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MC10SX1130 bảng dữ liệu(PDF) 5 Page - ON Semiconductor |
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MC10SX1130 bảng dữ liệu(HTML) 5 Page - ON Semiconductor |
5 / 8 page 5 MOTOROLA High Performance Frequency Control Products — BR1334 APPLICATIONS INFORMATION Introduction The MC10SX1130 is intended to be integrated into high performance fiber optic modules or used stand-alone to drive a packaged optical LED device. The wide frequency response of the device allows it to be used to support a variety of digital communication applications ranging from: • OC1/3 SONET/SDH Links • 100 MBit/s FDDI • 155 MBit/s ATM • 133/266 MBit/s FibreChannel To support such wide ranging application areas, the LED Driver incorporates a variety of unique features. These offer designers added flexibility that could not previously be realized in less integrated designs. LED Characteristics LED devices emit light when forward biased. The optical power emitted by an LED is determined by the amount of current flowing through the device. This relationship is a relatively linear function of the current, until the device saturates. In some ways, an LED device behaves much like a traditional small signal silicon diode, although the forward “ON” voltage of an LED is much larger and ranges from 1.0V to 2.0V. In addition, for a fixed amount of current, the optical power from the LED will decrease if the device junction temperature increases. Another behavior of most LED devices is that they have unequal turn-on and turn-off times. In developing an LED transmitter, the designer must wrestle with all these behaviors to develop a product that meets the design targets. LED Driver The MC10SX1130 LED Driver accepts a digital binary data stream which is processed by the driver circuitry to create a current waveform to modulate the LED device. The LED Driver contains circuitry to program the modulation current, pre-distort the input waveform to partially compensate for the LED turn-on/turn-off delay, and compensate for the negative optical output power tracking co-efficient. The LED Driver operates from a +5V supply for PECL applications or a –5.2V supply for traditional ECL systems. For further information on PECL, please consult “Designing with PECL Application Note”, AN1406/D available from a Motorola representative. Circuit Blocks Some of the key sub-circuits in the LED Driver are listed below: • Input Line Receiver • Pulse Stretcher • Bias Control Circuitry • Output Current Switch The data input circuitry has been realized as a traditional differential ECL line receiver. It can accept either differential 100K or 10KH style ECL or PECL depending on the supply voltage used. In addition, a VBB reference is provided for use in single ended applications. This reference is useful if the input signal must be AC coupled into the device. The pulse stretcher provides two choices of duty cycle pre-distortion. It is controlled by the input STRETCH signal. When the pin is left open, no pre-distortion is applied to the input waveform. If the pin is strapped to the upper or lower rail, then the output waveform low pulse width will be increased. In a +5V application, when the STRETCH pin is tied to +5V, the nominal pulse width increase is 155 ps and when it is connected to 0V, the nominal pulse width is increased by 310 ps. The bias control circuitry regulates the voltage supplied at the RSET pin of the output current switch. In addition, it implements a positive tracking circuit which provides open loop temperature compensation for the LED’s negative tracking coefficient. An external resistor connected between the RTCO1 and RTCO2 is used to select the rate of voltage change at the RSET pin. The output current switch is the final stage in modulating the LED. The emitter of the current source is pinned out so that an external resistor can be used to set the modulation current. This circuit is implemented using a fully differential gate where both collectors are brought out. As the LED is modulated on and off, the current switches from one collector to another. This architecture minimizes the switching noise inherent in some LED driver design topologies where the modulation current is actually turned on and off. Design Considerations Once the user has selected an LED, the driver circuitry should be optimized to match the characteristics of the LED. The three circuit blocks previously described allow the user to control the pulse width adjustment, LED drive current and temperature tracking rate. A very simple example may best illustrate the design process steps. An LED has been selected which has the desired optical output power when modulated with a waveform of 65mA. In addition, the LED has an output power tracking coefficient of –0.5%/ °C. Thus for every 1°C rise in the case temperature of the LED, the output power will decrease by 0.5% of the nominal value. In addition, the LED forward voltage is 1.5V. First, the RSET resistor must be chosen to set the desired nominal modulation current based on the following equation: RSET = VSET/IMOD (Equation 1) The voltage at VSET is a function of the RTCO tracking resistor, so the desired tracking rate (VTR) must also be chosen. To determine this, the equation must be normalized to correspond to how the LED has been specified. Temp Co = VTR/VSET (Equation 2) The data sheet has three temperature tracking rates for different values of the RTCO resistor. By using the VSET values at 25 °C and substituting those numbers into Equation 2, normalized tracking rates can be calculated. |
Số phần tương tự - MC10SX1130 |
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Mô tả tương tự - MC10SX1130 |
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