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AS3691
Datasheet
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
From main
DCDC Converter
for VDD
VDDx
Csup
100nF
supply
Voltage Feedback
Vfb
R1
R3
R4
UVx
input for DCDC
R2
C1
UV1
CURR1
UV2
CURR2
Feedback resistor divider
(part of DCDC converter circuit)
Vc
Vc
AS3691
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
is defined by the parameter V C,GAIN .)
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).
The minimum output voltage VDDx min can be set accurately by the resistors R1 and R2. The maximum output
voltage VDDx max is set by R1, R2, R3 and R4 (Vref is the internal voltage reference of the DCDC converter;
usually Vref = Vfb):
VDDx MIN = Vref
VDDx MAX = Vref
R 1 + R 2
R 2
R 1 + R 2 ( R 3 + R 4 )
R 2 ( R 3 + R 4 )
Therefore even if a led string is broken (then UVx is forced to 0V) or some leds are shorted, the supply always
stays within the limits VDDx MIN and VDDx MAX .
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