MP3312L － 2.7V-5.5V INPUT, 38V OVP, DUAL-CHANNEL WHITE LED DRIVER
MP3312L Rev. 1.0 www.MonolithicPower.com 14
8/11/2015 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 is set through the
current-setting resistor on FB using Equation (2).
=
Ω
ISET
ISET
V(V)
ILED(mA) *1020
R(k)
(2)
For V
ISET
= 1.232V and R
ISET
= 63.4k, the LED
current is set to 20mA. Please do NOT leave
ISET 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 1µF~4.7F ceramic
capacitor will suffice.
Selecting the Inductor
The MP3312L 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, which reduces stress on the
internal N-channel MOSFET. However, the larger
value inductor has a larger physical size, a higher
series resistance, and a 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
Equation (3) and Equation (4):
2
OUT
SW LOAD
V D(1D)
L
2 f I
×××−
≥
××
(3)
IN
OUT
V
D1
V
=−
(4)
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
4.7µH~10µH inductor will suffice for most
applications. Note that the system efficiency is
dependent on the DC resistance of the inductor,
and a larger DC resistance causes more 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. Please note that ceramic
capacitance is also dependent on the voltage
rating; DC bias voltage and the value can lose as
much as 50% of its capacitance at its rated
voltage rating. Please leave a high enough
voltage rating margin when selecting the
component. However, if the capacitance is too
low, it will cause loop instability. For most
applications, a 1F~4.7F ceramic capacitor will
suffice.
Selecting the External Schottky Diode
To optimize efficiency, a high-speed and low-
reverse recovery current Schottky diode is
recommended. Make sure the diode’s average
and peak current ratings exceed the output
average LED current and the peak inductor
current. In addition, the diode’s breakdown
voltage rating should be larger than the
maximum voltage across the diode. Usually,
unexpected high-frequency spikes in the voltage
can be seen across the diode when the diode
turns off. When selecting a diode, always leave a
sufficient voltage rating margin to guarantee
normal, long-term operation.
PCB Layout Guidelines
Efficient PCB layout is critical to prevent noise
and electromagnetic interference. If the loop of
MP3312L’s internal low-side MOSFET, Schottky
diode, and output capacitor flows with a high
frequency ripple current, it MUST be minimized.
The input and output capacitor should be placed
as close to the IC as possible.
