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MAX3946_10 1.0625Gbps to 11.3Gbps, SFP+ Laser Driver with Laser Impedance Mismatch Tolerance MaximIC
Maxim Integrated MaximIC
MAX3946_10 Datasheet PDF : 28 Pages
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1.0625Gbps to 11.3Gbps, SFP+ Laser Driver
with Laser Impedance Mismatch Tolerance
VCC
MAX3946
0.6V +
IBIAS
-
100
WARNING
FAULT
100kI
1.2V
BIAS
BMON
OR
IF BMAX IS
NOT USED
OR
IF BMON IS
NOT USED
IF BMAX AND
OR
BMON ARE
NOT USED
R1
R1
1kI
BMAX
R2
1kI
R2
Figure 3. BMON and BMAX Circuitry
After initialization the value of the SET_IMOD DAC reg-
ister should be updated using the MODINC register
to optimize cycle time and enhance laser safety. The
MODINC register is an 8-bit register where the first 5 bits
contain the increment information in two’s comple-
ment notation. Increment values range from -8 to +7
LSBs. If the updated value of SET_IMOD[8:1] exceeds
IMODMAX[7:0], the IMODERR warning flag is set and
SET_IMOD[8:0] remains unchanged.
Modulation current sent to the laser is actually the com-
bination of the current generated by the SET_IMOD reg-
ister and current subtracted from this by the SET_TXDE
register.
Output Driver
The output driver is optimized for a 5I to 50I differen-
tial load. The output stage also features programmable
deemphasis that can be set as a percentage of the mod-
ulation current. The deemphasis function is controlled by
the TXDE_MD[1] and TXDE_MD[0] bits (TXCTRL[5:4])
and SET_TXDE[5:0].
Power-On Reset (POR)
POR ensures that the laser is off until supply voltage has
reached a specified threshold (2.75V). After POR, bias
current and modulation current ramps are controlled to
avoid overshoot. In the case of a POR, all registers are
reset to their default values.
BMON and BMAX Functions
Current out of the BMON pin is typically 1/100th the
value of the current at the BIAS pin. The total resistance
to ground at BMON sets the voltage gain. An internal
comparator at the BMAX pin latches a fault if the voltage
on BMAX exceeds the value of 1.2V. The BMAX voltage-
sense pin is connected by means of a voltage-divider to
the BMON pin and ground. The full-scale range of the
BMON voltage is 1.2V x (R1/R2 + 1) (Figure 3). The ana-
log bias-current limit is determined by (1.2V/R2) x 100.
Eye Safety and Output Control Circuitry
The safety and output control circuitry includes the dis-
able pin (DISABLE) and disable bit (TX_EN), along with a
fault indicator and fault detectors (Figure 4). The device
has two types of faults, HARD FAULT and SOFT FAULT.
A HARD FAULT triggers the FAULT pin, and the output
to the laser is disabled. A SOFT FAULT operates as a
warning, and the outputs are not disabled. Both types of
faults are stored in the TXSTAT1 and TXSTAT2 registers.
The FAULT pin is a latched output that can be cleared
by toggling the DISABLE pin. Toggling the DISABLE
pin also clears the TXSTAT1 and TXSTAT2 registers.
A single-point fault can be a short to VCC or ground.
Table 2 shows the circuit response to various single-
point failures.
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