Datasheet

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MAX3946
1Gbps to 11.3Gbps, SFP+ Laser Driver
with Laser Impedance Mismatch Tolerance
Design Procedure
Programming Bias Current
1) IBIASMAX[7:0] = Maximum_Bias_Current_Value
2) SET_IBIAS
i
[8:1] = Initial_Bias_Current_Value
Note: The total bias current is calculated using the
SET_IBIAS[8:0] DAC value. SET_IBIAS[8:1] are the bits
that can be manually written. SET_IBIAS[0] can only be
updated using the BIASINC register.
When implementing an APC loop it is recommended to
use the BIASINC register, which guarantees the fastest
bias current update.
3) BIASINC
i
[4:0] = New_Increment_Value
4) If (SET_IBIAS
i
[8:1] P IBIASMAX[7:0]), then (SET_
IBIAS
i
[8:0] = SET_IBIAS
i-1
[8:0] + BIASINC
i
[4:0])
5) Else (SET_IBIAS
i
[8:0] = SET_IBIAS
i
-
1
[8:0])
The total bias current can be calculated as follows:
6) I
BIAS
= [SET_IBIAS
i
[8:0] + 16] x 200FA
Programming Modulation Current
1) IMODMAX[7:0] = Maximum_Modulation_Current_Value
2) SET_IMOD
i
[8:1] = Initial_Modulation_Current_Value x 1.06
Note: The total modulation laser current is calculated
using the SET_IMOD[8:0] DAC value and the SET_TXDE
register value. SET_IMOD[8:1] are the bits that can be
manually written. SET_IMOD[0] can only be updated
using the MODINC register.
When implementing modulation compensation, it is rec-
ommended to use the MODINC register, which guaran-
tees the fastest modulation current update.
3) MODINC
i
[4:0] = New_Increment_Value
4) If (SET_IMOD
i
[8:1] P IMODMAX[7:0]), then (SET_
IMOD
i
[8:0] = SET_IMOD
i
-
1
[8:0] + MODINCi[4:0])
5) Else (SET_IMOD
i
[8:0] = SET_IMOD
i
-
1
[8:0])
The following equations give the modulation current
(peak-to-peak) seen at the laser when driven differen-
tially. R
EXTD
is the differential load impedance of the
laser plus any added series resistance.
6a) TXDE_MD[1:0] = 00, then
[ ]
( )
[ ]
( )
MOD
LD
0.3mA SET_IMOD 8 : 0 16
50
I
50 R
0.15mA SET_IMOD 8 : 3 2
 
+
Ω
 
= ×
Ω +
 
− +
 
6b) TXDE_MD[1:0] = 01, then
[ ]
( )
[ ]
( )
MOD
LD
0.3mA SET_IMOD 8 : 0 16
50
I
50 R
0.15mA SET_IMOD 8 : 4 1
 
+
Ω
 
= ×
Ω +
 
− +
 
6c) TXDE_MD[1:0] = 10, then set SET_TXDE[5:0] can be
set to any value ≥ SET_IMOD[8:4] and
[ ]
( )
[ ]
( )
MOD
LD
0.3mA SET_IMOD 8 : 0 16
50
I
50 R
0.15mA SET_TXDE 5:0 1
 
+
Ω
 
= ×
Ω +
 
− +
 
When SET_TXDE[5:0] is increased, the deemphasis
current increases and the overall peak-to-peak modu-
lation current decreases. This effect saturates when
SET_TXDE[5:0] = 0.2 x (SET_IMOD[8:0] + 16) - 1, and
further increases to SET_TXDE[5:0] do not increase the
deemphasis current.
6d) TXDE_MD[1:0] = 11, then
[ ]
( )
MOD
LD
50
I 0.9 0.3mA SET_IMOD 8 : 0 16
50 R
Ω
 
= × + ×
 
Ω +
Note: When TXDE_MD[1:0] = 10 and the SET_TXDE
register is set by the user, the minimum allowed deem-
phasis is 3% and the maximum is 10%. These limits are
internally set by the MAX3946.
Programming Transmit Output Deemphasis
1) TXDE_MD[1:0] = Transmit_Deemphasis_Mode
2) SET_TXDE[5:0] = Transmit_Deemphasis_Value. If
TXDE_MD[1:0] = 00, 01, or 11, the value of SET_TXDE
is automatically set by the device and there is no
need to enter data to SET_TXDE.
For Transmit_Deemphasis_Mode:
00 = deemphasis is fixed at 6% of the modulation ampli-
tude (the device controls the SET_TXDE value), default
setting
01 = deemphasis is fixed at 3% of the modulation ampli-
tude (the device controls the SET_TXDE value)
10 = deemphasis is programmed by the SET_TXDE
register setting
11 = deemphasis is at its maximum of approximately 9%
(the device controls the SET_TXDE value)