AS3695C

austriamicrosystems

www.austriamicrosystems.com

Rev 1.11 / 2010-12-08

24 - 39

4.6

Dynamic feedback control

The output of pins “FB1” and “FB2” can be used to control any external power supply for best power efficiency.

Every power supply senses its output voltage with a resistive voltage divider. This voltage divider can be modified

to set the output voltage between a minimum output voltage VMIN and a maximum output voltage VMAX. The

design of the dynamic feedback control is done in 3 steps.

Step 1: Set the resistors R1,R2 in the power supply according to the minimum output voltage

Step 2: Add the Resistors R3 in the power supply according to the maximum output voltage

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Step 3: Connect R3 to the feedback pin “FB”.

C1 should be chosen according to the speed requirements of the feedback loop.

The characteristic of the feedback function can be seen in the following diagram. The final output voltage Vdcdc

is determined by the setting of “FBvoltage” and the current flowing into the FB pin.

R1

R2

SMPS

Vdcdc_MIN

sense

R1

R2

SMPS

Vdcdc_MAX

sense

R3

AS369x

R1

R2

SMPS

Vdcdc_MIN to Vdcdc_MAX

sense

R3

FB

1uF-10uF

Feedback speed

C1

Current

Into

FB-pin

200uA

Voltage Dx

FBvoltage

-2mA/V

FBvoltage -0.1V

200R