MP3378E

– 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER

MP3378E Rev. 1.02

www.MonolithicPower.com

14

5/26/2017

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© 2017 MPS. All Rights Reserved.

For analog dimming, either a PWM signal or a

DC signal can be applied to ADIM.

When a PWM signal is applied to ADIM, this

signal is filtered by the internal RC filter. The

amplitude. A 20kHz or higher PWM signal is

recommended for better filtering performance.

When a DC signal is applied to ADIM, the

voltage ranging from 0.41V to 1.49V sets the

LED current linearly from minimum to full scale

directly.

Open-String Protection

Open-string protection is achieved through OVP

and LEDX (1 to 4). If one or more strings are

open, the respective LEDXs are pulled to

ground,

and

the

WLED

controller

keeps

charging the output voltage until it reaches the

over-voltage protection (OVP) threshold. If the

OVP point has been triggered for >4µs, the

WLED controller stops switching and marks off

the strings which have an LEDX voltage lower

than 190mV. 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 resets.

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 LEDXs tolerate high-voltage stress.

If an LEDX voltage is higher than 6.3V, this

condition triggers the detection of a short string.

When a short-string fault (LEDX over-voltage

fault) remains for 4,096 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 boost part restarts.

In order to prevent mistriggering the short LED

protection when opening the LED string or

sharp ADIM, the short LED protection function

channels are higher than 1.5V.

Inductor/Diode Short Protection

To prevent WLED controller and external

MOSFET

damage

when

the

external

inductor/diode is shorted, the protection mode

operates in two methods.

When the inductor/diode is shorted, the output

cannot maintain enough energy to load the LED,

and the output voltage drops. Thus, the COMP

(error amplifier output) voltage rises until it can

be clamped high. If it lasts longer than 512

switching cycles, the WLED controller turns off

and latches.

However, in some cases, the COMP voltage

cannot

be

clamped

to

high

when

the

inductor/diode is shorted. The WLED controller

provides a protection mode by detecting the

current flowing through the power MOSFET. In

this mode, when the current senses voltage

across the sense resistor (connected between

value (lasting 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, a thermal

shutdown is implemented in this chip and

monitors the silicon die temperature. When the

die temperature exceeds the upper threshold

returns to normal operation when the die

temperature drops below the lower threshold.

Typically, the hysteresis value is 25°C.