AS3689 datasheet(17/72 Pages) AMSOSRAM

17 / 72 page

AS3689

austriami crosyst ems

Datasheet, Confidential

www.austriamicrosystems.com (mlg, ptr)

Revision 1.0.2 / 20070115

15 - 70

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

stepup_lowcur

R/W

Step Up DC/DC Converter coil current limit.

0 = Normal current limit

1 = Current limit reduced by approx. 33%

curr1_prot_on

R/W

0 = No overvoltage protection

1 = Pull down current switched on, if voltage exceeds 13.75V,

and step_up_on=1

curr2_prot_on

R/W

0 = No overvoltage protection

1 = Pull down current switched on, if voltage exceeds 13.75V,

and step_up_on=1

curr6_prot_on

R/W

0 = No overvoltage protection

1 = Pull down current switched on, if voltage exceeds 13.75V,

and step_up_on=1

step_up_fb_auto

R/W

0 = step_up_fb select the feedback of the DCDC converter

1 = 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). Don’t use automatic

feedback selection together with external PWM for the

current sources CURR1, CURR2 or CURR6.

7.3 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 (0.5Ω );

−

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 100mV 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)

AS3689 Datasheet(PDF) 17 Page - ams-OSRAM AG

AS3689 Datasheet(HTML) 17 Page - ams-OSRAM AG

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