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AS3644

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

8 Detailed Description

The AS3644 is a high performance DCDC step up converter with internal PMOS and NMOS switches. The switching

frequency of 4MHz allows the use of tiny coils. Its output is connected to a flash LED by an internal current source.

The AS3644 is controlled by an I

support following operating modes:

1. Flash mode (enabled by mode=11):

The LED current (260mA...320mA) is defined by register flash_current. A timer defines the output flash dura-

tion (30ms...480ms in 30ms steps defined by register flash_timer). The flash is started immediately after the

end of the I

2. Assist light mode (=video or torch light mode) (enabled by mode=10):

The LED current (51.6mA or 72.3mA) is defined by register assist_current. The current is enabled until another

mode is chosen by the interface.

3. Shutdown mode (mode=00), SCL=0V, SDA=0V:

The DCDC and the current source is disabled and the AS3644 is configured to draw minimum current.

Internal Circuit

The AS3644 includes a fixed frequency DCDC step-up with accurate startup control. Together with the output current

source (on LED_OUT) it includes protection and safety functions as shown in the following internal blockdiagram:

Figure 16. AS3644 internal circuit

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 15)=0. In this case pulseskip will be used.

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





 



 



 

 

 

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AS3644