MAX3646 Maxim Integrated Products, MAX3646 Datasheet - Page 12

• Manufacturer Part Number: MAX3646
• Description: 155Mbps to 622Mbps Sff/sfp Laser Driver
• Manufacturer: Maxim Integrated Products

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of R
at +25°C:
An external resistor at the MODBCOMP pin sets current
proportional to I
pin can turn off the interaction between I
If I
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
The temperature coefficient of thermal compensation
above T
MODTCOMP pin open disables additional thermal
compensation:
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Figure 6. Simplified Input Structure
12
T
TH
IN+
IN-
MOD
0.11pF
0.11pF
MODSET
TC = 1 / (0.5 + R
______________________________________________________________________________________
= -70°C + 1.45MΩ / (9.2kΩ + R
K = 1700 / (1000 + R
I
I
must be increased from I
K = (I
MODT
MODT
PACKAGE
TH
I
MOD
I
MODS
that corresponds to the required current
is set by R
0.7nH
0.7nH
MOD2
= TC x (T - T
= 0
= I
BIAS
MODS
= 268 x V
. Open circuiting the MODBCOMP
- I
MODTCOMP
MAX3646
MOD1
V
V
CC
CC
+ K x I
5kΩ
5kΩ
MODTCOMP
TH
) / (I
REF
MODBCOMP
16kΩ
24kΩ
)
BIAS
BIAS2
(kΩ)) mA/°C +10%
/ R
MODSET
V
MOD1
TH_TEMP
CC
+ I
| T > T
| T < T
- I
MODT
. Leaving the
BIAS
) +10%
BIAS1
to I
TH
TH
)°C +10%
and I
)
MOD2
MOD
to
:
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
For:
V
R
R
For compliance:
For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3646’s modulation output
can be AC-coupled to the cathode of a laser diode. An
external pullup inductor is necessary to DC-bias the
modulation output at V
laser forward voltage from the output circuitry and allows
the output at OUT+ to swing above and below the sup-
ply voltage (V
modulation current can be programmed up to 85mA.
Refer to Maxim Application Note HFAN 02.0: Interfacing
Figure 7. Simplified Output Structure
DIODE
L
D
= Diode bias point resistance (5Ω typ)
= Series matching resistor (20Ω typ)
V
OUT+
= Diode bias point voltage (1.2V typ)
Current Compliance (I
Current Compliance (I
= V
I
CC
BIAS
CC
). When AC-coupled, the MAX3646
- V
x R
DIODE
CC
L
≥ 0.7V
. Such a configuration isolates
V
CC
- I
MOD
x (R
MOD
MOD
0.7nH
0.7nH
MAX3646
PACKAGE
D
DC-Coupled
AC-Coupled
+ R
> 60mA),
≤ 60mA),
L
) -
0.11pF
0.11pF
OUT+
OUT-