MP3388S—55V, 8-STRING, STEP-UP, WHITE LED DRIVER
MP3388S Rev.1.02 www.MonolithicPower.com 10
9/28/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
V
PWM
V
IN
V
EN
V
PWM
V
IN
V
EN
V
PWM
V
IN
V
EN
(a)
(b)
(c)
Figure 4: Recommended Power-Off Sequence
Step-Up Converter
The converter operation frequency is selectable
(625kHz or 1.25MHz), which can optimize for
external component sizes and improve efficiency.
At the beginning of each cycle, the internal clock
turns on the power MOSFET. To prevent
sub-harmonic oscillations at D>50%, add a
stabilizing ramp to the output of the current-sense
amplifier; the output goes to the positive input of
the PWM comparator. When the PWM
comparator’s positive input voltage equals the
output voltage of the error amplifier (V
COMP
) the
power MOSFET turns off.
The internal error amplifier amplifies the
difference between the 600mV reference voltage
and the feedback voltage. The converter
automatically chooses the lowest active LEDx pin
voltage to provide the bus voltage to power all the
LED arrays.
If the feedback voltage drops below the 600mV
reference, the output of the error amplifier
increases to increase the current flowing through
the power MOSFET, thus increasing the power
delivered to the output. This forms a closed loop to
regulate the output voltage.
At light-load operation—or V
OUT
VIN—the
converter enters pulse-skipping mode where the
MOSFET turns on for a minimum ON-time of
approximately 100ns before the converter
discharges the power to the output. The MOSFET
repeats this cycle until the output voltage requires
a boost.
Dimming Control
The MP3388S provides two
PWM dimming
methods: external PWM signal from the PWMI pin
or DC-input PWM dimming mode (see Figure 5).
Figure 5: PWM Dimming
When applying a PWM signal to the PWMI pin, the
MP3388S generates a DC voltage on the PWMO
pin that is proportional to the duty cycle of the
PWMI signal. By comparing the PWMO pin signal
to the FPWM pin triangle waveform, the converter
gets a choppy low-frequency signal with a duty
cycle the same as the input’s. This choppy
low-frequency signal modulates the LED current.
Directly applying a DC analog signal to the PWMO
pin also modulates the LED current: the DC signal
is converted to a DPWM signal set at the
oscillation frequency. The polarity is negative. The
brightness of the LED array is proportional to the
duty cycle of the DPWM signal. The capacitor at
the FPWM pin sets the DPWM signal frequency.
