MP3399-12-STRING, CASCADING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3399 Rev. 1.0

www.MonolithicPower.com

11

3/27/2013

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When bias the BOSC pin to a DC level by a

resistor, applying an external PWM signal to the

DBRT pin to achieve the PWM dimming. A DC

analog signal can be directly applied to the DBRT

pin to modulate the LED current. And the DC

signal is then converted to a DPWM dimming

signal at the setting oscillation frequency.

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 BOSC

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 180mV. 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 MP3399 always tries to light at least one

string and if all strings in use are open, the

MP3399 shuts down the step-up converter. The

part will maintain mark-off information until the

part shuts down.

Short String Protection

The MP3399 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 6.5V, the short string condition is detected

on the respective string. When the short string

fault (LEDX over-voltage fault) continues for

greater than 2048 switching clocks, 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 will be shut off totally until

the part restarts. If all strings in use are short, the

MP3399 will shut down the step-up converter.