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:
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.
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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:
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Input Line Receiver
Pulse Stretcher
Bias Control Circuitry
Output Current Switch
High Performance Frequency
Control Products — BR1334
MOTOROLA
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MOTOROLA [ MOTOROLA, INC ]
