AP5724

WHITE LED STEP-UP CONVERTER

AP5724

Document number: DS31843 Rev. 3 - 2

10 of 17

www.diodes.com

July 2010

© Diodes Incorporated

Open-Circuit Protection

In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the

feedback voltage will be zero. The AP5724 will then switch at a high duty cycle resulting in a high output voltage,

which may cause the SW and OVP pin voltage to exceed the voltage rating of these pins. The OVP pin monitors

the output voltage. If the output voltage reaches the over voltage protection threshold at the OVP pin (Figure 2),

the over voltage protection is activated and SW pin stops switching.

V

IN

SW

EN

GND

FB

AP5724

C

OUT

1uF

5

L1

22uH

D1

V

IN

1uF

OVP

x

x

Figure 2. LED Driver with Open-Circuit Protection

Dimming Control

There are four different types of dimming control circuits:

1. Using a PWM Signal to EN Pin

With the PWM signal applied to the EN pin, the AP5724 is turned on or off by the PWM signal. The LEDs operate

at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM

signal. A 0% duty cycle will turn off the AP5724 and corresponds to zero LED current.

A 100% duty cycle

corresponds to full current. The typical frequency range of the PWM signal is below 2 kHz.