TW201919443A - Temperature compensation in optical sensing system - Google Patents

Abstract

Disclosed is a temperature compensation circuit for a light source (e.g., light emitting diode (LED)) whose radiant energy output decreases when ambient temperature increases. The circuit includes first means for sourcing a first current that increases proportional to an increase in ambient temperature, and second means for sourcing a second current that is first order independent of ambient temperature. The circuit further includes a weighted current adder for sourcing a third current by combining the first and second currents with first and second weights applied to the first and second currents respectively. The circuit further includes third means responsive to the third current for supplying a fourth current to the light source to maintain a radiant energy output of the light source constant independent of ambient temperature.

Description

Temperature compensation technology in optical sensing systems

[0001] The present invention is generally related to temperature compensation techniques in optical sensing systems.

[0002] Light-emitting diodes (LEDs) have been widely used in applications such as card reading, character recognition, proximity sensing, label printing, electro-optical switching, and the like. In particular, LEDs have been used in optical sensing systems in conjunction with photodiodes. For example, an optical sensing system can drive an LED to produce some radiant energy. This radiant energy is received by the photodiode after passing through the medium or being reflected by the surface. The photodiode converts the received radiant energy into a current that is further processed to detect, for example, the presence of a medium or surface. [0003] In the above optical sensing system, the resolution of the sensing (eg, the ability to distinguish the difference between a thin piece of paper and two thin sheets of paper) depends on a number of factors. One such factor is the ability of the LED to maintain its radiant energy output (or optical output power) substantially constant over the entire range of ambient temperatures at which the optical sensing system operates. Unfortunately, the radiant energy output of most LEDs varies considerably with ambient temperature. In particular, if the current input to the LED remains the same, the radiant energy output of most of the LEDs is considerably reduced as the temperature increases. Such variations are generally unacceptable in conventional high resolution optical sensing systems. Therefore, improvements in this area are necessary.

and

The following disclosure provides a number of different embodiments or examples for implementing the various features of the subject matter provided, and specific examples of components and configurations are described below to simplify the present invention. Of course, these are merely examples and are not intended to be limiting. Any alterations and further modifications to the described devices, systems, and methods, as well as any other applications of the principles of the present invention, have been considered in the ordinary. For example, features, components, and/or steps described in connection with one embodiment may be combined with features, components, and/or steps described in connection with other embodiments of the invention to form a device, system, or system in accordance with the present invention. Another embodiment of the method, such a combination, is not explicitly shown. Moreover, for the sake of brevity, in the examples, the same reference numerals are used to refer to the The present invention relates generally to optical sensing systems and methods, and more particularly to circuits, systems, and methods for compensating a light source of an optical sensing system to operate at an operating temperature range. It is an object of the present invention to provide a temperature compensated light source whose radiant energy output is typically kept constant over the entire operating temperature range. Another object of the present invention is to provide an LED driver circuit that can operate with most of the existing LEDs via a calibration procedure. After being calibrated, the LED drive circuit can operate to generate a current that compensates for the given LED such that when the ambient temperature changes, the output power of the LED typically remains constant. It is yet another object of the present invention to provide a calibration method that can be implemented in an optical sensing system to achieve the temperature compensation described above. Referring to Figure 1, there is shown a schematic partial view of an optical sensing system 10 constructed in accordance with an embodiment of the present invention. The optical sensing system 10 includes an LED 12 having an anode 11 coupled to a supply voltage V _DD and a cathode 13 coupled to a current source 28 via a switch 30 (discussed further below). The forward current I _LED flows through the LED 12 and causes it to generate light (or radiant energy). The LED 12 can produce visible or invisible light, which includes ultraviolet light and infrared light. LED 12 may comprise any suitable semiconductor material such as gallium arsenide (GaAs), gallium phosphide (GaP), aluminum gallium arsenide (AlGaAs), gallium arsenide (GaAsP), indium gallium nitride (InGaN), phosphorous Aluminum gallium (AlGaP), and aluminum gallium indium phosphide (AlGaInP). The optical sensing system 10 further includes a photodiode 14 having an anode 15 connected to ground ( _Vss ) and a cathode 17 connected to a switch (eg, a reset switch) connected to a supply voltage _VDD . The photodiode 14 receives at least a portion of the radiant energy of the LED 12, for example, by reflection away from the surface, by diffraction of the medium, or transmitted through the medium. In response, photodiode 14 produces a current from its cathode 17 to its anode 15, which is sensed by detection circuit 16 for further processing. [0009] The characteristic of the LED 12 is that if the current I _{LED is} kept constant, its radiant energy output decreases as the ambient temperature increases. Such an increase in the ability of the photodiode 14 and the detection circuit 16 to detect the radiant energy reliability as the ambient temperature changes. As a result, the sensitivity of the photodiode 14 is limited by how much variation is made in the radiant energy output of the LED 12. For applications operating over a wide range of ambient temperatures (for example, commercial grade temperatures from 0 ° C to 70 ° C, industrial grade temperatures from -40 ° C to 85 ° C, or from -55 ° C to 125 ° C The military grade temperature range) is highly desirable to keep the radiant energy output of LED 12 constant or generally constant throughout the operating temperature (eg, within a few percent). The present invention provides a solution to this problem as discussed further below. [0010] Still referring to FIG. 1, the optical sensing system 10 further includes a module 18 and a module 20, which is a temperature-dependent current generator, and the module 20 is not changed with temperature. (temperature-independent) current generator. Module 18 is operative to generate current I _S . Module 20 is operative to generate current I _R . In the present embodiment, the current I _S in response to ambient temperature increases (or decreases) and increase (or decrease). In another embodiment, when the ambient temperature increases (or decreases), a current I _S-order linear increase (or decrease). In the present invention, the term "first-order linearly" means that when the temperature is within a defined range (such as within a desired operating range), the current I _S can be in the following equation (1) The linear equation is modeled as an absolute temperature T, and any second or higher order effects of temperature T can be ignored. The same definition of "first-order linearly" applies to the discussion of other variables including voltage, current, power, and resistance, and circuit parameters. Where m and I ₀ are constant and m > 0 [0011] Conversely, the current I _R is independent of the ambient temperature, that is, it remains fairly constant over the entire operating ambient temperature range, and the environment Any second-order or higher-order effects of temperature can be ignored. [0012] The optical sensing system 10 further includes a current adder (or current mode adder) 22. Current adder 22 is operative to generate current I _B , which is the weighted sum of currents I _S and I _R . In the present embodiment, the current adder 22 applies the first weight W _S to the current I _S and the second weight W _R to the current I _R . Therefore, the current I _B is expressed in the following equation (2): [0013] In the present embodiment, the weights W _S and W _R are provided by the control unit 24 and they are each a vector of multiple bits. In some examples, control unit 24 is a special application integrated circuit (ASIC) or other processing circuit that operates to read computer executable instructions from a memory component and is provided herein by executing the instructions. The functionality described. In an embodiment, the weights W _S and W _R are programmable by the user to fine tune (or calibrate) the current I _B . As can be seen from equations (1) and (2) above, current I _B is a linear equation with an absolute temperature T of positive slope. Furthermore, when the weights W _S and W _R are normalized to a sum of 1, the temperature dependence is between the temperature dependence of I _S and I _R . For a given LED 12, the user can adjust the values of W _S and W _R to derive the appropriate I _B , which complements the temperature dependence of LED 12 . In other words, it causes the radiant energy output of LED 12 to generally remain constant over the entire operating ambient temperature range. Once the appropriate W _S and W _R values are found (or calibrated), the values can be stored in non-volatile memory (eg, as digital bits in control unit 24). This provides the optical sensing system 10 with the ability to resume proper operation after power-off and power-on without the need to repeat the calibration procedure. Still referring to FIG. 1, in the present embodiment, optical sensing system 10 further includes a current multiplier 26 that multiplies current I _B by control vector A _CTRL and produces current I _A as follows: [0015] In the present embodiment, the control vector A _CTRL is provided by the control unit 24. In application, the user can step manually or automatically via a set of values of A _CTRL , which ultimately produces a set of optical output power at LED 12. This can be used to calibrate LED 12 and photodiode 14 to find the desired operating conditions for optical sensing system 10. In an embodiment, once LED 12 and photodiode 14 are calibrated to a nominal temperature (eg, at room temperature), the value of A _CTRL is stored in non-volatile memory and is optically sensed by system 10 Used in subsequent operations. [0016] In an embodiment, current source 28 is a current mirror that replicates current I _A to the electrical path of LED 12. In fact, when switch 30 is turned "on" (or off), the I _LED is equal to I _A . The temperature dependence of I _A complements the temperature dependence of LED 12. In an embodiment, the switch 30 is controlled to open and close to periodically pulse the LED 12. For example, the LED 12 can be turned on for a period of 1 to 2 microseconds (μs) for 200 to 300 nanoseconds (ns). Referring to FIG. 2A, optical sensing system 10 further includes a bandgap voltage reference circuit 19 operative to provide a voltage potential V _R . The voltage V _R is independent of the first order of the ambient temperature. Voltage V _R is supplied to the current generator 18 (FIGS. 2A and 2B) and a current generator 20 (FIG. 3) as a reference voltage. [0018] Still referring to FIG. 2A, an embodiment of current generator 18 as a function of temperature is schematically illustrated. Current generator 18 comprises a bipolar transistor Q _1. In this embodiment, the bipolar transistor Q ₁ is a PNP transistor having an emitter coupled to the node 45 and a base and collector coupled to the node 50. In the present embodiment, node 50 is grounded (i.e., coupled to ground potential _Vss ). The voltage potential V _BE at node 45 is inversely proportional to the ambient temperature. In other words, when the temperature is within the defined range (such as within the desired operating range), the voltage V _BE can be modeled as a linear equation (4) of the absolute temperature T, and any of the temperatures T Second-order or higher-order effects can be ignored. Wherein n and V ₀ are constant and n > 0 [0019] The current generator 18 further includes resistors R ₁ and R ₂ connected in series between the reference voltage V _R and the ground V _SS . Resistors R ₁ and R ₂ are coupled to a common node 32. The resistance values of resistors R ₁ and R ₂ are independent of the ambient temperature. Thus, the potential at node ₃₂ V 32 based first-order independent of the ambient temperature, and is expressed in the following equation (5). [0020] The current generator 18 further includes an operational amplifier X ₁ . Operational amplifier X ₁ has a non-inverting input terminal coupled to node 32, an inverting input terminal coupled to node 36, and an output terminal coupled to node 38. The current generator 18 further includes a field effect transistor (FET) M ₃ , the gate of which is coupled to the node 38 , the source of which is coupled to the node 36 , and the drain of which is coupled to the node 42 . The negative feedback path of operational amplifier X ₁ is formed from node 38 via FET M ₃ to node 36. Upon reaching equilibrium current generator 18 during operation, the voltage potential at the node ₃₆ V 36 due to the negative feedback path is equal to the voltage level V _32. [0021] 18 further comprises a current generator coupled to the node 36 and the resistor R 45 between the nodes _3. The resistance value of the resistor R ₃ is independent of the first order of the ambient temperature. The current I _Q1 flowing through the resistor R ₃ is shown below in the equation (7). [0022] The result of combining equations (4) to (7) is the following equation (8). [0023] As can be seen from equation (8), the current I _Q1 is a first-order linear equation with an absolute temperature T of positive slope. In other words, when the ambient temperature T increases, the current I _Q1 increases linearly in the first order. [0024] Still referring to Figure 2A, since the input terminal of the operational amplifier ₁ X high impedance, the transistors M ₃ Suo current I _M3 is substantially equal to the current flowing I _Q1. Current generator 18 further includes a current mirror 47 that replicates current I _M3 to output current I _S at output node 46. The current I _S is shown below in equation (9). [0025] As can be seen from the equation (9) by the current generator 18 operable to generate a current line I _S, when the ambient temperature T increases, a current I _S increases linearly order. In the embodiment shown in FIG. 2A, current mirror 47 has two FETs M ₁ and M ₂ coupled to its gate terminals. In an alternate embodiment, current mirror 47 can be implemented using a bipolar transistor or FET, such as a metal oxide FET, and can be implemented using any architecture. [0027] Referring to FIG. 2B, shown therein is another embodiment of current generator 18. In addition to this embodiment uses the NPN bipolar transistor Q ₁ 'instead of the PNP bipolar transistor Q _1, this embodiment has the illustrated embodiment of FIG. 2A in substantially the same components. The emitter of transistor Q ₁ ' is coupled to node 50, while node 50 is grounded in this embodiment. The base and collector of transistor Q ₁ ' are coupled to node 36. The circuit analysis of this embodiment can be performed in the same manner as above. [0028] Referring to FIG. 3, an embodiment of current generator 20 that does not vary with temperature is schematically illustrated. The current generator 20 includes an operational amplifier X ₂ having a non-inverting input terminal coupled to the reference voltage V _R , an inverting input terminal coupled to the node 62 , and an output terminal coupled to the node 56 . 20 further comprises a current generator coupled between node 62 and ground potential V _SS resistor R _4. The resistance value of the resistor R ₄ is independent of the first order of the ambient temperature. The current generator 20 further includes a FET M ₆ having a gate coupled to the node 56, a source coupled to the node 62, and a drain coupled to the node 60 for generating a current I _M6 . Current generator 20 further includes a current mirror 65 that replicates current I _M6 to output current I _R . Current mirror 65 has two FETs M ₄ and M ₅ in this embodiment. In an alternate embodiment, current mirror 65 can be implemented using a bipolar transistor or FET, such as a metal oxide FET, and can be implemented using any architecture. [0029] When the current generator 20 reaches equilibrium during operation, the voltage potential V ₆₂ at node 62 due to the negative feedback path from node 56 to node 62 and a voltage potential equal to V _R. In fact, the following is true: [0030] As shown in equation (10), the current generator 20 is operative to generate a current I _R independent of the first order of ambient temperature. Referring to FIG. 4, a representative embodiment of the weighted current adder 22 is partially illustrated. In particular, Figure 4 illustrates the application of the weight W _R to the current I _R . The current adder 22 includes a FET M _{7 for} receiving the input current I _R and FETs M _8-1 , M _8-2 , ... M _8-i ... and M _{8 for} sourcing the output current I _WR . _Z. In an embodiment, Z can be any integer greater than zero (0). The plurality of FETs M _8-i (i in [1:Z]) each have their gate terminals coupled to the gate terminals of FET M ₇ via node 66. Basically, the plurality of FETs M _8-i are each operated to replicate the input current I _R . Furthermore, the plurality of FETs M _8-i are each connected to a switch which is controlled by one of the bits in the vector W _R [1:Z]. The current adder effectively produces I _WR = W _R . I _R . [0032] The weighted current adder 22 may include similar circuitry to the weight W _S I _S applied to the current so as to generate weighted current I _WS = W _S. I _S . [0033] The weighted current adder 22 can further include circuitry that operates to combine I _WR and I _WS to generate I _B such that: [0034] In various embodiments, the weighted current adder 22 can be implemented in any architecture. In a similar fashion, current multiplier 26 can be implemented to generate current I _A such that: Referring to FIG. 5, a representative embodiment of current source 28 is partially depicted along with switch 30 and LED 12. In this embodiment, the current source 28 having a current mirror FET M ₉ and M ₁₀ of. When the switch 30 is turned off, the current mirror operates to replicate its input current I _A to its output current I _LED . In an alternate embodiment, current source 28 can be implemented in any architecture. [0036] FIGS. 6A and 6B illustrate a portion of the operation of optical sensing system 10 in accordance with an embodiment of the present invention. Referring to Figure 6A, curve 70 depicts (at a given fixed input current) the radiant energy output of LED 12 as a function of ambient temperature prior to temperature compensation. The radiant energy output of LED 12 decreases as the ambient temperature increases from temperature t ₁ to temperature t ₂ . Referring to FIG. 6B, the relationship between the currents I _S , I _R , and I _LEDs is illustrated. The current I _S increases linearly with the first order as a function of ambient temperature (curve 74). The current I _R is independent of the ambient temperature (curve 76). The current I _LED increases linearly with first order as a function of ambient temperature (curve 78). In the example, the various components of the LED driver circuit of FIG. 1 and the control vectors W _S , W _R , and A _CRTL are selected such that curve 78 complements curve 70. [0038] FIG. 6A further illustrates the radiant energy output of LED 12 after being temperature compensated using the I _LED , which is shown in curve 72. As the ambient temperature increases from temperature t ₁ to temperature t ₂ , the radiant energy output of LED 12 remains constant or nearly constant (eg, the variation is within a user selected threshold), which overcomes the previously discussed temperature And the problem is changing. FIG. 7 illustrates a flow chart of a method 100 of temperature compensation for a given LED 12. FIG. 8 illustrates a flow chart of a method 150 of calibration of portions of optical sensing system 10. Each or all of methods 100 and 150 can be performed by optical sensing system 10. It is understood that additional operations may be provided before, during, and after each of methods 100 and 150, and that some of the operations may be substituted, eliminated, or changed in order for other embodiments of the methods. The methods 100 and 150 are merely examples, and are not intended to limit the invention to those specifically recited in the claims. [0040] Referring to FIG. 7, method 100 includes operations 102, 104, 106, 108, 110, and 112. These operations are further discussed below in conjunction with Figures 1 through 6B above. [0041] providing a first current I _S in operation 102, the method 100, in the first-order increase linearly as the ambient temperature increases (see FIG. 1, 2A, 2B, and 6B). At operation 104, method 100 provides a second current I _R that is independent of ambient temperature (see Figures 1, 3, and 6B). At operation 106, the method 100 first weight W _S is applied to the first current I _S, so as to generate a first weighted current I _WS (see FIG. 1 and 4). At operation 108, method 100 applies a second weight W _R to second current I _{R to} generate a second weighted current I _WR (see FIGS. 1 and 4). At operation 110, method 100 combines first and second weighted currents I _WS and I _{WR to} produce a combined weighted current I _B (see FIG. 1). At operation 112, method 100 applies a current I _LED derived from the combined weighted current I _B to LED 12 (see Figures 1, 5, and 6B). In an embodiment, the derived current I _LED is obtained by multiplying the combined weighted current I _B by the amplitude control vector A _CRTL (see Figure 1). Referring to FIG. 8, method 150 includes operations 152, 154, 156, 158, 160, 162, 164, and 166. These operations are further discussed below in conjunction with Figures 1 through 7 above. Measurement, calculation, and storage may be implemented by executing computer readable code processing logic, such as control unit 24, to provide the functionality described herein. In an embodiment, operations 152 through 166 are performed with optical sensing system 10 placed in a temperature chamber. The temperature chamber can be controlled to set the ambient temperature for the optical sensing system 10. [0043] At operation 152, method 150 initializes various components of optical sensing system 10 (FIG. 1). In an embodiment, operation 152 includes initializing an amplitude control vector A _CRTL . In an embodiment, this is implemented as part of a calibration procedure for LED 12 and photodiode 14. For example, the amplitude control vector A _CRTL can be set such that the LED 12 and the photodiode 14 operate properly for a desired detection sensitivity at a nominal operating temperature, such as room temperature. In an embodiment, operation 152 further comprises initializing the weights W _S and W _R, and the use of various weights W _S and W _R 7, and the sum is operable to provide an initial current I _LED. Operation 152 can include an initial value of a parameter set to a preset value that can be adjusted during method 150. [0044] At operation 154, method 150 measures the radiant energy output of LED 12 as a baseline power. Operation 154 may use a thermal detector, a quantum detector, or any other suitable detector. This measurement can be performed at a nominal operating temperature such as room temperature. [0045] At operation 156, method 150 sets an ambient temperature that is different than the nominal operating temperature. For example, this temperature can be the highest or lowest operating temperature that is intended for the optical sensing system 10. In an embodiment, the temperature chamber can be controlled to set different ambient temperatures. At operation 158, method 150 measures the radiant energy output of LED 12 at the temperature set at operation 156. In an embodiment, method 150 may repeat operations 156 and 158 multiple times at different selected temperatures. For example, the selected temperature can include the highest desired operating temperature, the lowest desired operating temperature, and the nominal operating temperature. In the example, the selected temperature comprises 0, 20, and 70 °C. Method 150 can select other temperatures and/or use more temperature points. However, at least two temperatures are selected in this calibration procedure. Method 150 records the radiant energy output of LED 12 as measured at each selected temperature. At operation 160, method 150 calculates a change (eg, percent change) in the radiant energy output of LED 12 at the selected ambient temperature point. [0048] At operation 162, method 150 checks if the change is within a critical value. The threshold can be designed by the user to be programmable and matched to the detection sensitivity in the photodiode 14. For example, the user can set the threshold to 5%. The threshold is one of the factors that affect the sensing resolution of optical sensing system 10. [0049] If the change in radiant energy output of LED 12 is within the threshold, method 150 stores various control information in non-volatile memory (eg, at control unit 24 or at another suitable location). Inside, it contains the amplitude control vector A _CRTL and the weights W _S and W _R . This is done by operation 164. When the optical sensing system 10 is again powered down or powered on, the stored values are automatically loaded and applied to individual components of the system without recalibration. [0050] If the change in the radiant energy output of the LED 12 is outside of the threshold, the method 150 changes one or both of the weights W _S and W _R at operation 166 and returns to operation 154. The above operations 154, 156, 158, 160, 162, and 166 are repeated until the appropriate weights W _S and W _R groups are found. If all of the weights W _S and W _R have been searched and no solution is found, the method 150 can take other actions, such as replacing the LED 12 or replacing another component of the optical sensing system 10. [0051] While not intending to be limiting, one or more embodiments of the present invention provide many benefits to an optical sensing system that uses LEDs as a light source. Because most LEDs have the characteristic that their relative radiant energy output decreases as the ambient temperature increases, embodiments of the present invention provide circuits, systems, and methods that compensate for the operation of such LEDs below the temperature range. When the temperature changes within the operating temperature range, the temperature compensated LED in accordance with aspects of the present invention maintains its radiant energy output constant or nearly constant, which greatly improves the sensitivity of the optical sensing system. Various embodiments are applicable to other types of light sources having emission characteristics that vary with temperature. [0052] In one representative aspect, the invention relates to a temperature compensation circuit for a light source, such as a light emitting diode. The source has a characteristic that if the current flowing through the source remains constant, the radiant energy output of the source decreases as the ambient temperature increases. The temperature compensation circuit includes a first mechanism that causes a first current to flow, the first current increasing in proportion to an increase in ambient temperature. The temperature compensation circuit further includes a second mechanism for causing the second current to flow out, the second current system being independent of the ambient temperature. The temperature compensation circuit further includes a weighted current adder for causing the third current to flow out by combining the first and second currents with the first and second weights applied to the first and second currents, respectively. The temperature compensation circuit further includes a third mechanism responsive to the third current for supplying the fourth current to the light source such that the radiant energy output of the light source remains constant regardless of ambient temperature. [0053] In another representative aspect, the invention relates to a temperature compensated light source. The temperature compensated light source includes a light emitting diode (LED) for generating a radiant energy output in response to a first current applied to the LED, the LED having a radiant energy when the ambient temperature increases and the first current remains constant Reduced output characteristics. The temperature compensated light source further includes a first current source, a second current source, a weighted current adder, and a third current source. The first current source group is configured to generate a second current, and the second current is linearly increased in a first order. When the ambient temperature increases, the second current source system constitutes a third current, and the third current system is independent of the first step of the ambient temperature. The weighted current adder system is configured by respectively summing the second and third currents respectively and applying A fourth current is generated at the first and second weights of the second and third currents, and the third current source is responsive to the fourth current and is configured to supply a temperature compensated first current to the LED. [0054] In yet another representative aspect, the present invention is directed to a method of compensating a light emitting diode (LED) to operate below an ambient temperature range. The method includes providing a first order linear increase in a first current when the ambient temperature is increased, providing a second current independent of ambient temperature, applying a first weight to the first current to generate a first weighted current, and a second A weight is applied to the second current to generate a second weighted current, and a third current is generated by summing the first weighted current and the second weighted current. The method further includes applying a third current to apply the fourth current to the LED. [0055] The above summary is a brief description of the features of several embodiments, and those skilled in the art may have a better understanding of the aspects of the invention. It will be appreciated by those skilled in the art that they can readily use the present invention as a basis for designing or modifying other processes and structures for performing the same purpose and/or achieving the same as the embodiments described herein. advantage. It is also to be understood by those skilled in the art that such equivalent constructions are not in the spirit and scope of the invention, and it is understood that they may make various changes, substitutions, and substitutions herein without From the spirit and scope of the invention.

[0056]

10‧‧‧ Optical sensing system

11‧‧‧LED anode

12‧‧‧Lighting diode (LED)

13‧‧‧LED cathode

14‧‧‧Light diode

15‧‧‧ anode of light diode

16‧‧‧Detection circuit

17‧‧‧ cathode of photodiode

18‧‧‧ Current generators that vary with temperature

19‧‧‧Differential voltage reference circuit

20‧‧‧ Current generators that do not change with temperature

22‧‧‧current adder

24‧‧‧Control unit

26‧‧‧ Current Multiplier

28‧‧‧current source

30‧‧‧ switch

32‧‧‧Common node

36‧‧‧ nodes

38‧‧‧ nodes

42‧‧‧ nodes

45‧‧‧ nodes

46‧‧‧ Output node

47‧‧‧current mirror

50‧‧‧ nodes

56‧‧‧ nodes

62‧‧‧ nodes

65‧‧‧current mirror

66‧‧‧ nodes

70, 72, 74, 76, 78‧‧‧ Curve

100, 150‧‧‧ method

102, 104, 106, 108, 110, 112, 152, 154, 156, 158, 160, 162, 164, 166‧‧

Q ₁ , Q ₁ '‧‧‧ Bipolar transistor

R ₁ , R ₂ , R ₃ , R ₄ ‧‧‧ resistors

X ₁ , X ₂ ‧‧‧Operational Amplifier

M ₁ , M ₂ , M ₃ , M ₄ , M ₅ , M ₆ , M ₇ , M _8-i , M ₉ , M ₁₀ ‧‧‧ field effect transistor

The aspects of the invention are best understood from the following detailed description. It is emphasized here that, according to the industry's standard practice, various features are not drawn to size. In fact, the dimensions of the various features may be arbitrarily expanded or reduced for clarity of discussion. 1 is a simplified block diagram of an optical sensing system in accordance with an aspect of the present invention; FIG. 2A is a partial schematic view of the optical sensing system including the current source module of FIG. 1 in accordance with an embodiment of the present invention; Another embodiment of the invention, FIG. 1 is a partial schematic diagram of an optical sensing system including a current source module; FIG. 3 is an optical sensing system including another current source module of FIG. 1 according to an embodiment of the present invention; FIG. 4 is a partial schematic view of a weighted current adder of the optical sensing system of FIG. 1 according to an embodiment of the present invention; FIG. 5 is a partial schematic view of the optical sensing system of FIG. 1 according to an embodiment of the present invention; 6A and 6B illustrate a portion of the operation of the optical sensing system of FIG. 1 in accordance with an embodiment of the present invention; and FIG. 7 illustrates a flow of a method for compensating an LED of the optical sensing system of FIG. 1 in accordance with an embodiment of the present invention; Figure 8 and Figure 8 show a flow chart of a method of calibrating some of the components of the optical sensing system of Figure 1 in accordance with an embodiment of the present invention.

Claims (20)

A temperature compensation circuit for a light source having a characteristic that a radiant energy output of the light source is reduced when the ambient temperature is increased if the current flowing through the light source is kept constant, the temperature compensation circuit comprising: a first mechanism through which current flows, the first current increases in proportion to an increase in ambient temperature; a second mechanism for causing the second current to flow out, the second current system being independent of ambient temperature; a three current flowing out weighted current adder by combining the first and second currents with first and second weights respectively applied to the first and second currents; and the third mechanism, responsive to the third current And a fourth current is supplied to the light source such that the radiant energy output of the light source remains constant regardless of ambient temperature. The temperature compensation circuit of claim 1, further comprising: a current multiplier coupled between the weighted current adder and the third mechanism, the current multiplier operating to multiply the third current by Amplitude control input. The temperature compensation circuit of claim 2, wherein the first weight, the second weight, and the amplitude control input are stored as digital bits by the non-volatile memory. The temperature compensation circuit of claim 1, wherein the first mechanism comprises: a bipolar transistor, the system is formed in a forward mode, and the bipolar electro-crystal system is connected to the first node and the second node. And providing a first voltage potential at the second node, the first voltage potential being linearly reduced when the ambient temperature is increased; an operational amplifier having first and second input terminals and an output terminal, the An input terminal is coupled to a second voltage potential independent of a first order of ambient temperature, the second input terminal is coupled to the third node; a current mirror, the current mirror is responsive to a fifth current applied thereto to generate the first a field effect transistor having a gate terminal coupled to the output terminal of the operational amplifier, a source terminal coupled to the third node, and a current mirror coupled to the fifth current And a first resistor coupled between the third node and the second node, the resistance value of the first resistor being independent of ambient temperature. The temperature compensation circuit of claim 4, wherein the second voltage potential is divided by the second and third resistors connected in series between the reference voltage and the first node. Provided, wherein the reference voltage is independent of the ambient temperature, wherein the resistance values of the second and third resistors are independent of the ambient temperature. The temperature compensation circuit of claim 5, further comprising: a differential voltage reference circuit for supplying the reference voltage, wherein the reference voltage is also supplied to the second mechanism. The temperature compensation circuit of claim 4, wherein the bipolar transistor is a PNP transistor, the emitter is coupled to the second node, and the base and the collector are coupled to the first node. The temperature compensation circuit of claim 4, wherein the bipolar transistor is an NPN transistor, the emitter is coupled to the first node, and the base and the collector are coupled to the second node. A temperature compensated light source comprising: a light emitting diode (LED) configured to generate a radiant energy output in response to a first current applied to the LED, the LED having an increase in ambient temperature and the first current remaining constant The radiant energy output is reduced in characteristics; the first current source is configured to generate a second current, the second current is linearly increased first-order when the ambient temperature is increased; and the second current source is configured to generate a third current, The third current system is independent of the first order of the ambient temperature; the weighted current adder is configured to respectively add the second and third currents to the first and second currents respectively applied to the second and third currents The fourth current is generated by weighting; and the third current source is responsive to the fourth current and is configured to supply a temperature compensated first current to the LED. The temperature compensated light source of claim 9 is further characterized by: a differential voltage reference circuit configured to supply the same reference voltage to the first and second current sources. The temperature compensated light source of claim 9, wherein the first current source comprises: a bipolar transistor, the system is formed in a forward mode, and the bipolar electro-crystal system is connected to the first node and the Between the two nodes, and configured to provide a first voltage potential at the second node, the first voltage potential is linearly reduced in a first order when the ambient temperature increases, wherein the first node is grounded; a first resistor between the second node and the third node, the resistance value of the first resistor is independent of ambient temperature; an operational amplifier having first and second input terminals and an output terminal The first input terminal is coupled to a second voltage potential that is independent of the first step of the ambient temperature, the second input terminal is coupled to the third node; the field effect transistor has a coupling to the operational amplifier a gate terminal of the output terminal, a source terminal coupled to the third node, and a group forming a 汲 terminal that provides a fifth current; and a current mirror coupled to the field effect transistor The drain terminal, and is set back to be configured to generate the fifth current and the second current. The temperature compensated light source of claim 11, further comprising: a second and a third resistor connected in series between the reference voltage and the first node, wherein the reference voltage is independent of ambient temperature first order The resistance values of the second and third resistors are first-order independent of the ambient temperature, and wherein the fourth node between the second and third resistors is coupled to the operational amplifier The first input terminal is also grouped to provide the second voltage potential. The temperature-compensated light source of claim 11, wherein the bipolar transistor is a PNP transistor, an emitter is coupled to the second node, and a base and a collector are coupled thereto. The first node. The temperature-compensated light source of claim 9 further comprising: a current multiplier coupled between the weighted current adder and the third current source, the current multiplier group forming the fourth The current is multiplied by the first control input. The temperature compensated light source of claim 14, wherein the first weight, the second weight, and the first control input are stored as digital bits by the non-volatile memory. A method of compensating a light emitting diode (LED) to operate below an ambient temperature range, the method comprising: providing a first order linear increase in a first current when the ambient temperature is increased; providing a second independent of ambient temperature a current is applied to the first current to generate a first weighted current; a second weight is applied to the second current to generate a second weighted current; and the first weighted current and the first The second current is generated by the two weighted currents; and the third current is applied to apply the fourth current to the LEDs. The method of claim 16, further comprising: measuring a radiant energy output of the LED under at least two ambient temperatures selected from the ambient temperature range; and calculating the LED under the at least two ambient temperatures The change in radiant energy output; and checking if the change is within a critical value. The method of claim 17, if the change is not within the threshold, further comprising: changing a value of at least one of the first and second weights; and repeating the following action: applying the first Weighting, applying the second weight, generating the third current, applying the fourth current, measuring the radiant energy output of the LED, calculating the change, and checking the change. For the method of claim 17, if the change is within the threshold, the method further comprises: writing the first weight and the second weight into the non-volatile memory. The method of claim 16, wherein applying the fourth current to the LED comprises multiplying the third current by a user input.