MP24895 – STEP-DOWN WHITE-LED DRIVER WITH 6-36V INPUT VOLTAGE
MP24895 Rev. 1.0 www.MonolithicPower.com 10
8/6/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 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:
R
SET
=100mV/I
LED
For R
SET
=0.2Ω, the LED current is set to 500mA
Selecting the Inductor
Lower value of the inductor results 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
0.4 I f
For higher efficiency, use inductors with low DC
resistance.
Selecting the Diode
The output diode supplies 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 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 40% 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
MP24895 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.
Figure 3: PWM Dimming External Circuit
MP24895
