MP3388- 50V, 8-STRING WHITE LED DRIVERS

MP3388 Rev. 1.0

www.MonolithicPower.com

10

12/16/2011

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© 2011 MPS. All Rights Reserved.

When applying a PWM signal to the PWMI pin,

the MP3388 generates a DC voltage on PWMO

pin which is reversely 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 to the

PWMO pin to modulate the LED current. And the

DC signal is then converted to a DPWM dimming

signal at the setting oscillation frequency. 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 MP3388 always tries to light at least one

string and if all strings in use are open, the

MP3388 shuts down the step-up converter. The

part will maintain mark-off information until the

part shuts down.

Short String Protection

The MP3388 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, 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 will be shut off totally until the part restarts.

If all strings in use are short, the MP3388 will shut

down the step-up converter.