Laser

VT5376

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6.2

Laser or led system managed by host (external micro)

The host must first select LED or LASER (bit [1] of register 0x05).

●

LED

The host must select if the LED is to be driven by the internal DAC or an external

current supply using bit [5] of register 0x05 (led_dac_driven). Bit [3] of register 0x05

(host_config_done) then needs to be set.

–

Case internal DAC drive: VT5376 sets the maximum current from the DAC and

the system starts running.

–

Case external drive: VT5376 powers down its laser_drive and the led_on signal

is present on the LASER_NEN pin.

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LASER

The host must first decide whether to perform LASER fault detection (described in

Section 6.3) then set bit [3] of register 0x05 (host_config_done).

If the system passes the laser fault detection (or laser fault detect was not performed),

the host can then adjust the LASER DAC current by writing a value to bits [0-6] in

register 0x0A AND writing its complementary value to bits [0-6] of register 0x0D, if the

values are not compatible the VT5376 applies the minimum DAC current.

6.3

Laser fault detection and safety feature

The sensor includes a set of diagnostic features that can be carried out at power-up (before

setting host_config_done). The tests listed below can be selected.

●

Check LASER_OUT is not shorted to VDD (LASER_OUT < 1.2V).

–

Enable DAC and disable OUT_HIGH switch by writing 0xF7 to register 0x0B (Top_

laser_setting), then make force_laser_out_on = 1, by writing 0x01 to bit 7 of

register 0x0A (Top_laser_DAC_setting), and 0 to it’s complementary bit (bit 7) in

register 0x0D (Top_laser_Dac_setting_C). Finally read bit 6 of register 0xC to

ensure that laser_low = 1.

External LASER_NEN switch must be fitted in order to make LASER_OUT go

below 0.4V.

●

Check LASER_OUT and RBIN are not shorted to GND (LASER_OUT and

RBIN > 0.4V).

–

Disable DAC and enable OUT_HIGH switch (force_laser_out_high = 1) by writing

0xFE to register 0x0B (Top_laser_setting). Then read register 0x0C

(Top_laser_diagnostics) to ensure that bit 7 (laser_high) is set to 1 and bit 5

(Rbin_low) is set to 0.

If the result of these tests is a pass then the MCU can set the laser system as follows:

1.

Set bit [3] of register 0x05 (host_config_done).

2.

Write 0x25 (laser_drive and laser_nen enable) in register 0x0B (Top_laser_setting).

3.

Write the required DAC value (bits 0-6) in register 0x0A (Top_laser_DAC_setting)

ensuring that force_laser_out_on = 0. Write the 1’s complement value of the above

setting in register 0x0D (Top_laser_Dac_setting_C).