Datasheet
DocID024346 Rev 1 17/42
LED2001 Application notes
As a consequence, the LED equivalent circuit gives the α
LED
(s) term correlating the output
voltage with the high impedance FB input:
Equation 14
6.5 Total loop gain
In summary, the open loop gain can be expressed as:
Equation 15
Example 1
Design specification:
V
IN
=12 V, V
FW_LED
=3.5 V, n
LED
= 2, r
LED
= 1.1
Ω
, I
LED
= 4 A, I
LED RIPPLE
= 2%
The inductor and capacitor value are dimensioned in order to meet the I
LED RIPPLE
specification (see Section 7.1.2 for output capacitor and inductor selection guidelines):
L=2.2 μH, C
OUT
=2.2 μF MLCC (negligible ESR)
Accordingly, with Section 7.1.1 the sensing resistor value is:
Equation 16
Equation 17
The gain and phase margin Bode diagrams are plotted respectively in Figure 9 and
Figure 10.
α
LED
n
LED
()
R
SENSE
n
LED
r
LED
R
SENSE
+⋅
----------------------------------------------------------=
Gs() G
CO
s()A
0
s()α
LED
n
LED
()⋅⋅=
R
S
100 mV
4A
-------------------- 25 mΩ≅=
α
LED
n
LED
()
R
SENSE
n
LED
r
LED
R
SENSE
+⋅
----------------------------------------------------------=
25 mΩ
21.1Ω⋅ 25 mΩ+
---------------------------------------------- 0.011==