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ZL40510 Datasheet(PDF) 6 Page - Zarlink Semiconductor Inc

Part No. ZL40510
Description  Dual Output DVD and CD 4 Channel Laser Diode Drivers
Download  30 Pages
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Maker  ZARLINK [Zarlink Semiconductor Inc]
Homepage  http://www.zarlink.com
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ZL40510 Datasheet(HTML) 6 Page - Zarlink Semiconductor Inc

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ZL40510/14
Data Sheet
6
Zarlink Semiconductor Inc.
1.4
Electrical and Optical Pulse Response
Figure 3 - Pulse Response Model
Figure 3 illustrates a simplified model of the typical ZL40510/14 and the application. The ZL40510/14 consists of an
ideal switched current source and an equivalent model of the ZL40510/14 output stage. The Electrical Model for the
Laser Diode is a Voltage source Vd (V_on) in series with the On Resistance Rd all in parallel with the Junction
Capacitance Cd. This simplified model approximately represents the Laser Diode Electrical load when operated
beyond the Laser Threshold. To a first approximation, the Optical output is proportional to the current flow in the
Resistor Rd.
The Laser Diode and the ZL40510/14 are connected together by interconnect tracks with the return current passing
through the supply decoupling bypass capacitor between ground and output Vcc.
The ZL40510/14 can be approximated to an ideal switched programmed current source with a propagation delay of
Iout_on (1.2 nS) and a switch transition time of 400 ps. The final output electrical pulse response parameters, Trise,
Tfall, Overshoot and Undershoot are determined by the combined electrical network as illustrated in Figure 3.
For example, the Rise Time and Fall Time for large current steps can be slew rate limited by the combined
interconnect and fixed interconnect inductance. The Fixed Inductance represents that associated with packaging
and minimum interconnect distance. The Interconnect Inductance is that associated with the additional tracking
between Laser Diode and the ZL40510/14 to accommodate application physical limitations.
For example, if a pulse of 360 mA amplitude (40 mA to 400 mA) is to be switched in a time of 1 nS with the Vd =
1.6 V, then the maximum volt drop across the interconnect inductance is approximately 3.5 V (maximum Vpin for
500 mA output) – 1.6 V (Vdiode) = 1.9 V. Consequently, L*di/dt < 1.9 V. Hence, L < 1.9/ (0.36A/1 nS) = 5.3 nH.
Small current step size Rise and Fall time will be determined by the Bandwidth of the combined network. This is
dominated by the Interconnect Inductance and the output Capacitance. Similarly, the overshoot and undershoot will
be determined by the Q of the network. This is a function of the Source Impedance from the ZL40510/14, the
Interconnect inductance and the Load impedance of the Laser Diode. Figure 3 includes example simplified
estimates of the Q and BW of the combined Laser Diode, ZL40510/14 and interconnect network for two different
interconnect inductance values (5 nH and 7 nH) and two different Diode On resistance (3 Ohm and 7 Ohm). This
simple analysis illustrates the change in BW and Q of the network depending on these parameters. This in turn
effects the Rise Time and Fall time and the Overshoot and Undershoot performance achieved in the application.
En
Iout
500
17p
15
2p
ZL40510 Model
Vd
Rd
Cd
Lint
C_out
Lfix = 3nH
Lint=5nH , BW = 460MHz, Rd=7, Q=j20/(15+7) =0.9
Lint=5nH, BW = 460MHz, Rd=3, Q=j20/(15+3) = 1.11
Lint=7nH, BW = 411MHz, Rd=7, Q=j18/(15+7) = 0.8
Lint=7nH, BW = 411MHz, Rd=3, Q=j18/(15+3) = 1.0
Lint
Lfix = 3nH
C_bypass
K
K
Vcc_A
OutA


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