AS3687 datasheet(14/55 Pages) AMSCO

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AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

1v3-4

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DCDC Control 2

Addr: 22h

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

CURR3x.

Bit

Bit Name

Default

Access

Description

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). Don’t use automatic

feedback selection together with external PWM for the

current sources CURR1, CURR2 or CURR6.

7.2 Charge Pump

The Charge Pump uses two external flying capacitors C6, C7 to generate output voltages higher than the battery

voltage. 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)

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

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