SML2120
Programmable Adaptive Laser Power Controller with Dual Lookup Tables
Preliminary Information
1
(See Last Page)
–
–
FEATURES AND APPLICATIONS
INTRODUCTION
The SML2120 is an advanced, programmable laser diode
power controller ideal for optical networking applications.
The integrated automatic power control (APC) circuit adapts
to variations in the laser’s power output as detected by a
photo diode.
The SML2120 drives two low current outputs derived from
values stored in independent 256x8 Lookup Tables. The low
current outputs are suitable for controlling the MODSET
input of typical laser driver IC’s. The input stimulus for each
Lookup Table can be configured as the laser temperature,
the bias current, or an external signal. Characteristics of the
laser’s performance over time and temperature are stored
in the lookup tables, allowing the outputs to adapt to system
conditions and optimize overall performance.
Programming of configuration and control values by the
user are simplified with the I
2
C interface adapter
(SMX3200) and Windows Programming software available
from Summit Microelectronics.
Features:
• Integrated Automatic Power Control (APC) circuit
• 100 mA bias current sink capability
• Dual low-current outputs (up to 2.5 mA) based on two
independent 256x8 Lookup Table values
• Bias current and/or temperature monitoring capability
• Dual high/low alert registers
• Advanced lookup algorithm eliminates unnecessary
output changes
• Flexible voltage operation;
0 to 5V, 0 to 3.3V, or -5.2V to +3.3V
2
C 2-wire serial bus interface for programming
• I
configuration, control values, monitoring, and
operational status - 100KHz and 400KHz
• Small 5X5 28-Pin QFN Package
Applications:
• Laser power management for Telecom/Datacom
motherboards
• Direct modulation and electro-absorptive modulation
applications
–
SIMPLIFIED APPLICATION DRAWING
DATA+
DATA–
+3.3V
MODSET
25
24
15
VHI
ILU0
13
OUT+
Typical
Laser Driver
CAPC
APCSET
BIASMAX
OUT–
V
DD
No Connects
V
DD
(+3.3V)
VDD
VDD
Address = 0
VBRIDGE
12
NTC
2
3
4
5
6
7
A1
A2
THERMISTOR
11
LD
SML2120
BIAS
EXT_TEMP
ILU1
MPD
C2
20
23
14
18
16
C
INT
Typical Laser
Diode with
Photo Diode
and Thermistor
MPD
SDA
SCL
AM#
ALERT#
VLO W
ENA#
VAPC
VSS
VSS
VSS
C1
17
R
INT
10
Figure 1. Typical SML2120 Connections to a Laser Driver and Laser Diode
Note: This is an applications example only. Some pins, components and values are not shown.
© SUMMIT
Microelectronics, Inc. 2004 • 1717 Fox Drive • San Jose CA 95131-2312 • Phone 408 436-9890 • Fax 408 436-9897 • www.summitmicro.com
2066 6.3 1/22/04
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22
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Functional Description
–\
SML2120
FUNCTIONAL DESCRIPTION
The SML2120 is an adaptive power controller for laser
diodes. The device contains an active feedback loop used
to calibrate and control the mean and modulation power of
high-speed high-power laser diodes.
Inherent manufacturing tolerances introduce variations of
performance in laser diodes. These variations, combined
with parametric changes over the laser's extreme operating
temperature range and laser aging, require an efficient
compensation solution. The SML2120, together with a
minimum number of external components, is designed to
compensate for these variations using a digital control loop
and dual programmable nonvolatile calibration lookup
tables.
Figure 2 shows the output light power of a typical laser
diode versus its operating current. Depicted in the graph are
laser diode characteristics at two different temperatures. At
the first temperature (T
Cold
), the laser requires an average
bias current of I
BIAS1
. The modulation current required to
switch the laser between its ON and OFF states is labeled
I
MOD1
.
The ratio of light power of its ON state divided by the light
power of its OFF state is referred to as the extinction ratio.
Ideally the laser requires a constant extinction ratio over its
entire operating temperature range, as the receiver module
is calibrated to this level. Operating the laser driver at a
higher extinction ratio indicates that power is being wasted,
whereas operating at a lower extinction ratio indicates that
data may possibly be lost.
The required bias current increases to I
BIAS2
when the laser
is operated at a second temperature (T
Hot
). The laser
requires a modulation of I
MOD2
to maintain a constant
extinction ratio as in the T
Cold
curve. The SML2120, with its
dual lookup table architecture (Figure 3), is capable of
providing variable output currents based on a function of
either the bias current or the external temperature, and
thereby enables the system designer to optimize the
extinction ratio of the laser driver module.
The SML2120 eliminates the need for any manual
calibration of the laser control circuit. All calibration values
are programmed through the I
2
C industry-standard 2-wire
communication interface whose protocol and functions can
be controlled by automated test equipment (ATE).
Figure 2. Laser Current Increase Caused by Temperature Increase, Constant Light Power Out
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2066 6.3 1/22/04
Summit Microelectronics
SML2120
–
Functional Block Diagram
FUNCTIONAL BLOCK DIAGRAM
VAPC
22
Internal
Reference
0.75V
MPD
18
C1
17
C2
16
ENA#
1
VBURST
19
-
Quick
Start
+
Current Generator
20
BIAS
Bias to Voltage
Converter
28
BIASMON
23
Bridge
Circuit
12
-
+
11
+
EXT_TEMP
CH1 Previous
Conversion
VBRIDGE
THERMISTOR
10-BIT ADC
Scale
&
Offset
Comparison
Logic
8
MPD to Voltage
Converter
26
POWERMON
CH1 High/Low
Limits
Alarm
Logic
A1
A2
SDA
SCL
AM#
ALERT#
2
3
4
5
6
7
I
2
C
2-Wire
Serial
Interface
CH2 High/Low
Limits
Address Decode
256 x 8
E
2
PROM
Lookup
Table
256 x 8
E
2
PROM
Lookup
Table
High Range
DAC
8-BIT DAC
256 x 8
E
2
PROM
Low Range
DAC
General
Purpose
VLOW
VSS
VSS
VSS
VDD
VDD
VHI
8
9
10
21
15
24
25
Power
Distribution
High Range
DAC
8-BIT DAC
Low Range
DAC
Figure 3. SML2120 Block Diagram
Summit Microelectronics
2066 6.3 1/22/04
-
VSS
Temp to Voltage
Converter
27
TEMPMON
Current Generator
13
ILU0
Current Generator
14
ILU1
CH2 Previous
Conversion
3
Package and Pin descriptions
–
SML2120
PACKAGE AND PIN DESCRIPTIONS
Pin 1
ENA#
A1
A2
SDA
SCL
AM#
ALERT#
BIASMON
TEMPMON
POWERMON
VHI
VDD
EXT_TMP
VAPC
VSS
BIAS
VBURST
MPD
C1
C2
VDD
VLOW
VSS
VSS
THERMISTOR
VBRIDGE
ILU0
ILU1
Figure 4. 28-Pin QFN Package Pinout (top view)
PIN DESCRIPTIONS
PACKAGE AND PIN DESCRIPTIONS
PIN DESCRIPTIONS
PIN DESCRIPTIONS
Pin Number Pin Type
Pin Name
Description
1
2
3
4
5
6
I
I
I
I/O
I
I
ENA#
*
A1*
A2
*
SDA
*
SCL
*
AM#
*
Active low input enables the BIAS, ILU0 and ILU1 output currents.
The address pins are connected to either the VHI or VLOW pins to provide a
mechanism for assigning a unique I
2
C bus address to the SML2120.
Bi-directional I
2
C serial data pin
I
2
C serial clock input
AM# is an active low input, when asserted the SML2120 is placed in the Auto-
Monitor mode. AM# must be high for programming the Configuration registers
and the Lookup Tables, ILU0 and ILU1 and the general purpose E
2
PROM.
Active low, open-drain output indicates when one of the monitored inputs
exceeds its user-programmable high or low alert levels.
In dual-rail supply voltage systems, VLOW is tied to the system logic low
potential. It is a logic low reference for all pins marked with an asterisk (*).
VSS must be tied to the lowest system voltage potential.
7
8
9
10
11
O
PWR
PWR
PWR
I
ALERT#
*
VLOW
VSS
VSS
THERMISTOR
Connect a thermistor to this pin to provide an alternative source of temperature
sensing (see VBRIDGE description).
* See VLOW and VHI pin descriptions.
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2066 6.3 1/22/04
Summit Microelectronics
SML2120
Pin Number Pin Type
Pin Name
Description
Pin Descriptions
12
O
VBRIDGE
Connection to an external Full-Bridge Sensor when used with the Thermistor pin.
Two of the full-bridge resistors are internal. See the Bridge Circuit Diagram in
Figure 1 and 3. Voltage level is 0.2V with respect to VSS.
Current output resulting from Lookup Table 0. User-programmable to either sink
(to VSS) or source (to VDD) up to 2.5mA.
Current output resulting from Lookup Table 1. User-programmable to either sink
(to VSS) or source (to VDD) up to 2.5mA.
VDD is the positive rail for most internal functions.
Place a capacitor (C
INT
) between C1 and C2 to increase the time constant for the
APC integrator.
Monitor Photo Diode anode input. Connect a resistor between MPD and C1 to
work in conjunction with C
INT
to establish the integrator time constant.
In burst mode, this input supplies a ballast current that allows the SML2120 to
quickly restart when ENA# is asserted.
Supplies the main laser current as controlled by the APC circuit; capable of
sinking up to 100mA to VSS.
VSS must be tied to the lowest system voltage potential.
APC Override pin.
This input can be configured to sense either a voltage or current generated from
an external temperature monitoring device.
VDD is the positive rail for most internal functions.
In dual-voltage rail systems, VHI is tied to the system logic high potential. It is the
logic high reference for all pins marked with an asterisk (*).
Analog output voltage indicating the state of the Monitor Photo Diode (MPD)
input.
Analog output voltage proportional to the temperature as sensed by the
EXT_TEMP or THERMISTOR input.
Analog output voltage proportional to BIAS current.
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
I/O
I/O
PWR
I
I
I
I
I
PWR
I/O
I
PWR
PWR
O
O
O
ILU0
ILU1
VDD
C2
C1
MPD
VBURST
BIAS
VSS
VAPC
EXT_TEMP
VDD
VHI
POWERMON
*
TEMPMON
*
BIASMON
*
* See VLOW and VHI pin descriptions.
Summit Microelectronics
2066 6.3 1/22/04
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