Rainbow Electronics MAX3646 - Manual (Page 12)

MAX3646
Current Compliance (I MOD ≤ 60mA),
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
For:
V
DIODE
= Diode bias point voltage (1.2V typ)
R L = Diode bias point resistance (5Ω typ)
R D = Series matching resistor (20Ω typ)
For compliance:
V
OUT+
= V
CC
- V
DIODE
- I
MOD
x (R
D
+ R
L
) -
I BIAS x R L ≥ 0.7V
Current Compliance (I MOD > 60mA),
AC-Coupled
For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3646’s modulation output
can be AC-coupled to the cathode of a laser diode. An
external pullup inductor is necessary to DC-bias the
modulation output at V
CC . Such a configuration isolates
laser forward voltage from the output circuitry and allows
the output at OUT+ to swing above and below the sup-
ply voltage (V
CC ). When AC-coupled, the MAX3646
modulation current can be programmed up to 85mA.
Refer to Maxim Application Note HFAN 02.0: Interfacing
Maxim’s Laser Drivers to Laser Diodes for more informa-
tion on AC-coupling laser drivers to laser diodes.
For compliance:
V
OUT+
= V
CC
- I
MOD
/ 2 x (R
D
+ R
L
) ≥ 0.75V
Determine C APC
The APC loop filter capacitor (C APC ) must be selected
to balance the requirements for fast turn-on and mini-
mal interaction with low frequencies in the data pattern.
The low-frequency cutoff is:
C APC (µF) ≅ 68 / (f 3dB (kHz) x (η x ρMON ) 1.1
High-frequency noise can be filtered with an additional
cap, C
MD , from the MD pin to ground:
C MD ≅ C APC / 4
The MAX3646 is designed so turn-on time is faster than
1ms for most laser gain values (η x ρ
MON ). Choosing a
smaller value of C APC reduces turn-on time. Careful
balance between turn-on time and low-frequency cutoff
may be needed at low data rates for some values of
laser gain.
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3646 laser driver. If dice are used,
replace package parasitic elements with bondwire par-
asitic elements.
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
12 ______________________________________________________________________________________
V CC
V CC
V CC
0.11pF
0.7nH
IN+
0.11pF
0.7nH
IN-
PACKAGE
5kΩ
5kΩ
24kΩ
16kΩ
MAX3646
Figure 6. Simplified Input Structure
MAX3646
PACKAGE
0.7nH OUT-
0.7nH OUT+
0.11pF
0.11pF
V CC
Figure 7. Simplified Output Structure