AS3691

Datasheet

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Revision 2.3

13 - 22

8.3 Automatic Supply Regulation

The purpose of the automatic supply regulation is to minimize the voltage supply to reduce the voltage across the

current sources of the AS3691 (CURR1-CURR4 to VSS) and therefore reduce the power dissipation of the

AS3691 and the complete system. The AS3691 automatically controls the minimum required supply voltage for

the different led strings to support very power efficient systems for lighting using the following circuit (any off-the-

shelf dcdc converter or ldo with adjustable output voltage can be used):

Figure

13 – Automatic Supply Regulation Circuit

The function of this circuit is as follows:

All channels, which are connected to the supply VDDx should have their respective UV pin connected together to

UVx (see above Figure and Section ‘Application Schematic’). If any of these current sources has a too low

voltage, it gradually pulls the wire UVx low. (The analog gain between the current source CURRx and output UVx

Therefore the feedback pin Vfb of the dcdc converter is pulled low and the dcdc converter compensates this by

increasing the voltage on VDDx to obtain the same feedback voltage as before.

To stabilize this regulation loop, the low pass filter build by C1 and R4 is used (this should be the dominant pole

for the regulation loop).

usually Vref = Vfb):

Therefore even if a led string is broken (then UVx is forced to 0V) or some leds are shorted, the supply always

2

2

1

R

R

R

Vref

VDDx

MIN

+

=

)

(

)

(

4

3

2

4

3

2

1

R

R

R

R

R

R

R

Vref

VDDx

MAX

+

+

+

=

UVx

VDDx

Voltage Feedback

input for DCDC

DCDC Converter

for VDD

From main

supply

Feedback resistor divider

(part of DCDC converter circuit)

R1

R2

R3

R4

C1

CURR1

CURR2

UV1

Vc

UV2

Vc

AS3691

Vfb

Csup

100nF