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11 of 13

SS6550

Layout Consideration

To ensure proper operation of the SS6550, the following

points should be considered:

1.

The input capacitor and VIN should be placed as

close as possible to each other to avoid the AC

current flow into the internal MOSFET.

2.

The output loop, which consists of the inductor,

Schottky diode and output capacitor, should be

kept as small as possible.

3.

The routes carrying large currents should be

kept short and wide.

4.

Logically the large current of the converter,

when SS6550 is on or off, should flow in the

same direction.

5.

The FB pin should connect to the feedback re-

sistors directly, and the route should be away

from any noise source, such as the inductance

of the LX line.

6.

Grounding all components at the same point

may effectively reduce the occurrence of loops.

A stable ground plane is very important to obtain

higher efficiency. When a ground plane is cut

apart, it may cause disturbed signals and noise.

If possible, two or three through-holes can en-

sure the stability of grounding. Fig.2 to 4 shows

the layout diagrams of the SS6550.

Example

Here is an example to illustrate the components selec-

tion guide lines above. Let’s assume the SS6550 is

to be used for a mobile phone application, which uses a

1-cell Li-Ion battery with 2.7V to 4.2V input voltage for

the power source. The required load current is 800mA,

H

23

.

8

V

2

.

4

V

8

.

1

1

mA

250

kHz

500

V

8

.

1

L

µ

=











 −

×

=

Therefore, 10

µH is appropriate for the inductor. The in-

ductor, series number SLF6025-100M1R0 from TDK,

with 57.3m

Ω series resistance is recommended for the

best efficiency.

For the output capacitor, the ESR is more important

than

its

capacitance.

Assuming

ripple

voltage

of 100mV, then the ESR can be calculated as:

Ω

=

=

∆

∆

=

4

.

0

mA

250

mV

100

I

V

ESR

Therefore, a 33

µF/10V capacitor, MCM series from

NIPPON, is recommended.

Schottky selection is calculated as following.

V

2

.

4

V

V

IN

)

OFF

(

MAX

,

D

=

=

)

ON

(

MAX

,

D

I

2

I

I

L

MAX

,

OUT

∆

+

=

2

mA

250

mA

800

+

=

mA

925

=

)

ON

(

avg

,

D

I

OUT

I

)

D

1

(

×

−

=

mA

800

)

2

.

4

8

.

1

1

(

×

−

=

mA

14

.

457

=

According to the data above, the Schottky diode, SS12,

from GS is recommended.

For feedback resistors, choose R2=390k

Ω, and then

R1 can be calculated as follow:

Ω

=

Ω

×













−

=

k

546

k

390

1

75

.

0

V

8

.

1

1

R

; use 560k

Ω

Fig. 22 shows this application circuit for the SS6550.

Rev.2.01 6/06/2003