AN2827

Operation modes

Doc ID 15047 Rev 1

17/30

5.5

Flash mode

This mode is intended to be used for high light intensities. The LED current in flash mode

can be adjusted up to 400 mA with the input voltage ranging from 2.5 to 5.5 V.

The flash mode is activated by writing the following data to the COMMAND register.

The DIMMING register value (FDIM) must be written in order to set the desired flash current,

while the FTIM_0 ~ 3 are used to set a maximum time duration for the flash. Values from

0 ~15 correspond to 0 ~1.5 s (100 ms steps). This allows a better safety of the system

providing a certain turn-off of the flash even in case of application software problems.

The activation of the flash mode requires the TRIG pin to be High. The flash mode is active

only when the TRIG_EN bit in the COMMAND register is set to 1 and the TRIG pin is High.

This gives the user the possibility to choose between a soft and a hard triggering of the flash

mode.

The soft triggering is done by writing data to the internal registers only, while the TRIG pin is

microcontroller, which can be used for a different purpose, but this way of triggering is less

accurate than the hard one. The second disadvantage of this solution is that the flash

duration can only be set in discrete steps of the internal timer (1 step = approx. 100 ms).

Hard triggering of the flash mode requires the microcontroller to manage the TRIG pin. The

COMMAND and DIMMING registers are loaded with data before the TRIG pin is set to High.

This allows the user to avoid the I²C bus latency. The flash mode then starts as soon as the

TRIG pin is set to High. It takes typically about 0.7 ms to ramp-up the LED current to the

adjusted value. This time may vary according to the LED current value and the battery

voltage.

If the TRIG pin is kept High for more than the FTIM value, the internal timer reaches zero

and the flash mode is terminated. As soon as the flash is timed out, the ATN pin is pulled

down for 11 µs to inform the microcontroller that the STATUS register has been updated and

that the flash is over. If the TRIG pin is set to Low before the internal timer reaches zero, the

flash mode will be interrupted and can be restarted by setting the TRIG pin High again. The

internal timer is stopped while the TRIG pin is Low. This means that the user can split the

flash into several pulses of a total length equal to the FTIM value. Figure 14 shows the

splitting of the flash into several shorter pulses. The cumulative length of all the pulses is

determined by the FTIM value. Figure 14 shows the case for

FTIM = 9 (900 ms flash time). The cumulative time when the TRIG pin is High is 1000 ms (5

pulses 200 ms long). The last flash pulse will be100 ms long only. The reason is that the

internal flash timer reaches zero and the TRIG_EN bit is set to 0.

Table 8.

COMMAND register data to enter flash mode

CMD_REG

PWR_ON

TRIG_EN

TCH_ON

NTC_ON

FTIM_3

FTIM_2

FTIM_1

FTIM_0

1

1

xxx

xxx

MSB

LSB