LINEAR OPTOCOUPLER FEATURES Couples AC and DC signals.% Servo Linearity Wide Bandwidth, > KHz High Gain Stability, ±.%/C Low Input-Output Capacitance Low Power Consumption, < mw Isolation Test Voltage, VAC RMS, sec. Internal Insulation Distance, >. mm for VDE Underwriters Lab File #E VDE Approval # (Optional with Option, Add -X Suffix) G Replaced by -X APPLICATIONS Power Supply Feedback Voltage/ Current Medical Sensor Isolation Audio Signal Interfacing Isolate Process Control Transducers Digital Telephone Isolation 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 LED's flux and generates a control signal (IP ) 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 (IP ) that is linearly related to the servo optical flux created by the LED. The time and temperature stability of the input-output coupler gain (K) is insured by using matched PIN photodiodes that accurately track the output flux of the LED. A typical application circuit (Figure ) 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, IP, will be of a magnitude to satisfy the relationship of (IP=V IN /R). Dimensions in inches (mm) DESCRIPTION (continued) The magnitude of this current is directly proportional to the feedback transfer gain (K) times the LED drive current (V IN /R=K I F ). The op-amp will supply LED current to force sufficient photocurrent to keep the node voltage (Vb) equal to Va 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 (V O =I F K R). Therefore, the overall transfer gain (V O /V 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 V O /V IN = (K R)/(K R). The overall transfer gain is completely independent of the LED forward current. The transfer gain (K) is expressed as the ratio of the ouput 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 [V O / V IN =K (R/R)]. Figure. Typical application circuit + Vin. (.). (.) R Typ..9 (9.9).9 (9.). (.). (.) Pin One I.D.. (.). (.) K Typ.. (.). (.) 9. (. ). (.). (.). (.) Typ.. (.) Va + Vb U - I F lp K K lp K. Typ. (.) Typ. V c R - U +. (.). (.9) V out
Terms KI Servo 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. K Transfer Gain The Transfer Gain is the ratio of the Forward Gain to the Servo gain, i.e., K = K/K. K Transfer Gain 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 dependant 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. Absolute Maximum Ratings Emitter Power Dissipation (T A = C) Symbol Min. Max. Unit P LED mw Derate Linearly from C. mw/ C Forward Current lf ma Surge Current (Pulse width <µs) lpk ma Reverse Voltage V R V Thermal Resistance Rth C/W Junction Temperature T J C Detector Power Dissipation P DET ma Derate linearly from C. mw/ C Reverse Voltage V R V Junction Temperature T J C Thermal Resistance Rth C/W Coupler Total Package Dissipation at C P T mw Derate linearly from C. mw/ C Storage Temperature T S C Operating Temperature T OP C Isolation Test Voltage VAC RMS Isolation Resistance V IO = V, T A = C V IO = V, T A = C Ω Ω
Characteristics (T A = C) Symbol Min. Typ. Max. Unit Test Condition LED Emitter Forward Voltage V F.. V I F = ma V F Temperature Coefficient V F / C -. mv/ C Reverse Current I R µa V R = V Junction Capacitance C J pf V F = V, f= MHz Dynamic Resistance V F / I F Ω I F = ma Switching Time t R tf µs µs I F = ma, I Fq = ma I F = ma, I Fq = ma Detector Dark Current I D na V det =- V, I F = µa Open Circuit Voltage V D mv I F = ma Short Circuit Current I SC µa I F = ma Junction Capacitance C J pf V F = V, f= MHz Noise Equivalent Power NEP x W/ Hz V det = V Coupled Characteristics K, Servo Gain (I P /I F ) K... I F = ma, V det =- V Servo Current, see Note, I P µa I F = ma, V det =- V K, Forward Gain (I P /I F ) K... I F = ma, V det =- V Forward Current I P µa I F = ma, V det =- V K, Transfer Gain (K/K) See Note, K... K/K I F = ma, V det =- V Transfer Gain Linearity K ±. % I F = to ma Transfer Gain Linearity K ±. % I F = to ma, T A = C to C Photoconductive Operation Frequency Response BW (- db) KHz I Fq = ma, MOD=± ma, R L = Ω, Phase Response at KHz - Deg. V det =- V Rise Time t R. µs Fall Time t F. µs Package Input-Output Capacitance C IO pf V F = V, f= MHz Common Mode Capacitance C cm. pf V F = V, f= MHz Common Mode Rejection Ratio CMRR db f= Hz, R L =. KΩ Notes. Bin Sorting: K (transfer gain) is sorted into bins that are ±%, as follows: Bin A=.. Bin B=..9 Bin C=.9. Bin D=..9 Bin E=..9 Bin F=.9. Bin G=.. Bin H=.9. Bin I=.9. Bin J=.. K=K/K. K is tested at I F = ma, V det = V.. Bin Categories: All s are sorted into a K 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 K. The category of the parts in the tube is marked on the tube label as well as on each individual part.. 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, four different bin option parts are offered. -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. -F: Order this part number to receive category F only
Figure. LED forward current vs. forward voltage Figure. Normalized servo photocurrent vs. LED current and temperature Normalized Photocurrent...... Normalized to:ip @ I F = ma, T A = C, C V D = V C C C.... VF - LED Forward Voltage - V.. Figure. LED forward current vs. forward voltage...... VF - LED Forward Voltage - V Figure. Servo photocurrent vs. LED current and temperature IP- Servo Photocurrent - µa. C C C C V D = V Figure. Servo photocurrent vs. LED current and temperaturefigure LED current and temperature IP- Servo Photocureent - µa C C C C V D = Vd = -V V. Figure. Normalized servo photocurrent vs. LED current and temperature IP- Normalized Photocurrent... C C C C Figure. Servo gain vs. LED current and temperature NK- Normalized Servo Gain........ Normalized to IP @ I F = ma, T A = C, V D = V Figure 9. Normalized servo gain vs. LED current and temperature NK- Normalized Servo Gain...... Normalized to: I F = ma, T A = C..
Figure. Transfer gain vs. LED current and temperature. Figure. Common mode rejection - K - Transfer Gain - (K/K)...99.99 C C C C CMRR - Rejection Ratio - db - - -9 - - - - F - Frequency - Hz Figure. Normalized transfer gain vs. LED current and temperature Figure. Photodiode junction capacitance vs. reverse voltage K - Transfer Gain - (K/K)....99 C C C C Normalized to I F = ma, T A = C Capacitance - pf.99 Figure. Amplitude response vs. frequency Amplitude Response - db - - - - I F = ma, Mod=± ma (peak) F - Frequency - Hz R L = KΩ R L = KΩ Figure. Amplitude and phase response vs. frequency Amplitude Response - db - - - - I Fq = ma Mod=± ma T A = C RL= Ω F - Frequency - Hz db PHASE - -9 - Ø - Phase Response - - Voltage - V det 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 Figure for the basic structure of the switch mode supply using the Siemens TDA9 Push-Pull Switched Power Supply Control Chip. Line isolation and insulation is 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. Figure more closely shows the basic function of the amplifier. The control amplifier consists of a voltage divider and a noninverting 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 +V signal. Given this information, the amplifier circuit topology shown in Figure is selected. The power supply voltage is scaled by R and R so that there is + V at the non-inverting input (Va) of U. This voltage is offset by the voltage developed by photocurrent flowing through R. This photocurrent is developed by the optical flux created by current flowing through the LED. Thus as the scaled monitor voltage (Va) varies it will cause a change in the LED current necessary to satisfy the differential voltage needed across R at the inverting input. The first step in the design procedure is to select the value of R given the LED quiescent current (I Fq ) and the servo gain (K). For this design, I Fq = ma. Figure shows the servo photocurrent at I Fq is found to be µa. With this data R can be calculated. V b V R= ----- = I Pl µa ----------------- = KΩ Figure. Switch mode power supply Figure. Isolated control amplifier To Control Input ISO AMP + + - Voltage Monitor For best input offset compensation at U, R will equal R. The value of R can easily be calculated from the following. V MONITOR R= R -------------------------- V a KΩ KΩ V = V ------ The value of R depends upon the Transfer Gain (K). K is targeted to be a unit gain device, however to minimize the part to part Transfer Gain variation, Siemens offers K graded into ±% bins. R can determined using the following equation, V OUT R ( R + R) R= V-------------------------- ------------------------------------ MONITOR RK Or if a unity gain amplifer is being designed (VMONI- TOR=VOUT, R=), the euation simplifies to: R R= K ------ R R / MAIN AC/DC RECTIFIER SWITCH XFORMER AC/DC RECTIFIER DC OUTPUT SWITCH MODE REGULATOR TDA9 CONTROL ISOLATED FEEDBACK Figure. DC coupled power supply feedback amplifier V monitor R KΩ R KΩ R KΩ Va Vb + U LM - R Ω pf K K V out R KΩ To control input
Table gives the value of R given the production K bins. Table. R selection Bins Min. Max. K Typ. R Resistor KΩ % KΩ A...9.. B..9... C.9.9... D.9.... E..9.9.. Figure. Linearity error vs. input voltage Linearity Error - %...... -. -. -.. LM... Vin - Input Voltage - V. F.9.... G..... H....9. I..9..9. J.9.. 9. 9. 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, or. V. With an LED quiescent current of ma the typical LED (V F ) is. V. Given this and the operational output voltage, R can be calculated.. V opamp V F.V.V R= ------------------------------- I = ----------------------------- Fq ma = Ω 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 charateristics are shown in Figure 9. The amplifier was designed to have a gain of. and was measured to be.. Greater accurracy 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.% over the input voltage range of V V in a servo mode; see Figure. Figure 9. Transfer gain. Vout - Ooutput Voltage - V..... Vout =. mv +. x Vin LM Ta = C The AC characteristics are also quite impressive offering a - db bandwidth of KHz, with a - phase shift at KHz as shown in Figure. Figure. Amplitude and phase power supply control Amplitude Rresponse - db - - - db PHASE - - - F - Frequency - Hz The same procedure can be used to design isolation amplifiers that accept biploar signals referenced to ground. These amplifiers circuit configurations are shown in Figure. In order for the amplifier to respond to a signal that swings above and below ground, the LED must be prebiased from a separate source by using a voltage reference source (Vref). In these designs, R can be determined by the following equation. V ref V ref R= ------------ I = P KI -------------- Fq - -9 Phase Response -..... Vin - Input Voltage - V.
Figure. Non-inverting and inverting amplifiers Input Vin + R Ω R + pf R Vref Input Output +Vref R R Vo + R Output Vin R + Ω R + pf R +Vref +Vref + Vout R Table. Optolinear amplifiers Amp[ifier Input Output Gain Offset V OUT K R R = V IN R (R+R) V ref = V ref R K R V OUT K R R (R+R) = V IN R R (R +R) V ref = V ref R (R+R) K R R V OUT K R R (R+R) = V IN R R (R +R) V ref = V ref R (R+R) K R R V OUT K R R = V IN R (R +R) V ref = V ref R K R 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 Vref 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.