Electrical and Optical Pulse Response
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
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.
Zarlink Semiconductor Inc.