AN2827 Datasheet, PDF (16/30 Pages) STMicroelectronics – Driver for double flash LED with I²C interface

Operation modes
AN2827
Table 6. COMMAND register data to enter READY mode
PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3
CMD_REG
1
0
0
1
x
MSB
FTIM_2
x
FTIM_1
x
FTIM_0
x
LSB
As soon as the NTC feature is activated, the internal switch connects the NTC resistor to the
RX resistor, there by creating a voltage divider. The voltage on this divider can be, if desired,
monitored by the A/D converter of the microcontroller. An external voltage reference must be
connected to the NTC to use this feature. The bits NTC_W and NTC_H of the STATUS
register will not be properly set if there is no external reference voltage connected to the
NTC.
If the NTC feature is not going to be used, neither the negative thermistor nor the external
reference needs to be connected. In this case, it is recommended to ground the RX pin. As
the NTC feature is automatically activated during the flash and torch mode, leaving the RX
pin floating could lead to unwanted interruptions of the light due to non-defined voltages on
the RX pin.
5.4
Torch mode
This mode is intended to be used for low light intensities. The LED current in torch mode can
be adjusted in a range from 15 mA up to 120 mA.
The torch mode is activated by writing the following data to the COMMAND register.
Table 7. COMMAND register data to enter torch mode
PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3
CMD_REG
1
0
1
x
x
MSB
FTIM_2
x
FTIM_1
x
FTIM_0
X
LSB
The DIMMING register value (TDIM) must be set as well, unless it has already been set
during a previous operation. If the TDIM register is not set, then the default output current
value will be at the minimum.
As soon as the torch mode is activated, it remains active until a new mode is entered by
writing new data to the COMMAND register.
If the torch mode was terminated by entering READY or flash mode, it can be restarted by
writing the corresponding data to the COMMAND register only, because entering any of the
READY and flash modes does not influence the TDIM value. If the torch mode was
terminated by entering SHUTDOWN mode, then the TDIM value must be set again during
the restart of the torch mode because entering the SHUTDOWN mode clears the TDIM
value.
As soon as the torch mode is activated, the NTC feature is automatically activated too so as
to protect the LED from overheating. The NTC feature will be activated even if the NTC_ON
bit in the COMMAND register is set to zero.
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Doc ID 15047 Rev 1