MP3397—4-STRING, MAX 350mA/STRING WHITE LED DRIVER

MP3397 Rev. 1.01

www.MonolithicPower.com

10

9/4/2012

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© 2012 MPS. All Rights Reserved.

For external PWM dimming, ground the BOSC

pin through a resistor, and apply an external

PWM signal to the DBRT pin.

For DC-input PWM dimming, apply a DC analog

signal to the DBRT pin, and connect a capacitor

from BOSC to ground. The DC signal is then

converted to a DPWM dimming signal with a

proportional 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 capacitor from the

BOSC pin to ground.

Open String Protection

Open string protection is achieved through the

OVP pin and the LED (1 to 4) pins. 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 reaches the over-voltage

protection (OVP) threshold. Then the chip marks

which strings have an LEDX pin voltage lower

than 196mV. Once marked, the remaining LED

strings force the output voltage back into tight

regulation. The string with the largest voltage

drop determines the output regulation.

The MP3397 will always attempt to light at least

one string. If all strings are open, the MP3397

shuts down the step-up converter. The strings

will remain in this marked state until the chip

reset.

Short String Protection

The MP3397 monitors the LEDX pin voltages to

determine if a short string fault has occurred. If

one

or

more

strings

are

shorted,

the

corresponding LEDX pins tolerate this higher

voltage. If an LEDX pin voltage is higher than

6.3V, this condition triggers the detection of a

short string. When a short string fault (LEDX

over-voltage fault) continues for 4096 switching

cycles, the fault string is marked OFF and

disabled. Once a string is marked OFF, it

disconnects from the output voltage loop. The

marked LED strings shut off completely until the

part restarts. If all strings are shorted, the

MP3397 will shut down the step-up converter.

The strings remain marked OFF until the chip

resets.