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SS6550 Folha de dados(PDF) 11 Page - Silicon Standard Corp.

Nome de Peças SS6550
Descrição Electrónicos  Low-Noise Synchronous PWM Step-Down DC/DC Converter
PDF  13 Pages
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Fabricante Electrônico  SSC [Silicon Standard Corp.]
Página de início  http://www.siliconstandard.com
Logo SSC - Silicon Standard Corp.

SS6550 Folha de dados(HTML) 11 Page - Silicon Standard Corp.

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SS6550
n APPLICATION INFORMATION (cont.)
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,
and the output voltage is 1.8V. Substituting VOUT=1.8V,
VIN=4.2V, ripple =250mA, and f=500 kHz to equation (1)
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



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