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ZL40514LCE Datasheet(PDF) 6 Page - Zarlink Semiconductor Inc |
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ZL40514LCE Datasheet(HTML) 6 Page - Zarlink Semiconductor Inc |
6 / 30 page 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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