HV9921/22

NR040805

7

switching currents of the converter. As a rule of

thumb, this capacitor should be approximately 0.1-

0.2

µF/W of LED output power. A recommended

input filter is shown in Figure 2 for the following

design example.

Design Example 1

Let us design an HV9921 LED lamp driver meeting

the following specifications:

Input:

Universal AC, 85-264VAC

Output Current: 20mA

Load:

String of 10 LED (LW541C by

OSRAM

Step 1.

Calculating L1.

equation (1) assuming a 30% peak-to-peak ripple.

mH

mA

s

V

L

72

20

3

.

0

5

.

10

41

1

=

⋅

⋅

=

µ

Select L1 68mH, I=30mA. Typical SRF=170KHz.

Calculate the coil capacitance.

pF

KHz

mH

SRF

L

C

L

13

)

170

2

(

68

1

)

2

(

1

1

2

2

≈

⋅

⋅

=

⋅

⋅

=

π

π

Step 2.

Selecting D1

Usually, the reverse recovery characteristics of ultra-

manufacturer’s data books. The designer may want

to experiment with different diodes to achieve the

best result.

Step 3.

Calculating total parasitic capacitance using

(3)

pF

pF

pF

pF

pF

C

P

31

8

13

5

5

=

+

+

+

=

Step 4.

Calculating the leading edge spike duration

using (4), (5)

)

(

136

20

100

31

2

264

MIN

BLANK

SPIKE

T

ns

ns

mA

pF

V

T

<

≈

+

⋅

⋅

=

Step 5.

Estimating power dissipation in HV9921 at

264VAC using (8) and (10)

Let us assume that the overall efficiency

η = 0.7.

Switching power loss:

()

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

⋅

+

⋅

⋅

≈

7

.

0

41

264

20

100

2

31

264

5

.

10

2

1

V

V

ns

mA

pF

V

s

P

SWITCH

µ

mW

P

SWITCH

120

≈

Minimum duty ratio:

16

.

0

)

2

264

7

.

0

/(

41

≈

⋅

⋅

=

V

V

D

m

Conduction power loss:

(

)

mW

V

A

mA

P

COND

55

264

200

63

.

0

210

20

25

.

0

2

≈

⋅

⋅

+

Ω

⋅

⋅

=

µ

Total power dissipation in HV9921:

mW

mW

mW

P

TOTAL

175

55

120

=

+

=

Step 6.

mW

mA

V

820

20

41

Power

Output

=

⋅

=

400V, Metalized Polyester Film).

Design Example 2

Let us design an LED lamp driver using the HV9922

that would meet the following specifications:

Input:

Universal AC, 85-135VAC

Output Current: 50mA

Load:

String of 12 LED (Power TOPLED

®

Step 1.

Calculating L1.

equation (1) assuming a 30% peak-to-peak ripple.

mH

mA

s

V

L

21

50

3

.

0

5

.

10

30

1

=

⋅

⋅

=

µ

Select L1 22mH, I=60mA. Typical SRF=270KHz.

Calculate the coil capacitance.

pF

KHz

mH

SRF

L

C

L

15

)

270

2

(

22

1

)

2

(

1

1

2

2

≈

⋅

⋅

=

⋅

⋅

=

π

π

Step 2.

Selecting D1