MP24893 – STEP DOWN WLED DRIVER WITH WIDE 6V-TO-36V INPUT VOLTAGE

MP24893 Rev.1.02

www.MonolithicPower.com

8

12/7/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

The LED current is set by the current-setting

resistor between the IN and RS pins, where:

Selecting the Inductor

Lower values of the inductor result in higher

switching frequencies, leading to larger switch

loss. For most applications, select a switching

frequency

between

200kHz

and

600kHz.

Estimate the inductor value based on the desired

switching frequency, where:

OUT

OUT

IN

LED

SW

V

(1-

) V

V

L

0.3 I

f

×

=

××

For higher efficiency, use inductors with low DC

resistance.

Selecting the Diode

The output diode supplies current to the inductor

when the internal MOSFET is off. To reduce

losses due to the diode forward voltage and

recovery time, use a Schottky diode. Select a

diode rated with a reverse voltage greater than

the input voltage. The average current rating

must be 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. Select capacitor value

between 10µF and 22µF for most applications.

The voltage rating should exceed the input

voltage.

Use

a

low-ESR

capacitor

input

decoupling.

Selecting the Output Capacitor

For most applications, an output capacitor is not

necessary. But if the peak-to-peak ripple LED

current must be less than 30% of the average

current, add a capacitor across the LEDs, where

a higher capacitor value leads to a proportionally

lower ripple. A 2.2µF capacitor will meet most

requirements.

Selecting Soft-Start Capacitor

The delay time with the soft-start capacitor can

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

select a C<2.2nF to eliminate its effect on the

average LED current.

Selecting Dimming Control Mode

MP24893 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. 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

EN/DIM

GND

MP24893

VIN

IN

TP4

R6

0

C5

1nF

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 the PWM duty cycle. Add an NPN-

transistor on/off circuit to separate the PWM

signal from the current reference (see Figure 3)

because this pin is pulled up by the 1.25V

internal source as the inductor current reference.

The minimum PWM amplitude is 1.5V.

EN/DIM

GND

MP24893

VIN

IN

EN/DIM

TP4

R6

NC

R4

1k

Q1

R5

NS

C5

82pF

Figure 3: PWM Dimming External Circuit