Rainbow Electronics MAX3795 - Manual (Page 10)

MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
10 ______________________________________________________________________________________
Modulation Circuit
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
3). The modulator drives up to 15mA of modulation into
a 50
Ω VCSEL load.
The amplitude of the modulation current is set with
resistors at MODSET and temperature coefficient (TC1,
TC2) pins. The resistor at MODSET (R
MODSET ) pro-
grams the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (R
TC )
programs the temperature coefficient of the modulation
PIN NAME
CIRCUIT RESPONSE TO
V
CC
SHORT
CIRCUIT RESPONSE TO
GND SHORT
CIRCUIT RESPONSE TO
OPEN
FAULT Does not affect laser power. Does not affect laser power. Does not affect laser power.
TX_DISABLE
Modulation and bias current are
disabled. Normal condition for circuit
operation. Modulation and bias current are
disabled.
IN+ Does not affect laser power. Does not affect laser power. Does not affect laser power.
IN- Does not affect laser power. Does not affect laser power. Does not affect laser power.
SQUELCH Does not affect laser power. Does not affect laser power. Does not affect laser power.
TC1 Does not affect laser power. Does not affect laser power.
The laser modulation is decreased,
but average power is not affected.
TC2
The laser modulation is increased,
but average power is not affected.
Modulation current is disabled.
The laser modulation is decreased,
but average power is not affected.
MODSET Modulation current is disabled.
The laser modulation is increased,
but average power is not affected. The laser modulation is decreased,
but average power is not affected.
PEAKSET Does not affect laser power. Does not affect laser power. Does not affect laser power.
OUT+ Modulation current is disabled. Modulation current is disabled. Modulation current is disabled.
OUT- Does not affect laser power. Does not affect laser power. Does not affect laser power.
BIASSET Laser bias is disabled. Fault state* occurs. Laser bias is disabled.
BIAS
Fault state* occurs. Note that VCSEL
emissions may continue. Care must
be taken to prevent this condition.
This disables the VCSEL. This disables the VCSEL.
BIASMON Fault state* occurs. Does not affect laser power. Fault state* occurs.
COMP
The bias current is reduced, and the
average power of the laser output is
reduced.
I BIAS increases to the value
determined by R BIASSET . If the bias-
monitor fault threshold is exceeded,
a fault is signaled.
APC loop will be unstable.
If the bias-monitor fault threshold is
exceeded, a fault is signaled.
MD
I BIAS increases to the value
determined by R BIASSET . If the bias-
monitor fault threshold is exceeded,
a fault is signaled.
The bias current is reduced, and the
average power of the laser output is
reduced.
I BIAS increases to the value
determined by R BIASSET . If the bias
monitor fault threshold is exceeded,
a fault is signaled.
REF
I BIAS increases to the value
determined by R BIASSET . If the bias-
monitor fault threshold is exceeded,
a fault is signaled.
The bias current is reduced, and the
average power of the laser output is
reduced. The bias current is reduced, and the
average power of the laser output is
reduced.
PWRMON Fault state* occurs. Does not affect laser power. Does not affect laser power.
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)