MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER
MP3389 Rev. 1.04 www.MonolithicPower.com 9
3/27/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OPERATION
The MP3389 employs a programmable constant
frequency, peak current mode step-up converter
and 12-channels regulated current sources to
regulate the array of 12 strings white LEDs. The
operation of the MP3389 can be understood by
referring to the block diagram of Figure 1.
Internal 5V Regulator
The MP3389 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.
The MP3389 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 MP3389 is enabled, the chip checks
the topology connection first. The VFAULT pin
drives the external Fault Disconnection PMOS to
turn on slowly. Then 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 MP3389 will also check other
safety limits, 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
programmable (from 100kHz to 500kHz) with a
external set resistor on OSC pin, which is helpful
for optimizing the external components sizes and
improving the efficiency.
At the beginning of each cycle, the external
MOSFET is turned with the internal clock. 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 result voltage reaches the output
voltage of the error amplifier (V
COMP
) the external
MOSFET is turned off.
The voltage at the output of the internal error
amplifier is an amplified signal of the difference
between the 550mV 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 below the 550mV
reference, the output of the error amplifier
increases. It 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 remain
period. The external MOSFET will keep off until
the output voltage needs to be boosted again.
Dimming Control
The MP3389 provides two PWM dimming
methods: external PWM signal or DC input PWM
Dimming mode (see Figure 2). Both methods
results in PWM chopping of the current in the
LEDs for all 12 channels to provide LED control.
+
-
DPWM
Oscillator
DPWM
Comparator
DBRT
BOSC
Ex-PWM Input
C
BOSC
DPWM Output
Figure 2—PWM Dimming Method
When bias the BOSC pin to a DC level, applying
a PWM signal to the DBRT pin to achieve the
PWM dimming. A DC analog signal can be
