MAX3798ETJ+ Maxim Integrated Products, MAX3798ETJ+ Datasheet - Page 27

• Manufacturer Part Number: MAX3798ETJ+
• Description: IC LIMITING AMP/VCSEL DVR 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX3798ETJ+

• Amplifier Type: Limiting
• Number Of Circuits: 1
• Output Type: Differential
• Current - Supply: 97mA
• Voltage - Supply, Single/dual (±): 2.85 V ~ 3.63 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Operating Temperature Range: + 85 C
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output / Channel: -
• -3db Bandwidth: -
• Slew Rate: -
• Gain Bandwidth Product: -
• Current - Input Bias: -
• Voltage - Input Offset: -
• Lead Free Status / Rohs Status: Details

Bits 3 to 0: SET_TXDE[3:0]. This is a 4-bit register used to control the amount of deemphasis on the transmitter out-
put. When calculating the total modulation current, the amount of deemphasis must be taken into account. The
deemphasis is set as a percentage of modulation current.
1) IBIASMAX[7:0] = Maximum_Bias_Current_Value
2) SET_IBIAS
Note: The total bias current value is calculated using
the SET_IBIAS[8:0] register. SET_IBIAS[8:1] are the bits
that can be manually written. SET_IBIAS[0] can only be
updated using the BIASINC[4:0] register.
When implementing an APC loop it is recommended to
use the BIASINC[4:0] register, which guarantees the
fastest bias current update.
3) BIASINC
4) If (SET_IBIAS
5) Else (SET_IBIAS
The total bias current can be calculated as follows:
6) I
1) IMODMAX[7:0] = Maximum_Modulation_Current_Value
2) SET_IMOD
Note: The total modulation current value is calculated
using the SET_IMOD[8:0] register. SET_IMOD[8:1] are
the bits that can be manually written. SET_IMOD[0] can
only be updated using the MODINC[4:0] register.
When implementing modulation compensation, it is rec-
ommended to use the MODINC[4:0] register, which
guarantees the fastest modulation current update.
3) MODINC
4) If (SET_IMOD
5) Else (SET_IMOD
The following equation is valid with assumption of 100Ω
on-chip and 100Ω external differential load (Rextd). The
maximum value that can be set for SET_TXDE[3:0] = 11.
Bit #
Name
Default Value
Power SFP+ Limiting Amplifier and VCSEL Driver
then (SET_IBIAS
then (SET_IMOD
BIAS
= [SET_IBIAS
1.0625Gbps to 10.32Gbps, Integrated, Low-
i
i
[4:0] = New_Increment_Value
[4:0] = New_Increment_Value
Programming Modulation Current
i
i
[8:1] = Initial_Bias_Current_Value
[8:1] = Initial_Modulation_Current_Value
i
[8:1] ≤ IBIASMAX[7:0]),
i
[8:1] ≤ IMODMAX[7:0]),
i
i
[8:0] = SET_IBIAS
[8:0] = SET_IMOD
______________________________________________________________________________________
i
[8:0] = SET_IBIAS
7
X
X
i
[8:0] = SET_IMOD
Programming Bias Current
i
[8:0] + 20] x 40μA
Design Procedure
6
X
X
i-1
i-1
[8:0] + MODINC
[8:0] + BIASINC
i-1
i-1
[8:0])
[8:0])
5
X
X
i
i
[4:0])
[4:0])
4
X
X
Transmitter Deemphasis Control Register (SET_TXDE)
SET_TXDE
6)
For general Rextd, the modulation current that is
achieved using the same setting of SET_IMOD
for Rextd = 100Ω is shown below. It can be written as a
function of I
chip load.
7)
The TXDE_EN bit must be set to 1 to enable the deem-
phasis function. The SET_TXDE register value is used
to set the amount of deemphasis, which is a percent-
age of the modulation current. Deemphasis percentage
is determined as:
Where the maximum SET_TXDE[3:0] = 11.
For an I
deemphasis value is approximately 20%. Maximum
deemphasis achievable for full I
limited to 15%.
With deemphasis enabled, the value of the modulation
current amplitude is reduced by the calculated deem-
phasis percentage. To maintain the modulation current
amplitude constant, the SET_IMOD[8:0] register must
be increased by the deemphasis percentage. If the sys-
tem conditions like temperature, required I
etc., change during the transmit operation, the deem-
phasis setting might need to be readjusted. For such an
[3] (MSB)
I
3
0
MOD
I
MOD(Rextd=100Ω)
MOD
(Re
LOS
DE
SET_TXDE
MOD(Rextd=100Ω)
xtd
(%)
value of 10mA, the maximum achievable
TH
×
[2]
)
2
0
=
⎡
⎢
⎣
Programming Transmit Output
=
= (SET_LOS[5:0] x 1.5mV
1
2
−
Programming LOS Threshold
100
×
2
I
MOD
= [(20 + SET_IMOD
SET_TXDE
+
×
SET TXDE
(
[1]
2
1
0
(Re
+
_
, still assuming a 100Ω on-
64
SET TXDE
xtd
64
MOD
=
SET_TXDE
_
100
[0] (LSB)
[ : ]
3 0
Ω
0
0
range of 12mA is
)
⎡
⎢
⎣ ⎣
Deemphasis
[ : ]
i
⎤
⎥
⎦
Re
3 0
[8:0]) x 40μA]
P-P
Re
xt
MOD
ADDRESS
)
)
+
H0x10
xt
i
[8:0] as
100
value,
⎤
⎥
⎦
27