MAX3795ETG Maxim Integrated Products, MAX3795ETG Datasheet - Page 9

• Manufacturer Part Number: MAX3795ETG
• Description: Laser Driver 1Gbps to 4.25Gbps Mu ltirate VCSEL Driver
• Manufacturer: Maxim Integrated Products
• Type: Transmitterr

Specifications of MAX3795ETG

• Operating Supply Voltage: 3.3 V
• Package / Case: TQFN-24 EP
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT

The MAX3795 contains a bias generator with APC,
safety circuit, and a laser modulator with optional peak-
ing compensation (see the Functional Diagram).
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:
where V
Figure 1. Bias Generator
PWRMON
R
PWRSET
I
PD
V
1Gbps to 4.25Gbps Multirate VCSEL Driver
PWRMON
= 0.4V (typ)
REF
_______________________________________________________________________________________
MD
Detailed Description
= 2 x I
SMOOTH-
START
C
COMP
(2VBE + 0.2)
PD
1.8V
x R
PWRSET
Bias Generator
1.6V
(2V
BE
2X
)
PWRMON
AMPLIFIER
CONTROL
POWER-
COMP
BIAS GENERATOR
1V
MAX3795
with Diagnostic Monitors
The BIASMON output provides a current proportional to
the laser bias current given by:
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 pin and GND. When a closed-loop configura-
tion is used, connect a 1.7kΩ resistor between ground
and BIASSET to set the maximum bias current.
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
be pulled high with a 4.7kΩ to 10kΩ resistor.
Table 1. Fault Conditions
BIAS
BIASMON
PWRMON
PIN
I
BIAS
34
AMPLIFIER
CURRENT
BIASSET
I
BIASMON
200Ω
R
BIASSET
I
V
V
V
BIAS
ENABLE
9
BIAS
BIASMON
PWRMON
= I
> V
BIAS
CC
FAULT CONDITION
CC
> 0.8V
> 0.8V
BIASMON
BIAS
x G
- 0.2V
. The FAULT pin should
BIASSET
R
BIASMON
BIASMON
Safety Circuit
) between the
FERRITE
BEAD
9