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SS6550

100% Duty Cycle Operation

When the input voltage approaches the output voltage,

the converter continuously turns Q1 on. In this mode,

the output voltage is equal to the input voltage minus

the voltage-drop across Q1.

Components Selection

Inductor

The inductor selection depends on the operating fre-

quency of the SS6550. The internal switching fre-

quency is 500 kHz, and the external synchronized fre-

quency ranges from 500 kHz to 1 MHz. A higher fre-

quency allows the use of smaller inductor and capacitor

values. However, higher frequency also results in lower

efficiency due to the internal switching loss.

The ripple current

lower inductor value creates a higher ripple current. A

inductor value can be calculated from the following for-

mula:

( )( )









−

∆

=

IN

OUT

OUT

L

V

V

1

V

I

f

1

...(1)

Users can define the acceptable ripple current to

obtain a suitable inductor value.

Output Capacitor

The selection of output capacitor depends on the ac-

ceptable ripple voltage. Lower ripple voltage corre-

sponds to lower ESR (equivalent-series-resistance) of

the output capacitor. Typically, once the ESR is deter-

mined from the ripple voltage, the value of the capaci-

tor is adequate for filtering. The formula for ripple volt-

age is:









+

∆

=

∆

OUT

L

OUT

fC

8

1

ESR

I

V

For more reduction in the ripple voltage, a 15pF ce-

ramic capacitor can be used in parallel with the output

capacitor.

External Schottky Diode

The SS6550 has an internal synchronous rectifier,

instead of the Schottky diode usually found in a buck

converter. However, a blank period occurs at each

switching cycle when both the main switch, Q2, and the

synchronous rectifier, Q3, are off. This results in a de-

crease in efficiency. Therefore, an external Schottky

diode is needed to reinforce the efficiency.

Since the diode conducts during the off time, the peak

current and voltage of the converter must not exceed

the diode ratings. The ratings of the diode can be cal-

culated from the following formulae:

IN

)

OFF

(

MAX

,

D

V

V

=

)

ON

(

MAX

,

D

I

2

I

I

L

MAX

,

OUT

∆

+

=

)

ON

(

avg

,

D

I

OUT

OUT

IN

OUT

I

D

I

I

I

×

−

=

−

=

OUT

I

D)

(1

×

−

=

Adjustable Output Voltage

The SS6550 presents a 0.75V reference voltage at

the FB pin. The output voltage, ranging from 0.75V to

)

2

R

1

R

1

(

V

75

.

0

+

×

=

Applying a 15

µF capacitor in parallel with R1 can pre-

vent stray pickup. This should sit as close to the

SS6550 as possible. However, the load transient

response is degraded by this capacitor.

Rev.2.01 6/06/2003