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AS3643

Datasheet, Confidential - D e ta i l e d D e s c r i p t i o n

Flash Current Reductions

TXMASK

Usually the flash current is defined by the register led_current . If the TXMASK/TORCH input is used and (configured

by ext_torch_on=01), the flash current is reduced to flash_txmask_current if TXMASK/TORCH=1.

Current Reduction by VIN measurements in Flash Mode

Due to the high load of the flash driver and the ESR of the battery (especially critical at low temperatures), the voltage

on the battery drops. If the voltage drops below the reset threshold of the system would reset. To prevent this condition

the AS3643 monitors the battery voltage and keeps it above vin_low_v_run as follows:

Before a flash is started the voltage on VIN is measured. If the voltage is below the setting of vin_low_v the fault_uvlo

(see page 25) is set and the flash is disabled (driver stays in shutdown) if vin_low_v_shutdown=1. The flash current is

reduced to flash_txmask_current if vin_low_v_shutdown=0.

During flash, if the voltage on VIN drops below the threshold defined by vin_low_v_run, the flash current is reduced (or

ramping of the current is stopped during flash current startup) and fault_uvlo is set. The timing for the reduction of the

current is 8µs/LSB current change.

During the flash pulse the actual used current can be readout by the register led_current_actual.

After the flash pulse the minimum current can be readout by the register led_current_min - this allows to adjust the

camera sensitivity (gain or iso-settings) for the subsequent flash pulse (e.g. when using a pre-flash and a main flash

pulse).

The internal circuit for low voltage current reductions are shown in Figure 20:

Figure 20. Low Voltage current Reduction Internal Circuit

A mobile phone camera flash system can trigger a diagnostic flash and a main-flash:

The diagnostic flash is initiated by the processor. After this diagnostic flash, the determined maximum flash current can

be read back through the I

main flash. Therefore the current in the main-flash is constant and additionally the camera system can use this current

for picture quality adjustments - the waveforms for this concept are shown in Figure 21:



















  

 



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AS3643