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ISL58831CRZ-T13 Scheda tecnica(PDF) 9 Page - Intersil Corporation |
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ISL58831CRZ-T13 Scheda tecnica(HTML) 9 Page - Intersil Corporation |
9 / 12 page ISL58831 9 FN7440.1 January 28, 2016 Submit Document Feedback Applications Information Enable and Read Operation The ENABLE line powers up the chip and supplies bias to all the circuits. After being enabled, read current can be obtained by applying a current to the IINR input. The read power is usually operated in an automatic power control loop, by varying the current in the IINR pin in response to the monitored laser light power. Equation 2 is the defining equation for each amplifier: Oscillator Operation Usually a laser will be noisy due to mode-hopping often caused by variable optical feedback into the laser. RF current can be applied to reduce this noise effect by bringing the OSCEN pin high. The amplitude of the RF is set by the RAMP resistor and the frequency is set by the RFREQ resistor. See the “Typical Performance Curves” on page 8 for resistor set values. RF current is applied in a on/off fashion. Thus, if the RF amplitude is 50mAP-P, 50mA will be added to the read current for half the RF cycle, and then 0mA will be added to the read current for half the RF cycle. In this case, if the threshold current is only 40mA, the average laser power could exceed the intended read laser power by about 2mW, due to the 50% duty cycle current of 10mA above threshold. Therefore, in order to regulate the read power, it is necessary to make sure that the RF amplitude is not much more than the required DC read current. The circuit has a feature to increase the ability to turn off the laser for low threshold currents. At low read currents, the amplitude of the RF will be reduced as the amplitude of the read current is reduced. Write Levels Typical applications will have at least two write powers. The recommended method to control the write power level is to assign Channel 2 to the lowest power level above read and add in Channel 3 to obtain the highest write power level. This spreads the gain over the most amplifiers, allows the largest current level to the laser, reduces the sensitivity of each input and provides the most protection to the laser in case of erroneous input commands. Write Switching Waveforms The WEN lines are applied to a fast comparator set to 1.67V. This makes it possible to have predictable rise and fall propagation delays from the WEN write pulse inputs to the laser. Power Supply Decoupling Due to the high values of current being switched rapidly on and off, it is important to ensure that the power supply is well decoupled to ground. During switching, the VDD undergoes severe current transients, thus every effort should be made to decouple the VDD as close to the package as possible, and to route the laser cathode to the decoupling capacitor with a short wide trace. Symptoms that could arise include poor rise/fall times, current overshoot and poor settling response. Since even a well placed bypass capacitor will have a response limitation due to the lead inductance, it might be necessary to also place a lossy bead and a second decoupling capacitor on the supply side of the bead to prevent switching currents on the supply line from generating EMI. Laser Diode Routing It is very important to minimize the inductance of the trace between the IOUT pin and the laser diode. This trace acts as an antenna for EMI, inhibits the flow of RF and pulse current to the laser and absorbs RF current into ground. The ground return from the laser cathode to the chip and decoupling capacitors is best as a wide plane on both sides of the trace leading to the laser anode. Ringing of the waveform might be observed on the IOUT pin. The best way is to check the optical output of the laser with an optical probe. If ringing is confirmed that cannot be reduced by an improved layout, the addition of an RC snubber network right at the output of the laser driver may be helpful. Be aware however, that the rise time might be affected and that the pulse power might be affected by pattern dependent voltage build-up on the snubber capacitor. Users should expect to lose 0.5ns of tr/tf for every 1cm of distance from IOUT to the laser diode and back to the VDD decoupling capacitor. Power Consumption Issues The ISL58831 has been designed for low power consumption. When disabled, the part takes negligible power consumption, regardless of the state of the other pins. In addition, for VDD <3.5V, the ISL58831 will shut down to less than 1mA of supply current. When in normal operation, the ISL58831 total power consumption depends strongly on the laser diode current and voltage. Since the total power consumption under worst case conditions could approach one watt, the burden is on the user to dissipate the heat into the board ground plane or chassis. An in-depth discussion of the effects of ground plane layout and size can be found in application note AN1091. An approximate equation for the device power consumption is shown in Equation 3 (users must adjust accordingly for any duty cycle issues): Where: IS = IS2 when oscillator off, or IS3 when oscillator on (see page 5) IIN = Sum of all the IIN currents VDD = Device power supply voltage IDIODE = Laser diode current VDIODE = Forward voltage of laser diode at current of IDIODE When using the ISL58831, the user must take extreme care not to exceed the maximum junction temperature of +150°C. Since the case-to-ambient thermal coefficient will dominate, and since this is very much defined by the user’s thermal engineering, it is not practical to define a strict limit on power consumption. IOUT VDAC RSET RINx + ----------------------------------- GAIN = (EQ. 2) PDISS I S 14 I IN VCC IDIODE VCC - VDIODE + + = (EQ. 3) |
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