MP3378

– 4-CHANNEL WLED CONTROLLER PLUS BUCK CONVERTER

MP3378 Rev. 1.01

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14

5/26/2017

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(OVP) threshold. If the OVP point has been

triggered for >4 µs, the WLED controller stops

switching and marks off the strings that have an

LEDX voltage lower than 190 mV. Once

marked off, the remaining LED strings force the

output voltage back into tight regulation. The

string with the largest voltage drop determines

the output regulation. If all strings are open, the

WLED controller shuts down until the WLED

controller re-sets.

Short-String Protection

The WLED

controller

monitors

the

LEDX

voltages to determine if a short-string fault has

occurred. If one or more strings are shorted, the

respective LEDX pins tolerate high-voltage

stress. If an LEDX pin voltage is higher than

6.3 V, this condition triggers the detection of a

short string. When a short-string fault (LEDX

over-voltage fault) remains for 4096 switching

clocks, 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

boost

part

re-starts.

In

order

to

prevent

mis-triggering

short

LED

protection

when

opening an LED string or sharp ADIM, the short

LED protection function is disabled when the

Vledxs of all the used LED channels are higher

than 1.5 V.

Inductor/Diode Short Protection

To prevent damage to the WLED controller and

external

MOSFET

when

the

external

inductor/diode is shorted, the protection mode

operates in the following ways:

1. When the inductor/diode is shorted, the

output cannot maintain enough energy

to load the LED, causing the output

voltage to drop. Thus, the COMP (the

error amplifier output) voltage tends to

rise until it is clamped high. If it lasts

longer than 512 switching cycles, the

WLED controller turns off and latches.

2. However, in some cases the COMP

voltage cannot be clamped high when

the inductor/diode is shorted, so the

WLED controller provides the protection

mode by detecting the current flowing

through the power MOSFET. In this

mode, when the current sense voltage

across the sense resistor (connected

limit value and lasts for 4 switching

cycles, the WLED controller

turns off

and latches.

Thermal Shutdown Protection

To prevent the WLED controller from operating

at

exceedingly

high

temperatures,

thermal

shutdown detects the die temperature. When

the

die

temperature

exceeds

the

upper

threshold (150°C), the WLED controller shuts

down. The controller resumes normal operation

when the die temperature drops below the

lower threshold. Typically, the hysteresis value

is 25°C.