AT9933LG-G datasheet(10/16 Pages) MICROCHIP

10 / 16 page

AT9933

DS20005597A-page 10

 2016 Microchip Technology Inc.

4.5

Current Limits

The current sense resistor RCS2, combined with the

other resistors RS2 and RREF2, determines the output

current limits.

The resistors can be chosen using Equation 4-1 and

Equation 4-2.

EQUATION 4-1:

IRCS



1.2V

RREF

-------------



 0.05V

–



Where I is the current (either IO or IIN) and ∆I is the

peak-to-peak ripple in the current (either

∆IO or

∆IIN).

EQUATION 4-2:

IRCS





0.1V

RREF

-------------



 0.1V



Where I is the current (either IO or IIN) and ∆I is the

peak-to-peak ripple in the current (either

∆IO or

∆IIN).

For the input side, the current level used in the

equations should be larger than the maximum input

current, so that it does not interfere with the normal

operation of the circuit. The peak input current can be

computed as shown in Equation 4-3.

EQUATION 4-3:

IIN PK



IIN MAX



IIN



----------





1.706A

Assuming a 30% peak-to-peak ripple when the

converter is in Input Current Limit mode, the minimum

value of the input current is calculated as seen in

Equation 4-4.

EQUATION 4-4:

ILIM MIN



0.85 IIN LIM





Setting

ILIM MIN



1.05 IIN PK





The current level to limit the converter can then be

computed. See equation Equation 4-5.

EQUATION 4-5:

IIN LIM



1.05

0.85

----------



 I

IN PK





2.1A

Using IO = 350 mA and ∆IO = 87.5 mA in Equation 4-1

and Equation 4-2, RCS2 = 1.78Ω and 

RS2/RREF2 = 0.5625.

Before the design of the output side is complete,

overvoltage protection has to be included in the design.

For this application, choose a 33V Zener diode. This is

the voltage at which the output will clamp in case of an

Open LED condition. For a 350 mW diode, the

maximum current rating at 33V works out to about

10 mA. Using a 2.5 mA current level during Open LED

conditions, and assuming the same RS2/RREF2 ratio,

the Zener current limiting resistor can be determined as

illustrated in Equation 4-6.

EQUATION 4-6:

RCS RS2A

120



Choose the following values for the resistors:

RCS2 = 1.65Ω, 1/4W, 1%

RREF2 = 10 kΩ, 1/8W, 1%

RS2A = 100Ω, 1/8W, 1%

RS2B = 5.23 kΩ, 1/8W, 1%

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

1% resistor.

Similarly, using IIN = 2.1A and ∆IIN = 0.3 x IIN = 0.63 in

Equation 4-1 and Equation 4-2, the following values

can be determined:

RS1

RREF1

---------------

0.442

PRCS1

IN LIM



RCS1



Choose the following values for the resistors:

RCS1 = parallel combination of three 0.68Ω, 1/2W, 5%

resistors

RREF1 = 10kΩ, 1/8W, 1%

RS1 = 4.42kΩ, 1/8W, 1%

RCS1

0.228



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