AK2574 Datasheet, PDF (20/30 Pages) Asahi Kasei Microsystems – 156M Laser Diode Driver + APC for Burst Mode

ASAHI KASEI
[AK2574]
3. Alarm
AK2574 has 2 alarm functions as shown in Table 3-1.
Table 3-1 Alarm function
ALM
Detection condition
TXACT
When the PDIN voltage (monitor PD current) is
beyond the reference voltage (txact_ref). TXACT is
kept once TXACT detection until BRST=”L”.
CLKALM When the detected 0s or 1s sequential CLK input
(100ns typical). When detected CLKALM,
AK2574 goes into “Shutdwon”.
Release condition
BRST=”L”
Polarity of CLK is
changed
Detected Time
Depens on the condition.
See “3.1 TXACT” for more
information
100ns (typ)
200ns (max)
3.1 TXACT
Fig 3-1illustrates TXACT block diagram. TXACT is detected when the PDIN voltage is beyond the reference voltage
(txact_ref), and TXACT is held during BRST = “H”. The txact_ref is set with RE_DAC_TXACT. TXACT detection
time depends on the Monitor PD current (i_pd) and capacitance (CMPD), external resistor (RPD), AK2574 PDIN
capactance (CPDIN) and txact_ref voltage (see Fig 3-2 for the reference).
Table 3-2 shows the PDIN capacitance (CPDIN) and Table 3-3 shouws the relationship between txact_ref and
RE_DAC_TXACT. The polarity of TXACT can be set by RE_TXACT_POL (0: “H” at detection, 1: “L” at detection).
The TXACT detection time (t_act) is estimated under the 50% mark data input and average monitor current as
follows:
t _ act = −RPD * (CMPD + CPDIN ) * LN (1 − ^{txact _ ref} )
RPD * i _ pd / 2
Note: TXACT detection time varies with input data pattern.
Fig 3-1 TXACT block diagram
Monitor
PD
i_pd
PDIN
RPD
CPDIN
BRST
DAC
(TXACT)
COMP
txact_ref
comp_out
SET/
RESET
AND
TXACT
Fig 3-2 Reference figures for TXACT detection time
i_pd
RPD
i_pd @ DATA=1
0 @ DATA=0
CMPD
vpd
CPDIN
COMP
txact_ref
comp_out
<MS0266-E-00>
-20-
2003/12