ZLED7022 Datasheet

© 2016 Integrated Device Technology, Inc.

11

April 20, 2016

Cin

Cout

2.8V to 5.5V

1

2

n<=14

1

μF

2.2

μF

2.2

μF

ZLED7022

Cpump

VCC

EN/SET

GND

C+

C-

Vout

D0

D1

D2

D3

D4

D5

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 0.47

μF 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.