ZLED7720

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.

21 of 27

nF

367

1

.

0

*

V

24

s

67

.

2

*

A

33

.

0

1

C

MIN

=

µ

=

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 2.67

µs. 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.

3.3.5

Capacitor C3 for Reducing Output Ripple

If required, the C3 can be used to reduce peak-to-peak ripple current in the LED string. Low ESR capacitors

should be used because the efficiency of C3 largely depends on its ESR and the dynamic resistance of the LEDs.

For an increased number of LEDs, using the same capacitor will be more effective. Lower ripple can be achieved

with higher capacitor values, but this will increase start-up delay by reducing the slope of the LED voltage as well

as cause increased current during converter start-up. The capacitor will not affect operating frequency or effici-

ency. For a simulation or bench optimization, C3 values of a few

µF are an applicable starting point for the given

configuration. Ripple current reduction is approximately proportional to the value of C3.

3.3.6

Diode D1

The flyback diode D1 must have a continuous current rating greater than the maximum output load current and a

peak current rating higher than the peak L1 coil current. Important: Use a low-capacitance, fast Schottky diode

that has low reverse leakage at the maximum operating temperature and maximum operating voltage for the

application to avoid excess power dissipation and optimize performance and efficiency. For silicon diodes, there is

a concern that the higher forward voltage and increased overshoot from reverse recovery time could increase the

3.4

Application Circuit Layout Requirements

The following guidelines are strongly recommended when laying out application circuits:

ZLED7x20 to minimize parasitic inductance and resistance, which can compromise efficiency. Use low

node.

pickup.