Example 3 On & Off Times

Three Series LEDs

V

V

R

ON

t

ON

t

OFF

36

10.4

113 k

Ω

5.92E-07

1.09E-07

48

10.4

113 k

Ω

4.03E-07

1.12E-06

60

10.4

113 k

Ω

3.06E-07

1.14E-06

Four Series LEDs

36

13.8

113 k

Ω

6.83E-07

7.78E-07

48

13.8

113 k

Ω

4.43E-07

8.21E-07

60

13.8

113 k

Ω

3.28E-07

8.41E-07

Five Series LEDs

36

17.2

113 k

Ω

8.06E-07

5.77E-07

48

17.2

113 k

Ω

4.92E-07

6.34E-07

60

17.2

113 k

Ω

3.54E-07

6.59E-07

Calculate Switching Frequency

Example 3 Switching Frequency

V

V

f

Three Series LEDs

36

10.4

595

48

10.4

656

60

10.4

692

Four Series LEDs

36

13.8

685

48

13.8

791

60

13.8

855

Five Series LEDs

36

17.2

723

48

17.2

888

60

17.2

987

Calculate Inductor Value

Therefore:

You can quickly see one benefit of the modified circuit. The

improved circuit eliminates the input and output voltage vari-

ation on RMS current.

•

I

•

Δi

•

R

•

L = 59 µH (68 µH standard value)

•

Δi

Calculate R

SNS

Original R

Substitute improved circuit t

Simplified:

Typical Application:

•

V

•

I

•

R

•

L = 68 µH

•

t

R

This equation shows that only variations in V

average current over the entire application range. These vari-

ations should be very minor even with large variations in

output voltage.

Calculate Average Current through LED

Modified application circuit average forward current equation.

Simplified:

9

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