Abstract:
The infrared LED of an IrDA module transmits IR energy with a peak wavelength (for example, 875 nm) appropriate for IrDA communication. This peak wavelength is lower than is the wavelength (for example, 940 nm) used in ordinary IR remote controls (RC). The IrDA LED does, however, transmit some energy at the wavelength of the peak sensitivity of an RC receiver. When making an IrDA transmission, the IrDA LED is driven with a lower amount of current. When making an RC transmission, the IrDA LED is driven with an increased amount of current such that higher wavelength emissions received by the RC receiver are of adequate power to realize RC communication. A passive circuit is disclosed for automatically increasing IrDA LED current during RC transmissions. The circuit involves an inductor that shunts current around a current-limiting resistor used to limit LED drive current.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to IrDA transceivers.  
       BACKGROUND INFORMATION  
       [0002]     Personal digital assistants (PDAs), cell phones, digital cameras, laptop computers and other portable. electronic devices often use a type of infrared transceiver called an IrDA (Infrared Data Association) transceiver to transfer information.  FIG. 1  illustrates a typical IrDA transceiver  1 . Transceiver  1  is a multi-chip module that is enclosed in a metal shield  2 .  FIG. 2  is a cross-sectional view of the IrDA transceiver.  FIG. 3  is a top-down cross-sectional view of the circuitry within the IrDA module. The IrDA module includes three dice: 1) an IR transmitter diode die  3 , 2) an IR receiver PIN diode die  4 , and 3) a controller die  5 . The three dice are mounted on a very small integrated circuit board  6 . Molded plastic  7  covers the printed circuit board and die assembly and forms two semi-spherical lenses  8  and  9 . Lens  8  is to focus incoming IR radiation  10  onto the PIN receiver diode  4 . Lens  9  is redirect the radiation from the IR transmitter diode die  3  into a beam  11 .  
         [0003]     Electronic consumer devices in the home such as televisions, VCRs, DVDs, stereo equipment, home theatre equipment, CD players and so forth are typically controlled by remote control devices that transmit IR signals. Although the various manufacturers of electronic consumer devices use different coding and modulation schemes to control their electronic consumer devices, the overall use of IR to control such electronic consumer devices is referred to here as “RC”.  
         [0004]     Although both IrDA and RC both use IR to communicate, the two use different wavelengths of IR. IrDA typically uses an infrared signal in the range of 850 nm to 900 nm (for example, 875 nm). RC remote control devices typically use an infrared signal of 900 nm to 950 nm (for example, 940 nm). In addition to the IR wavelength, IrDA and RC use very different protocols. For this reason, IrDA and RC have been considered two different communication schemes.  
         [0005]     It has been recognized, however, that the IrDA transceivers so common in PDAs, cell phones, digital cameras, and laptop computers all include IR transmitter diodes. It would be advantageous if these IrDA IR transmitters could be used to control electronic consumer devices in the home. One PDA manufacturer has endeavored to have the IrDA module in its PDAs be capable of controlling electronic consumer devices in the home. PDAs are currently on the market with this functionality.  
         [0006]      FIGS. 1-4  illustrate the design employed by this PDA manufacturer. For additional information, see: 1) “Agilent HSDL-3003 IrDA Data Compliant Low Power 115.2 kbit/s with Remote Control Infrared Transceiver—Data Sheet,” 21 pages, June 2003, 2) “Interfacing the Agilent HSDL-3002 for Remote Control Operation—Application Note 1314,38 16 pages, February 2003, and 3) “Agilent IrDA Data Link Design Guide,” 67 pages, March 2003.  
         [0007]     Because RC electronic consumer devices typically respond to IR signals at 940 nm, an 940 nm RC transmitter diode  12  is included along with the 875 nm IrDA transmitter diode  3 . When an IRDA transmission is to be made, then the IrDA driver portion of controller die  5  pulls current through the 875 nm IrDA transmitter diode  3  in standard IrDA fashion. When an RC transmission is to be made, then an external FET  13  is made conductive to pull current through the added 940 nm RC transmitter diode  12 .  
         [0008]     In order to be able to provide the coding information needed to control all the many electronic consumer devices on the market, the PDA manufacturer has consulted a manufacturer of IR remote controls. The remote control manufacturer evidently will not provide the PDA manufacturer the coding information independently of hardware. The remote control manufacturer is evidently requiring the PDA manufacturer to buy microcontrollers from the remote control manufacturer that have the coding information programmed into them. The PDA manufacturer is therefore forced to provide a microcontroller  14  (that includes the coding information) in each PDA it manufacturers.  
         [0009]     Although it might seem that the functionality of FET  13  could be included on controller die  5 , the PDA manufacturer has evidently chosen not to do this. The reason may be that FET  13 , in order to pull the required amount of current through the 940 nm diode, has to have a low on-resistance. Realizing a FET with such a low on-resistance would take up so much integrated circuit area that controller die  5  would be so large that it could not any longer fit into the confining space of the standard IrDA form factor. Providing the two transmitter diodes into the form factor is difficult even without increasing the die size of controller die  5 . Accordingly, the design employed on the PDAs involves at least three components external to the IrDA module: 1) the FET  13 , 2) the microcontroller  14 , and 3) a current-limiting resistor  15 .  FIG. 1  illustrates the external FET component  13  as well as external resistor  15 . External microcontroller  14  is not pictured.  
         [0010]     Not only does the PDA design set forth above involve an external microcontroller, a FET and a resistor, but the design also requires an extra IR transmitter LED  12  to be provided in the IrDA module. Providing this IR LED die adds cost to the system. In addition, IrDA module assembly can be more expensive because the placement of the two IR LEDs is somewhat difficult. The two IR LED diode dice need to be placed close to one another in order to be satisfactorily aligned with respect to the optics of lens  9 . Very importantly, the PDA manufacturer is forced to buy and power a microcontroller just to get the coding information needed to interface to RC electronic consumer devices.  
         [0011]     It has been recognized that an 886 nm IrDA transmitter LED within an IrDA module can be used as a transmitter both for IrDA and RC applications. Although the peak wavelength of the 886 nm IrDA transmitter LED and the wavelength of peak sensitivity of a remote control receiver are not matched, radiation transmitted by the 886 nm transmitter LED is nonetheless received at the remote control receiver. Due to the mismatch, however, it may be necessary for the IrDA transmitter LED power to be increased in order for it to work properly in a remote control application. A recent application note from Vishay Semiconductors (Vishay Semiconductor Application Note entitled “Utilizing a Vishay IrDA Transceiver for Remote Control”, document number 82606, 14 pages, Feb. 20, 2004) discloses reducing the value of a current-limiting resistor so as to increase IrDA LED peak current and thereby to increase emission intensity of the IrDA LED transmitter. Laboratory test results are described, but a workable and cost-effective mechanism or circuit for reducing the value of the current-limiting resistor is not set forth.  
       SUMMARY  
       [0012]     Rather than providing a first infrared LED that has a peak emission suited for IrDA communication and a second infrared LED that has a peak emission suited for remote control (RC) communication, a single IrDA LED having a peak emission suited for IrDA communication (for example, 875 nm) is used to transmit both an IrDA transmission as well as an RC transmission. By using a single infrared LED for both IrDA and RC transmission purposes, the field effect transistor (FET) of the prior art solution of  FIG. 4  is omitted. RC codeset information is made available to the central processing unit (CPU) of the device of which the IrDA module is a part, thereby allowing the CPU to generate RC signals. This eliminates the need for the separate microcontroller of the prior art solution of  FIG. 4  and further reduces overall system cost, size and power consumption.  
         [0013]     In an IrDA mode, the IrDA 875 nm LED is driven with a lower amount of average LED drive current. This amount of average LED drive current is, in some embodiments, determined at least in part by the resistance of a current-limiting resistor that is disposed in the path of the LED drive current.  
         [0014]     In an RC mode, the IrDA 875 nm LED is driven with an increased amount of current such that higher wavelength emissions received by an RC receiver (for example, an RC receiver on a television or other electronic consumer device) will be of adequate power to realize RC communication at a distance of ten meters. A passive circuit is disclosed for automatically increasing average IrDA LED drive current when an RC transmission is being transmitted from the IrDA LED. In one embodiment, the passive circuit involves an inductor that shunts current around the current-limiting resistor, thereby increasing IrDA LED drive current. In one embodiment, current flow through the inductor saturates in the RC mode, thereby fixing the maximum average LED drive current with which the IrDA 875 nm LED is driven in the RC mode.  
         [0015]     Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.  
         [0017]      FIG. 1  (Prior Art) is a perspective view of a conventional IrDA module.  
         [0018]      FIG. 2  (Prior Art) is a cross-sectional side view of the IrDA module of  FIG. 1 .  
         [0019]      FIG. 3  (Prior Art) is a cross-sectional top-down view of the IrDA module of  FIG. 1 .  
         [0020]      FIG. 4  (Prior Art) is a simplified schematic view of the IrDA module of  FIG. 1 .  
         [0021]      FIG. 5  is a simplified schematic view of a system involving an IrDA module in accordance with one embodiment of the present invention.  
         [0022]      FIG. 6  is a chart that shows: 1) a normalized spectral emission curve of the 875 nm LED of the IrDA module of  FIG. 5 , and 2) a composite normalized spectral sensitivity curve for numerous remote control (RC) receivers.  
         [0023]      FIG. 7  is an equation that sets forth how to determine the peak power required to be emitted from the 875 nm LED in order for the IrDA module of  FIG. 5  to work for RC applications at a distance of ten meters.  
         [0024]      FIG. 8  is a chart showing how LED drive current should be increased until the 875 nm LED emits adequate radiation at 900 nm.  
         [0025]      FIG. 9  is a diagram showing an IrDA transmission and an RC transmission, and how the average LED drive current is controlled by the passive current-limiting circuit in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0027]      FIG. 5  is a diagram of a system  100  in accordance with one embodiment of the invention. System  100  includes an IrDA transceiver module  101 , a current-limiting resistor  102 , an inductor  103 , a central processing unit (CPU)  111 , and an operating system  104  that has access to IR remote control (RC) codeset information  105 . Rather than providing two LED transmitter diode dice of different wavelengths in IrDA module  101 , only one 875 nm transmitter LED diode die  106  is provided. IrDA module  101  includes the 875 nm transmitter LED diode die  106 , a controller die  107 , and an IrDA receiver PIN diode die (not shown). IrDa module  101  is of standard IrDA module construction and includes a molded plastic transmitter lens disposed over the IrDA transmitter diode  106  and another receiver lens disposed over the IrDA receiver diode. IrDA module  101  may be an IrDA module of conventional construction such as an IrDA module available from Zilog, Inc. of San Jose, Calif.  
         [0028]     Inductor  103  and resistor  102  are disposed in parallel between a source of supply voltage VCC and an LED drive current supply terminal  108  of IrDA module  101 . Inductor  103  and resistor  102  are both discrete components that are disposed on the printed circuit board upon which IrDA module  101  is mounted. In one example, this printed circuit board is the printed circuit board of a personal digital assistant (PDA).  
         [0029]     When an IrDA transmission is to occur, the 875 nm transmitter LED diode  106  is controlled in the conventional IrDA manner. In one example, an IrDA signal is supplied to the IrDA module via an IrDA/RC TX terminal  109  on the IrDA module, and an IrDA driver portion  110  of controller die  107  pulls current through LED diode  106  in conventional fashion. The amount of LED drive current initially when current starts to flow through LED  106  is limited by current-limiting resistor  102 . In one embodiment, the resistance of resistor  102  is approximately one ohm, the LED drive current is approximately 265 mA average (over the period of one IR burst of the IrDA transmission), and the resulting intensity of radiation emitted from IrDA module  101  is approximately 40 mW/sr at 875 nm.  
         [0030]     When a remote control (RC) transmission is to occur, however, an RC control signal is generated by software executing on the CPU  111  of the PDA using the codeset information  105 . No microcontroller that stores codeset information need be provided. Rather, codeset information  105  is stored in main memory on the PDA such that software executing on the CPU of the PDA can access the codeset information and use it to drive the IrDA/RC TX terminal  109  with the RC signal. This RC signal is provided to IrDA module  101  via the same IrDA/RC TX terminal  109  that is used to supply the IrDA signal for transmission. Although an RC signal is to be transmitted, transmitter diode  106  has a transmission peak at 875 nm. An IR remote control receiver (not shown) on an electronic consumer device (not shown) that is to be controlled by the RC transmission has a peak spectral sensitivity at a wavelength of approximately 940 nm. In accordance with the invention, it is recognized that most IR remote control receivers in electronic consumer devices will work satisfactorily if they receive 40 mW/steradian at 940 nm.  
         [0031]      FIG. 6  is a graph that shows the spectral emission distribution curve of 875 nm IrDA transmitting LED  106  as well as a composite spectral sensitivity curve for numerous typical IR remote control (RC) receivers. Note that the amount of radiation emitted from the 875 nm transmitting diode  106  drops from its normalized high at 875 as wavelength increases to the right of its peak. The amount of radiation emitted is down to sixty percent of its normalized high at 900 nm. Also note that the spectral sensitivity of the composite IR remote control (RC) receiver curve is at its normalized high at 940 nm, but that it is down to seventy percent of its high at 900 nm to the left of its peak.  
         [0032]     In accordance with one embodiment of the present invention, the amount of drive current used to drive the 875 nm IrDA transmitting LED  106  is increased in RC mode in comparison to the IrDA mode. The LED drive current is increased in RC mode such the emissions at 940 nm from the IrDA module  101  are adequately strong to be received by an RC receiver at a distance of ten meters.  
         [0033]      FIG. 7  sets forth the calculation of the peak radiated power to be output by the IrDA module  101  at 875 nm. The equation takes into account the fact that the composite RC receiver at 900 nm is only seventy percent as efficient as it is at its peak spectral efficiency of 940 nm. The equation takes into account the fact that the IrDA module  101  at 900 nm transmits only sixty percent of the radiated power that it does at 875 nm. The equation also takes the inverse square law into account and the fact that the IrDA transmitter is located ten meters away from the RC receiver. In accordance with the equation of  FIG. 7 , the IrDA module  101  is to emit 95 mW/sr of peak power at 875 nm when in RC mode.  
         [0034]      FIG. 8  illustrates the emission distribution curve of IrDA module  101 . As illustrated, IrDA module  101  outputs 57 mW/sr at 900 nm when it is outputting 95 wW/sr at 875 nm. At this power, the 875 nm IrDA transmitting LED  106  within IrDA module  101  transmits adequate power to communicate IR remote control (RC) communications to an electronic consumer device located ten meters away from the IrDA module.  
         [0035]     The amount of LED drive current that results in IrDA module  101  emitting 95 mW/sr at 875 nm is determined. In one embodiment, this is done by building IrDA module  101 , driving LED  106  with different drive currents, and measuring the emitted radiation at 875 nm. In this way, a relationship of LED drive current to emitted radiation at 875 nm is obtained. In other embodiments, optical design parameters affecting IrDA module emission efficiency are taken into account to determine the relationship between LED drive current and IrDA module emitted radiation at 875 nm. Such design parameters may, for example, include the distance the transmitting LED  106  is from its lens, the diameter of its lens, and the material of which its lens is made. In the present example, an LED drive current of 400 mA corresponds to approximately 95 mW/sr emitted from IrDA module  101  at 875 nm.  
         [0036]     Whereas the amount of transmitted radiation is to be 95 mW/sr at 875 nm for RC applications, the amount of transmitted radiation should be less at 875 nm for IrDA applications. In the present example, an LED drive current of 265 mA corresponds to the amount of radiation to be emitted from IrDA module  101  when it is operating as an IrDA transmitter.  
         [0037]     The circuit of  FIG. 5  works to drive LED  106  with the lower amount of LED drive current (approximately 265 mA) when in IrDA mode and with the higher amount of LED drive current (approximately 400 mA) during RC mode by taking advantage of differences between the IrDA and RC protocols.  FIG. 9  illustrates some of these protocol differences. The left half of  FIG. 9  illustrates a typical IrDA transmission. The IrDA transmission is not modulated with a sub-carrier. The right half of  FIG. 9  illustrates a typical RC transmission. Note that the mark times of the RC signal are modulated with a sub-carrier. The second row of waveforms shows this modulation of the RC signal in greater detail. Individual bursts of IR are approximately 1.6 microseconds long in the IrDA example on the left, whereas individual bursts of IR are approximately 10 microseconds long in the RC example on the right.  
         [0038]     Inductor  103  (see  FIG. 5 ) operates to shunt current flow around current-limiting resistor  102 , but the current flow through inductor  103  increases with time in accordance with the inductor equation V=Ldi/dt. Accordingly, in the case of the shorter 1.6 microsecond IR burst duration of the IrDA signal, the LED drive current is as illustrated in the lower left of  FIG. 9 . LED drive current increases over time from its starting value (determined at least in part by resistor  102 ), but the duration of the IR burst is short and there is not very much current increase. As illustrated in  FIG. 9 , the average LED current is about 265 mA.  
         [0039]     In the case of the longer 10 microsecond IR burst of the RC signal, however, the LED drive current increases for a longer period of time. Inductor  103  shunts current around current-limiting resistor  102 . In one embodiment, the inductor is made, to saturate such that the combined parallel impedance of resistor  102  and inductor  103  limits peak current flow to approximately 500 mA. Accordingly, the LED current waveform in the lower right of.  FIG. 9  shows the current increasing over time during the beginning part of the IR burst period, and then leveling off at the saturated 500 mA level. The average LED drive current is approximately 400 mA over the IR burst period.  
         [0040]     It is therefore seen that the 875 nm IrDA transmitter LED  106  is driven with a lower 265 mA average drive current during an IR burst of an IrDA transmission, but is driven with a higher 400 mA average drive current during an IR burst of an RC transmission. The 400 mA average drive current results in the IrDA module  101  outputting the desired 95 mW/sr peak power at 875 nm when IrDA transmissions are made.  
         [0041]     In the present example, inductor  103  has an inductance of approximately 30 microhenrys and has a DC series resistance of approximately 1.5 ohms. It is approximately 0.18 inches in diameter. It has a powdered iron toroidal core and is mounted on a 0802 component size coil platform. The platform allows inductor  103  to be placed on the PDA printed circuit board using automatic component placement equipment.  
         [0042]     In another embodiment, the inductance of inductor  16  is provided in the form of a printed circuit board trace and associated magnetic core material on the printed circuit board of the IrDA module. Known techniques for integrating and miniaturizing an inductor are applicable to further reduce size and cost. Rather than a single inductor component  103  being disposed in parallel with resistor  102 , a second resistor component may be placed in series with inductor  103  such that current shunted around resistor  102  passes through both the inductor and the second resistor.  
         [0043]     The system of  FIG. 5  has considerable advantages in comparison to the system employed by the PDA manufacturer as set forth above in the background information section of this patent document. The size of the controller chip within the IrDA module does not need to be increased. The relatively expensive external FET of the circuit set forth in the background information section can be dispensed with and replaced with a relatively inexpensive inductor (a passive component). The overall solution of  FIG. 5  is far less expensive than the circuit set forth in the background information section that requires a microcontroller. It consumes less power. Assembly of the IrDA module is not complicated by having to place two LED dice in close proximity to one another under a small lens. IrDA module cost does not involve the cost of having to provide a second LED die. The extra terminal on the IrDA module required to couple to the external FET in the circuit set forth in the background information section is also not required. A standard IrDA module can be used. The amount of LED drive current is controlled using only passive components (i.e., a resistor and an inductor).  
         [0044]     Although the present invention is described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Although an LR filter involving current-limiting resistor  103  and inductor  102  is described above, other circuits can be used to change the amount of LED drive current to accommodate dual IrDA and RC use of LED  106 . The IRDA driver  110  may, for example, include an internal drive current control circuit that controls LED  106  to be driven with more drive current when in an RC mode and controls LED  106  to be driven with a lesser amount of drive current when in an IrDA mode. Controller die  107  may include a terminal for receiving a control signal indicative of whether an IrDA transmission is to occur, or whether an RC transmission is to occur. A circuit with a first resistance when in an IrDA mode and a second resistance when in an RC mode can be disposed in the LED drive current path, where the resistance of the circuit is determined by a control signal received onto a control input lead of the circuit. The circuit may, for example, include a first resistor and a second resistor connected in series, where a transistor is disposed in parallel with the second resistor. In an RC mode, the transistor is controlled to be non-conductive such that the resistance of the circuit is the combined resistance of the two resistors. In an IrDA mode, the transistor is controlled to be conductive such that the transistor shorts out the second resistor and such that the resistance of the circuit is the resistance of the first resistor. Such a variable resistance circuit can be disposed on a PDA printed circuit board or can be included on an IrDA module. Such a circuit can be integrated into controller die  110 . Although an embodiment of an IrDA module is set forth above that only has one 875 nm LED die, it is to be understood that more than one 875 nm LED dice can be disposed in parallel under the same transmitter lens. Only one of the two LEDs is used in the IrDA mode, whereas both LEDs are used in the RC mode. The two 875 nm LEDs can be disposed on a single die rather than being disposed on two different dice. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.