6 EVM Operation Out-of-the-Box

The EVM, out-of-the-box, has a place to put a resistor load and a place to put a laser diode. The purpose of the resistor load is to set up the appropriate pulses required for a laser diode application. When operating the EVM, out-of-the-box, follow the procedure to properly fine tune the gate drive pulses before powering on a laser diode:

1. Size the resistor load, R5-R8, to represent the application's typical laser diode resistance
2. Populate the resistor load, R5-R8, by soldering 4 parallel 0603 size, 100 mW resistors
3. Achieve the required pulses for the laser diode application
4. Take the resistors, R5-R8, off before powering a laser diode
5. Populate a laser diode

When using LMG1020EVM out-of-the-box, the resistor load, R5 - R8, is not populated. With R5 - R8 not populated, the power loop is open and therefore the K waveform is not switching. For full functionality of the power loop without a laser diode connected, a resistor load R5 - R8 must be soldered on. The resistor load can be soldered by populating the four parallel resistors R5 - R8 as seen in Figure 6-2. To properly solder R5 - R8, use a soldering iron by hand and also hot air directed to the bottom of the board if needed. When selecting a resistor load, use 4 parallel 0603 size, 100 mW resistors and a typical laser diode resistance value of 1 Ω to 20 Ω. To achieve nanosecond pulses, a 1 Ω resistor load is recommended, in which four 4 Ω parallel resistors for R5 - R8 would be needed. The higher the resistor load value, the longer the switching rise and fall times as well as lower peak current. To avoid excessive power dissipation, damaging the R5 - R8 load resistors as seen in Figure 6-1, start by testing the EVM without a bus voltage to achieve the required pulse width, frequency, and repetition rate on the gate test point, Vg (TP4). When first testing the LMG1020EVM, start without a bus voltage. Try to achieve a short 1–2 ns gate pulse width to limit the peak current and thermal dissipation in the load resistors to a safe point when a bus voltage would be present. The on-board pulse shortener can be used to create a 1–2 ns pulse. The pulse shortener is explained in more detail in Section 10.2. After the required gate pulses are achieved, apply a small bus voltage. Begin with 5–10 V and gradually increase. When operating the EVM at 75 V while using the R5 - R8 resistor load, make sure to proceed with high voltage safety and caution. Operating at 75 V, the EVM can achieve high switching frequencies by using very short pulse widths and repetition rates. If the load resistors become damaged, the load resistance can be increased to an appropriate amount to limit the peak current or higher wattage resistors can be used. After achieving the required pulses on the gate, switch out the resistor load, R5-R8, for a laser diode of choice.

Figure 6-1 Schematic View of Unpopulated Load Resistors R5–R8

Figure 6-2 Populate Load Resistors R5–R8 Out-of-the-Box and Depopulate Before Testing Laser Diode