ZLED7020

High Current 40V LED Driver with Internal Switch

Data Sheet

August 12, 2011

© 2011 Zentrum Mikroelektronik Dresden AG — Rev. 1.2

All rights reserved. The material contained herein may not be reproduced, adapted, merged, translated, stored, or used without

the prior written consent of the copyright owner. The information furnished in this publication is subject to changes without notice.

19 of 27

kHz

8

.

121

s

56

.

5

s

64

.

2

1

t

t

1

f

OFF

ON

LX

=

µ

+

µ

=

+

=

3

.

0

s

56

.

5

s

64

.

2

s

64

.

2

t

t

t

V

12

V

4

.

3

V

V

D

OFF

ON

ON

IN

LED

LX

≈

µ

+

µ

µ

=

+

≈

=

=

(

)

s

56

.

5

26

.

0

30

.

0

*

A

333

.

0

V

36

.

0

V

4

.

3

A

333

.

0

*

3

.

0

*

H

220

t

OFF

µ

=

Ω

+

Ω

+

+

µ

=

(

)

s

64

.

2

27

.

0

26

.

0

3

.

0

*

A

333

.

0

V

4

.

3

V

12

A

333

.

0

*

3

.

0

*

H

220

t

ON

µ

=

Ω

+

Ω

+

Ω

−

−

µ

=

the same reason, the inductance has no influence on the duty cycle, for which the relationship of the summed

factor in the on time, variations in the input voltage affect the switching frequency and duty cycle.

To achieve optimum performance, duty cycles close to 0.5 at the nominal average supply voltage are preferable

for improving the temperature stability of the output current.

Example:

(5)

(6)

(7)

(8)

For the L1 inductor, use a coil with a continuous current rating higher than the required mean output current and a

saturation current that exceeds the peak output current by 30% to 50% for robustness against transient con-

ditions; e.g., during start-up.

3.3.3

Bypass Capacitor C1

The bypass capacitor C1 has two functions: maintaining operating voltage and bypassing the current ripple of the

switching converter. In general low ESR capacitors must be used.

If the circuit is supplied by rectified line voltage, C1 must provide enough charge to maintain the ZLED7x20’s

minimum operating voltage as well as the forward voltage of the LED string to keep the application working even

if the rectified supply voltage periodically drops below these values. A rough estimate for the minimum capacity

needed can be calculated with equation (9).