DS1875 datasheet(19/92 Pages) MAXIM

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quick-trip alarm that is detected by default remains

active until a subsequent comparator sample shows the

condition no longer exists. A second bias-current moni-

tor (BIAS MAX) compares the DS1875’s BIAS DAC’s

code to a digital value stored in the MAX BIAS register.

This comparison is made at every bias-current update

to ensure that a high bias current is quickly detected.

Monitors and Fault Detection

Monitors

Monitoring functions on the DS1875 include a power-on

analog (POA) VCC comparison, five quick-trip com-

parators, and ADC channels. This monitoring combined

with the interrupt masks determine if the DS1875 shuts

down its outputs and triggers the TX-F and FETG out-

puts. All the monitoring levels and interrupt masks are

user programmable with the exception of POA, which

trips at a fixed range and is nonmaskable for safety

reasons.

Power-On Analog (POA)

POA holds the DS1875 in reset until VCC is at a suitable

level (VCC > VPOA) for the part to accurately measure

with its ADC and compare analog signals with its quick-

trip monitors. Because VCC cannot be measured by the

ADC when VCC is less than VPOA, POA also asserts the

VCC low alarm, which is cleared by a VCC ADC conver-

sion greater than the customer-programmable VCC low

ADC limit. This allows a programmable limit to ensure

that the head room requirements of the transceiver are

satisfied during slow power-up. The TX-F and FETG

outputs do not latch until there is a conversion above

the VCC low limit. The POA alarm is nonmaskable. The

TX-F and FETG outputs are asserted when VCC is

below VPOA. See the Low-Voltage Operation section for

more information.

Five Quick-Trip Monitors and Alarms

Five quick-trip monitors are provided to detect potential

laser safety issues. These monitor:

1) High Bias Current (HBIAS)

2) Low Transmit Power (LTXP)

3) High Transmit Power (HTXP)

4) Max Output Current (BIAS MAX)

5) MON3 Quick Trip (M3QT)

The high- and low-transmit power quick-trip registers

(HTXP and LTXP) set the thresholds used to compare

against the BMD voltage to determine if the transmit

power is within specification. The HBIAS quick trip com-

pares the MON1 input (generally from the MAX3643

bias monitor output) against its threshold setting to

determine if the present bias current is above specifica-

tion. The BIAS MAX quick trip is a digital comparison

that determines if the BIAS DAC indicates that the bias

current is above specification. IBIAS is not allowed to

exceed the value set in the MAX BIAS register. When

the DS1875 detects that the bias is at the limit, it sets

the BIAS MAX status bit and clamps the bias current at

the MAX BIAS level. In the closed-loop mode, if the

recalled value from the BIAS LUT is greater than MAX

BIAS then, the update is not done and IBIAS reverts to

the previous IBIAS value. The quick trips are routed to

the TX-F and FETG outputs through interrupt masks to

allow combinations of these alarms to be used to trigger

these outputs. When FETG is triggered, the DS1875 also

disables the MOD and BIAS outputs. See the

BIAS and

MOD Output During Power-Up section for details.

MON3 Quick Trip

One additional quick trip is used to protect the APD

from overcurrent. MON3P is used to monitor the current

through the APD. When MON3P exceeds a threshold

set by the M3QT DAC register (Table 02h, Register

C3h), the PWM is shut down by blocking SW pulses.

The MON3 comparison is single-ended referenced to

ground. In the case where MON3 is used differentially

and not referenced to ground, this must be considered

when setting the MON3 quick-trip threshold.

Additionally, the D2 pin can be driven either high or low

as determined by INV M3QT and MUX M3QT bits in

Lower Memory, Register 79h. An external switch con-

trolled by pin D2 may be used to clamp the converter’s

output when MON3 quick trip occurs. This external

switch discharges the output voltage much faster than

allowing the load to discharge the rail. The MON3

quick-trip alarm can be latched by enabling M3QT LEN

in Table 02h, Register 89h. The latch is reset by setting

M3QT RESET in Lower Memory, Register 78h. A soft

quick trip is performed by setting SOFT M3QT in Lower

Memory, Register 78h (see Figure 4).

ADC Monitors and Alarms

The ADC monitors six channels that measure tempera-

ture (internal temp sensor), VCC, and MON1–MON4

using an analog multiplexer to measure them round

robin with a single ADC. Each channel has a customer-

programmable full-scale range and offset value that is

factory programmed to default value (see Table 3).

Additionally, MON1–MON4 can right-shift results by up

to 7 bits before the results are compared to alarm

thresholds or read over the I2C bus. This allows cus-

tomers with specified ADC ranges to calibrate the ADC

full scale to a factor of 1/2n their specified range to

measure small signals. The DS1875 can then right-shift

the results by n bits to maintain the bit weight of their

specification.

PON Triplexer and SFP Controller

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