MP2370 – 1.2A, 24V, 1.4MHz STEP-DOWN WHITE LED DRIVER IN A TSOT23-6

MP2370 Rev. 0.96

www.MonolithicPower.com

6

9/21/2007

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2007 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting LED Current

The current sense resistor is inserted between

the anode of WLED and GND. The current

sense resistor value is calculated as:

LED

I

V

15

.

0

1

R

=

For 1A WLED current, choose R1 = 150mΩ

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor’s DC resistance should be less than

200mΩ. Refer to Table 2 for suggested surface

mount inductors. For most designs, the required

inductance value can be derived from the

following equation.

SW

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose the inductor ripple current to be 30% of

the maximum load current. The maximum

inductor peak current is calculated from:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, a

larger inductance is recommended to improve

efficiency. See Table 2 for suggested inductors.

Also note that the maximum recommended load

current is 1A if the duty cycle exceeds 35%.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. The input capacitor

impedance at the switching frequency should be

less than the output impedance of the input source

to prevent high frequency switching current from

passing through the input. Ceramic capacitors with

X5R or X7R dielectrics are highly recommended

because of their low ESR and small temperature

coefficients. For most applications, a 4.7µF

capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output current

ripple small and ensures feedback loop stability.

The output capacitor impedance should be low

at the switching frequency. Ceramic capacitors

with X5R or X7R dielectrics are recommended

for their low ESR characteristics. For most

applications, a 2.2µF ceramic capacitor will be

sufficient.

The MP2370 can also work properly without

output capacitor by adding a small capacitor in

the feedback loop (see Figure 2). The small

capacitor is recommended 680pF.

PC Board Layout

The high current paths (GND, IN and SW) should

be placed very close to the device with short,

direct and wide traces. The input capacitor needs

to be as close as possible to the IN and GND pins.

The external feedback resistors should be placed

next to the FB pin. Keep the switch node traces

short and away from the feedback network.