AT9933 datasheet(9/16 Pages) MICROCHIP

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DS20005597A-page 9

AT9933

4.0

APPLICATION INFORMATION

4.1

Overvoltage Protection

Overvoltage protection can be added by splitting the

output side resistor RS2 into two components and

adding a Zener diode D3. (Refer to Figure 4-1 below.)

When there is an Open LED condition, the diode D3 will

clamp the output voltage and the Zener diode current

will be regulated by the sum of RS2A and RCS2.

4.2

Damping Circuit

The Ćuk converter is inherently unstable when the

output current is being controlled. An uncontrolled input

current will lead to an undamped oscillation between L1

and C1, causing excessively high voltages across

capacitor C1. To prevent these oscillations, a damping

circuit consisting of RD and CD is applied across the

capacitor C1. This damping circuit will stabilize the

circuit and help in the proper operation of the converter.

4.3

Design and Operation of the

Boost-buck Converter

For details on the design for a boost-buck converter

using the AT9933 and the calculation of the damping

components, refer to Application Notes AN-H51 and

AN-H58.

4.4

Design Example

The choice of the resistor dividers to set the input and

output current levels is illustrated by means of the

design example given below.

The parameters of the power circuit are:

VIN MIN



9.01V

VIN MAX



16V

28V

0.35A

fS MIN



300kHz

Using these parameters, the values of the power stage

inductors and capacitor can be computed. (See figures

below.) Refer to Application Note AN-H51 for more

details.

H

150

H

0.22

F

The input and output currents for this design are:

IIN MAX



1.6A



0.21A

350mA



87.5mA

VIN

GATE

CS1

GND

VDD

PWMD

CS2

REF

VDC

CS1

S2A

REF2

REF1

2 (optional)

CS2

AT9933

S2B

FIGURE 4-1:

Design Example Circuit.



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