MP1528 – 9 LAMP, 36V PRECISION WHITE LED DRIVER

MP1528 Rev. 1.7

www.MonolithicPower.com

7

2/21/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

Setting the LED Current

The LED current is set by the voltage at BRT

0.27V to 1.2V. Over this range, the LED current

can be calculated as:

1

R

3

V

I

BRT

LED

×

=

Voltages greater than 1.2V can be applied to

the BRT pin is left open.

For example, the LED current is set to 20mA if

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.