MP2489 – STEP DOWN WLED DRIVER WITH WIDE 6V–60V INPUT VOLTAGE

MP2489 Rev. 1.23

www.MonolithicPower.com

8

10/11/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

The LED current is identical and set by the

current setting resistor between the IN pin and

RS pin.

Selecting the Diode

The output diode supplies current to the inductor

when the internal MOSFET is off. To reduce

losses due to diode forward voltage and recovery

time, use a Schottky diode with the MP2489. The

diode should be rated for a reverse voltage

greater than the input voltage. The average

current rating must be greater than the maximum

load current expected, and the peak current

rating must be greater than the peak inductor

current.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. A 10µF–22µF capacitor is

enough for most application. The voltage rating

should be greater than input voltage. A low ESR

capacitor should be used for input decoupling.

Selecting the Output Capacitor

For most applications, output capacitor is not

necessary. But if peak to peak ripple LED current

is required less than 30% of average current, add

a capacitor across the LEDs. Proportionally lower

ripple can be achieved with higher capacitor

value. A value of 2.2µF will meet most

requirements.

Selecting Soft-Start Capacitor

With the soft-start capacitor, the delay time can

be estimated by 0.2ms/nF. In PWM dimming,

C<2.2nF is recommended to eliminate its effect

on LED average current.

Selecting Dimming Control Mode

MP2489 provides two dimming methods: DC

analog dimming and PWM dimming.

1. DC analog dimming mode

Apply a 0.3V–2.5V DC voltage to the EN/DIM pin.

The voltage from 0.3V to 2.5V changes the

inductor current reference directly and linearly

controls the inductor current range from 25% to

200%. (See Figure 2)

EN/DIM

VIN

R6

0

C5

1nF

3

6

5

2

IN

GND

GND

EN/DIM

MP2489

TP4

Figure 2—Analog Dimming External Circuit

2. PWM dimming mode

Apply a 100Hz to 2kHz square waveform to the

EN/DIM pin. The average LED current is

proportional to PWM duty. Because this pin is

pulled up by 1.25V internal source as inductor

current reference, a NPN-transistor on/off circuit

is recommended to separate PWM signal from

the current reference. (See Figure 3). The

minimum amplitude of the PWM signal is 1.5V.

EN/DIM

VIN

R6

NC

R4

10k

R7

10k

Q1

R5

NS

C5

1nF

3

6

5

2

IN

GND

GND

EN/DIM

MP2489

TP4

Figure 3—PWM Dimming External Circuit

Selecting the Inductor

Lower value of inductance can result in higher

switch frequency, which causes larger switch loss.

So switch frequency between 200 kHz to 600

kHz is highly recommended for most application.

According to switch frequency, inductor value

can be estimated as below:

OUT

OUT

IN

LED

SW

V

(1-

) V

V

L

0.3 I

f

×

=

××

For high efficiency, the inductor’s DC resistance

should be as small as possible.

For 1A application and 40

°C temperature rising

restriction, Figure 4 and 5 can provide a

reference for selecting proper value of inductor.

PCB Size: double layer 5cm x 4.6cm