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SS6845G датащи(PDF) 9 Page - Silicon Standard Corp.

номер детали SS6845G
подробное описание детали  Regulated 5V Charge Pump
PDF  13 Pages
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производитель  SSC [Silicon Standard Corp.]
домашняя страница  http://www.siliconstandard.com
Logo SSC - Silicon Standard Corp.

SS6845G датащи(HTML) 9 Page - Silicon Standard Corp.

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SS6845G
7/21/2005 Rev.3.01
D)T
(1
I
DT
I
AVE
OFF
AVE
ON
−
×
=
×
−
−
(1)
External Capacitor Selection
D)
(1
I
D
I
AVE
-
OFF
AVE
-
ON
−
×
=
×
(2)
Three external capacitors, CIN, COUT and CFLY,
determine SS6845G performance, in the area of
output ripple voltage, charge pump strength and
transients. Optimum performance can be obtained
by the use of ceramic capacitors with low ESR.
Due to their high ESR, tantalum and aluminum
capacitors
are not recommended for charge-pump
applications.
D)
-
(1
I
2
D
I
2
D)
(1
I
D
I
I
AVE
-
OFF
AVE
-
ON
AVE
-
OFF
AVE
-
ON
IN
×
×
=
×
×
=
−
×
+
×
=
(3)
)
D
(1
I
I
AVE
-
OFF
OUT
−
×
=
OUT
IN
2I
I
=
For the SS6845G, the controller uses the PSM
(Pulse
Skipping Modulation) control strategy. When
the
duty cycle is limited to 0.5, then:
T
0.5)
1
(
I
T
0.5
I
AVE
-
OFF
AVE
-
ON
×
−
×
=
×
×
AVE
-
OFF
AVE
-
ON
I
I
=
..........(5)
To reduce noise and ripple, a low ESR ceramic
capacitor, ranging from 2.2
µF to 10µF, is
recommended for CIN and COUT. The value of
COUT determines the amount of output ripple
voltage. An output capacitor with a larger value
results in smaller ripple.
According to the equation (4), we know that as
long as the flying capacitor C1 is at steady state,
the input current is twice the output current. The
efficiency of charge pump is given below:
IN
OUT
OUT
IN
OUT
OUT
IN
IN
OUT
OUT
2V
V
2I
V
I
V
I
V
I
V
η
=
×
×
=
×
×
=
..(6)
CFLY is critical to the performance of a charge pump.
The larger CFLY is, the larger the output current and
the
smaller the resulting ripple voltage. However,
a large CFLY requires large CIN and COUT.
The ratio of CIN (as well as COUT) to CFLY should
be approximately 10:1.
C1
Q1
Q3
Q2
Q4
VOUT
VIN
COUT
CIN
RDS-ON
RDS-ON
ION
ESR
The values of the capacitors used under operating
conditions, determine the performance of the charge
pump converter, and two factors, described below,
affect the value of the capacitors.
Fig. 20 The on-state of charge pump circuit
1. Material: Ceramic capacitors of different
materials, such as X7R, X5R, Z5U and Y5V,
have different tolerances to temperature and
capacitance can vary significantly. For example,
X7R
or X5R types of capacitor retain their
capacitance over temperatures from -40
°C
to
85
°C, but a Z5U or Y5V type will change a
lot over
that temperature range.
C1
Q1
Q3
Q2
Q4
VOUT
VIN
COUT
CIN
RDS-ON
RDS-ON
IOFF
ESR
Fig. 21 The off-state of charge pump circuit
..........(4)



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