MP3394S—4-STRING, MAX 200mA/STRING WHITE LED DRIVER
MP3394S Rev. 1.03 www.MonolithicPower.com 9
3/21/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OPERATION
Employing the peak-current mode and a
programmable constant frequency, the MP3394S
is a step-up LED driver with 4 channels or
regulated current sources to drive an array of up
to 4 strings of white LEDs.
Internal Regulator
The MP3394S includes an internal linear
regulator (VCC). When VIN is greater than 6V,
this regulator outputs a 5.8V power supply to the
external MOSFET switch’s gate driver and the
internal control circuitry. The VCC voltage drops
to 0V when the chip shuts down. The MP3394S
features under-voltage lockout (UVLO). The chip
is disabled until VCC exceeds the UVLO
threshold. The UVLO hysteresis is approximately
200mV.
System Startup
When enabled, the MP3394S checks the
topology connection first. The chip monitors the
over-voltage protection (OVP) pin to see if the
Schottky diode is connected or if the boost output
is shorted to GND. An OVP voltage of less than
75mV will disable the chip. The MP3394S also
checks other safety limits, including UVLO and
over-temperature protection (OTP) after passing
the OVP test. If all the protection tests pass, the
chip then starts boosting the step-up converter
with an internal soft-start.
It is recommended that the enable signal is given
after the establishment of the input voltage and
PWM dimming signal during the start-up
sequence.
Step-Up Converter
The converter’s operating frequency is
programmable (from 100kHz to 500kHz) with an
external set resistor on the OSC pin. This
flexibility helps to optimize the size of external
components and improve the efficiency.
At the beginning of each cycle, the internal clock
output a signal to turn on the external MOSFET
(minimum turn on time is 100ns). A stabilizing
ramp added to the output of the current sense
amplifier prevents sub-harmonic oscillations for
duty cycles greater than 50 percent. This result is
fed into the PWM comparator. When this
resulting voltage reaches the output voltage of
the error amplifier (V
COMP
), the external MOSFET
turns off.
The output voltage of the internal error amplifier
is an amplified signal of the difference between
the reference voltage and the feedback voltage.
The converter automatically chooses the lowest
active LEDX pin voltage to provide a high-
enough bus voltage to power all the LED arrays.
If the feedback voltage drops below the reference,
the output of the error amplifier increases. This
result in more current flowing through the
MOSFET, thus increasing the power delivered to
the output. This forms a closed loop that
regulates the output voltage.
Under light-load operation—where V
OUT
VIN—
,the converter runs in pulse-skipping mode where
the MOSFET turns on for a minimum on-time of
approximately 100ns, and then the converter
discharges the power to the output for the
remaining period. The external MOSFET remains
off until the output voltage needs to be boosted
again.
Dimming Control
The MP3394S provides two PWM dimming
methods: external PWM signal or DC-input PWM
dimming mode (see Figure 2).
+
-
DPWM
Oscillator
DPWM
Comparator
DBRT
BOSC
Ex-PWM Input
C
BOSC
DPWM Output
Figure 2—PWM Dimming Method
