MP3385A – 4-STRING WLED CONTROLLER WITH I

MP3385A Rev. 1.01

www.MonolithicPower.com

14

8/29/2017

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If the MODE bits = 10b when the dimming duty

cycle is larger than the 25% threshold, the IC

works in analog dimming mode. Otherwise, the

IC works in PWM dimming mode, and the LED

current follows the PWM ratio. If the MODE bits

= 11b when the dimming duty cycle is larger

than the 50% threshold, the IC works in analog

dimming mode. Otherwise, the IC works in

PWM dimming mode.

In mixed dimming mode, when INTERFACE = 0,

the LED current amplitude is proportional to the

ratio of the external PWM signal if the duty

cycle of PWM is larger than the transfer point

(25% or 50%). The LED current amplitude is

fixed to 100% brightness while the LED current

duty is equal to the input PWM ratio (if the duty

cycle of PWM is lower than the transfer point).

When INTERFACE = 1, the LED current

amplitude and the duty cycle are set by

registers 03H and 04H.

Regardless of the INTERFACE setting (1 or 0),

the LED current frequency is fixed at 23kHz in

mixed dimming mode when the duty cycle is

lower than the transfer point (25% or 50%).

In analog and mixed dimming mode, to avoid

noise at the small dimming ratio, the IC turns off

the current sources if the current is less than

1.2% times the full scale current.

For external analog and mixed dimming modes,

the minimum pulse width of the PWM signal is

limited to 2µs to avoid noise interruption.

Operation Switching Frequency

The operation frequency of the converter can

be changed by the OSC resistor and the FS0-3

bit in register 01H. The switching frequency

register bits when a 100kΩ resistor is

connected to OSC. This optimizes the size of

the external components and system efficiency.

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. The IC continues

charging the output voltage until it reaches the

over-voltage protection (OVP) threshold. If the

OVP point has been triggered, the chip stops

switching and marks the strings that have an

LEDX pin voltage lower than 206mV. 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 MP3385A always attempts to light at least

one string. If all strings are open, the MP3385A

shuts down the step-up converter. The strings

remain in this marked state until the chip resets.

Unused LED String Auto-Detection

For the MP3385A, if an LED string is open or

unused before start-up, the IC detects and

marks off the open channel automatically to

prevent the output from charging to the OVP

value. This function prevents a start-up failure,

which

is

caused

by

LED

short

string

misprotection due to OVP triggering. This is

useful for application use and test board

debugging. The unused LED string auto-

detection function is disabled if the OVP point is

The MP3385A disables the unused LED string

by disabling the corresponding register control

bit for each 4-channel current source. In some

applications, if fewer than four LED strings are

needed, the unused LED current sources can

be disabled by setting the LED4:0 bit to 0 in

register 00H.

Short-String Protection

The MP3385A monitors the LEDX pin 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 the

protection

threshold,

which

can

be

programmable by the LEDS2:0 bits in 01H, this

condition triggers the detection of a short-string

fault. When a short-string fault remains for

longer than 10ms, the fault string is marked off

and disabled. Once a string is marked off, it

disconnects from the output voltage loop until

VIN or EN restarts.

Cycle-by-Cycle Current Limit

To prevent the external components from

exceeding the current stress rating, the IC uses

a cycle-by-cycle current limit protection. The

limit value is programmable from 150mV to