MP2489 – STEP DOWN WLED DRIVER WITH WIDE 6V–60V INPUT VOLTAGE
MP2489 Rev. 1.23 www.MonolithicPower.com 8
10/11/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the LED Current
The LED current is identical and set by the
current setting resistor between the IN pin and
RS pin.
R
SET
=202mV/I
LED
For R
SET
=0.36Ω, the LED current is set to 550mA
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 with the MP2489. The
diode should be rated for a reverse voltage
greater than the input voltage. The average
current rating must be greater than the maximum
load current expected, and the peak current
rating must be greater than 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. A 10µF–22µF capacitor is
enough for most application. The voltage rating
should be greater than input voltage. A low ESR
capacitor should be used for input decoupling.
Selecting the Output Capacitor
For most applications, output capacitor is not
necessary. But if peak to peak ripple LED current
is required less than 30% of average current, add
a capacitor across the LEDs. Proportionally lower
ripple can be achieved with higher capacitor
value. A value of 2.2µF will meet most
requirements.
Selecting Soft-Start Capacitor
With the soft-start capacitor, the delay time can
be estimated by 0.2ms/nF. In PWM dimming,
C<2.2nF is recommended to eliminate its effect
on LED average current.
Selecting Dimming Control Mode
MP2489 provides two dimming methods: DC
analog dimming and PWM dimming.
1. DC analog dimming mode
Apply a 0.3V–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
VIN
R6
0
C5
1nF
3
6
52
IN
GNDGND
EN/DIM
MP2489
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. Because this pin is
pulled up by 1.25V internal source as inductor
current reference, a NPN-transistor on/off circuit
is recommended to separate PWM signal from
the current reference. (See Figure 3). 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
52
IN
GNDGND
EN/DIM
MP2489
TP4
Figure 3—PWM Dimming External Circuit
Selecting the Inductor
Lower value of inductance can result in higher
switch frequency, which causes larger switch loss.
So switch frequency between 200 kHz to 600
kHz is highly recommended for most application.
According to switch frequency, inductor value
can be estimated as below:
OUT
OUT
IN
LED SW
V
(1- ) V
V
L
0.3 I f
×
=
××
For high efficiency, the inductor’s DC resistance
should be as small as possible.
For 1A application and 40°C temperature rising
restriction, Figure 4 and 5 can provide a
reference for selecting proper value of inductor.
PCB Size: double layer 5cm x 4.6cm
