ML4770

7

In applications where the ML4770 is operated at or near

the maximum output current, it is recommended to add a

optimum value of the high frequency bypass capacitor is

dependent on the layout and the value of the bulk output

capacitor selected.

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

ML4770 control circuitry, and also improves the

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 ML4770 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.

SHUTDOWN

The input levels of the SHDN pin are CMOS compatible.

To guarantee proper operation, SHDN must be pulled to

dissipation and possible oscillations.

SETTINGTHE OUTPUT VOLTAGE

The adjustable output of the ML4770 requires an external

can be determined from the following equation:

V

RR2

R2

+

257

1

.

()

(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 ML4770. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4770

• Use a single point ground for the ML4770 PWR GND

pin and the input and output capacitors, and connect

the GND pin to PWR GND using a separate trace

• Separate the ground for the converter circuitry from the

ground of the load circuitry and connect at a single

point

avoid noise pickup

• Route the high frequency bypass capacitor from a

to the GND pin

DESIGN EXAMPLE

In order to design a boost converter using the ML4770,

it is necessary to define the values of a few parameters.

First, it must be determined whether the ML4770 is

capable of delivering the output current. This is done

using Equation 1:

IA

mA

OUT MAX

()

..

.



!

"

$#

-=

3

5

0 0972 5

0 486

0 144

439

05

27

Next, select an inductor:

As previously mentioned, it is 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. Since the 400mA output current is close to

GND is recommended.

Finally, the values of R1 and R2 are calculated using

equation 3. Assuming R2 = 250k

W:

R

V

kk

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

strongly recommended.