tza3050 NXP Semiconductors, tza3050 Datasheet - Page 16

• Manufacturer Part Number: tza3050
• Description: Tza3050 30 Mbits/s Up To 1.25 Gbits/s Burst Mode Laser Driver
• Manufacturer: NXP Semiconductors

Philips Semiconductors
9397 750 14806
Product data sheet
12.2 Burst mode application
In burst mode application, data flow is not constant, data (the ‘burst‘) is interrupted with
significant idle times when no data is present and the laser is shut-down.
When using average loop control, the control is only done during a burst of data and the
average value should be stored during idle time.
The TZA3050 requires only one external capacitor to perform this storage. A typical
100 nF lossless capacitor connected to pin CBIAS will define the time constant of the loop
during bursts (typical 5 ms) and will also define the accuracy of the value stored between
bursts. Tuning of the external capacitor allows tuning of the average loop time constant
depending on the duty cycle and the burst duration.
When pin ENABLE is LOW, an internal switch is opened and the external capacitor is
connected to a high-impedance point. When pin ENABLE is HIGH, the internal switch is
closed and the external capacitor is connected to the internal average loop control circuit.
The ENABLE pin also controls the on and off switch state of the bias and modulation
current output stages resulting in burst-to-idle and idle-to-burst times below 100 ns on the
full current range available, with a typical load of 25
It is not recommended to use the TZA3050 without an external capacitor on pin CBIAS as
this would result in a too small time constant, with the risk of pattern dependent behavior.
Table 7
Table 7:
Using a smaller CBIAS capacitor allows a faster loop recovery in short burst period
applications, but it also means a shorter storage period. A 100 nF is considered as a
convenient value, even in applications with short burst time (minimum 30 s), and large
idle time (maximum 8 ms) applications as shown in
At power-up, the memorized value on pin CBIAS is reset by connecting pin CBIAS
internally to ground. The timing in
of the storage circuit. This initial timing is directly proportional to the value of the CBIAS
capacitor and to the duty cycle settings of the application.
Capacitor C
10 nF
100 nF
470 nF
Fig 7. Timing between burst and Idle mode
shows time constants for different CBIAS capacitors.
Typical time constant of the average loop
CBIAS
Enable
Rev. 03 — 7 April 2005
30 Mbits/s up to 1.25 Gbits/s burst mode laser driver
burst > 30 s
Figure 7
does not take into account the initial charging
Time constant (typ.)
<1 ms
5 ms
30 ms
idle < 8 ms
Figure
on pins LA and LAQ.
© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
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TZA3050
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