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SY88904 Datasheet(PDF) 4 Page - Micrel Semiconductor

No. de Pieza. SY88904
Descarga  8 Pages
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Fabricante  MICREL [Micrel Semiconductor]
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SY88904 Datasheet(HTML) 4 Page - Micrel Semiconductor

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Micrel, Inc.
M9999-081005 or (408) 955-1690
The SY88904 is an integrated control circuit for laser
diode modules intended for high-frequency fiber-optic
applications. The device is designed to operate with the
SY88902 laser diode driver providing Automatic Power
Control (APC) which provides bias and modulation current
control, laser diode driver enable and monitor diode interface.
The device also interfaces with the SY88903 high-speed
limiting amplifier providing chatter-free Loss of Signal
function. The system functions provided include laser diode
over-current detection, module fault signal and transmit
disable control. The SY88904 also provides power up cycling
of laser diode current and the fault detect signal.
Laser Diode Supply Current
The SY88904 provides the adjustable drive current for
the laser diode, DC bias.
Laser Diode Over-Current Detection
The SY88904 provides the ability to set an upper limit
fail point for the laser diode. ILIMIT SET sets an upper limit
for the average current present at the IBIAS pin.
Monitor Diode
The MD pin on the SY88904 provides low frequency
light intensity, laser power out feedback from the monitor
diode in the laser diode package. This feedback is used to
adjust the IBIAS level through the Automatic Power Control
circuit to maintain a near constant power output from the
laser diode. The SY88904 will also generate a fault if the
MD pin receives photo current less than IPINSET or is no
longer connected to the monitor diode.
Laser Diode Driver Enable
The SY88904 provides a TX_Out pin used to control the
output of the laser diode driver, through the Enable pin , of
the SY88902. This is a single ended PECL output.
Module Fault
The SY88904 provides an open collector output,
TX_Fault, that indicates to the host system that the module
is in a fault condition. The SY88904 will assert TX_Fault
when a low VCC condition is detected, when the monitor
diode is not connected or when a laser diode over-current
condition is detected. The SY88904 will attempt to clear a
TX_Fault condition if the device is power cycled or a reset
cycle is initiated with TX_Disable. When the SY88904 is in
a TX_Fault condition Ibias will be removed, TX_Out will be
negated and fault detection will be disabled.
Power Up Sequence, TX_Disable asserted
The SY88904 will power up in a TX_Fault condition and
will remain in a fault condition while TX_Disable is asserted.
Power Up Sequence, TX_Disable negated
The SY88904 will power up in a TX_Fault condition and
will remain in a fault condition until VCC is within operational
limits, at which time an initialization sequence will be started.
TX_Disable Cycle, TX_Fault Negated
When the SY88904 is not in a TX_Fault condition, a
rising edge of TX_Disable will cause the SY88904 to enter
a TX_Fault condition. The falling edge of TX_Disable will
cause a power up sequence to be started.
TX_Disable Cycle, TX_Fault Asserted
When the SY88904 is in a TX_Fault condition, the falling
edge of TX_Disable will cause an initialization sequence to
be started.
TX_Disable Cycle, during an initialization cycle
A rising edge of TX_Disable will cause the SY88904 to
terminate the current initialization cycle and remain in a
TX_Fault condition. The falling edge of TX_Disable will cause
the initialization sequence to be restarted.
Initialization Sequence
The initialization sequence consists of a delay, set by
the RC time constant on the Rwin input, followed by a
power up sequence.
Power Up Sequence
A power up sequence will negate TX_Fault and then
ramp up Ibias, controlled by the capacitor on the Rwin input
and an internal 1mA current, followed by TX_OUT being
negated and fault checking. If a fault occurs at this time the
module will enter a TX_Fault conditions, otherwise the
module will be ready for operation.

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