Abstract:
A driving circuit for a light emitting diode may include two transistors and an operational amplifier. The operational amplifier may act to cause the output voltages of the drain terminals of the transistors to be substantially equal, making the light emitting diode forward current substantially equal to the reference current. This current may provide a steady drive current, even when the supply voltage varies over a wide range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention is directed to driving circuits for light emitting diodes. 
     2. Description of Related Art 
     Light emitting diodes (LEDs) are driven by applying a high potential to the anode terminal of the light emitting diode and a ground potential to the cathode terminal. Current flowing between the terminals creates electron-hole pairs, which recombine to emit light over a spectral range defined by the band gap of the diode. If the light emitting diode is made of the appropriate materials and placed between reflecting surfaces to form an optical cavity, a light emitting diode may emit coherent light and thereby form a light emitting diode laser. In many cases, the cleaved edges of the device have sufficient reflectivity to form the optical cavity and allow the diode to operate as a laser. 
     In printing system applications, laser light intensity control of 1% accuracy or greater is required. The light intensity of a laser varies linearly with the current flowing through the laser, once the laser is lasing. Semiconductor lasers are diodes, and lase at a forward bias voltage level. The forward bias voltage changes little with current, but may vary from laser to laser. Hence, laser drivers are current drivers delivering a current to the laser that is independent of voltage conditions, ideally. Voltage conditions can vary due to power supply fluctuations and the differing forward bias voltages of the laser diodes. Furthermore, heating of the current driver increases the internal resistance of the current driver, causing the current driver to require more voltage to maintain the same current. If a forward bias voltage of the laser diode is small and the power supply voltage is large, the laser driver has an ample supply voltage range to provide the level of current needed. If the forward bias voltage of the laser diode takes up more of the power supply range and/or the power supply voltage becomes less, and/or if the current source heats up, the laser driver must supply the same current with the reduced voltage supply range. A laser driver that can deliver the same current over a wide range of voltages, particularly lower voltages, is desirable. 
     Typically, diodes are driven by, for example, PMOS transistors, as shown, for example, in  FIG. 1 .  FIG. 1  shows a light emitting diode  30  coupled to a drain terminal  24  of a PMOS transistor  20  and to ground  14 . A source terminal  22  of PMOS transistor  20  is coupled to a supply voltage source  10 , which supplies a voltage through PMOS transistor  20  sufficient to forward bias light emitting diode  30 . The amount of current delivered by PMOS  20  is controlled by the voltage at a gate terminal  26 , which is coupled to a reference voltage  12 . Raising the gate voltage to the supply voltage, for example, will switch off PMOS transistor  20 , whereas reducing the gate voltage back to the reference voltage level will turn PMOS transistor  20  back on. 
     SUMMARY OF THE INVENTION 
     However, a number of disadvantages are present in the light emitting diode driver circuit shown in  FIG. 1 . For example, light emitting diode  30 , as part of an optical system implementation, may need replacement, or may be updated to a different model or type of light emitting diode, thus introducing a different forward bias voltage which directly affects the voltage drop across PMOS transistor  20 , affecting the current available for the light emitting diode  30 . For example, the PMOS transistor  20  as part of an optical device driver may be used in several optical systems, each employing a different light emitting diode  30  with its own forward bias voltage. In each optical system, the PMOS transistor  20  will be left with a different amount of the power supply range for operation, resulting in a different current through the light emitting diode  30 . For example, upon switching on, PMOS transistor  20  heats up causing its resistance to rise, which increases the voltage drop across PMOS transistor  20 , and reduces the output voltage, and therefore the current available for light emitting diode  30 . 
     Also, variations in the voltage from supply voltage source  10  may directly affect the amount of current delivered to light emitting diode  30 , because the voltage delivered to input terminal  32  of light emitting diode  30  depends directly on the voltage from supply voltage source  10  through PMOS transistor  20 . 
     Each of these sources of variability of the supply current may affect the light output properties of light emitting diode  30 . 
     Accordingly, the driving circuit shown in  FIG. 1  suffers from a number of disadvantages. Therefore, it would be advantageous to design a driving circuit for light emitting diodes which avoids these disadvantages. 
     A driving circuit may be provided for a light emitting diode which delivers a steady amount of driving current to the light emitting diode regardless of changes in the supply voltage, changes in the forward bias voltage of the light emitting diode, or changes in the internal resistance of the driving PMOS transistor. 
     A circuit may be provided for driving a light emitting diode that includes an operational amplifier which controls a gate voltage of two transistors. Inputs to the operational amplifier may be coupled to drains of the transistors, such that voltages on the drains may be kept equal by the operational amplifier. A reference current may be coupled to the drain of a first transistor, so that the operational amplifier operates to deliver a substantially equal amount of current through the drain terminal of a second PMOS transistor. The light emitting diode may be further coupled to the drain terminal of the second transistor. The first and the second transistors may be PMOS transistors. The light emitting diode may be, for example, a laser diode. 
     A strobe signal may be connected to the operational amplifier, which, when the strobe signal is high, disables the operational amplifier, for example, by driving the output of the operational amplifier to its positive rail, which shuts off the PMOS transistor whose drain is coupled to the light emitting diode. The strobe signal, when low, may enable the operational amplifier so that current is delivered to the light emitting diode. 
     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary details are described with reference to the following figures, wherein: 
         FIG. 1  is a schematic diagram of a known driving circuit for a light emitting diode; and 
         FIG. 2  is a schematic diagram of an exemplary driving circuit for a light emitting diode, using a reference current. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, a driving circuit for a light emitting diode includes a balancing circuit having inputs coupled to drain terminals of two transistors, and an output coupled to control gates of the two transistors. The two transistors may be PMOS transistors, and the balancing circuit may be an operational amplifier. The drain terminal of one PMOS transistor may also be coupled to a reference current source, and the drain terminal of the other PMOS transistor may be coupled to the light emitting diode. Because the operational amplifier operates to equalize its inputs, the output of the operational amplifier may adjust the control gates of the PMOS transistors until its inputs are substantially equal. This causes the current delivered to the light emitting diode to be substantially equal to the reference current supplied by the reference current source, for example, regardless of the changes in the operating voltage of the driving circuit. 
       FIG. 2  is a schematic diagram of an exemplary driving circuit  100  for a light emitting diode. The circuit  100  includes two PMOS transistors  110  and  120 , an operational amplifier  150 , and light emitting diode  180 . Source terminals of PMOS transistors  110  and  120  may be coupled to supply voltage source  105 . The supply voltage from supply voltage source  105  may be referred to hereinafter as V DD  and may be, for example, 5 volts. 
     An input voltage at the input voltage node  194  may be coupled to a negative (inverting) input terminal  154  of the operational amplifier  150 . This input voltage may be the forward bias voltage  174  of a light emitting diode  180 . The reference current  182  on node  192  is defined by the reference current source  130  consisting of two series NMOS transistors; NMOS transistor  131  drain terminal connected to node  192 , gate terminal connected to bias voltage  136 , Bias 1 , and source terminal connected to node  162 ; and NMOS transistor  132  drain terminal connected to node  162 , gate terminal connected to bias voltage  137 , Bias 2 , and source terminal connected to ground  000 . This reference current may be input to the drain terminal of PMOS transistor  110  through node  192 . The resulting voltage is connected to the positive (non-inverting) input terminal  152  of the operational amplifier  150  by means of node  192 . 
     The operational amplifier acts to substantially equalize the voltage on its inputs,  152  and  154 , by changing the voltage at its output  156 . Changing the output voltage  156  changes the voltage at the gate inputs of PMOS transistors  110  and  120 , changing the drain currents, Id 110  ( 111 ) and Id 120  ( 121 ), of PMOS transistors  110  and  120 , delivered to nodes  192  and  194 , respectively. The positive input  152  of operational amplifier  150  accepts no input current, so drain current Id 110  ( 111 ) of PMOS transistor  110  is equal to the reference current Ics  182 . As PMOS transistors  110  and  120  have the same source voltage  105 , and the same gate voltage  156 , the currents outputted from their drains are equal except for the effect of differing drain voltages on PMOS transistors  110  and  120 . The negative input  154  of operational amplifier  150  accepts no input current, so the forward diode current Iled  184  is equal to the drain current Id 120  ( 121 ) of PMOS transistor  120 . As the characteristic of a light emitting diode is that the forward bias voltage changes little as the forward current through the light emitting diode changes, the negative input terminal  154  of the operational amplifier  150  is equal to forward bias voltage  174  and is relatively stable. 
     The characteristic of a current reference source is that its reference current changes little as the voltage across it changes. 
     If the drain current Id 110  ( 111 ) of PMOS transistor  110  is larger than the reference current Ics  182 , charge is accumulated onto node  192  raising the voltage on node  192  to higher levels than the voltage on the negative input terminal  154  of the operational amplifier  150 . This causes the output voltage  156  of the operational amplifier  150  to increase, decreasing the drain currents Id 110  ( 111 ) and Id 120  ( 121 ) of PMOS transistors  110  and  120  by means of their gate terminals, until the drain current Id 110  ( 111 ) of PMOS transistor  110  is made equal to the reference current Ics  182 . 
     If the drain current Id 110  ( 111 ) of PMOS transistor  110  is smaller than the reference current Ics  182 , charge is drained off of node  192  lowering the voltage on node  192  to lower levels than the voltage on the negative input terminal  154  of the operational amplifier  150 . This causes the output voltage  156  of the operational amplifier  150  to decrease, increasing the drain currents Id 110  ( 111 ) and Id 120  ( 121 ) of PMOS transistors  110  and  120  by means of their gate terminals, until the drain current Id 110  ( 111 ) of PMOS transistor  110  is made equal to the reference current Ics  182 . 
     When the drain current Id 110  ( 111 ) of PMOS transistor  110  is made equal to the reference current Ics  182 , voltage on the operational amplifier inputs,  152  and  154 , are made substantially equal by means of the gain of the operational amplifier  150 . With the operational amplifier inputs,  152  and  154 , made substantially equal, the drain voltages of the PMOS transistors  110  and  120  are also substantially equal. With the gate and source voltages of PMOS transistors  110  and  120  being equal, their drain currents Id 110  ( 111 ) and Id 120  ( 121 ) are substantially equal, causing the forward diode current Iled  184  to be substantially equal to the reference current Ics  182 . 
     It should be appreciated that the exemplary current driver circuit  100  functions to keep the forward diode current Iled  184  substantially equal to the reference current Ics  182  despite changes or transients in the voltage from supply voltage sources  105  and  115 . 
     It should be appreciated that the exemplary current driver circuit  100  functions to keep the forward diode current Iled  184  substantially equal to the reference current Ics  182  despite changes in the forward bias voltage. 
     When Bias 1   136  and Bias 2   137  of the reference current source  130  are adjusted, the current flowing through reference current source  130  can also be adjusted. According to the operation of operational amplifier  150  as explained above, this amount of current is then also delivered to light emitting diode  180 . Therefore, Bias 1   136  and Bias 2   137  may be used to control the current, and therefore, the light output level of light emitting diode  180 . 
     Because of the configuration of current driver circuit  100  shown in  FIG. 2 , the level of reference current Ics  182  selected by Bias 1   136  and Bias 2   137  will be delivered to light emitting diode  180  throughout a wide range of operating conditions. For example, the current delivered to light emitting diode  180  may be determined by Bias 1   136  and Bias 2   137  for a wide range of supply voltages V DD  applied to PMOS transistors  110  and  120  at supply terminal  105 . 
     Although a particular implementation of a reference current source is shown, it is only illustrative and any implementation of reference current source may be used. 
     It should be appreciated that the exemplary current driver circuit provides an advantageous output impedance compared to the typical driver of  FIG. 1  mentioned earlier. The typical driver of  FIG. 1  supplies current to the light emitting diode through a PMOS transistor  20 . The output impedance of an output node is the change in its voltage divided by the change in current drawn from the output node. A large output impedance is indicated by a large voltage change at the drain  24  of the PMOS transistor  20  causing only a small change in current drawn from the drain  24 . The output impedance of a PMOS device is typically less than that of an NMOS device and much less than that of the series arrangement  130  of NMOS devices  131  ad  132  in  FIG. 2 . The operational amplifier  150  of  FIG. 2  acts to make the magnitude of the output impedance of the drain terminal of the PMOS transistor  120  equal to the output impedance of the reference current source. If the forward bias voltage  174  changes, the operational amplifier acts to change the voltage across the reference current generator  192  by an equal change. The large output impedance of the reference current generator causes the reference current to change very little. The operational amplifier then acts to change the drain current Id 110  ( 111 ) of PMOS  110  to be equal to the reference current  182  which changes the drain current Id 120  ( 121 ) of PMOS transistor  120  to also be equal to the reference current  182 . Although the forward bias voltage  174  changed, the forward diode current Iled  184  changed only as much as the reference current source would allow given its output impedance. The lower output impedance of the PMOS transistor  120  is preempted. If transistors  110  and  120  are in close proximity as to share thermal environments, lower output impedance of the PMOS transistor  120  is preempted, regardless of heating effects on PMOS transistor  120 . 
     In addition to controlling the amount of forward diode current Iled  184  to light emitting diode  180  using reference current source  130 , the forward diode current Iled  184  to light emitting diode  180  may also be turned on and off by the addition of a strobe signal  160  to the circuit, as shown in  FIG. 2 . In various exemplary implementations, a strobe signal  160  may be connected to the operational amplifier  150 , which, when the strobe signal  160  is high, disables the operational amplifier  150 , for example, by driving the output node  156  of the operational amplifier  150  to its positive rail, which shuts off the PMOS transistor  120  whose drain is coupled to the light emitting diode  180 . The strobe signal  160 , when low, may enable the operational amplifier  150  so that the drain current Id 120  ( 121 ) is delivered to the light emitting diode  180 . 
     While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. The driving circuit may be applies to any light emitting diode, such as a laser diode, a visible light diode, a vertical cavity surface emitting laser, and so on. More generally, the driving circuit may be applied to any network or networks, having a high impedance node for connection to node  192 , and a low impedance node for connection to node  194 , where the current versus voltage characteristics of the high and low impedance nodes have a non-zero correspondence. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.