ZLED7x20 Datasheet

© 2016 Integrated Device Technology, Inc.

19

April 20, 2016

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

=

LX

MAX

LX

F

MAX

D

AVG

MIN

f

*

V

D

*

I

V

t

*

I

1

C

∆

=

∆

=

Equations (5), (6), (7), and (8) provide an example of calculating tON, tOFF, operating frequency fLX, and duty cycle

DLX when using a 220

μH inductor for L1 and V

IN=12V, Rs = 0.30

Ω, r

L

=0.26Ω, V

LED=3.4V, IAVG =333mA, VD=0.36V,

and RLX

=0.27Ω.

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).

(9)

Where:

Symbol

Description

IAVG

Average nominal LED string current assuming that the contribution of the IC supply current is negligible.

tD

Discharge time at given AC frequency. Will be a maximum of 10ms (½ period duration) at 50Hz.

ΔVMAX

Peak rectified supply voltage minus LED string forward voltage or minimum ZLED7x20 supply voltage, whichever is greater.