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SS6550COTB Fiches technique(PDF) 10 Page - Silicon Standard Corp.

No de pièce SS6550COTB
Description  Low-Noise Synchronous PWM Step-Down DC/DC Converter
Download  13 Pages
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Fabricant  SSC [Silicon Standard Corp.]
Site Internet  http://www.siliconstandard.com
Logo SSC - Silicon Standard Corp.

SS6550COTB Fiches technique(HTML) 10 Page - Silicon Standard Corp.

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SS6550
n APPLICATION INFORMATION (cont.)
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
∆ IL is related to the inductor value. A
lower inductor value creates a higher ripple current. A
higher VIN or VOUT can also create the same result. The
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
VIN, can be set by connecting two external resistors, R1
and R2. VOUT can be calculated as:
)
2
R
1
R
1
(
V
75
.
0
VOUT
+
×
=
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


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