MP3384L – 50V, 4-STRING, WHITE LED DRIVER
MP3384L Rev. 1.0 www.MonolithicPower.com 7
12/27/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OPERATION
The MP3384L is a step-up converter with
peak-current mode that regulates the LED current
in each of its 4 channels. The block diagram
(Figure 1) illustrates its operation.
Internal 5V Regulator
The MP3384L employs an internal linear
regulator (VCC) to supply the internal MOSFET
gate driver and the internal control circuitry. VCC
drops to 0V when the chip shuts down. If VIN is
lower than 5.5V, tie VCC and VIN together. The
MP3384L features under-voltage lockout to
disable the chip until VCC exceeds the UVLO
threshold. UVLO hysteresis is approximately
200mV.
System Startup
When the MP3384L is enabled, the chip checks
the topology connection first. The chip checks for
a Schottky diode on the OVP pin. A V
OVP
below
70mV disables the chip. The MP3384L then
checks for UVLO and OTP threshold crossings
before an internal soft-start drives the step-up
converter.
For bests results, apply the enable signal after
applying both the input voltage and the PWM
dimming signal.
Step-Up Converter
The MP3384L’s selectable operating frequency
(1.25MHz or 625kHz) optimizes for external
component sizes and efficiency.
When each cycle starts, the MOSFET turns ON
after the internal clock. A stabilizing ramp added
to the output of the current-sense amplifier
prevents sub-harmonic oscillations at duty cycles
greater than 50% before it enters the PWM
comparator. When this summed voltage equals
the error amplifier’s output voltage (V
COMP
), the
MOSFET turns OFF.
The output of the internal error amplifier is the
amplified difference between the 600mV
reference voltage and the feedback voltage. The
converter automatically chooses the lowest active
LEDx pin voltage as the boost-control loop’s
feedback to regulate the output voltage to all the
LED arrays.
If the feedback voltage drops below the 600mV
reference, the output of the error amplifier
increases, increasing the current flowing through
the MOSFET to increase the output power for
closed-loop, output-voltage regulation.
At light-loads or when VoutVIN , the converter
runs in pulse-skipping mode to turn the MOSFET
on for the minimum ON time （approximately
100ns). Then the converter powers the output in
the remaining time. The MOSFET will remain
OFF until the output voltage requires another
boost.
Dimming Control
The MP3384L provides PWM dimming. PWM
dimming uses the PWM signal to determine the
size of the current signals from each of the 4 LED
channels. The brightness of the LED array is
directly proportional to the duty cycle of the PWM
signal.
Open-String Protection
Open-string protection relies on over-voltage
protection (OVP) to monitor an LED
X
pin is pulled
to ground and V
OUT
continues to rise until it
reaches the OVP threshold. Then the part records
the LEDx pin voltages less than 180mV before
forcing the remaining LED strings back into tight
regulation. The string with the highest voltage
drop is the reference string during output
regulation.
The MP3384L tries to light at least one string, and
if all strings in use are open, the MP3384L shuts
down the step-up converter. The part retains the
short information until the part shuts down.
Short-String Protection
The MP3384L monitors the LEDx pin voltages for
LED string shorts. If one or more strings are
shorted, the respective LEDx pin rises to the
boost output and tolerates high voltage stress. If
the LEDx pin voltage exceeds 5.5V, this even is
considered a short string. When a short string
fault continues for more than 1.6ms, the string is
recorded and disabled. Once a string turns OFF,
