10

Single Fault Detection

The sensor is able to detect a short circuit or fault condi-

tion at its internal laser’s cathode node, which could lead

to excessive laser output power. A leakage path to ground

on this node will trigger the fault detection circuit, which

will turn off the laser drive current source and set the in-

ternal signal of LASER_NEN output high. The system will

prevent excessive laser power by shutting off the laser.

In addition to the ground path fault detection described

above, the fault detection circuit is periodically checking

for proper operation by internally generating a path to

ground with the laser turned off via LASER_NEN. If the la-

ser cathode is shorted to VDD3, this test will fail and will

be reported as a fault.

Figure 7. Single Fault Detection and Eye-safety Feature Block Diagram

22nF

-VCSEL

VDD3

VDD3

GND

+VCSEL

Serial Port

LASER DRIVER

VCSEL

LASER_NEN

P_MOSFET

voltage sense

fault control

block

current set

Microcontroller

S

G

D

PixArt Imaging Inc.

Track-on-Glass Laser Sensor

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PixArt Imaging Inc.