ZL40511/15

Data Sheet

6

Zarlink Semiconductor Inc.

1.4

Electrical and Optical Pulse Response

Figure 2 - Pulse Response Model

Figure 3 illustrates a simplified model of the typical ZL40511/15 and the application. The ZL40511/15 consists of an

ideal switched current source and an equivalent model of the ZL40510/15 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 ZL40511/15 are connected together buy interconnect tracks with the return current

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

The ZL40511/15 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 ZL40511/15 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 ZL40511/15, 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, ZL40511/15 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