Datasheet

______________________________________________________________________________________ 13
MAX3946
1Gbps to 11.3Gbps, SFP+ Laser Driver
with Laser Impedance Mismatch Tolerance
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 -16 to +15
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 sin-
gle-point fault can be a short to V
CC
or ground. Table 2
shows the circuit response to various single-point faults.
Figure 3. BMON and BMAX Circuitry
V
CC
WARNING
0.6V
BIAS
BMON
BMAX
OR
IF BMAX IS
NOT USED
+
-
FAULT
100kI
R1
R1
R2
1.2V
I
BIAS
100
MAX3946
1kI
OR
IF BMON IS
NOT USED
R2
OR
IF BMAX AND
BMON ARE
NOT USED
1kI