ZLED7022

Low-Voltage Six-Channel LED Driver

Data Sheet

November 9, 2010

© 2010

Zentrum Mikroelektronik Dresden AG - Rev. 1.0

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.

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4

Application Circuit Design

4.1.

Applications

The ZLED7022 is designed for white or other color LED applications requiring a controlled LED current that is generated from

a varying input voltage. Typical applications include main/sub LCD display backlighting, cell phones (including smartphones),

personal digital assistants (PDAs), MP3/MP4 devices, portable media players (PMPs), and general purpose low-voltage

industrial applications. See section 4.2.3 for details and requirements for the LEDs.

Figure 4.1

Basic Application Circuit using D4:D0

4.2.

External Components

4.2.1.

External Capacitors Cout and Cin

The values and type of capacitors used for Cin and Cout can affect several circuit parameters including transient performance,

turn-on time, and output ripple. Use low equivalent series resistance (ESR) ceramic capacitors (internal resistance < 100mΩ)

for best performance, lower costs, and component size reduction. Low ESR capacitors can enable applications at the highest

LED forward voltage with the lowest supply voltage. Tantalum and aluminum electrolytic are not generally recommended

because the ESR is typically in the range of several ohms.

Note: The recommended starting value when determining the optimal value for Cout is 2.2μF. There is often a design trade-off

between low ESR ratings and smaller size because physically larger ceramic capacitors usually have a relatively lower ESR,

which can improve the operating range of the circuit.

Cin is a bypass capacitor (

 2.2μF) essential to proper operation because it must deliver the dynamic current for the charge

pump.

4.2.2.

External Pump Capacitor

The external pump capacitor Cpump connected between the C+ and C- pins is an essential part of the ZLED7022’s internal

charge pump circuit. Because the ZLED7022 operates at a constant charge-pump frequency of 1MHz (typical), it is important

that the pump capacitance is greater than 0.47μF so it can transfer enough charge each cycle to supply sufficient load current.

To reduce voltage fluctuations on the supply line, do not use a value for the pump capacitor that is greater than the bypass

capacitor, Cin.