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AN2259 Datasheet(PDF) 9 Page - STMicroelectronics

Part # AN2259
Description  High intensity LED driver using
PDF  38 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2259 Datasheet(HTML) 9 Page - STMicroelectronics

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AN2259
3 CURRENT FEEDBACK LOOP
9/38
3
CURRENT FEEDBACK LOOP
To drive LEDs in a constant current mode, the feedback for the regulator is taken by sensing
the voltage drop across the current sense resistor, Rs, as shown in Figure 2. The voltage
divider between the sense resistor and the feedback pin (R1 and R2) scales the voltage at the
feedback pin so that it equals the internal reference voltage at the desired current level.
Figure 2.
Current feedback
In order to get Io = 350 mA, the values of R1, R2 and Rs are selected based on the following
values.
Vref = 3.3V ; Vfb = 1.235V ; Ifb_bias = 2.5 10-6 A ; Rs=0.68
Ω
Vsense=Rs.Io
Using the superposition method:
Since Vref and Vfb come from same band gap, they are directly correlated. K=Vref/Vfb=2.672.
Therefore, the equation can be simplified to:
For 350mA output the selected values are:
R1 = 2.74k
Ω,
R2 =1.30k
Ω and
Rs = 0.68
Ω.
2
R
1
R
2
R
1
R
bias
_
Ifb
2
R
1
R
1
R
Vsense
2
R
1
R
2
R
Vref
Vfb
+
⋅
⋅
+
+
⋅
+
+
⋅
=
2
R
bias
_
Ifb
1
R
2
R
)
Vfb
Vref
(
Vfb
Vsense
⋅
−
⋅
−
−
=
Rs
Vsense
Io
=
Rs
2
R
bias
_
Ifb
1
R
2
R
)
Vfb
Vref
(
Vfb
Io
⋅
−
⋅
−
−
=
Rs
2
R
bias
_
Ifb
Vfb
1
R
2
R
)
1
K
(
1
Io
⋅
−
⋅


⋅
−
−
=



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