MP3310—50V, 1.3A, STEP-UP WHITE LED DRIVER WITH TRUE PWM DIMMING

MP3310 Rev.1.0

www.MonolithicPower.com

8

7/5/2011

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© 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:

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.

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.