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测距 传感器 低成本 欧司朗SFH 7770 external driver 50cm

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SFH 7770 E6 – Driving an External LED Application Note 1 Introduction This application note describes technical details and guidelines to use the SFH 7770 E6 (see Fig. 1) to drive an external LED. The main goal is to either increase the detection distance by applying higher LED drive currents or by separating the LED from the SFH 7770 E6 sensor beyond the recommended maximum spacing. The SFH 7770 E6 contains an integrated ambient light (ALS) and proximity sensor (PS) which is capable of simultaneously operating up to three individual LEDs (see Tab. 1) and provide the combined proximity information within 10 ms. This unique capability makes the SFH 7770 E6 highly suited for i.e. gesture recognition. An additional feature of the SFH 7770 E6 is the logarithmic output, ideally suited to provide an almost linear output signal vs. target distance – especially important if large Pin # 1 2 3 4 5 6 7 8 9 10 Pin label LED 3 LED 2 LED 1 GND_LED INT VDD GND SCL SDA N.C. Description Cathode of LED 3 Cathode of LED 2 Cathode of LED 1 Separate LED ground Interrupt pin Supply voltage VDD Ground I²C bus clock line I²C bus data line Not connected Fig. 1: Ambient Light and Proximity Sensor SFH 7770 E6. detection distances are intended. More general information about the SFH 7770 E6 can be found in the SFH 7770 E6 application note [1], available from OSRAM OS or your local OSRAM partner. 2 Background There can be various reasons to operate the LED via an external driver. One might be the need for larger detection distances, thus operating the LED beyond the sensors driving capability of 200 mA. An other reason might be to position and operate the LED further away from the sensor. Tab. 1: SFH 7770 E6 pinning. Fig. 2: Circuitry of the SFH 7770 E6 September, 2012 Page 1 of 10 LED Package Package Type SFH 4650 SFH 4655 SFH 4451 MIDLED (qualified for use in automotive) MiniMIDLED SFH 4059 SFH 4059S SFH 4555 SFH 4550 ChipLED ChipLED w. stacked emitter Radial Radial Gain in Detection Distance (approx.) x 1.0 x 1.0 x 1.2…1.3 x 1.3...1.6 x 1.8 x 1.8 Half- Typ. Rad. Component Angle / Intensity Height deg / mW/sr / mm ± 15 65 @ 100 mA 1.60 ± 17 60 @ 100 mA 0.90 ± 10 100 @ 70 mA 1.85 ± 15 130 @ 70 mA 1.85 ±5 550 @ 100 mA 7.9 ±3 900 @ 100 mA 9.0 Tab. 2: Table of recommended 850 nm emitter devices. The relative gain in detection distance is a rough approximation and assumes driving the LEDs with identical current. (Note that the achievable gain depends also on target size, reflectivity and distance. The above LEDs also have different radiation characteristics and radiant intensity.) Proximity Signal vs. Target Distance vs. LED Type 220 Target: Kodak White (90%), 100 x 100 mm2 SFH 7770E6 setting: IF = 200 mA, Tint = 1500 μs 200 180 SFH 4650 SFH 4059 160 SFH 4059S SFH 4550 140 SFH 4550 Proximity Signal vs. Target Distance vs. LED Type 220 Target: Kodak Grey (18%), 100 x 100 mm2 SFH 7770E6 setting: IF = 200 mA, Tint = 1500 μs 200 180 SFH 4650 SFH 4059 160 SFH 4059S SFH 4550 140 SFH 4555 PS Signal / counts PS Signal / counts 120 100 typ. threshold level for interrupt 80 SFH 7770 E6 noise level 60 0 50 100 150 200 250 300 Target Distance / mm 120 typ. threshold level 100 for interrupt 80 SFH 7770 E6 noise level 60 0 50 100 150 200 250 300 Target Distance / mm Fig. 3: Target distance vs. proximity sensor signal for a target reflectivity of 18 %. Please note the logarithmic PS count vs. PS signal relationship which leads to an almost linear dependency between PS count and target distance. The maximum detection distance depends – besides target properties - on LED current, PS integration time (Tint) and LED type (in above graph the typ. LED – SFH 7770 E6 spacing is 10 mm). The signal level at small distances depends also on the separator design between the LED and the SFH 7770 to prevent optical crosstalk. Important Note: The SFH 7770 E6 noise level is at around 50-60 counts. Thus ensuring a sufficient signal-to-noise margin OSRAM recommends setting the (interrupt) threshold not below 80 counts. Please refer to the SFH 7770 E6 Application Note for more details [1]. September, 2012 Page 2 of 10 Target Size Target Size TARGET Target Distance IR-LED Optical SFH 7770 E6 Isolation Fig. 4: Typical proximity setup. 2.1 High LED Currents (> 200 mA) As the SFH 7770 E6 driving capability is limited to 200 mA it is recommended to use an external circuit to drive the LED with more than 200 mA. Alternative options to increase the detection distance without external driver (higher LED current) is to use more than one LED in series (see Fig. 2) or LEDs with stacked emitter. Depending on the desired target size and distance the use of narrow angle / high radiant intensity LEDs might also help to improve the situation. These LEDs include but are not limited to - SFH 4059S (stacked LED) and SFH 4555 (radial), see Tab. 2. Depending on the setup, i.e. the angle α (refer to Fig. 4 for definition), the SFH 4555 provides the best solution, whereas SFH 4550 only provides some additional gain in signal level (= PS counts) if α < 7° (e.g. target size 100 x 100 mm2 at 50 cm distance, realized with external drivers). The SFH 4059S contains a stacked emitter chip – which increases the intensity but also doubles the forward voltage. Tab. 2 and Fig. 3 present estimations on the gain in detection distance one can achieve using different LED types. Please note that this is only a rough guideline. To achieve maximum performance it is mandatory to avoid any optical crosstalk between emitter and the SFH 7770 sensor (please refer to the SFH 7770 Application Note [1] for details on this topic). OSRAM recommends evaluating the impact of different LEDs as target size and target distance strongly impact the detection distance. A further option is to increase the default PS integration time (Tint = 750 μs) of the SFH 7770 E6 to e.g. 1500 μs or 2500 μs. Increasing the integration time increases the signal level resp. PS counts (e.g. increasing Tint from 750 μs to 1000 μs, 1500 μs or 2500 us increases the absolute PS count by around 7, 17 or 30 counts). The integration time is accessible via register 0x20 and can be set in register 0x27 (see SFH 7770 E6 Datasheet or Application Note [1]). 2.2 Wide Emitter – Detector Spacing The basic operation principle of the SFH 7770 E6 are 667 kHz burst pulses with up to 200 mA (user configurable). This requires fast electrical rise and fall times (tens of ns) of the pulse current (dI/dt). To achieve the required dI/dt rate and avoid damage of the SFH 7770 internal circuitry a maximum load inductance is allowed. This limit is at around 40 nH for the 200 mA LED drive current and 400 nH for the 20 mA setting. To overcome the e.g. 40 nH limitation at 200 mA sink currents into the SFH 7770 (typ. up to around five to twenty centimetres of pcb board or twisted pair) one can trigger a far away drive circuit with i.e. 20 mA to source the LED with high LED burst currents. This is recommended in e.g. tablet devices where LEDs are located at far away positions from the SFH 7770 E6. September, 2012 Page 3 of 10 Fig. 5: The LED pins of the SFH 7770 E6 sink different currents over time, e.g. during the LED burst off-state the current is 0.5 mA. 3 The SFH 7770 LED Driver The general dynamic requirements for a driving circuit for the SFH 7770 are: This section presents some background necessary to develop an external LED driver with maximum efficiency. “Off-the-shelf” solutions recommended by OSRAM can be found in Sec. 4. Fig. 5 presents the LED drive current of the SFH 7770 E6 during operation. During the 667 kHz bursts the LED-off current is around 0.5 mA. This must be considered when designing any drive circuit with simple thresholds. It is therefore recommended that a decisive switching threshold current ITH is set at I TH = (I _ LED + 0.5 mA) 2 Eq. (1) • symmetric pulse shape • low propagation delay (tpd) • fast rise / fall times (tr, tf) The propagation delay / distortion between the SFH 7770 E6 electrical output pulses and the optical LED pulses should be minimized to be ideally below 50 ns (practically up to 70 ns still works with negligible impact). It is also recommended to keep the LED-on duty cycle (DC) between 45 % < DC ≤ 50 %. Fig. 6 illustrates these requirements. Desirable rise resp. fall times are in the 50 ns region. Additionally the anatomy of the integrated Fig. 6: Recommended ideal relationship between external driven LED (optical pulse) and the electrical drive signal of the SFH 7770 E6. The system works also at less than ideal circumstances with negligible impact on the overall performance as long as the pulse shape is symmetrical and the delay time < 70 ns. September, 2012 Page 4 of 10 Fig. 7: Drive circuit with two NMOS stages: Optimized to drive an external LED via the SFH 7770 with I_LED = 20 mA setting. For alternative NMOS components please refer to the end note of Sec. 4. LED driver of the SFH 7770 E6 is susceptible to the type of load. It is designed to drive LEDs and causes unacceptable pulse distortions if it is used to drive high ohmic loads (kΩ range) depending on the selected I_LED current. This symmetry degeneration leads to functional failures of the proximity sensor (lower detection distance, increased noise level). Thus testing any desired external circuitry is mandatory in any case. Considering that the signal bursts are at 667 kHz requires a high speed circuit to achieve symmetrical pulses as signal deviation from the above description might lead to lower detection distance. 4 External LED Driver Solutions 4.1 Dual NMOS Driver This section presents practical solutions driving an external LED with maximum efficiency. The proposed circuits are just examples with the goal to be a low cost solution. There are other options, e.g. using high speed comparators or operational amplifiers by setting a threshold according to Eq. (1), followed e.g. by a switching FET to drive high LED currents. The schematic in Fig. 7 allows working with two independent voltage supplies: One, V_Logic, for the SFH 7770 LED output (which could be the same as the SFH 7770 supply voltage) and V_Drive for driving the LED. The combination of the NMOS-FET and R1 (330 Ω) in the circuit works best if I_LED of the SFH 7770 E6 is set to 20 mA, which grounds the gate of the first FET and therefore makes the FET non-conductive. Other I_LED settings might result in pulse asymmetries which deteriorate the system performance. The above circuitry is designed to match I_LED, R1 and VGS(th) of the NMOS. Variations on either one of these settings need to be experimentally verified as they impact the pulse shape. R2 (470 Ω) is selected to guarantee low turn-on delay of the second FET and determines the off-state (standby) current (as during the off-state the FET 1 is in conducting mode). The R2 value strongly depends on the gate-source charge QGS necessary to turn the FET 2 on and can be selected according to: R2 << 50 ns ⋅V _ Logic QGS Eq. (2) September, 2012 Page 5 of 10 Fig. 8: Drive circuit with a single PMOS stage: Optimized to drive an external LED via the SFH 7770 with I_LED = 20 mA setting. For alternative PMOS components please refer to the end note of Sec. 4. The LED current itself is controlled via V_Drive, the resistor R3 and the onresistance of the NMOS FET 2. To ensure low serial inductances in the high current LED path R3 could be realized by using several parallel resistors. This circuit works fine for the operating range of the SFH 7770 E6 (V_Logic = 2.3 V up to 4.25 V). 4.1 Single PMOS Driver The circuit in Fig. 8 presents a solution which works with a single PMOS device, but is limited to a common supply voltage V_LED for the drive rail and the SFH 7770 output stage. Compared to the previous circuit (featuring two NMOS devices) this design has no standby off-current. It features a high-speed PMOS-FET as a logic switch with a threshold level set between the 20 mA LEDon current and the 0.5 mA LED-off current. R1 adjusts the threshold according to VGS(th) of the PMOS-FET. For the circuit according to Fig. 8 and a VGS < -2.0 V the FET should be sufficiently conductive to support the desired LED current (R1 = 100 Ω ensures the symmetry of the pulses with the SFH 7770 E6 setting of I_LED = 20 mA). In general, R1 and I_LED together with VGS(th) of the FET need to be matched to ensure the correct pulse shapes according to the criteria stated in Fig. 6. The LED peak pulse current is simply set via V_LED (limited by the SFH 7770 E6 between 2.3 V and 4.25 V), R2 and the onresistance of the FET. These values need to be adjusted to match the VF-IF characteristics of the LED. To drive high currents once again a low inductive LED current path is recommended (i.e. using several R2 in a parallel configuration to reduce the inductance). Note: The stated NMOS / PMOS devices are just an example, representing low VGS(th) and fast switching times (tr, tf and td(on), td(off)) where the circuit works. Considering Fig. 6, a selection criterion for the FET will be: td( on ) + tr ≤ 50 ns Eq. (3) td( off ) + t f ≤ 50 ns Eq. (4) As another “rule-of-thumb” criteria for the maximum gate charge possible to bring VGS above / below VGS(th) within 50 ns should be: QGS < 1 nC Eq. (5) QGD < 1 nC Eq. (6) Viable PMOS options include e.g. NTA4151P (e.g. OnSemi), FDN336P, FDY102PZ (e.g. Fairchild). Suitable dual package NMOS options are e.g. PMGD400 September, 2012 Page 6 of 10 PS Signal vs. Target Distance 220 Target: 90% White, 100 x 100 mm2 LED: SFH 4650, Tint = 1500 μs 200 200 mA 180 500 mA 1000 mA 160 PS Signal vs. Target Distance 220 Target: 18% Grey, 100 x 100 mm2 LED: SFH 4650, Tint = 1500 μs 200 180 200 mA 500 mA 160 1000 mA PS Signal / counts PS Signal / counts 140 140 120 120 100 100 80 80 60 0 100 200 300 400 500 Target Distance / mm 60 0 100 200 300 400 500 Target Distance / mm Fig. 9: Operating the SFH 7770 E6 with SFH 4650 vs. different IF currents for target reflectivity of 90 % resp. 18 % (realized with a setup according to Fig. 8). PS Signal vs. Target Distance 220 Target: 90% White, 100 x 100 mm2 LED: SFH 4059, Tint = 1500 μs 200 200 mA 180 500 mA 160 PS Signal vs. Target Distance 220 Target: 18 % Grey, 100 x 100 mm2 LED: SFH 4059, Tint = 1500 μs 200 180 200 mA 500 mA 160 PS Signal / counts PS Signal / counts 140 140 120 120 100 100 80 80 60 0 100 200 300 400 500 Target Distance / mm 60 0 100 200 300 400 500 Target Distance / mm Fig. 10: Operating the SFH 7770 E6 with SFH 4059 vs. different IF currents for target reflectivity of 90 % resp. 18 % (realized with a setup according to Fig. 8). PS Signal vs. Target Distance 220 Target: 90% White, 100 x 100 mm2 LED: SFH 4059S, Tint = 1500 μs 200 200 mA 180 500 mA 1000 mA 160 140 PS Signal vs. Target Distance 220 Target: 18 % Grey, 100 x 100 mm2 LED: SFH 4059S, Tint = 1500 μs 200 180 200 mA 500 mA 160 1000 mA 140 PS Signal / counts PS Signal / counts 120 120 100 100 80 80 60 0 100 200 300 400 500 Target Distance / mm 60 0 100 200 300 400 500 Target Distance / mm Fig. 11: Operating the SFH 7770 E6 with SFH 4059S vs. different IF currents for target reflectivity of 90 % resp. 18 % (realized with a setup according to Fig. 8). (e.g. NXP), FDG1024NZ (e.g. Fairchild), BSD840N (e.g.Infineon). To ensure circuit stability OSRAM recommends using capacitors to buffer V_Drive, V_Logic and V_LED. For simplicity these components are not included in above schematics. September, 2012 Page 7 of 10 PS Signal vs. Target Distance 220 Target: 90% White, 100 x 100 mm2 LED: SFH 4555, Tint = 1500 μs 200 200 mA 180 500 mA 1000 mA 160 140 PS Signal vs. Target Distance 220 Target: 18 % Grey, 100 x 100 mm2 LED: SFH 4555, Tint = 1500 μs 200 180 200 mA 500 mA 160 1000 mA 140 PS Signal / counts PS Signal / counts 120 120 100 100 80 80 60 0 100 200 300 400 500 Target Distance / mm 60 0 100 200 300 400 500 Target Distance / mm Fig. 12: Operating the SFH 7770 E6 with SFH 4555 vs. different IF currents for target reflectivity of 90 % resp. 18 % (realized with a setup according to Fig. 8). PS Signal vs. Target Distance 220 Target: 90% White, 100 x 100 mm2 LED: SFH 4550, Tint = 1500 μs 200 200 mA 180 500 mA 1000 mA 160 140 PS Signal vs. Target Distance 220 Target: 18 % Grey, 100 x 100 mm2 LED: SFH 4550, Tint = 1500 μs 200 180 200 mA 500 mA 160 1000 mA 140 PS Signal / counts PS Signal / counts 120 120 100 100 80 80 60 0 100 200 300 400 500 Target Distance / mm 60 0 100 200 300 400 500 Target Distance / mm Fig. 13: Operating the SFH 7770 E6 with SFH 4550 vs. different IF currents for target reflectivity of 90 % resp. 18 % (realized with a setup according to Fig. 8). The maximum allowed LED pulse current is limited and depends – among other issues – on the burst length and PS measurement / repetition rate. Please verify the operating conditions with the pulse derating diagram of the employed LED. 5 Software Options There are several options to increase the sensitivity limit (resp. reduce noise floor) of the SFH 7770 E6 by simple software algorithms. This sporadic noise of the SFH 7770 E6 allows applying simple - but effective - filtering strategies. Below two proven options are discussed: 5.1 Interrupt Persistence This process can be implemented if the interrupt functionality of the SFH 7770 E6 is used. Here the microcontroller / digital signal processor applies a persistence filter onto the interrupt signal, e.g. a valid interrupt only occurs if within m time slots at least n-times an interrupt event occurs. Typ. values here are e.g. 4 out of 4 (m, n = 4). In essence it allows the user to set the PS threshold limit below 80 counts resulting in a more robust interrupt alert and increased detection range. Please note to set the interrupt mode in register (0x92) accordingly (latched vs. nonlatched mode). September, 2012 Page 8 of 10 PS Signal / counts Proximity Signal vs. Target Distance Target: 18% Grey, 100 x 100 mm^2 200 Reduction of PS noise due to averaging 180 LED: SFH 4555, IF = 200 mA, Tint = 1500 ms 160 no averaging 140 averaging (5 measurements) 120 averaging (10 measurements) 100 80 60 40 20 0 0 100 200 300 400 500 Target Distance / mm Fig. 14: Impact of averaging on the noise floor. The averaging of e.g. 10 values allows a reduction of the possible threshold from e.g. 90 counts to e.g. below 60 counts. This increases the potential detection distance by a factor of two. 5.1 Data Averaging To further increase the detection distance (sensitivity) of the SFH 7770 E6 the following strategy to reduce the noise floor up to a factor of 10 (roughly 50 counts) is recommended. This requires the continuous readout of the proximity data into a microcontroller resp. digital signal processor and do some simple (continuous) averaging algorithm. The simplest one could be: ∑ PSavg ( n ) = 1 k n PSdata ( i i=n−k −1 ) Eq. (7) The number of samples size (k ≥ 1) could be chosen to be e.g. k = 5 or 10 (see Fig. 13). This implies some delay in online PS monitoring, thus requires the PS repetition rate to be set accordingly if timing is critical (alternative: use triggered mode which provides the measured data immediately after the integration time is finished). Please also note that the first k data sets are only of limited value. The above equation is applied whenever the PS signal count is below the typ. SFH 7770 E6 noise limit of around 60 counts. The software noise averaging can roughly add an additional gain of 50 % in detection distance on top of the above mentioned measures (e.g. using external driver to power the LED beyond the 200 mA). The graph in Fig. 14 shows this principle applied on the combination of SFH 7770 E6 and SFH 4555 powered with ‘only’ 200 mA. Note: Although the readout of the SFH 7770 E6 is in logarithmic counts vs. the detectors irradiation it is the easiest and recommended option to simply average the PS signal counts. 6 Summary With the above discussed measures the potential maximum detection distance of the SFH 7770 E6 can be extended beyond its datasheet listed values. These techniques include the proper selection of the right IRLED, arranging several of them in a serial connection, an external circuit for higher LED burst current and software based data averaging. The combination of all of the above allows total target detection distances of 50 cm and more. 7 Literature [1] Application notes (e.g. general application note of the SFH 7770E6) and the data sheet can be downloaded from http://www.osram-os.com/osram_os/EN September, 2012 Page 9 of 10 Appendix Don't forget: LED Light for you is your place to be whenever you are looking for information or worldwide partners for your LED Lighting project. www.ledlightforyou.com Author: Dr. Hubert Halbritter ABOUT OSRAM OPTO SEMICONDUCTORS OSRAM AG (Munich, Germany) is a wholly-owned subsidiary of Siemens AG and one of the two leading light manufacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications. OSRAM Opto Semiconductors has production sites in Regensburg (Germany) and Penang (Malaysia). Its headquarters for North America is in Sunnyvale (USA), and for Asia in Hong Kong. OSRAM Opto Semiconductors also has sales offices throughout the world. For more information go to www.osram-os.com. All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents. September, 2012 page 10 of 10

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