7

ML4771

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

ML4771 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 ML4771 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.

SETTING THE OUTPUT VOLTAGE

The adjustable output of the ML4771 requires an external

can be determined from the following equation:

V

RR

R

+

257

12

2

.

16

(3)

where R1 and R2 are connected as shown in Figure 2. The

value of R2 should be 250k

W or less to minimize bias

current errors. Choose an appropriate value for R2 and

calculate R1.

LAYOUT

Good layout practices will ensure the proper operation of

the ML4771. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4771

• Use a single point ground for the ML4771 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.

DESIGN EXAMPLE

In order to design a boost converter using the ML4871, 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.

Then, 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.

Finally, the values of R1 and R2 are calculated using

equation 3, assuming that R2 = 250k

W:

Rk

k

k

1

50

257

250

250

236

=™



.

.

WW

W

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

ML4771

100µF

100µF

VIN

10µH

(Sumida CD75)

VL1

VIN

GND

SENSE

PWR GND

NC

VL2

VOUT

250k

Ω

236k

Ω

VOUT