QUICK START GUIDE
Demonstration System EPC9126xx
EPC – THE LEADER IN GaN TECHNOLOGY |
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| COPYRIGHT 2019 | | 3
OPERATING PRINCIPLE
The EPC9126xx is shipped as a capacitive discharge laser diode driver.
Please refer to the block diagram (figure 2) and the schematic (figure 4).
It has several possible modifications (section MODIFICATIONS), but only
the basic operation will be covered in this section. The EPC9126xx basic
operating principle is to discharge energy storage capacitance {C10,
C11, C12, C13, C14} through the laser diode, and then recharge {C10,
C11, C12, C13, C14} through the resistor bank {R2, R3, R5, R6}.
The discharge is controlled via an input pulse that is delivered to SMA
connector J5, which is terminated on the demo board with 50 Ω. As
shipped, SW1_6 is turned on, and this pulse is delivered to gate driver
U5. When the input goes high, the gate driver turns on Q1, allowing {C10,
C11, C12, C13, C14} to discharge through the laser diode U2. When the
input goes low, Q1 turns off. If there is current remaining in the power
loop, diodes D1, D2 (if added by user) can conduct and help prevent
overvoltage of the laser and FET.
Measurements of many of the main waveforms can be made through
the SMA test points provided. These test points can provide waveform
measurements with equivalent bandwidths > 3 GHz. As a result, they
have requirements and properties that differ from most conventional
oscilloscope probes. More details on the usage of these test points is
provided in section MEASUREMENT CONSIDERATIONS.
For further details on lidar driver circuits, it is recommended to read
the application note Getting the Most out of eGaN FETs and Your
EPC9126 Laser Driver (AN027), available at
www.epc-co.com
. While this
application note refers to V2.5 of the EPC9126xx, the basic principles
and design methods are still applicable.
LASER DIODE OR LOAD CONSIDERATIONS
The EPC9126xx can be used as is to mount a laser diode or other
load. Figure 3 highlights the output pad locations. However, many
laser suppliers have different mounting footprints, making it difficult
to optimize the performance of the driver and still maintain the
desired flexibility. The use of an interposer PCB provides a solution
to this problem with only a small added performance penalty.
The EPC9126xx Rev. 3 ships with the EPC9989 interposer PCB,
shown in figure 4. The EPC9989 has an assortment of 5 mm square
interposer PCBs that can be snapped off the board. These interposers
have various footprints on the top side that can accommodate
several surface mount laser diodes, an MMCX connector, and several
patterns designed to accommodate a wide variety of possible loads.
These interposers mount between the EPC9126xx and the laser diode
or other load. Figure 5 shows an example of an Excelitas SMD laser
diode mounted with one of the interposers.
Figure 1: Block diagram of EPC9126xx development board
C10, 11, C12, C13, C14
R14, R15, R16, R17, R18
Shunt (J6)
V
BUS
D
1
,D
2
V
OUT
(J7)
S1_1 – S1_5
S1_6
Cap (J3)
Input (J5)
Narrow pulse
generator
V
GS
(J9)
V
GDIN
V
Logic
U
2
Q
1
+
+
–
–
Figure 2: Connection and measurement setup
Figure 3: Output terminals of EPC9126xx
Figure 4: EPC9989 interposer PCB
Top
Bottom
SMD lasers MMCX
Alternate loads
Breakaway
V-grooves
Laser cathode
(FET drain)
Laser anode
(FET drain)
GND (for alternate
applications)
V7
IN
Laser
diode
or load
Signal generator
V
BUS
+
–
+
–
Oscilloscope
(50
Ω
inputs)