MP3310—50V, 1.3A, STEP-UP WHITE LED DRIVER WITH TRUE PWM DIMMING
MP3310 Rev.1.0 www.MonolithicPower.com 8
7/5/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the Switching Frequency
The resistor on FSET pin is used to set the
switching frequency. The relationship of the
operating frequency vs. the FSET resistor is
showed as the curve in page 5. A 20kΩ to 100kΩ
FSET resistor, which sets the operating
frequency from around 1.2MHz to 300kHz, is
recommended. Leaving the FSET pin open sets
the switching frequency to 1MHz as default.
Setting the LED Current
The regulated total LED current is identical and
set through the current setting resistor on the
ISET pin through the use of the equation:
I
LED
= 4605V / R
ISET
For R
ISET
=60.4kΩ, the LED current is set to about
76.2mA. The ISET pin can not be open.
With internal 500mV reference, the current flows
into LED pin should be less than 120mA for
normal operation.
PWM Dimming
Apply a 200Hz to 2kHz square waveform to the
EN pin to implement PWM dimming of the LEDs.
The minimum recommended amplitude of the
PWM signal is 2V.
Setting the Over Voltage Protection
The open load protection is achieved through the
over voltage protection (OVP) setting. In some
cases, LED strings failure results in the feedback
voltage is always zero. The part then keeps
boosting the output voltage higher and higher. If
the output voltage reaches the programmable
OVP threshold, the protection will be triggered.
To make sure the chip functions properly in
normal operation, the OVP setting resistor divider
must be set with a proper value. The
recommended OVP point is about 1.3 times
higher than the output voltage for normal
operation.
V
OV
=1.24V x (R1+R2)/R2
Selecting the Inductor
A 10μH to 22µH inductor with a DC current rating
of at least 40% higher than the maximum input
current is recommended for most applications at
wide input range. For highest efficiency, the
inductor’s DC resistance should be as small as
possible.
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
less than the input source impedance to prevent
high frequency switching current from passing
through the input. Ceramic capacitors with X5R
or X7R dielectrics are highly recommended
because of their low ESR and small temperature
coefficients. For most applications, two 4.7μF
capacitors paralleled is sufficient.
Selecting the Output Capacitor
The output capacitor keeps the output voltage
ripple small and ensures feedback loop stability.
The output capacitor impedance should be low at
the switching frequency. Ceramic capacitors with
X7R dielectrics are recommended for their low
ESR characteristics. For most applications, a 1μF
ceramic capacitor is sufficient.
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 MP3310 SW to GND pin,
output diode, and output capacitor is flowing with
high frequency pulse current. It must be as short
as possible. The IC exposed pad is internally
connected to GND pin. Lay the GND plane
widely for proper thermal performance. The VCC
pin is the power supply input for the internal
MOSFET switch gate driver and the internal
control circuitry. It must be locally bypassed.
