MP3386- 50V, 6 STRINGS WHITE LED DRIVERS
MP3386 Rev. 1.0 www.MonolithicPower.com 9
1/19/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
The MP3386 employs a constant frequency,
peak current mode step-up converter and 6-
channels regulated current sources architecture
to regulate the array of 6 strings white LEDs. The
operation of the MP3386 can be understood by
referring to the block diagram of Figure 1.
Internal 5V Regulator
The MP3386 includes an internal linear regulator
(VCC). When VIN is greater than 5.5V, this
regulator offers a 5V power supply for the internal
MOSFET switch gate driver and the internal
control circuitry. The VCC voltage drops to 0V
when the chip shuts down. In the application of
VIN smaller than 5.5V, tie VCC and VIN together
and connect them to an external 5V power
supply. The MP3386 features Under Voltage
Lockout. The chip is disabled until VCC exceeds
the UVLO threshold. And the hysteresis of UVLO
is approximately 200mV.
System Startup
When the MP3386 is enabled, the chip checks
the topology connection first. The VFAULT pin
drives the external Fault Disconnection PMOS to
turn on slowly. And after 400us delay, the chip
monitors the OVP pin to see if the Schottky diode
is not connected or the boost output is short to
GND. If the OVP voltage is lower than 70mV, the
chip will be disabled and the external PMOS is
turned off together. The MP3386 will also check
other safety limit, including UVLO and OTP after
the OVP test is passed. If they are all in function,
it then starts boosting the step-up converter with
an internal soft-start.
It is recommended on the start up sequence that
the enable signal comes after input voltage and
PWM dimming signal established.
Step-up Converter
The converter operation frequency is selectable
(1.25MHz or 625kHz), which is helpful for
optimizing the external components sizes and
improving the efficiency.
At the beginning of each oscillator cycle, the
power FET is turned on. To prevent sub-
harmonic oscillations at duty cycles greater than
50 percent, a stabilizing ramp is added to the
output of the current sense amplifier and the
result is fed into the PWM comparator. When this
voltage equals to the output voltage of the error
amplifier (V
COMP
) the power FET is turned off.
The voltage at the output of the internal error
amplifier is an amplified signal of the difference
between the 600mV reference voltage and the
feedback voltage. The converter automatically
chooses the lowest active LEDX pin voltage for
providing enough bus voltage to power all the
LED arrays.
If the feedback voltage drops to smaller than the
600mV reference, the output of the error amplifier
increases. This results in more current flowing
through the power FET, thus increasing the
power delivered to the output. In this way it forms
a close loop to make the output voltage in
regulation.
At light-load or Vout near to Vin operation, the
converter runs into the pulse-skipping mode, the
FET is turned on for a minimum on-time of
approximately 100ns, and then the converter
discharges the power to the output in the
remnant period. The FET will keep off until the
output voltage needs to be boosted again.
Dimming Control
The MP3386 provides several PWM dimming
methods: external PWM signal from PWMI pin or
DC input PWM Dimming mode (see Figure 2).
These methods results in PWM chopping of the
current in the LEDs for all 6 channels to provide
an average LED current.
+
-
DPWM
Oscillator
DPWM
Comparator
400 k?
1.2V
0.2V
PWMOPWMI
FSET
Ex-PWM Input
CPWM O
CFSET
DPWM Output
1.2V
0.2V
DC Input
Figure 2—PWM Dimming Method
