Rainbow Electronics MAX3795 - Manual (Page 13)

MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
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The low frequency cutoff of a transmitter using APC is
given by:
Input Termination Requirements
The MAX3795 data inputs are SFP MSA compatible. On-
chip, 100
Ω differential input impedance is provided for
optimal termination (Figure 4). Because of the on-chip
biasing network, the MAX3795 inputs self-bias to the
proper operating point to accommodate AC-coupling.
Applications Information
Interface Models
Figures 4 and 5 show simplified input and output circuits
for the MAX3795 laser driver. Figure 6 shows the fault cir-
cuit interface.
Layout Considerations
To minimize inductance, keep the connections between
the MAX3795 output pins and laser diode as short as
possible. Use multilayer boards with uninterrupted
ground planes to minimize EMI and crosstalk.
Exposed-Pad (EP) Package
The exposed pad on the 24-pin thin QFN provides a very
low thermal resistance path for heat removal from the IC.
The pad is also electrical ground on the MAX3795 and
must be soldered to the circuit board ground for proper
thermal and electrical performance. Refer to Maxim
Application Note HFAN-08.1: Thermal Considerations for
QFN and Other Exposed-Pad Packages for additional
information.
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions from
fiber-optic transmitters. IEC 825 defines the maximum
light output for various hazard levels. The MAX3795 pro-
vides features that facilitate compliance with IEC 825. A
common safety precaution is single-point fault tolerance,
whereby one unplanned short, open, or resistive connec-
tion does not cause excess light output. Using this laser
driver alone does not ensure that a transmitter design is
compliant with IEC 825. The entire transmitter circuit and
component selections must be considered. Customers
must determine the level of fault tolerance required by
their applications, recognizing that Maxim products are
not designed or authorized for use as components in
systems intended for surgical implant into the body, for
applications intended to support or sustain life, or for any
other application where the failure of a Maxim product
could create a situation where personal injury or death
may occur.
I
V V
R
V
R
PD
REF MD
PWRSET PWRSET
=
−
≈
0 2 .
Figure 6. Fault Circuit Interface
MAX3795
V CC
FAULT
V CC
MAX3795
PACKAGE
1nH
1nH
0.5pF
0.5pF
OUT-
OUT+
50W 50W
Figure 5. Simplified Output Structure
f
I
I C
dB
PD
LASER APC
3
1
2 50
≈ Ч
Ч Ч Ч
∆
∆ π