ZLED7x20 Datasheet

© 2016 Integrated Device Technology, Inc.

20

April 20, 2016

F

135

V

2

.

3

*

3

2

*

V

24

ms

10

*

A

33

.

0

1

C

MIN

µ

=

−

=

RIPPLE

ON

AVG

MIN

V

t

*

I

1

C

=

nF

367

1

.

0

*

V

24

s

67

.

2

*

A

33

.

0

1

C

MIN

=

µ

=

Example: For an application with 3 LEDs with 3.2V forward voltage each driven at 0.33A and supplied with

rectified 24VAC, a min

imum bypass capacitor C1 of 220μF or 330μF might be adequate. Compared to the

calculation, a safety margin of about 50% must be added to consider temperature effects and aging.

(10)

A second function of C1 is to bypass the current ripple of the switching converter and thus prevent it from

disturbing a stable IC supply or backlash on the power supply circuit. For this reason, even in DC-supplied

applications, the use of an adequate C1 might be useful. The defining parameters are now as shown in

equation (11):

(11)

Where:

Symbol

Description

IAVG

Average nominal LED string current.

tON

On time of the internal MOSFET output switch.

Note: tON must be longer than tONmin=200ns.

VRIPPLE

Permissible voltage ripple on the supply voltage.

Example: For an application of 3 LEDs driven at 0.33A and supplied with 24VDC, a maximum ripple of 10% is

allowed. The ZLED7x20 is operated at 150kHz with a duty cycle of 0.4 leading to an on

time of 2.67μs. As

calculated in equation 12, a capacitor C1 of 470nF may be adequate, again including a safety margin of about

50%.

(12)

To achieve maximum stability over temperature and voltage, an X7R, X5R, or better dielectric is recommended

while Y5V must be avoided.

3.3.4

De-bouncing Capacitor C2

External capacitor C2 minimizes ground bounce during switching of the internal MOSFET output switch. Ground

bounce is typically caused by parasitic inductance and resistance due to the distance between the grounds for the

power supply and the ZLED7x20 GND pin. Use a 0.1

μF, X7R ceramic capacitor to ground for C2.