LTC5100 datasheet(29/52 Pages) LINER

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LTC5100

sn5100 5100fs

fault has occurred. For example, the software in the host

system may need to evaluate the cause of the fault before

shutting down the laser. If Auto_shutdn_en = 1, the

LTC5100 automatically disables the transmitter after a

fault. If Auto_shutdn_en = 0, data transmission continues

after a fault. The transmitter is not disabled until the host

system drives the EN pin inactive or clears the Soft_en bit.

Low power supply voltage and memory load errors are

considered hard faults and always disable the transmitter,

regardless of the setting of Auto_shutdn_en.

The LTC5100 implements the GBIC protocol for prevent-

ing software from repeatedly re-enabling a faulted trans-

mitter. When a first fault is detected, it can be cleared by

disabling the transmitter. If the transmitter is re-enabled

and a second fault occurs within 25ms after fault detection

is enabled, the transmitter is permanently disabled. Only

cycling power to the LTC5100 can clear this condition.

This feature is called “Repeated Fault Inhibit” and can be

overridden by setting the Repeated_flt_inhibit bit to zero.

The FAULT pin can be configured active high or active low

with the Flt_pin_polarity bit. The FAULT pin can be pro-

grammed for open drain, 330

µA internal pull-up, 500µA

internal pull-up or complementary (push-pull) drive with

the two Flt_drv_mode bits. Refer to Figure 8 for an

equivalent circuit of the FAULT pin.

The FAULT pin can be overridden in software for testing

purposes or to allow a microprocessor in the transceiver

module to fully control the module’s fault output. If the

Flt_pin_override bit is set, then the Force_flt bit fully

controls the state of the FAULT pin.

The state of the LTC5100 can be monitored by reading the

FLT_STATUS register. See Table 21 for a description of the

status bits.

EYE SAFETY INFORMATION

Communications lasers can emit levels of optical power

that pose an eye safety risk. While the LTC5100 provides

certain fault detection features, these features alone do

not ensure that a laser transmitter using the LTC5100 is

compliant with IEC 825 or the regulations of any particu-

lar agency. The user must analyze the safety require-

ments of their transceiver module or system, activate the

appropriate laser safety features of the LTC5100, and take

any

additional precautions needed to ensure compliance

of the end product with the requirements of the relevant

regulatory agencies. In particular, the LTC5100 produces

laser currents in response to digitally programmed com-

mands. The user must ensure software written to control

the LTC5100 does not cause excessive levels of radiation

to be emitted by the laser.

POWER CONSUMPTION AND POWER MANAGEMENT

The power consumption of the LTC5100 is dependent on

several variables, including the modulation current range

(set by Im_rng), the laser bias and modulation levels, and

the state of the transmitter (whether enabled or disabled.)

If Power_down_en = 1, the LTC5100 turns off its high

speed amplifiers when the transmitter is disabled, reduc-

ing supply current to less than 5mA (typical). See the

Typical Performance Chacteristics for further information.

HIGH SPEED PEAKING CONTROL

The LTC5100 has the ability to selectively peak the falling

edge of the modulation waveform to accelerate the turn-off

of the laser diode. The 5-bit PEAKING register controls this

function. See the Typical Performance Chacteristics for

further information. Lower values in the PEAKING register

increase the falling edge peaking.

ANALOG-TO-DIGITAL CONVERSION

Overview

The ADC in the LTC5100 is a 10-bit, dual slope integrating

converter with excellent linearity and noise rejection. A

multiplexer allows digitizing six quantities:

• SRC pin current, IS

• Average modulation current, IM

• Laser diode voltage, VLD

• Monitor diode current, IMD

• Termination resistor voltage, VTERM

• Die temperature, T

All of these measurements are available to the user via the

I2C serial bus.

OPERATIO

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