8

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

(5)

Setting

I

(6)

The current level to limit the converter can then be com-

puted.

(7)

Using IO = 350mA and ΔI

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

R

(8)

Choose the following values for the resistors:

R

R

R

R

The current sense resistor needs to be at least a 1/4W, 1%

resistor.

Similarly, using I

(2):

R

P

2

Choose the following values for the resistors:

R

0.68Ω, 1/2W, 5%

R

R

0.85

= 2.1A

1.05

= 1.78Ω

= 0.5625

= 0.442