6 Implementing precise laser pulse control

Generating accurate laser pulses requires more than delivering current into the diode. The driver must deliver high peak currents with fast edges, predictable delay and repeatable pulse amplitude. TI's LMH13000 high-speed laser driver generates pulses by converting the input voltage at the V_SET pin into a precisely regulated sink current at I_OUT, as described by Equation 3. A digital-to-analog converter (DAC) or reference source sets V_SET, while the device's internal current mirror and control circuitry regulate the current through the laser diode, as shown in Figure 3. Careful selection of V_SET, R_SET and the laser anode bias voltage (VLD) allows designers to tune the pulse amplitude, timing and overall pulse stability.

Here are the design steps for setting pulse current and speed.

1. Define the target output current (I_OUT). Begin with the optical power that the laser diode requires. Equation 3 expresses the peak output current, set by the laser's slope efficiency:
   Equation 3. ${\mathrm{I}}_{\mathrm{O}\mathrm{U}\mathrm{T}}=\frac{{\mathrm{P}}_{\mathrm{O}\mathrm{P}\mathrm{T}}}{\mathrm{\eta }}$

   where P_OPT is the desired optical output power and η is the laser's slope efficiency (watts per ampere). For example, if P_OPT = 1W and η = 0.5W/A, then I_OUT = 2A.

   Because the LMH13000 supports pulsed currents up to 5A, the selected laser diode must achieve the target optical power at or below this limit. Accurately setting I_OUT is paramount for minimizing t_pp and reducing amplitude-driven timing errors.

2. Select R_SET and V_SET. The LMH13000 sets the output current using the ratio of V_SET to R_SET, scaled by an internal gain factor k (Equation 4):
   Equation 4. ${\mathrm{I}}_{\mathrm{O}\mathrm{U}\mathrm{T}}=\frac{{\mathrm{V}}_{\mathrm{S}\mathrm{E}\mathrm{T}}}{{\mathrm{R}}_{\mathrm{S}\mathrm{E}\mathrm{T}}}\times \mathrm{k}$

   In high-current mode (MODE = 1), k ≈ 50k. For example, with R_SET = 20kΩ and V_SET = 0.8V:

   ${\mathrm{I}}_{\mathrm{O}\mathrm{U}\mathrm{T}}=\frac{0.8}{20\mathrm{k}}\times 50\mathrm{k}\approx 2.0\mathrm{A}$

   It is possible to make fine adjustments by trimming V_SET with a DAC. Because the LMH13000 regulates current on-chip, this approach minimizes sensitivity to temperature and supply variations, helping keep t_pp small within the timing budget.

3. Set the VLD. VLD must be high enough to support the laser forward voltage and dynamic voltage required during fast current transitions. The LMH13000 data sheet provides Equation 5 as a sizing guideline:
   Equation 5. $\mathrm{V}\mathrm{L}\mathrm{D}={\mathrm{V}}_{\mathrm{O}\mathrm{U}\mathrm{T}\left(\mathrm{M}\mathrm{I}\mathrm{N}\right)}+{\mathrm{V}}_{\mathrm{F}}\mathrm{L}\times \frac{\mathrm{d}\mathrm{I}}{\mathrm{d}\mathrm{t}}+{\mathrm{I}}_{\mathrm{O}\mathrm{U}\mathrm{T}}\times \left({\mathrm{R}}_{\mathrm{L}\mathrm{A}\mathrm{S}\mathrm{E}\mathrm{R}}+{\mathrm{R}}_{\mathrm{D}\mathrm{A}\mathrm{M}\mathrm{P}}\right)$

   where:

   • V_IOUT is the minimum compliance voltage at I_OUT
   • V_F is the forward voltage of the laser at I_OUT
   • L is the total loop inductance (package and PCB)
   • dI/dt is the current slew rate (amperes per second) from rise and fall requirements
   • R_LASER is the dynamic resistance of the laser diode
   • R_DAMP is the external resistance of the laser diode

   For example, with:

   VIOUT(MIN) = 6V

   VF = 2V

   L = 3nH

   $\frac{\mathrm{d}\mathrm{I}}{\mathrm{d}\mathrm{t}}=\frac{2\mathrm{A}}{1\mathrm{n}\mathrm{s}}=2\times 109\mathrm{A}/\mathrm{s}$

   RLASER = 0.3Ω

   RDAMP = 1Ω

   $\mathrm{V}\mathrm{L}\mathrm{D}\approx 6+2+\left(3\times {10}^{-9}\right)\left(2\times {10}^9\right)+2\left(0.3+1.0\right)\approx 16.6\mathrm{V}$

   A starting value of 17V is therefore appropriate. Increasing VLD improves the edge speed but can increase overshoot, thus requiring careful tuning. Proper VLD selection ensures fast transitions while limiting overshoot, directly reducing the rise and fall time (t_r/f) contribution to the overall total timing variation (t_total) budget.

4. Optimize rise and fall times and damping. Both driver capability and circuit parasitics set the rise and fall times. Without proper damping, fast current pulse transitions can excite ringing in the laser and PCB loop, causing overshoot and unstable optical pulses. Designers commonly address this by adding a damping resistor and snubber network at the I_OUT node. Together, the resistor and snubber suppress parasitic ringing, preserve fast edges, and prevent t_r/f from unnecessarily increasing t_total.

   Select snubber capacitors based on the output capacitance of the driver, calculated using Equation 6:

   Equation 6. ${\mathrm{C}}_{\mathrm{S}\mathrm{N}\mathrm{U}\mathrm{B}}\approx 5\times {\mathrm{C}}_{\mathrm{I}\mathrm{O}\mathrm{U}\mathrm{T}}$

   where C_IOUT is the effective capacitance at the I_OUT pin. If C_IOUT = 40pF, then C_SNUB ≈ 200pf.

   Adding a small damping resistor in series with the laser and snubber network suppresses unwanted oscillations. As shown in Figure 4, typical values for R_DAMP and R_SNUB are in the 5Ω to 10Ω range, with the snubber capacitor sized to the output node capacitance. Select C_SNUB for the worst-case (highest) C_IOUT, trimming during validation to balance overshoot and edge speed. As illustrated in Figure 5, this approach reduces ringing from fast transitions and PCB parasitics, while preserving the sub-nanosecond t_r/f required for precise pulse control.

   Figure 4 Damping resistor and snubber network circuit
   
   

   

   
   Figure 5 The LMH13000 pulse with and without a snubber circuit or R_DAMP
5. Control the propagation delay. Unlike rise and fall times, propagation delay is not defined by a formula but instead depends on these layout and interface practices:

• Input routing. Use differential routing for EP pin and EN pin with 100Ω termination, or route a single-ended input with controlled impedance and proper termination at the LMH13000 input.
• Output loop. Keep the high-current I_OUT loop short and tightly coupled to PGND to minimize inductive delay and ringing.
• System calibration. Account for any residual system delay by including the driver-laser path in the ToF measurement budget.

As shown in Figure 6, minimizing the trace inductance and ensuring consistent input termination reduces variation in t_pd, keeping this contribution small and predictable. For applications that require even higher accuracy or where temperature-based calibration is not practical, Section 6.3.2 of the LMH13000 datasheet presents a technique for high-accuracy start-pulse generation by directly monitoring the laser stage.