Rainbow Electronics MAX3795 - Manual (Page 9)

Detailed Description
The MAX3795 contains a bias generator with APC,
safety circuit, and a laser modulator with optional peak-
ing compensation (see the Functional Diagram).
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 x I PD x R PWRSET
where V
PWRMON
= 0.4V (typ)
The BIASMON output provides a current proportional to
the laser bias current given by:
I BIASMON = I BIAS x G BIASMON
When APC is not used (no monitor diode), connect the
COMP and PWRMON pins to GND. In this mode, bias
current is set by the resistor (R
BIASSET ) between the
BIASSET pin and GND. When a closed-loop configura-
tion is used, connect a 1.7k
Ω 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 4.7k Ω to 10kΩ resistor.
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
_______________________________________________________________________________________ 9
C COMP
COMP BIASSET
R PWRSET
MD
1.8V
(2VBE + 0.2)
REF
CURRENT
AMPLIFIER
POWER-
CONTROL
AMPLIFIER
ENABLE
200 Ω
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
MAX3795
1V
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