MP3312 － 2.7V-5.5V INPUT, 38V OVP, DUAL-CHANNEL WHITE LED DRIVER
MP3312 Rev. 1.02 www.MonolithicPower.com 13
1/13/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the LED Current
The full scale LED current can be set through the
current setting resistor on the FB pin.
=
Ω
ISET
ISET
V(V)
ILED(mA) *1020
R(k)
For V
ISET
=1.232V, R
ISET
=63.4k, the LED current
is set to 20mA. Please do not leave ISET pin
open.
Selecting the Input Capacitor
The input capacitor reduces the surge current
drawn from the input supply and the switching
noise from the device. The input capacitor
impedance at the switching frequency should be
much less than the input source impedance to
prevent the high-frequency switching current
from passing through to the input. Use ceramic
capacitors with X5R or X7R dielectrics for their
low ESR and small temperature coefficients. For
most applications, a 1uF~4.7F ceramic
capacitor is ok.
Selecting the Inductor
The MP3312 requires an inductor to supply a
higher output voltage while being driven by the
input voltage. A larger value inductor results in
less ripple current, resulting in lower peak
inductor current and reducing stress on the
internal N-channel MOSFET. However, the larger
value inductor has a larger physical size, higher
series resistance, and lower saturation current.
Choose an inductor that does not saturate under
the worst-case load conditions. Select the
minimum inductor value to ensure that the boost
converter works in continuous conduction mode
with high efficiency and good EMI performance.
Calculate the required inductance value using the
equation:
2
OUT
SW LOAD
V D(1D)
L
2 f I
×××−
≥
××
IN
OUT
V
D1
V
=−
Where V
IN
and V
OUT
are the input and output
voltages, f
SW
is the switching frequency, I
LOAD
is
the total LED load current, and is the efficiency.
The switching current is used for the peak current
mode control. In order to avoid hitting the current
limit, the worst-case inductor peak current should
be less than 80% of the current-limit I
LIM
.
Generally, a 4.7uH~10uH inductor is ok to cover
most of the applications. Note that the system
efficiency is dependent on the DC resistance of
inductor, and larger DC resistance causes larger
power loss.
Selecting the Output Capacitor
The output capacitor keeps the output voltage
ripple small and ensures feedback loop stability.
The output capacitor impedance must be low at
the switching frequency. Ceramic capacitors with
X7R dielectrics are recommended for their low
ESR characteristics. Care must be taken that
ceramic capacitance is also dependent on the
voltage rating; DC bias voltage and the value can
loss as much as 50% of its capacitance at its
rated voltage rating. Please leave enough
voltage rating margin when select the component.
In addition, too low capacitance will cause the
loop instability. For most applications, a
1F~4.7F ceramic capacitor is ok.
Selecting the External Schottky Diode
To optimize the efficiency, a high-speed and low
reverse recovery current schottky diode is
recommended. Make sure the diode’s average
and peak current rating exceeds the output
average LED current and the peak inductor
current. In addition, the diode’s break-down
voltage rating should be large than the maximum
voltage across the diode. Usually, unexpected
high frequency spike voltage can be seen across
the diode when the diode turns off. So, leaving
some voltage rating margin is always needed to
guarantee normal long term operation when
selecting a diode.
Layout Considerations
Careful attention must be paid to the PCB board
layout and components placement. Proper layout
of the high frequency switching path is critical to
prevent noise and electromagnetic interference
problems. The loop of MP3312’s internal low side
MOSFET, schottky diode, and output capacitor is
flowing with high frequency ripple current, it must
be minimized. So the input and output capacitor
should be placed to IC as close as possible.
