ZLED7x30 Datasheet

17

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

A

V

V

A

H

t

OFF

µ

µ

56

.

5

26

.

0

30

.

0

333

.

0

36

.

0

4

.

3

333

.

0

3

.

0

220

=

Ω

+

Ω

∗

+

+

∗

∗

=

(

)

s

A

V

V

A

H

t

ON

µ

µ

64

.

2

27

.

0

26

.

0

3

.

0

333

.

0

4

.

3

12

333

.

0

3

.

0

220

=

Ω

+

Ω

+

Ω

∗

−

−

∗

∗

=

The inductance value has an equivalent effect on tON and tOFF and therefore affects the switching frequency. For the

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

forward voltages n * VF to the input voltage VIN is a reasonable approximation. Because the input voltage is a 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.

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 conditions;

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 ZLED7x30’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).