AT9933
Assuming a 30% peak-to-peak ripple when the converter is
in input current limit mode, the minimum value of the input
current will be:
I
LIM,MIN
= 0.85 • I
IN,LIM
Setting
I
LIM,MIN
= 1.05 • I
IN,PK
(6)
(5)
R
CS2
+ R
S2A
= 120Ω
Choose the following values for the resistors:
R
CS2
= 1.65Ω, 1/4W, 1%
R
REF2
= 10kΩ, 1/8W, 1%
R
S2A
= 100Ω, 1/8W, 1%
R
S2B
= 5.23kΩ, 1/8W, 1%
The current sense resistor needs to be at least a 1/4W, 1%
resistor.
Similarly, using
I
IN
= 2.1A
and
ΔI
IN
= 0.3xI
IN
= 0.63
in (1) and
(2):
R
S1
= 0.442
R
REF1
R
CS1
= 0.228Ω
P
RCS1
= I
2IN,LIM
• R
CS1
= 1W
Choose the following values for the resistors:
R
CS1
= parallel combination of three
0.68Ω, 1/2W, 5%
R
REF1
= 10kΩ, 1/8W, 1%
R
S1
= 4.42kΩ, 1/8W, 1%
(8)
The current level to limit the converter can then be com-
puted.
I
IN LIM
=
1.05
• I
IN PK
0.85
(7)
= 2.1A
Using
IO = 350mA
and
ΔI
O
= 87.5mA
in (1) and (2),
R
CS2
= 1.78Ω
R
S2
= 0.5625
R
REF2
Before the design of the output side is complete, over voltage
protection has to be included in the design. For this applica-
tion, choose a 33V zener diode. This is the voltage at which
the output will clamp in case of an open LED condition. For
a 350mW diode, the maximum current rating at 33V works
out to about 10mA. Using a 2.5mA current level during open
LED conditions, and assuming the same
R
S2
/R
REF2
ratio,
8
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