MP24894 – STEP-DOWN LED-CURRENT CONTROLLER WITH WIDE 6V TO 60V INPUT VOLTAGE

MP24894 Rev 1.01

www.MonolithicPower.com

10

12/20/2012

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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 sense resistor between the IN pin and RS

pin.

Selecting the Inductor

Lower value of inductance can result in a higher

switching frequency, which causes a larger

switching loss. Choose a switch frequency

between 100kHz to 600kHz for most application.

According to switching frequency, inductor value

can be estimated as:

OUT

OUT

IN

LED

SW

V

(1-

) V

V

L

0.3 I

f

×

=

××

For higher efficiency, choose an inductor with a

DC resistance as small as possible.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. Choose a capacitor value

between 10µF and 22µF for most applications.

The voltage rating should be greater than the

input voltage. Use a low ESR capacitor for input

decoupling.

Selecting the Output Capacitor

For most applications, the output capacitor is not

necessary. For applications that require that the

peak-to-peak LED ripple current falls below 30%

of the average current, add a capacitor across the

LEDs. Higher capacitor values will result in

proportionally lower ripple. A value of 2.2µF will

meet most requirements.

Selecting Soft-Start Capacitor

When selecting a soft-start capacitor, the delay

time can be estimated as 0.2ms/nF. For PWM

dimming, select C < 2.2nF to eliminate its effect

on the average LED current.

Dimming Control

MP24894 provides 1:2500 high-ratio PWM

dimming. Apply a 100Hz to 20kHz square

waveform to the EN/DIM pin. The average LED

current is proportional to PWM duty cycle.

Layout Consideration

Pay careful attention to the PCB layout and

close to the IN pin and RS pin in order to minimize

current sense error. The input loop—including

input capacitor, Schottky diode, and Mosfet

—should be as short as possible.

Design Example

Below is a design example following the

application guidelines for the specifications:

Table 1: Design Example

24V

10V

Iout

1A

The detailed application schematic is shown in

Figure 2. The typical performance and circuit

waveforms have been shown in the Typical

Performance Characteristics section. For more

device applications, please refer to the related

Evaluation Board Datasheets.