Linear Optocoupler, High Gain Stability, Wide Bandwidth

Transcription

1 Linear Optocoupler, High Gain Stability, Wide Bandwidth i9 DESCRIPTION The linear optocoupler consists of an AlGaAs IRLED irradiating an isolated feedback and an output PIN photodiode in a bifurcated arrangement. The feedback photodiode captures a percentage of the LEDs flux and generates a control signal (I P ) that can be used to servo the LED drive current. This technique compensates for the LED s non-linear, time, and temperature characteristics. The output PIN photodiode produces an output signal (I P ) that is linearly related to the servo optical flux created by the LED. The time and temperature stability of the input-output coupler gain (K3) is insured by using matched PIN photodiodes that accurately track the output flux of the LED. C A C A i9_ 3 K K D E NC NC C A FEATURES Couples AC and DC signals. % servo linearity Wide bandwidth, > khz High gain stability, ±. %/ C typically Low input-output capacitance Low power consumption, < mw Isolation rated voltage RMS Internal insulation distance, >. mm Material categorization: for definitions of compliance please see APPLICATIONS Power supply feedback voltage / current Medical sensor isolation Audio signal interfacing Isolated process control transducers Digital telephone isolation AGENCY APPROALS UL file no. E cul tested to CSA. bulletin A DIN EN -- (DE -) available with option BSI FIMKO ORDERING INFORMATION I L 3 - D E F G - X # # T DIP- Option PART NUMBER K3 BIN PACKAGE OPTION TAPE AND REEL. mm Option. mm Option 9 AGENCY CERTIFIED/ PACKAGE UL, cul, BSI, FIMKO Note () Also available in tubes, do not put T on the end K3 BIN. to.. to.. to.. to.9. to..9 to.. to.9.9 to. DIP- -DEFG - - -EF - -E -F DIP-, mil, option -X -DEFG-X - - -EF-X -FG-X -E-X -F-X SMD-, option -XT () -DEFG-XT () -EFG-X -DE-XT -EF-XT () - -E-XT -F-X SMD-, option 9 -X9T () -DEFG-X9T () - - -EF-X9T () - - -F-X9T () DE, UL, BSI, FIMKO. to.. to.. to.. to.9. to..9 to.. to.9.9 to. DIP- -X -DEFG-X - - -EF-X - -E-X -F-X DIP-, mil, option -X -DEFG-X - - -EF-X - - -F-X SMD-, option -X -DEFG-XT () - - -EF-XT () - -E-XT -F-XT () SMD-, option F-X9T () >. mm >. mm Rev..9, 3-May- Document Number: 3

2 OPERATION DESCRIPTION A typical application circuit (Fig. ) uses an operational amplifier at the circuit input to drive the LED. The feedback photodiode sources current to R connected to the inverting input of U. The photocurrent, I P, will be of a magnitude to satisfy the relationship of (I P = IN /R). The magnitude of this current is directly proportional to the feedback transfer gain (K) times the LED drive current ( IN /R = K x I F ). The op-amp will supply LED current to force sufficient photocurrent to keep the node voltage (b) equal to a. The output photodiode is connected to a non-inverting voltage follower amplifier. The photodiode load resistor, R, performs the current to voltage conversion. The output amplifier voltage is the product of the output forward gain (K) times the LED current and photodiode load, R ( O = I F x K x R). Therefore, the overall transfer gain ( O / IN ) becomes the ratio of the product of the output forward gain (K) times the photodiode load resistor (R) to the product of the feedback transfer gain (K) times the input resistor (R). This reduces to O / IN = (K x R)/(K x R). The overall transfer gain is completely independent of the LED forward current. The transfer gain (K3) is expressed as the ratio of the output gain (K) to the feedback gain (K). This shows that the circuit gain becomes the product of the transfer gain times the ratio of the output to input resistors O / IN = K3 (R/R). K-SERO GAIN The ratio of the input photodiode current (I P ) to the LED current (I F ) i.e., K = I P /I F. K-FORWARD GAIN The ratio of the output photodiode current (I P ) to the LED current (I F ), i.e., K = I P /I F. K3-TRANSFER GAIN The transfer gain is the ratio of the forward gain to the servo gain, i.e., K3 = K/K. K3-TRANSFER FAIN LINEARITY The percent deviation of the transfer gain, as a function of LED or temperature from a specific transfer gain at a fixed LED current and temperature. PHOTODIODE A silicon diode operating as a current source. The output current is proportional to the incident optical flux supplied by the LED emitter. The diode is operated in the photovoltaic or photoconductive mode. In the photovoltaic mode the diode functions as a current source in parallel with a forward biased silicon diode. The magnitude of the output current and voltage is dependent upon the load resistor and the incident LED optical flux. When operated in the photoconductive mode the diode is connected to a bias supply which reverse biases the silicon diode. The magnitude of the output current is directly proportional to the LED incident optical flux. LED (LIGHT EMITTING DIODE) An infrared emitter constructed of AlGaAs that emits at 9 nm operates efficiently with drive current from µa to ma. Best linearity can be obtained at drive currents between ma to ma. Its output flux typically changes by -. %/ C over the above operational current range. APPLICATION CIRCUIT + R a + CC in U b - I F CC lp 3 K K Fig. - Typical Application Circuit lp CC C R - + CC U out Rev..9, 3-May- Document Number: 3

3 ABSOLUTE MAXIMUM RATINGS (T amb = C, unless otherwise specified) PARAMETER TEST CONDITION SYMBOL ALUE UNIT INPUT Power dissipation P diss mw Derate linearly from C.3 mw/ C Forward current I F ma Surge current (pulse width < µs) I PK ma Reverse voltage R Thermal resistance R th K/W Junction temperature T j C OUTPUT Power dissipation P diss mw Derate linearly from C. mw/ C Reverse voltage R Thermal resistance R th K/W Junction temperature T j C COUPLER Total package dissipation at C P tot mw Derate linearly from C. mw/ C Storage temperature T stg - to + C Operating temperature T amb - to + C Note Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. Functional operation of the device is not implied at these or any other conditions in excess of those given in the operational sections of this document. Exposure to absolute maximum ratings for extended periods of the time can adversely affect reliability ELECTRICAL CHARACTERISTICS (T amb = C, unless otherwise specified) PARAMETER TEST CONDITION SYMBOL MIN. TYP. MAX. UNIT INPUT (LED EMITTER) Forward voltage I F = ma F -.. F temperature coefficient F / C m/ C Reverse current R = I R - - µa Junction capacitance F =, f = MHz C j - - pf Dynamic resistance I F = ma F / I F - - Ω OUTPUT Dark current det = -, I F = A I D - na Open circuit voltage I F = ma D - - m Short circuit current I F = ma I SC - - µa Junction capacitance F =, f = MHz C j - - pf Noise equivalent power det = NEP - x - - W/ Hz COUPLER Input-output capacitance F =, f = MHz - - pf K, servo gain (I P /I F ) I F = ma, det = - K... Servo current ()() I F = ma, det = - I P - - µa K, forward gain (I P /I F ) I F = ma, det = - K... Forward current I F = ma, det = - I P - - µa K3, transfer gain (K/K) ()() I F = ma, det = - K3.. K/K Transfer gain stability I F = ma, det = - K3/ T A - ±. ±. %/ C Transfer gain linearity I F = ma to ma K3 - ±. - % I F = ma to ma, T amb = C to C - ±. - % Rev..9, 3-May- 3 Document Number: 3

4 ELECTRICAL CHARACTERISTICS (T amb = C, unless otherwise specified) PARAMETER TEST CONDITION SYMBOL MIN. TYP. MAX. UNIT PHOTOCONDUCTIE OPERATION Frequency response I Fq = ma, MOD = ± ma, R L = Ω BW (-3 db) - - khz Phase response at khz det = Deg. Notes Minimum and maximum values were tested requirements. Typical values are characteristics of the device and are the result of engineering evaluation. Typical values are for information only and are not part of the testing requirements () Bin sorting: K3 (transfer gain) is sorted into bins that are ± %, as follows: Bin A =. to. Bin B =. to.9 Bin C =.9 to.3 Bin D =. to.9 Bin E =. to.9 Bin F =.9 to. Bin G =. to. Bin H =.9 to.3 Bin I =.9 to. Bin J =. to. K3 = K/K. K3 is tested at I F = ma, det = - () Bin categories: All s are sorted into a K3 bin, indicated by an alpha character that is marked on the part. The bins range from A through J. The is shipped in tubes of each. Each tube contains only one category of K3. The category of the parts in the tube is marked on the tube label as well as on each individual part (3) Category options: standard orders will be shipped from the categories that are available at the time of the order. Any of the ten categories may be shipped. For customers requiring a narrower selection of bins, the bins can be grouped together as follows: -DEFG: order this part number to receive categories D, E, F, G only -EF: order this part number to receive categories E, F only -E: order this part number to receive category E only SWITCHING CHARACTERISTICS PARAMETER TEST CONDITION SYMBOL MIN. TYP. MAX. UNIT Switching time I F = ma, I Fq = ma t r - - µs t f - - µs Rise time t r -. - µs Fall time t f -. - µs COMMON MODE TRANSIENT IMMUNITY PARAMETER TEST CONDITION SYMBOL MIN. TYP. MAX. UNIT Common mode capacitance F =, f = MHz C CM -. - pf Common mode rejection ratio f = Hz, R L =. kω CMRR db SAFETY AND INSULATION RATINGS PARAMETER TEST CONDITION SYMBOL ALUE UNIT Climatic classification According to IEC part / / Comparative tracking index CTI Maximum rated withstanding isolation voltage t = min ISO RMS Maximum transient isolation voltage IOTM peak Maximum repetitive peak isolation voltage IORM 9 peak IO =, T amb = C R IO Ω Isolation resistance IO =, T amb = C R IO Ω Output safety power P SO mw Input safety current I SI ma Safety temperature T S C Creepage distance mm Clearance distance mm Insulation thickness DTI. mm Note As per IEC --,..3.., this optocoupler is suitable for safe electrical insulation only within the safety ratings. Compliance with the safety ratings shall be ensured by means of protective circuits Rev..9, 3-May- Document Number: 3

5 TYPICAL CHARACTERISTICS (T amb = C, unless otherwise specified) I F - LED Current (ma) 3 3. iil3_...3 F - LED Forward oltage (). Amplitude Response (db) I Fq = ma Mod = ±. ma T A = C R L = Ω db Phase iil3_3 F - Frequency (Hz) Ø - Phase Response ( ) Fig. - LED Forward Current vs. Forward oltage Fig. - Amplitude and Phase Response vs. Frequency Normalized Photocurrent Normalized to: I P at I F = ma C C C C T A = C D = -. iil3_ I F - LED Current (ma) CMRR - Rejection Ratio (db) iil3_ F - Frequency (Hz) Fig. 3 - Normalized Servo Photocurrent vs. LED Current and Temperature Fig. - Common-Mode Rejection K3 - Transfer Gain - (K/K) C C C C Normalized to: I F = ma T A = C Capacitance (pf).99 iil3_ I F - LED Current (ma) iil3_ oltage ( det ) Fig. - Normalized Transfer Gain vs. LED Current and Temperature Fig. - Photodiode Junction Capacitance vs. Reverse oltage Rev..9, 3-May- Document Number: 3

6 APPLICATION CONSIDERATIONS In applications such as monitoring the output voltage from a line powered switch mode power supply, measuring bioelectric signals, interfacing to industrial transducers, or making floating current measurements, a galvanically isolated, DC coupled interface is often essential. The can be used to construct an amplifier that will meet these needs. The eliminates the problems of gain nonlinearity and drift induced by time and temperature, by monitoring LED output flux. A pin photodiode on the input side is optically coupled to the LED and produces a current directly proportional to flux falling on it. This photocurrent, when coupled to an amplifier, provides the servo signal that controls the LED drive current. The LED flux is also coupled to an output PIN photodiode. The output photodiode current can be directly or amplified to satisfy the needs of succeeding circuits. ISOLATED FEEDBACK AMPLIFIER The was designed to be the central element of DC coupled isolation amplifiers. Designing the into an amplifier that provides a feedback control signal for a line powered switch mode power is quite simple, as the following example will illustrate. See Fig. for the basic structure of the switch mode supply using the Infineon TDA9 push-pull switched power supply control cchip. Line isolation are provided by the high frequency transformer. The voltage monitor isolation will be provided by the. The isolated amplifier provides the PWM control signal which is derived from the output supply voltage. Fig. 3 more closely shows the basic function of the amplifier. The control amplifier consists of a voltage divider and a non-inverting unity gain stage. The TDA9 data sheet indicates that an input to the control amplifier is a high quality operational amplifier that typically requires a + 3 signal. Given this information, the amplifier circuit topology shown in Fig. is selected. The power supply voltage is scaled by R and R so that there is + 3 at the non-inverting input ( a ) of U. This voltage is offset by the voltage developed by photocurrent flowing through. This photocurrent is developed by the optical flux created by current flowing through the LED. Thus as the scaled monitor voltage ( a ) varies it will cause a change in the LED current necessary to satisfy the differential voltage needed across at the inverting input. The first step in the design procedure is to select the value of given the LED quiescent current (I Fq ) and the servo gain (K). For this design, I Fq = ma. Fig. shows the servo photocurrent at I Fq is found to be ma. With this data can be calculated. b 3 = = = 3 kω I PI µa To control input iil3_ ISO AMP Fig. - Isolated Control Amplifier oltage monitor For best input offset compensation at U, R will equal. The value of R can easily be calculated from the following. The value of R depends upon the Transfer Gain (K3). K3 is targeted to be a unit gain device, however to minimize the part to part Transfer Gain variation, Infineon offers K3 graded into ± % bins. R can determined using the following equation, OUT x R R R = x ( + ) MONITOR R x K3 or if a unity gain amplifier is being designed ( MONITOR = OUT, R = ), the equation simplifies to: R R R R x MONITOR = a R = K3 Rev..9, 3-May- Document Number: 3

7 / main AC/DC rectifier Switch Xformer AC/DC rectifier DC output Switch mode regulator TDA9 Control Isolated feedback iil3_ Fig. 9 - Switching Mode Power Supply monitor R kω R 3 kω 3 kω a b 3 + U LM - R CC Ω CC 3 pf K K CC R 3 kω out To control input iil3_ Fig. - DC Coupled Power Supply Feedback Amplifier Rev..9, 3-May- Document Number: 3

8 Table. Gives the value of R given the production K3 bin. TABLE - R SELECTION BIN K3 R RESISTOR MIN. MAX. TYP. % kω A B C D E F.9. 3 G... H..3. I J The last step in the design is selecting the LED current limiting resistor (R). The output of the operational amplifier is targeted to be % of the CC, or.. With an LED quiescent current of ma the typical LED ( F ) is.3. Given this and the operational output voltage, R can be calculated. opamp - F. -.3 R = = = Ω I Fq ma The circuit was constructed with an LM differential operational amplifier using the resistors selected. The amplifier was compensated with a pf capacitor connected between pins and. The DC transfer characteristics are shown in Fig.. The amplifier was designed to have a gain of. and was measured to be.3. Greater accuracy can be achieved by adding a balancing circuit, and potentiometer in the input divider, or at R. The circuit shows exceptionally good gain linearity with an RMS error of only.33 % over the input voltage range of to in a servo mode; see Fig.. out - Output oltage () Linearity Error (%) iil3_ out =. m +.3 x in LM T a = C.. Fig. - Transfer Gain LM iil3_... in - Input oltage (). Fig. - Linearity Error vs. Input oltage The AC characteristics are also quite impressive offering a -3 db bandwidth of khz, with a - phase shift at khz as shown in Fig.. Rev..9, 3-May- Document Number: 3

9 Amplitude Response (db) db Phase Phase Response ( ) The same procedure can be used to design isolation amplifiers that accept bipolar signals referenced to ground. These amplifiers circuit configurations are shown in Fig.. In order for the amplifier to respond to a signal that swings above and below ground, the LED must be pre biased from a separate source by using a voltage reference source ( ref ). In these designs, can be determined by the following equation. ref = = I P ref KI Fq iil3_ F - Frequency (Hz) Fig. 3 - Amplitude and Phase Power Supply Control Non-inverting input Non-inverting output in R 3 + cc R - cc pf - ref - cc Ω +cc 3 IL 3 cc + ref R R cc 3 + R o - cc Inverting input Inverting output in R iil3_ 3 + cc R cc + ref Ω + cc pf 3 - cc IL 3 + ref 3 + cc cc - cc R out Fig. - Non-inverting and Inverting Amplifiers TABLE - OPTOLINEAR AMPLIEFIERS AMPLIFIER INPUT OUTPUT GAIN OFFSET Non-inverting Inverting Inverting Non-inverting Non-inverting OUT IN OUT IN K3 x R x R = ref x R x K3 x ( R x R) ref = K3 x R x R x ( R + R) - = ref x R x ( R + R) x K3 x R x ( R x R) ref = x R Inverting Inverting Non-inverting Non-inverting Inverting OUT IN - K3 x R x R x ( R + R) = ref x R x ( R + R) x K3 x ( R x R) ref = x R OUT IN - K3 x R x R - = ref x R x K3 x ( R x R) ref = Rev..9, 3-May- 9 Document Number: 3

10 These amplifiers provide either an inverting or non-inverting transfer gain based upon the type of input and output amplifier. Table shows the various configurations along with the specific transfer gain equations. The offset column refers to the calculation of the output offset or ref necessary to provide a zero voltage output for a zero voltage input. The non-inverting input amplifier requires the use of a bipolar supply, while the inverting input stage can be implemented with single supply operational amplifiers that permit operation close to ground. For best results, place a buffer transistor between the LED and output of the operational amplifier when a CMOS opamp is used or the LED I Fq drive is targeted to operate beyond ma. Finally the bandwidth is influenced by the magnitude of the closed loop gain of the input and output amplifiers. Best bandwidths result when the amplifier gain is designed for unity. PACKAGE DIMENSIONS (in millimeters) Pin one ID ref.. typ.. ref.. ref. 3 9 ISO method A i Option Option Option typ ref min.. min..3 max... min.. typ. max. PACKAGE MARKING (example of -E-X) -E X YWW H Rev..9, 3-May- Document Number: 3