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SS6845G Datasheet(PDF) 11 Page - Silicon Standard Corp.

Part # SS6845G
Description  Regulated 5V Charge Pump
PDF  13 Pages
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Manufacturer  SSC [Silicon Standard Corp.]
Direct Link  http://www.siliconstandard.com
Logo SSC - Silicon Standard Corp.

SS6845G Datasheet(HTML) 11 Page - Silicon Standard Corp.

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SS6845G
7/21/2005 Rev.3.01
]
D)
D(1
1
I
ESR)
(2R
V
[2V
C
)
D
-
1
I
ESR
-
D
I
ESR
-
D
1
I
2R
D
I
2R
V
(2V
C
)
I
ESR
-
I
ESR
-
I
2R
-
I
2R
-
V
-
(2V
C
)
V
(V
C
Q
-
Q
Q
OUT
ON
DS
OUT
IN
1
OUT
OUT
OUT
ON
DS
OUT
ON
DS
OUT
IN
1
AVE
-
OFF
AVE
ON
AVE
-
OFF
ON
-
DS
AVE
-
ON
ON
-
DS
OUT
IN
1
AVE
OFF
C1
AVE
ON
C1
1
OFF
ON
−
×
×
+
−
−
×
=
×
×
−
×
−
×
−
−
×
=
×
×
×
×
×
=
−
×
=
=
∆
−
−
−
−
−
−
−
−
………(13)
Thus the output current can be written as
]
D)
-
D(1
1
I
)
ESR
(2R
-
V
[2V
C
f
)
Q
(Q
f
Q
f
I
OUT
ON
-
DS
OUT
IN
1
OFF
ON
OUT
×
×
+
−
×
×
=
−
×
=
∆
×
=
(14)
When the duty cycle is 0.5, the output current can be written as:
]
I
4ESR)
(8R
V
[2V
fC
]
0.5)
0.5(1
1
I
ESR)
(2R
V
[2V
C
f
I
OUT
ON
DS
OUT
IN
1
OUT
ON
DS
OUT
IN
1
OUT
×
+
−
−
×
=
−
×
×
+
−
−
×
×
=
−
−
(15)
And equation (15) can be re-written as:
OUT
ON
DS
OUT
1
OUT
IN
I
4ESR)
(8R
I
fC
1
V
2V
×
+
+
×
=
−
−
(16)
VOUT
2VIN
COUT
1/fC1
8RDS-ON
IOUT
LOAD
4ESR
Fig. 22 The equivalent circuit of charge pump
According to equation (16), when the duty cycle
is 0.5, the equivalent circuit of the charge pump is
shown in Fig. 22. The term 8RDS-ON is the total
effect of switching resistance, 1/fC1 is the effect
of flying capacitor and 4ESR is its equivalent
resistance.
From the equivalent circuit shown in Fig. 22, it is
seen that the terms 1/fC1, 4ESR and 8RDS-ON
should be as small as possible to get large output
current. However, since the R DS-ON is internal to
the SS6845G
, all that can be done is to lower the
values of
1/fC1 and ESR. However even if the
values
of 1/fC1 and ESR can be kept as small as
possible, the term 8RDS-ON still dominates the
limit of the maximum output current.
Layout Considerations
With the high
switching frequency and transient
currents of the SS6845G, careful consideration of
PCB
layout is important. To achieve the best
performance, it is necessary to minimize the distance
between every component and also to minimize
the length of
every connection and maximize the
trace width. Make sure each device connects to an
immediate ground plane. Fig. 23 to Fig. 25 show
a
recommended layout.



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