Datasheet

Application notes ST1CC40
16/37 DocID18279 Rev 5
Figure 8 shows the equivalent circuit of the LED constant current generator.
Figure 8. Load equivalent circuit
As a consequence, the LED equivalent circuit gives the 
LED
(s) term correlating the output
voltage with the high impedance FB input:
Equation 13
6.5 Total loop gain
In summary, the open loop gain can be expressed as:
Equation 14
Example
Design specifications:
V
IN
= 12 V, V
FW_LED
= 3.5 V, n
LED
= 2, r
LED
= 1.1 , I
LED
= 700 mA, I
LED RIPPLE
= 2%
The inductor and capacitor value are dimensioned in order to meet the I
LED RIPPLE
specifications (see Section 7.1.2 for output capacitor and inductor selection guidelines):
L = 10
 H, C
OUT
= 2.2  F MLCC (negligible ESR)
9,1
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
LED
n
LED

R
SENSE
n
LED
r
LED
R
SENSE
+
--------------------------------------------------------- -=
Gs G
CO
sA
0
s
LED
n
LED
=