7

ML4872

INPUT CAPACITOR

Due to the high input current drawn at startup and

possibly during operation, it is recommended to decouple

the input with a capacitor with a value of 47µF to 100µF.

This filtering prevents the input ripple from affecting the

ML4872 control circuitry, and also improves the

efficiency by reducing the I squared R losses during the

charge cycle of the inductor. Again, a low ESR capacitor

(such as tantalum) is recommended.

It is also recommended that low source impedance

batteries be used. Otherwise, the voltage drop across the

source impedance during high input current situations will

cause the ML4872 to fail to start-up or to operate

unreliably. In general, for two cell applications the source

impedance should be less than 200m

W, which means that

small alkaline cells should be avoided.

LAYOUT

Good layout practices will ensure the proper operation of

the ML4872. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4872

• Use a single point ground for the ML4872 ground pin,

and the input and output capacitors

• Separate the ground for the converter circuitry from the

ground of the load circuitry and connect at a single

point

A sample layout is shown in Figure 7.

Figure 7. Sample PC Board Layout

DESIGN EXAMPLE

In order to design a boost converter using the ML4872, it

is necessary to define a few parameters. For this example,

First, it must be determined whether the ML4871 is

capable of delivering the output current. This is done

using Equation 1:

I

V

V

AA

OUT MAX

()

.

.

.

..

125

30

50

07

0 53

Next, select an inductor. As previously mentioned, the

recommended inductance is 10µH. Make sure that the

peak current rating of the inductor is at least 1.5A, and

that the DC resistance of the inductor is in the range of 50

to 100m

W.

Finally, the value of the output capacitor is determined

using Equation 2:

C

H

V

F

™

=

44 10

50

88

m

m

.

The closest standard value would be a 100µF capacitor

with an ESR rating of 100m

W. If such a low ESR value

cannot be found, two 47µF capacitors in parallel could

also be used.

The complete circuit is shown in Figure 8. As mentioned

previously, the use of an input supply bypass capacitor is

highly recommended.

Figure 8. Typical Application Circuit

ML4872

VOUT

100µF

100µF

VIN

10µH

(Sumida CD75)

VL1

VIN

GND

SHDN

PWR GND

NC

VL2

VOUT