MP4690 – SMART BYPASS FOR LED OPEN PROTECTION

MP4690 Rev. 1.0

www.MonolithicPower.com

8

7/7/2011

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Figure 3 LED Driver with Output Capacitor

For most current mode PWM control Buck

converters like MP2488, the output capacitor

C1 is necessary to reduce the output voltage

ripple and stabilize the control loop. Without the

external inductor L2, two things happen when

LED1 enters open circuit. Firstly, the voltage of

C1 can’t change rapidly so U1 can’t turn on until

current path to maintain the LED2, LED3

current constant so the LED current collapses

to zero rapidly and it’s discontinuous. Secondly,

the Vout voltage has large voltage ripple. As a

result, the primary LED driver controller can’t

work at stable state unless remove the Vout

ripple voltage.

The added inductor L2 can avoid above two

issues without influencing the original LED

driver circuit’s normal operation. The L2 acts

like a current source to boost up the U1 drain

voltage for hiccup operation. When the U1

switch is off, LED current drops but the inductor

will try to push the current into the LED string,

hence charge up the MP4690 drain voltage and

initiates next hiccup cycle within 120ns. The

inductor L2 also eliminates the voltage ripple of

A larger value inductor results in less LED

current drop during which the MP4690 switch is

off, keeping the LED current more constant.

However, a larger value inductor has a larger

physical size, higher series resistance, and

lower saturation current.

A 1µH inductor is recommended for 2W LED

applications

and

a

2.2µH

inductor

is

recommended for most 1W LED applications.

However, a more exact inductance value can

be calculated. A good rule of thumb is to allow

the LED current drop to zero at the moment the

MP4690 switch turns on.

Figure 4 External Inductor Calculation

As figure 4 shows, the external inductor value

can be calculated by following equation:

LED

7V

T

L

I





LED output current and ΔT is the MP4690

protection delay time, typical value is about

100~150ns.

If the LED string is long enough, the inductor

can probably be saved even there exists output

capacitor. The LED forward voltage change will

make up the voltage change to initiate MP4690

hiccup mode. Generally speaking, for 1W LED

application when the series LED quantity is

more than 5, the external inductor can be saved.

Optional Decoupling Capacitor and PCB

Layout

An optional ceramic decoupling capacitor

across MP4690 is recommended for large size

LED application. The decoupling capacitor can

reduce the voltage spike which is produced by

the parasitic inductance and high di/dt from the

open circuit LED. A 3.3nF ceramic capacitor is

recommended for most applications where the

PCB trace from MP4690 to protected LED is

too long. In order to remove the decoupling

capacitor, we recommended improved the PCB

layout as following figure shows.