SML2120 datasheet(10/32 Pages) SUMMIT

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Applications Information

SML2120

2066 6.3 1/22/04

Summit Microelectronics

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APPLICATIONS INFORMATION

Power-Up Sequence

When power is first applied to the SML2120, the device

reads configuration register information from the array and

loads it to volatile latches. This procedure begins once the

supply voltage exceeds the power-on reset voltage (VPOR)

and requires about 3 ms to complete. During this

initialization period, all current outputs (BIAS, ILU0, ILU1)

are placed into a high impedance state.

At the conclusion of the initialization period, the ENA# pin

determines the state of the current output. However, the

autonomous monitoring (auto-monitor) function is not yet

allowed to activate, regardless of the state of the AM# pin.

A holdoff timer period (tHOLDOFF) is counted off at the

conclusion of the initialization period to allow the laser

control loop to stabilize. After tHOLDOFF, the AM# pin

determines whether the device begins its auto-monitor

sequence or not. If AM# is asserted, the device enters auto-

monitor mode.

Automatic Power Control (APC) Loop

The SML2120 provides an efficient feedback mechanism

for controlling the light output of the laser. Once the power-

on initialization period has expired, current to the laser is

driven by the BIAS pin, which can sink up to 100 mA of

current to Vss. An on-board amplifier controls the bias

output circuitry. The Monitor Photo Diode (MPD) pin is

driven from the anode of a photo-diode that monitors the

light power being given off by the laser. Adding an

integration capacitor between C1 and C2, and a

corresponding resistor between MPD and C1, completes

and stabilizes the loop, as is shown in Figure 5.

The APC loop attempts to drive the C1 node to the internal

VAPC voltage reference of 0.75V. If the voltage on the MPD

pin exceeds VAPC, the SML2120 reduces the amount of

current on the BIAS pin accordingly, thereby reducing the

amount of current to the laser. Conversely, if the voltage on

MPD falls below VAPC, the amount of current supplied to

the laser is increased. Note that the VAPC pin may be over-

driven using an external reference if a different steady state

value is desired.

The integration time constant of the APC loop is typically set

to a relatively long period (such as 1ms) in order to reduce

pattern dependent jitter. Unfortunately, a long time constant

also leads to a long start-up time. The SML2120 provides an

internal 'Quick Start' transistor, shown in Figure 5, that can

be used to accelerate the turn-on time of the laser. When

this feature is enabled, the Quick Start transistor effectively

shorts out the external resistor for a brief time period after

the outputs are first enabled.

Figure 5. Bias Control Circuit

Bias Output

The user may limit the maximum BIAS current in increments

of 12.5mA (up to a maximum of 100mA), using register 14

bits 2:0. This feature is useful in preventing damage to the

laser even during start-up or open-loop conditions.

Additionally, the BIAS output may be configured to provide

a fixed current. The value of current is based on the ILU1

output and scales proportionally. This fixed bias current may

be configured so that it is available at power-up only, or

whenever ENA# is disabled.

This feature is useful in

systems without a monitor diode, or in systems where feed-

forward current is desired. A fixed current is also useful for

providing reasonable BIAS when no data is yet flowing, and

for diagnostics and board debugging.

Autonomous Monitoring Function (Auto-

Monitor)

The Autonomous Monitoring (Auto-Monitor) Function

provides a control loop for setting two independent output

currents (ILU0 and ILU1) based on various programmable

input stimuli within the system. For each channel, the user

selects: 1) an input stimulus for the Analog/Digital Converter

(ADC), 2) scale and offset values for the conversion to

maximize the usable input range, 3) lookup table values that

translate the resulting ADC conversion to an output current,

and 4) minimum and maximum limits for the output current

driver. Refer to Figure 6.

GND

Laser

MPD

Photodiode

Quick

Start

0.75

Internal

Reference

Voltage

VAPC

BIAS

SML2120

20K

Ω

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