MP24895A

– 6V - 36V, STEP-DOWN, WHITE, LED DRIVER

MP24895A Rev. 1.0

www.MonolithicPower.com

11

2/3/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

The LED current is set by the current-setting

resistor between IN and RS, which can be

calculated with Equation (1):

(1)

Selecting the Inductor

A

lower

inductor

value

results

in

higher

switching frequencies and larger switching loss.

For

most

applications,

select

a

switching

frequency

between

200kHz

and

600kHz.

Estimate the inductor value based on the

desired switching frequency with Equation (2):







OUT

OUT

IN

LED

SW

V

(1-

) V

V

L

0.4 I

f

(2)

For higher efficiency, use inductors with a low

DC resistance.

Selecting the Diode

The output diode supplies a current flowing

path 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 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 surge current

drawn from the input supply and the switching

noise from the device. For most applications,

select a capacitor value between 10µF and

22µF. The voltage rating should exceed the

input voltage. Use a low ESR capacitor input for

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 40% of the average

current, add a capacitor across the LEDs. A

higher capacitor value leads to a proportionally

lower ripple. A 2.2µF capacitor meets most

requirements.

Selecting the Soft-Start Capacitor

The delay time with a soft-start capacitor can be

estimated by 0.2ms/nF. In PWM dimming,

select a capacitor <2.2nF to eliminate its effect

on the average LED current.

Selecting the Dimming Control Mode

The MP24895A provides two dimming methods:

DC analog dimming and PWM dimming.

For DC analog dimming mode, apply a 0.3V to

2.5V DC voltage to EN/DIM. The voltage from

0.3V to 2.5V changes the inductor current

reference directly and controls the inductor

current range linearly from 25% to 200% (see

Figure 2).

Figure 2: Analog Dimming External Circuit

For PWM dimming mode, apply a 100Hz to

2kHz square waveform to EN/DIM. 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 since this EN/DIM is pulled up by the

1.28V internal source as the inductor current

reference (see Figure 3 and Figure 4). The

minimum PWM amplitude is 1.5V.

Figure 3: PWM Dimming External Circuit