duty cycles (including 0%), or a 12.5% brightness change per step. 
     A simple digital converter (not shown) may be used to convert the 3-bit word into a pattern of 1&#39;s and 0&#39;s corresponding to the high and low states of the PWM signal. In the example used in the figures, a duty cycle of 62.5% is set by the 3-bit word (011), and the converter converts this code into the bit pattern 11111000, representing that the PWM brightness control signal is high for five bit-clock cycles and low for three bit-clock cycles. 
     In  FIG. 2A , the 8-bit shift register  40  is loaded with the 11111000 bit pattern in parallel. In another embodiment, the bit pattern is shifted into the shift register  40  as a serial stream. A shift clock is applied to the shift clock terminal of the register  40  to shift the bit pattern to the right for every shift clock cycle. The shift clock has a frequency of 8× the PWM frequency. The PWM frequency should be above 100 Hz to avoid noticeable flicker in the LEDs. The bit shifted out of the last position in the shift register  40  is fed back into the first position, so the 8-bit pattern recirculates through the shift register  40 . 
     The PWM brightness control signals for each parallel path of LEDs are tapped from different positions along the shift register  40  so that the PWM brightness control signals are identical but staggered. The spacing of the taps may be equal, random, or shaped, depending on the number of parallel paths and the current drawn by each path. The spacing and order of the taps should be that required to minimize ripple in the power supply. In the example shown, there are three taps outputting the staggered PWM signals: PWM 1 , PWM 2 , and PWM 3 . Each PWM signal will have the pattern 11111000, representing a 62.5% duty cycle. The taps may be in a different order, such as PWM 1 , PWM 3 , and PWM 2 . 
       FIG. 2B  shows the staggering of the three PWM signals, where the pattern is repeated as long as the bit clock continues to shift the bits in the register  40 . In other words, the PWM signals are out of phase with each other. In one example, if the duty cycle is low enough, no high state PWM signal overlaps another high state PWM signal, so the maximum current supplied by the power supply is the highest current in any one LED path. 
     If the driver drives many parallel paths, some paths may be driven in phase and other paths driven out of phase. 
     As shown by the LED driver of  FIG. 4 , described in greater detail later, the staggered PWM signals are applied to their corresponding current set circuit for each parallel path. 
     In another embodiment, shown in  FIG. 3 , the bits of the PWM brightness control signal bit pattern 1111000 are continuously and serially loaded into the first bit position of the shift register  46  and then shifted by the load/shift clock. In this embodiment, there is no feeding back of any bits in the shift register, and the shift register acts as a clocked tapped delay line. The PWM brightness control signals (PWM 1 , PWM 2 , PWM 3 ) for each parallel path of LEDs are tapped from different positions along the shift register  46  so that the PWM brightness control signals are identical but staggered, as shown in  FIG. 2B . 
       FIG. 4  illustrates an LED driver  50 . All circuitry, except for inductors, large capacitors, and the LEDs may be formed on the same integrated circuit. All elements shown in  FIG. 4 , except the LEDs, may be within the same package, with pins for connection to the LEDs, power, and an external dimming control circuit. 
     One or more LEDs  12  may be connected in each parallel current path. If more than one LED is to be driven by a single parallel path, such LEDs may be connected in series, parallel, or a combination to achieve the desired brightness and forward voltage drop for the path. There may be any number of parallel paths for driving LEDs, such as 2-10. Three paths are advantageous for controlling red, green, and blue LEDs to create a variety of colors. 
     The DC voltage regulator controller  52  may be a boost regulator controller, a buck regulator controller, or any other type of regulator controller needed to generate an output voltage (Vout) that exceeds the forward voltage drop for each parallel path of LEDs to turn the LEDs on. A conventional switching transistor and output circuit block  54  is shown. The voltage regulator controller  52  generates high frequency (e.g., 1 MHz) PWM signals that turn the switching transistor on and off, where the duty cycle controls the level of Vout. An inductor and diode (in block  54 ) connected to the switching transistor apply ramping currents to the smoothing capacitor  18 , which supplies a relatively constant output voltage (Vout). The topology of the switching transistor, inductor, and diode contained in the block  54  determines whether the regulator is a step up or step down regulator. A step up regulator will typically be needed if many LEDs are connected in series. Examples of topologies for step up and step down regulators are found in Micrel&#39;s published data sheets for LED drivers and other products, and in U.S. Pat. No. 7,307,614, assigned to Micrel, all incorporated herein by reference. The regulator portion may also use isolated components, such as in a flyback or forward converter. 
     All LED paths are energized with the same voltage but may draw different currents, set by the current set circuits  56 ,  57 , and  58 . The current set circuits  56 - 58  are effectively enabled and disabled by their associated staggered PWM brightness control signal PWM 1 , PWM 2 , or PWM 3 . There may be any number of current set circuits, each driven by a staggered PWM signal. The PWM brightness control unit  40 / 46  may be either of the shift registers  40  or  46  in  FIGS. 2A and 2B , or may be any other circuit that operates to stagger a PWM signal, such as a delay line. 
       FIG. 5  illustrates one current controller circuit  62  that may serve as each of the current set circuits  56 - 58  in  FIG. 4 . A low value sense resistor (Rsense) and MOSFET  64  are connected in series with the one or more LEDs  12  in a single parallel path of the driver  50 . A feedback voltage (Vfb), proportional to the current through the LEDs, is applied to an inverting input of an op amp  66 . The non-inverting input is coupled to a reference voltage (Vref). The op amp  66  controls the gate voltage of the MOSFET  64  to cause Vfb to equal Vref. The value of Rsense is selected to establish the peak current through the LEDs  12  when the MOSFET  64  is on. There may be a different Rsense value for each of the current set circuits  56 - 58  in  FIG. 4 . A pass transistor  68  is controlled to be on or off by the PWM brightness control signal. This turns the MOSFET  64  on at the PWM duty cycle. The transistors may be any type of suitable transistor. 
       FIG. 6  illustrates another type of current set circuit  70  that may serve as each of the current set circuits  56 - 58  in  FIG. 4 . A current set resistor Rset and MOSFET  72  are connected in series with the one or more LEDs  12  in a single parallel path of the driver  50 . Since Vout is substantially constant, the value of Rset is selected to establish the peak current through the LEDs  12 . There may be a different Rset value for each of the current set circuits  56 - 58  in  FIG. 4 . The MOSFET  72  is controlled to be on or off by the PWM brightness control signal. 
     In addition to there being less power supply output voltage ripple by staggering the on-times of the different parallel paths of LEDs, there is less overall flickering of the light, since there is a shorter time (or even no time) that all the LEDs are off at the same time, compared to the prior art techniques. This is advantageous in certain situations and reduces eye fatigue. 
     The various components may be coupled together by direct wiring, or via resistors, or via buffers, or via level shifters, or via inverters, or via other components in order to properly operate. 
     Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.