MP4690 – SMART BYPASS FOR LED OPEN PROTECTION

MP4690 Rev. 1.0

www.MonolithicPower.com

7

7/7/2011

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

Open LED protection serves as an electronic

shunt to provide a current bypass for a single

LED that fails due to an open circuit. A typical

forward voltage drop of a normally on-state LED

is approximately 3V, which is below the

MP4690 turn-on threshold. When an LED is

open, the voltage across the opened LED is

boosted up by LED controller, causing the

protection device to turn on.

The MP4690 integrates a low 220mΩ on-

resistance N-channel MOSFET to reduce the

turn-on power loss. This feature makes the

MP4690 suitable for one-, two-, or three-watt

LEDs that have a nominal 3V forward voltage

drop.

The MP4690 is a two terminal device, which

automatically resets if the LED heals itself or is

replaced.

Applying the Open LED Protection

The protection applies to any designs and it is

easy to implement. By connecting one MP4690

in parallel with each LED in a string, the string

is protected from an open-circuit failure.

Figure 1 shows a typical LED driver circuit with

open LED protection device, MP4690.

Figure 1. LED Open Protection Circuit

The LED driver can be any kind of topology that

can provide constant current output, such as

Buck, Boost or Buck-Boost converter.

Adding an Inductor to Smooth LED Current

There are many kinds of LED driver circuits.

Some require output capacitors while others do

not. When the MP4688 LED driver is used,

there is no need to add an external inductor.

Figure 2 No Output Capacitor LED Driver

As shown in Figure 2, since there is no output

capacitor, the LED string is connected to the

buck output inductor L1 directly. In case an

open circuit happens at LED1, for example, the

inductor L1 still maintains the LED current. At

the same time, the voltage across the open

circuit LED1 is boosted up immediately. U1

clamps the voltage at 7V. After about 120ns

delay, the internal MOSFET of U1 turns on,

thus the bypass circuit is established. During

the 120ns period, the current flow in LED2,

LED3 is nearly unchanged. That’s a benefit of

the output inductor L1, which acts like a current

source.

For LED drivers that require output capacitors,

maybe an additional inductor with small value is

needed for short LED strings (<5 LEDs).

Otherwise, the LED current will drop to zero

rapidly when any LED open circuit happens and

the MP4690 doesn’t have enough time to turn

on the switch. Figure 3 is an example of an LED

driver circuit with an output capacitor.