MP1528 – 9 LAMP, 36V PRECISION WHITE LED DRIVER
MP1528 Rev. 1.9 www.MonolithicPower.com 7
1/25/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2010 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the LED Current
The LED current is set by the voltage at BRT
(V
BRT
) and the sense resistor (R1) at FB. The
V
BRT
range for adjusting the LED current is
0.27V to 1.2V. Over this range, the LED current
can be calculated as:
1R3
V
I
BRT
LED
×
=
Voltages greater than 1.2V can be applied to
BRT. Use 1.2V in the equation if V
BRT
1.2V or
the BRT pin is left open.
For example, the LED current is set to 20mA if
V
BRT
1.2V (or left open) and R1 = 20.
Capacitor Selection
A 4.7µF to 10µF ceramic input capacitor (C1)
and a 0.47µF to 1µF ceramic output capacitor
(C2) are recommended for most applications. A
capacitor (C3, typically 1nF to 10nF) in parallel
with the LED string improves stability, input
ripple and output ripple, especially when large
inductance values (10µH or above) are used.
For the best stability over a wide temperature
range, use capacitors with a X5R or X7R
dielectric.
Inductor Selection
The MP1528 has a 950mA inductor current limit
and can drive up to 9 LEDs at 20mA.
Inductance values (for L1) from 4.7µH to 22µH
will work as well. 6.2µH or 6.8µH is a good
choice for high efficiency and small size. To
prevent saturation, use an inductor with a
saturation current rating that is higher than the
device current limit.
Diode Selection
The MP1528’s high switching frequency
demands a high speed rectifier (D1) for
optimum efficiency. A Schottky diode is
recommended due to its fast recovery time and
low forward voltage drop. Ensure the diode’s
average and peak current ratings exceed the
average output current and peak inductor
current. In addition, the diode’s reverse
breakdown voltage must exceed the output
voltage.
Layout
Careful PC board layout is required due to fast
switching. All components must be placed as
close to the IC as possible. Keep the path
between the inductor L1, diode D1, and output
capacitor C2 extremely short for minimal noise
and ringing. The feedback components such as
the sense resistor R1 must be kept close to the
FB pin to prevent noise injection on the FB pin
trace. The ground return of C1 and C2 should
be tied close to the GND pin. See the MP1528
demo board layout for reference.
MP1528
BRT
GND
EN
FB
SW
BIAS
2
5
6
1
3
4
LED1
LED8
8 LEDs
per string
D1
B160B
V
IN
5V-18V
EN
BRT
C3
10nF
Figure 2—Driving 3 Strings of 8 LEDs in Series (60mA Total Current)
