STCF04TBR Datasheet, PDF (32/43 Pages) STMicroelectronics – High power white LED SuperCap™ driver with I²C interface

Theory of operation
STCF04
10.8
External (microprocessor) temporization using the
FLASH_ON bit
Even though it is possible, it is not recommended to use the FLASH_ON bit to start and stop
the flash operation, because of I²C bus latencies: this may result in inaccurate flash timing.
Nevertheless, if this operation mode is chosen, the FLASH pin must be kept HIGH (logic
level or wired to V_BAT), leaving the whole flash control to the I²C bus. Also in this operation
mode the time counter times out flash operation and keeps the energy dissipated by the
LED within safe limits in case of software deadlock.
10.9
Single flash using internal temporization
Flash triggering can be obtained either by the FLASH pin (hard-triggering) or by I²C
commands (soft-triggering). The first solution is recommended for an accurate start time,
while the second is less accurate because of the I²C bus time latency. Stop time is defined
by the STCF04 internal temporization and its accuracy is determined by the internal
oscillator. For hard-triggering, it is necessary to set the FLASH_ON bit in advance. For soft-
triggering, the FLASH pin must be kept High (logic level or wired to V_BAT) and the flash can
be started by setting the FTIM and the TRIG_EN through I²C (both are located in the CMD
REG). There is a delay time between the moment the flash is triggered and when it appears.
This delay is caused by the time necessary to charge up the output capacitor, which
depends on battery voltage. Once triggered, the flash operation is stopped when the time
counter reaches zero. As soon as the flash is finished, the F_RUN bit is reset, the ATN pin is
pulled down for 11 µs to inform the microprocessor that the STAT_REG has been updated
and the device goes back to Idle mode.
10.10
Light sensor feature
This function works as an optional feature, which is able to optimize the length of the flash
according to the light conditions in the flashed area. It uses an external capacitor C_INT,
which is charged by a current coming from the external voltage reference, which is limited by
the light sensor (generally made by a phototransistor). Before the start of the flash
operation, the C_INT capacitor is discharged by the internal switch to the zero voltage during
200 µs. During the flash operation the C_INT capacitor integrates the charging current
according to the light conditions in the flashed area. When the voltage level on the C_INT
capacitor reaches the internal reference voltage, which is set by the light sensor register
(R4, LS_0~2), the STCF04 stops the operation of the flash and discharges the C_INT
capacitor through an internal switch.
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Doc ID 022927 Rev 3