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AS3675

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

8.3

Charge Pump

The Charge Pump uses two external flying capacitors C3, C4 to generate output voltages higher than the battery volt-

age. There are three different operating modes of the charge pump itself:

1:1 Bypass Mode

- Battery input and output are connected by a low-impedance switch

- battery current = output current.

1:1.5 Mode

- The output voltage is up to 1.5 times the battery voltage (without load), but is limited to VCPOUTmax all the time

- battery current = 1.5 times output current.

1:2 Mode

- The output voltage is up to 2 times the battery voltage (without load), but is limited to VCPOUTmax all the time

- battery current = 2 times output current

As the battery voltage decreases, the Charge Pump must be switched from 1:1 mode to 1:1.5 mode and eventually in

1:2 mode in order to provide enough supply for the current sinks. Depending on the actual current the mode with best

overall efficiency can be automatically or manually selected:

Examples:

Battery voltage = 3.7V, LED dropout voltage = 3.5V. The 1:1 mode will be selected and there is 200mV drop on the

current sink and on the Charge Pump switch. Efficiency 95%.

Battery voltage = 3.5V, LED dropout voltage = 3.5V. The 1:1.5 mode will be selected and there is 1.5V drop on the

current sink and 250mV on the Charge Pump. Efficiency 66%.

Battery voltage = 3.8V, LED dropout voltage = 4.5V (Camera Flash). The 1:2 mode can be selected and there is

600mV drop on the current sink and 2.5V on the Charge Pump. Efficiency 60%.

The efficiency is dependent on the LED forward voltage given by:

Eff=(V_LED*Iout)/(Uin*Iin)

(EQ 4)

The charge pump mode switching can be done manually or automatically with the following possible software settings:

Automatic up all modes allowed (1:1, 1:1.5, 1:2)

- Start with 1:1 mode

- Switch up automatically 1:1 to 1:1.5 to 1:2

Automatic up, but only 1:1 and 1:1.5 allowed

- Start with 1:1 mode

- Switch up automatically only from 1:1 to 1:1.5 mode; 1:2 mode is not used

Manual

- Set modes 1:1, 1:1.5, 1:2 by software

Step up_lowcur

1R/W

Step Up DC/DC Converter coil current limit

Normal current limit

Current limit reduced by approx. 33%

step_up_fb_auto

0R/W

step_up_fb select the feedback of the DCDC

converter

The feedback is automatically chosen within the

current sinks CURR1, CURR2 and CURR6

(never DCDC_FB). Only those are used for this

selection, which are enabled (currX_mode must

not be 00) and not connected to the charge

pump (currX_on_cp must be 0).

Table 12. DCDC control2

Addr: 22h

DCDC control2

This register controls the Step Up DC/DC Converter and low-voltage current

sinks CURR3x.

Bit

Bit Name

Default Access

Description

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