8

Disabling the LASER

LASER_NEN is connected to the gate of a P-channel

MOSFET transistor which when ON connects RefB to the

LASER. In normal operation, LASER_NEN is low. In the

case of a fault condition (ground or RefB at XYLASER),

LASER_NEN goes high to turn the transistor off and dis-

connect RefB from the LASER.

Single Fault Detection

ADNS-7550 is able to detect a short circuit or fault

condition at the XYLASER pin, which could lead to

excessive laser power output. A path to ground on this

pin will trigger the fault detection circuit, which will turn

off the laser drive current source and set the LASER_NEN

output high. When used in combination with external

components as shown in the block diagram below, the

system will prevent excess laser power for a resistive

path to ground at XYLASER by shutting off the laser. In

addition to the ground path fault detection described

above, the fault detection circuit is continuously

checking for proper operation by internally generating a

path to ground with the laser turned off via LASER_NEN.

will fail and will be reported as a fault.

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

LASER

DRIVER

fault control

block

VCSEL

VDD3

GND

voltage sensor

Microcontroller

Serial port

VCSEL+VE

VCSEL-VE

XYLASER

LASER_NEN

Ref B

current set

ADNS-7550

D

S

G

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PixArt Imaging Inc.

PixArt Imaging Inc.

ADNS-7550 Integrated Molded Lead-Frame DIP Sensor