MP24892 – STEP DOWN WLED DRIVER WITH WIDE 6V–45V INPUT VOLTAGE

MP24892 Rev. 1.0

www.MonolithicPower.com

8

11/3/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

The LED current is set by the current sense

resistor between the IN pin and RS pin.

Selecting the Inductor

A lower inductance value results in a higher

switching frequency, which causes greater

switching losses. Select a switching frequency

between 200kHz to 600kHz for most application.

Estimate the inductor value using the equation

OUT

OUT

IN

LED

SW

V

(1-

) V

V

L

0.3 I

f







For higher efficiency, select an inductor with the

lowest DC resistance possible.

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 rated for a reverse

voltage greater than the input voltage. The

average

current

rating

must

exceed

the

maximum expected load current, and the peak

current rating must exceed 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. Use a 10µF–22µF

capacitor with a voltage rating that exceeds the

input voltage for most applications. Select a

capacitor with low ESR.

Selecting the Output Capacitor

An output capacitor is not necessary for most

applications, but if the peak-to-peak ripple LED

current must be less than 30% of the average

current, add a capacitor across the LEDs. The

ripple current is inversely proportional to the

capacitor value. Use a 2.2µF capacitor for most

applications.

Selecting Soft-Start Capacitor

The delay time can be estimated as 0.2ms/nF

with the soft-start capacitor. In PWM dimming,

on the LED average current.

Selecting Dimming Control Mode

MP24892 provides two dimming methods: DC

analog dimming and PWM dimming.

1. DC analog dimming mode

Apply a 0.3V-to-2.5V DC voltage to the EN/DIM

pin. This voltage linearly changes the inductor

current reference 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

MP24892

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 cycle. Because this pin

is pulled up by the 1.25V internal source as the

inductor current reference, use an NPN-transistor

on/off circuit to separate the PWM signal from the

current reference (see Figure 2). 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

MP24892

TP4

Figure 3: PWM Dimming External Circuit

Layout Consideration

Pay close attention to the PCB board layout and

IN and RS pins to minimize the set current error.

The input loop—including the input capacitor, the

Schottky diode, and the internal MOSFET—

should be as short as possible.