ZLED7000

40V LED Driver with Internal Switch

Data Sheet

August 12, 2010

© 2010 Zentrum Mikroelektronik Dresden AG — Rev. 1.1

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.

14 of 19

The following calculation example yields an operating frequency of 122kHz and a duty cycle of 0.33:



s

47

.

5

3

.

0

48

.

0

A

333

.

0

V

36

.

0

V

4

.

3

A

333

.

0

3

.

0

H

220

























(5)

And



s

73

.

2

9

.

0

48

.

0

3

.

0

A

333

.

0

V

4

.

3

V

12

A

333

.

0

3

.

0

H

220

T

ON





























(6)

3.2

External Component – Capacitor C1

To improve system efficiency, use a low-equivalent-series-resistance (ESR) capacitor for input decoupling

because this capacitor must pass the input current AC component. The capacitor value is defined by the

target maximum ripple of the supply voltage; the value is given by equation (7).

MAX

ON

F

MIN

V

T

I

C







(7)

Where

Value of output current

Maximum ripple of power supply

Maximum ON time of MOSFET

In the case of an AC supply with a rectifier, the capacitor value must be chosen high enough to make sure

that the DC voltage does not drop below the maximum forward voltage of the LED string plus some margin for

the voltage drops across the coil resistance, shunt resistor, and ON resistance of the switching transistor.

Recommendation: Use capacitors with X5R, X7R, or better dielectric for maximum stability over temperature

and voltage. Do not use Y5V capacitors for decoupling in this application. For higher capacitance values,

aluminum electrolytic caps with high

switching capability should be used. In this case, improved perfor-

mance can be reached by an additional X7R/X5R bypass capacitor of at least 100nF.

3.3

External Component – Diode D1

For the rectifier D1, select a high-speed, low-capacitance Schottky diode with low reverse leakage at the

maximum operating voltage and temperature to ensure maximum efficiency and performance.

Important: Choose diodes with a continuous current rating higher than the maximum output load current and a

peak current rating above the peak coil current. When operating above 85°C, the reverse leakage of the diode

must be addressed because it can cause excessive power dissipation in the ZLED7000.

Note: Silicon diodes have a greater forward voltage and overshoot caused by reverse recovery time, which

can increase the peak voltage on the LX output. Ensure that the total voltage appearing on the LX pin,

including supply ripple, is within the specified range (see Table 1.1).