MP24833 – 3A, 55V WHITE LED DRIVER

MP24833 Rev. 1.0

www.MonolithicPower.com

10

05/05/2010

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APPLICATION INFORMATION

Setting the LED Current

The external resistor sets the maximum LED

current (refer to TYPICAL APPLICATION

CIRCUIT) and value can be determined using

the equation:

SENSE

LED

0.198V

R

I

=

Setting Over-Voltage Protection

The voltage divider sets the over-voltage

protection

point

(refer

to

TYPICAL

APPLICATION CIRCUIT) through the equation:

OVP1

OVP2

OVP

OVP2

RR

V1.2V

R

+

=×

Normally, the OVP point is setting about 10%-

30% higher than LED voltage.

Selecting

the

Inductor

(Step-Down

Application, refer to Figure 3. Please refer to

the

design

example

for

buck-boost

application)

Include an inductor with a value ranging from

10µH to 220µH with a DC current rating higher

than the maximum inductor current for most

applications. Include the DC resistance of the

inductor when estimating the output current and

the power consumption on the inductor.

For Buck converter designs, derive the required

inductance value from the following equation.

OUT

IN

OUT

IN

L

S

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose the inductor ripple current to be 30%

(usually in range of 30% to 60%) of the

maximum load current. The maximum inductor

peak current is calculated from:

L

L_peak

L_AVG

ΔI

II

2

=+

the inductor, it is equal to the output load

current (LED current) for buck application.

Under light-load conditions below 100mA, use a

larger inductor for improved efficiency.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. Chose an input capacitor

with a switching-frequency impedance that is

less than the input source impedance to

prevent high-frequency switching current from

passing through the input. Use ceramic

capacitors with X5R or X7R dielectrics if

possible because of their low ESR and small

temperature coefficients.

Select a capacitance that can limit the input

voltage ripple

5% to 10% of the DC value. For buck

application, it is:

LED

OUT

IN

OUT

IN

2

IN

s

IN

IV

(V

V

)

C

Vf

V

××

−

>

Δ× ×

For most applications, use a 4.7µF capacitor.

Please refer to the design example for buck-

boost application.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures a stable feedback loop.

Select an output capacitor with low impedance

at the switching frequency. Use ceramic

capacitors with X5R or X7R dielectrics for their

low ESR characteristics. For buck application,

the output capacitor is selected as following

equation:

L

OUT

OUT

s

I

C

8V

f

Δ

>

Δ×

A 2.2µF to 10µF ceramic capacitor will suffice

for most applications.

Please refer to the design example for the

buck-boost application.

PC Board Layout

Place the high current paths (VSS, VDD and

SW) close to the device with short, direct and

wide traces. Place the input capacitor as close

as possible to the VDD and VSS pins as

possible. Place the external feedback resistors

next to the FB pin. Keep the switch node traces

short and away from the feedback network.