CPC9909

8

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R03

The switching frequency is determined by:

Where:

In general, switching frequency selection is based on

the inductor size, controller power dissipation, and the

input filter capacitor.

is between 30 kHz and 120 kHz.

This operating range gives the designer a reasonable

compromise between switching losses and inductor

size. The internal off-time one-shot has an accuracy of

selection for the desired off-time.

2.8 Inductor Design

The inductor value is defined by the LED/inductor

ripple current, minimum off time, and the output

voltage. The minimum off time is determined by the

duty cycle and switching frequency. The duty cycle is

given by:

Where:

average LED current.

The minimum inductor value for a given ripple current

is:

Where:

•

I

The inductor peak current is given by:

2.9 Gate Output Drive

The CPC9909 uses an internal gate drive circuit to

turn on and off an external power MOSFET. The gate

driver can drive a variety of MOSFETs. For a typical

off-line application, the total MOSFET gate charge will

be less than 25nC.

1 D

–

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=

R

0

5

10

15

20

25

30

35

40

45

0

500

1000

1500

2000

2500

3000

R

t

off

(

μS)

D



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=

I

L

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

=

+

=