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M66512P Datasheet(PDF) 3 Page - Mitsubishi Electric Semiconductor |
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M66512P Datasheet(HTML) 3 Page - Mitsubishi Electric Semiconductor |
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3 / 10 page  3 MITSUBISHI 〈DIGITAL ASSP〉 M66512P/FP LASER-DIODE DRIVER OPERATION 1. Setting for Laser Drive Current The M66512 has 3 built-in constant current sources, IL1, IL2, and IL3. Each output current can be controlled independently. The following (1) to (3) describe the method for how to set IL1,IL2, and IL3. (1) IL1 setting method The value of IL1 is determined by the voltage on the VL1 pin and the resistor (RC1) connected between the 1RC pin and GND. The following equation is used for approxi- mation. VL1[V] RC1 [k Ω] provided that 0 ≤VL1≤VCC–1.8V and IL1(max.) =60mA (2) IL2 setting method The value of IL2 is determined by the voltage on the VL2 pin and the resistor (RC2) connected between the 2RC pin and GND. The following equation is used for approxi- mation. VL2[V] RC2 [k Ω] provided that 0 ≤VL2≤VCC–1.8V and IL2(max.) =30mA (3) IL3 setting method The value of IL3 is determined by the internal reference voltage (Vref) and the resistor (RC3) connected between the 3RC pin and GND. The following equation is used for approximation. Vref[V] RC3 [k Ω] provided that Vref =1.4V (typ.) and IL3(max.) =30mA Note: Each of the above equations is a typical on for ob- taining IL1–IL3. In practice values vary by a few per- cent due to some reasons such as differences of ICs from lot to lot and variations in operation tem- peratures. 2. Switching Operation The laser turns on if DATA =“L”. The laser drive current at that moment is IL1 +IL2 + IL3. If DATA = “H” the laser is turned off, and the laser drive cur- rent is almost zero irrespective of the values of IL1 to IL3. 3. Use of the VOFF Input When DATA = “H” the current flowing through the laser is zero so the laser is turned off, but the internal current sources are in operation. When VOFF = “L” in contrast, the internal current sources are turned off. Accordingly, it is possible to prevent an excessive current from flowing through the laser by, for example, fixing the VOFF input to “L” until VCC, after turned on, reaches 3.5V(typ.). (See the section dealing with internal reset.) 4. Laser Power Monitoring Operation At the MO and CO pins, the M66512 outputs data obtained by the monitoring photodiode (PD) contained in the laser, in the sequence explained below. (1) A current equal to the PD current generated by laser light flows through the resistor (RM) connected between 1RM and 2RM. Then, a potential difference (VM) proportional to the output of laser light occurs at RM. (2) The VM is output at the MO pin as an analog signal through a buffer amp. At the same time, VM is compared with the internal reference voltage Vref(1.4V typ.) by the comparator, thence the result of the comparison is output at the CO pin in TTL level. IL1 [mA] = 12 × IL2[mA] = 6 × IL3[mA] = 10 × Condition CO output VM<Vref “L” VM>Vref “H” A Schematic Diagram of Monitor Circuits LD PD RM VM Vref + _ _ + MO output CO output Buffer amp Comparator |
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