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AS3642

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

The DCDC converter always operates in PWM mode (exception: PFM mode is allowed during startup) to reduce EMI

in EMI sensitive systems. For flash and assist light mode and high duty cycles close to 100% on-time (maximum duty

cycle) of the PMOS, the DCDC converter can switch into a 1MHz operating mode and maximum duty cycle to improve

efficiency for this load condition. The DCDC converter returns back to its normal 4MHz operating frequency when load

or supply conditions change. Due to this switching between two fixed frequencies the noise spectrum of the system is

exactly defined and predictable. If improved efficiency is required, the fixed switching between 1MHz / 4MHz can be

disabled by freq_switch_on (see page 16)=0

The internal circuit for switching between these two frequencies is shown in Figure 17:

Figure 17. Internal circuit of 4MHz/1Mhz selection

External Strobe Input

To start the flash operation by an hardware input using an external strobe input, use schematic shown in Figure 18:

Figure 18. External strobe input

To enable this function, program the flash timer with the exact flash duration (by programming flash_timer) and set

external_strobe=1. The AS3642 waits for an external strobe signal on pin LED_OUT and starts the flash pulse with a

duration defined by flash_timer.

2. If current_mode=11 (500mA operating mode), the AS3642 will always use only 4Mhz.



 







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