MC2062
3.3 to 5V Laser/VCSEL Driver IC for Applications from 100Mbps to 1.25Gbps
MC2062_FPRELIMINAR Y INFORMATION
DESIGN INFORMATION
Typical Closed Loop Application
Setting Bias Current for Closed Loop
Operation
When the VCSEL includes a monitor photodiode, the
closed loop scheme should be adopted. The voltage
at IPIN is equal to VCC-1.2V, so the v oltage across RB1
is 1.2V. The automatic power control loop increases
or decreases IBIASOUT such that the current from the
monitor photodiode (and the VCSEL output power)
remains constant. Knowing the monitor photodiode
current (IPD) at the desired output power, RB1 is set as
follows:
RB1 = 1.2V/IPD
Typical Open Loop Application
Setting Bias Current for Open Loop
Operation:
When driving a device in open loop operation with-
out a monitor photodiode, refer to the arrangement
on page 9. The MC2062 can be configured to set
VCSEL bias current with or without temperature com-
pensation.
First calculate the nominal bias current:
BIAS TH Pavg
II η
=+
where Ith is the threshold current (in mA), Pavg is the
average power (in mW), and η is the VCSEL slope
efficiency (in mW/mA).
Next, the required temperature coefficient of bias
current must be calculated. Note that with the modu-
lation output AC coupled, the temperature coefficient
of the VCSEL slope efficiency also affects the over-
all desired temperature coefficient of bias current:
()
avg
TH
BIAS
P
I
I
ITH
IBIAS IBIASOUT
TC TC
TC TC
ηη
×− ×
=−
where TCITH is the temperature coefficient of thresh-
old current (in ppm/°C), TCη is the slope efficiency
temperature coefficient (in ppm/°C), and TCIBIASOUT is
the temperature coefficient of IBIASOUT with no pro-
grammed compensation – see DC Characteristics.
When not programming temperature compensation,
the typical temperature coefficient of bias current is
-1000ppm/°C. For this condition (no additional tem-
perature compensation), connect RB1 between IPIN
and VCC (RB2 and RB3 are not required). Knowing the
desired VCSEL bias current, RB1 is calculated as fol-
lows:
IBIAS
B1 BIAS- IBIAS
1.2V M
RIC
×
=
where MIBIAS is a bias current multiplier and CIBIAS is an
offset – see DC Characteristics.
As noted, programming the bias current as listed
above yields a temperature coefficient of -1000ppm/
°C . By connecting pin B1 to VCC and connecting RB1
between pin E1 and IPIN, the bias current is typically
≈ 2000ppm/°C . With this configuration, RB1 is set as
follows (RB2 and RB3 are not required):
IBIAS
B1 BIAS- IBIAS
0.55V M
RIC
×
=
Finally , when more temperature compensation is re-
quired for IBIAS, use the configuration as shown on
page 9. The procedure for selecting the values for
RB1, RB2, and RB3 is as follows:
1. Calculate the voltage at pin E1:
E1 1.2V
V3000
IBIAS
IBIAS
TC
TC
×
=+
where VE1 is the number of volts belo w VCC.
2. Calculate I(IBIASSET):
BIAS- IBIAS
BIASSET IBIAS
IC
I(I ) M
=
where MIBIAS is the bias current multiplier and CIBIAS is
an offset– see DC Characteristics.
3. Calculate RB1:
E1
B1 BIASSET
1.2-V
RI(I )
=
4. Calculate the values for the temperature compen-
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