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AS3691
Datasheet
A typical AS3691 for single color leds can be done as follows using automatic supply regulation (feedback path
UV):
Figure 15 – Typical AS3691 system for single color leds and supply regulation loop; serveral AS3691 can be cascaded
VDD
Csup
Main
supply
DCDC Converter
for VDD
100nF
...
...
Voltage Feedback
input for DCDC
UV
...
D1
Vc
Vc
Vc
Vc
D1
...
PWM
5V powered
system
(optional)
Overtemp
AS3691
Overtemp
AS3691
VSS
PWM
(optional)
Note: Csup (100nF) is only required, if there are long wires (>0.3m) between the DCDC converter and the
AS3691. The wire length between the Csup capacitor and the CURRx pin on AS3691 should not exceed 0.3m.
If this cannot be guaranteed, add additional capacitors of 100nF to the pins CURRx.
9.1 Design Example
Assume a single color leds application (4 times 3 leds in series, each 100mA with Uf ranging from Uf min =3.2V to
Uf max = 3.8V) with a fixed supply. First choose the external current set resistor with the following formula:
Ri 1 ? 4 =
250 mV
ICURR 1 ? 4
So for a current of 100mA, use a resistor of 2.5 Ω ; 1/8W rated resistors are suitable (even up to 400mA).
Then calculate the required voltage of the power supply. The minimum voltage on the current sink for
guaranteed operation is 0.41V (V CURR @100mA) and the maximum forward voltage of the LEDs is assumed to
be Uf max = 3.8V. Therefore 3*3.8V + 0.41V = 11.81V.
As this is the required minimum voltage of the power supply, add all the tolerances on top. Assumed +/-10%
supply tolerance results in a power supply with nominal 13V (to have at least 11.81V in worst case).
Using the following formula to calculate the external shunt resistor
Rvdd =
VDD MIN ? VREGINT MAX
IVREG MAX
=
VDD MIN ? 5 . 4 V
2 . 5 mA
VDD MIN is the minimum voltage of the
power supply, where Rvdd is connected
obtains 2564 Ω . The nearest lower(!) available value is 2.4k Ω . For Cvdd use
Cvdd = 100 nF
Csup (100nF) is only required, if there are long connections between the DCDC converter and the AS3691
(>0.3m). The wire length between the Csup capacitor and the CURRx pin on AS3691 should not exceed 0.3m. If
this cannot be guaranteed, add additional capacitors of 100nF to the pins CURRx.
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