MP1518 – FIXED FREQUENCY WHITE LED DRIVER
MP1518 Rev. 1.61 www.MonolithicPower.com 6
4/18/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
TM
APPLICATION INFORMATION
OFF ON
MP1518
V
IN
2.5V to 6V
LED1
LED2
LED3
CMDSH-3
IN
EN
GND
OV
FB
SW
Figure 2—Circuit for Driving 3 WLEDs
A typical application circuit can be seen in
Figure 2. The 3 white LEDs can be driven from
a voltage supply range of 2.5V to 6V at an
output current of 20mA. A 0.22µF output
capacitor is sufficient for most applications but
an output capacitor up to 1µF may be used. A
22µH inductor with low DCR (Inductor
resistance) is recommended to improve
efficiency. A 1µF ceramic capacitor is
recommended for the input capacitance in the
real system. Schottky diodes have fast recovery
and a low forward voltage and are
recommended. Schottky diodes rated with
100mA to 200mA are sufficient for the MP1518.
The switching characteristics during normal
operation can be seen in Figure 3. The MP1518
has internal soft-start to limit the amount of
current through VIN at startup and to also limit
the amount of overshoot on the output. The
current limit is increased by a fourth every 40µs
giving a total soft-start time of 120µs.
V
SW
5V/div.
V
OUT AC
50mV/div.
I
L
100mA/div.
200ns/div.
Steady State Operation
V
IN
= 3.6V, 3 LEDS, 20mA
Figure 3—Steady State Operation
Figure 4 shows the startup behavior of the
MP1518. The ramped voltage that is added to
the current sense amplifier reduces the current
output as the duty cycle increases. As more
LEDs are added, the output voltage rises but
the current that can be delivered to the load is
reduced as well.
