Datasheet

Detailed Description
The MAX3740A contains a bias generator with automat-
ic power control (APC), safety circuit, and a laser mod-
ulator with optional peaking compensation.
Bias Generator
Figure 1 shows the bias generator circuitry that contains
a power-control amplifier and smooth-start circuitry. An
internal PNP transistor provides DC laser current to bias
the laser in a light-emitting state. The APC circuitry
adjusts the laser-bias current to maintain average power
over temperature and changing laser properties. The
smooth-start circuitry prevents current spikes to the laser
during power-up or enable, ensuring compliance with
safety requirements and extending the life of the laser.
The MD input is connected to the cathode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the anode of the laser through an
inductor or ferrite bead. The power-control amplifier dri-
ves a current amplifier to control the laser’s bias current.
During a fault condition, the bias current is disabled.
The PWRMON output provides a voltage proportional to
average laser power given by:
V
PWRMON
= 2 ✕ I
PD
✕ R
PWRSET
The BIASMON output provides a current proportional to
the laser bias current given by:
I
BIASMON
= I
BIAS
/ 9
When APC is not used (no monitor diode, open-loop
configuration) connect the COMP and PWRMON pins
to GND. In this mode, the bias current is set by the
resistor R
BIASSET
. When a closed-loop configuration is
used, connect a 1.7kΩ resistor between ground and
BIASSET to set the maximum bias current.
Safety Circuit
The safety circuit contains an input disable
(TX_DISABLE), a latched fault output (FAULT), and fault
detectors (Figure 2). This circuit monitors the operation
of the laser driver and forces a shutdown (disables
laser) if a fault is detected (Table 1). Table 2 contains
the circuit’s response to various single-point failures.
The transmit fault condition is latched until reset by a
toggle of TX_DISABLE or V
CC
. The FAULT pin should
be pulled high with a 4.7kΩ to 10kΩ resistor.
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
_______________________________________________________________________________________ 9
C
COMP
COMP BIASSET
R
PWRSET
MD
1.8V
REF
CURRENT
AMPLIFIER
POWER-
CONTROL
AMPLIFIER
ENABLE
200Ω
R
BIASSET
PWRMON
BIAS GENERATOR
SMOOTH-
START
R
BIASMON
BIASMON
FERRITE
BEAD
BIAS
1.6V
(2V
BE
)
2X
I
BIAS
34
I
PD
I
BIAS
9
MAX3740A
1.2V
Figure 1. Bias Generator
PIN FAULT CONDITION
BIAS V
BIAS
> V
CC
- 0.2V
BIASMON V
BIASMON
> 0.8V
PWRMON V
PWRMON
> 0.8V
Table 1. Fault Conditions