MP3386- 50V, 6 STRINGS WHITE LED DRIVERS

MP3386 Rev. 1.0

www.MonolithicPower.com

10

1/19/2012

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© 2012 MPS. All Rights Reserved.

When applying a PWM signal to the PWMI pin,

the MP3386 generates a DC voltage on PWMO

pin which is proportional to the duty cycle of the

PWMI pin signal. By comparing the PWMO pin

signal with the FSET pin triangle waveform, the

converter gets a low frequency chopping signal

that the duty cycle is same as the input signal.

This low frequency chopping signal will modulate

the LED current.

A DC analog signal can be directly applied at the

PWMO pin to modulate the LED current. And the

DC signal is translated into the duty-cycle of the

low frequency chopping signal. The polarity is

negative.

The brightness of the LED array is proportional to

the duty cycle of the DPWM signal. The DPWM

signal frequency is set by the cap at the FSET

pin.

Open String Protection

The open string protection is achieved through

the over voltage protection. If one or more strings

are open, the respective LEDX pins are pulled to

ground and the IC keeps charging the output

voltage until it reach OVP threshold. Then the

part will mark off the open strings whose LEDX

pin voltage is less than 175mV. Once the mark-

off operation completes, the remaining LED

strings will force the output voltage back into tight

regulation. The string with the highest voltage

drop is the ruling string during output regulation.

The MP3386 always tries to light at least one

string and if all strings in use are open, the

MP3386 shuts down the step-up converter. The

part will maintain mark-off information until the

part shuts down.

Short String Protection

The MP3386 monitors the LEDX pin voltage to

judge if the short string occurs. If one or more

strings are short, the respective LEDX pins will

be pulled up to the boost output and tolerate high

voltage stress. If the LEDX pin voltage is higher

than 5.5V when the internal dimming control logic

is on, the short string condition is detected on the

respective string. When the short string fault

(LEDX over-voltage fault) continues for greater

than 1.6ms, the string is marked off and disabled.

Once a string is marked off, its current regulation

is forced to disconnect from the output voltage

loop regulation. The marked-off LED strings are

shut off totally until the part restarts. If all strings

in use are short, the MP3386 shuts down the

step-up converter.