LTC5100EUF datasheet(28/52 Pages) LINER

28 / 52 page

LTC5100

sn5100 5100fs

The EN pin and the Soft_en bit must both be active to

enable the transmitter, providing an extra degree of safety

and allowing full software control of the transmitter enable

function. As shown in Figure 6, the EN pin has a weak 10

µA

current source that pulls it to the inactive state in case of

an accidental open on the pin. The EN and Soft_en bits are

inhibited until the LTC5100 has successfully loaded its

registers from an EEPROM or the Operating_mode bit has

been set, signaling that a microprocessor has assumed

control of the chip.

The first time the transmitter is enabled after initial power

up, the servo loops find the correct DAC settings for bias

and modulation current through a feedback process.

Initial settling is typically within 300ms. If the transmitter

is disabled and subsequently re-enabled, the previously

determined DAC settlings are restored. In this case set-

tling occurs typically within 1ms. This feature is called

“Rapid Restart” and can be overridden by setting the

Rapid–restart_en bit to zero.

The LTC5100 has sophisticated eye safety and fault han-

dling features. Five types of faults are detected: low supply

voltage, excessive laser bias current, overpower,

underpower and EEPROM memory load failure. Table 1

summarizes these five faults and how they are handled in

the LTC5100.

Faults are latched in compliance with GBIC requirements.

Faults can be independently enabled (except for low sup-

ply voltage and memory load failure) and are recorded in

an internal register for readout over the serial bus. If two

faults occur simultaneously, the fault with the highest

priority (see Table 1) is recorded in the FLT_STATUS

is cleared only when read (not when the fault itself is

cleared.) Low supply voltage and memory load failure are

considered hard faults and cannot be masked or overrid-

den. They prevent the transmitter from begin enabled until

they are cleared.

Normally, a fault automatically disables the transmitter

and shuts down the laser. In some systems it may be

desirable to allow data transmission to continue after a

OPERATIO

Table 1. Fault Detection and Handling

FAULT TYPE

LASER

EEPROM MEMORY

POWER SUPPLY

SOFTWARE

OVERCURRENT

OVERPOWER

UNDERPOWER

LOAD FAULT

UNDERVOLTAGE

FORCED FAULT

Fault Occurs When

Laser Bias Current

Monitor Diode

EEPROM Load

VDD Drops

The Flt_pin_override

Exceeds the Value

Current is 50%

Starts But Fails

Below 2.8V

and Force_flt Bits

in the IB_LIMIT

Greater Than the

Less than the

to Complete

are Set

Set Point

Priority

Cleared by

Yes

Power-On Reset

Latched in the

Yes

No (Not Part of the

FLT_STATUS

FLT_STATUS Register)

Cleared from the

Yes

No (Not Part of the

FLT_STATUS

FLT_STATUS Register)

Latched at the

Yes

No (The FAULT Pin

Yes (Actually Latched

FAULT Pin

Signals a Fault as

in the FLT_CONFIG

Long as the Supply

Voltage Remains

Too Low)

Enabled by

Over_current_en

Over_pwr_en

Under_pwr_en

Always Enabled

Flt_pin_override

and Apc_en

Glitch Rejection

µs4µs4µs

200mV Typical

Hysteresis

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