Electrical and Optical Pulse Response

ZL40539

Data Sheet

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ZL40539 Model

Figure 3 - Pulse Response Model

Figure 3 illustrates a simplified model of the ZL40539 output and the application. The ZL40539 consists of an ideal

switched current source and an equivalent model of the ZL40539 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 ZL40539 are connected together buy interconnect tracks with the return current passing

through the supply decoupling bypass capacitor between ground and output Vcc.

The ZL40539 will typically switch the programmed output current in 400 ps and can be approximated to an ideal

switch with a propagation delay of Iout_on (1.2 nS). The 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 ZL40539 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/1nS) = 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 ZL40539, 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, ZL40539 and interconnect network for two different

interconnect inductance values (5 nH & 7 nH) and two different Diode On resistance (3 Ohm & 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.

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Zarlink Semiconductor Inc.