MP2483 – 2.5A, 55V, PROGRAMABLE FREQUENCY WHITE LED DRIVER

MP2483 Rev. 1.0

www.MonolithicPower.com

10

12/25/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

The external resistor is used to set the

maximum LED current (see the schematic on

front page) through the use of the equation:

S E N S E

L E D

0 . 1 9 8 V

R

I

=

Setting the Operating Frequency

The resistor on RSET pin is used to set the

operating frequency. A 1nF capacitor is

recommended to bypass this pin to GND.

The

relationship

between

the

operating

frequency and the RSET resistor is as the

following curve. A 20kΩ to 150kΩ RSET

resistor is recommended, which sets the

operating frequency from around 1.3MHz to

350kHz. Leaving the RSET pin open will set the

operating frequency to the default operating

frequency 1.35MHz.

Frequency vs. RSET

0

200

400

600

800

1000

1200

1400

0

50

100

150

200

250

Series1

Selecting the Inductor

(Step-Down Applications, see Figure 3)

A 1µH to 47µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For high efficiency, the

inductor’s DC resistance should be less than

200mΩ. Refer to Table 2 for suggested surface

mount inductors. For most designs, the required

inductance value can be derived from the

following equation.

SW

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose the inductor ripple current to be 30% of

the maximum load current. The maximum

inductor peak current is calculated from:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, a

larger inductance is recommended for improved

efficiency.

Also note that the maximum recommended load

current is 2A if the duty cycle exceeds 35%.

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, a 4.7µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stable.

The output capacitor impedance should be low

at the switching frequency. Ceramic capacitors

with X5R or X7R dielectrics are recommended

for their low ESR characteristics. For most

applications, a 2.2µF ceramic capacitor is

sufficient.

PC Board Layout

The high current paths (VSS, VDD and SW)

should be placed very close to the device with

short, direct and wide traces. The input

capacitor needs to be as close as possible to

the VDD and VSS pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switch node traces short and away

from the feedback network.