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SC470 Arkusz danych(PDF) 13 Page - Semtech Corporation

Numer części SC470
Szczegółowy opis  Synchronous Buck Controller for Dynamic Load-Voltage Applications
PDF  31 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC470 Arkusz danych(HTML) 13 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
SC470
POWER MANAGEMENT
Application Information (Cont.)
R
ESR_TR(MAX) = 10.4mΩ for a full 6A load transient
Wewill select a value of 12.5m
Ω maximum for our
design, which would be achieved by using two 25m
Ω
output capacitors in parallel.
Note that for constant-on converters there is a mini-
mum ESR requirement for stability which can be
calculated as follows:
SW
OUT
)
MIN
(
ESR
f
C
2
3
R
•
•
π
•
=
This criteria should be checked once the output
capacitance has been determined.
Now that we know the output ESR we can calculate
the output ripple voltage:
P
P
)
MAX
(
VBAT
_
RIPPLE
ESR
)
MAX
(
VBAT
_
RIPPLE
V
I
R
V
−
•
=
and,
P
P
)
MIN
(
VBAT
_
RIPPLE
ESR
)
MIN
(
VBAT
_
RIPPLE
V
I
R
V
−
•
=
For our example:
V
RIPPLE_VBAT(MAX) = 27mVP-P and VRIPPLE_VBAT(MIN) = 22mVP-P
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, V
FB, should be approximately 15mVP-P at minimum
V
BAT, and wo rst-case no smaller than 10mVP-P. If
V
RIPPLE_VBAT(MIN) is less than 15mVP-P the above component
values should be revisited in order to improve this. Quite
often a small capacitor, C
TOP, is required in parallel with
the top feedback resistor, R
TOP, in order to ensure that
V
FB is large enough. CTOP should not be greater than
100pF. The value of C
TOP can be calculated as follows,
where R
BOT is the bottom feedback resistor. Firstly
calculating the value of Z
TOP required:
()Ohms
015
.
0
V
015
.
0
R
Z
)
MIN
(
VBAT
_
RIPPLE
BOT
TOP
−
•
=
Secondly calculating the value of C
TOP required to achieve
this:
For our example we will use R
TOP = 20.0kΩ and
R
BOT = 14.3kΩ, therefore:
Z
TOP = 6.67kΩ and CTOP = 60pF
We will select a value of C
TOP = 56pF. Calculating the
value of V
FB based upon the selected CTOP:
P
P
TOP
)
MIN
(
VBAT
_
SW
TOp
BOT
BOT
)
MIN
(
VBAT
_
RIPPLE
)
MIN
(
VBAT
_
FB
V
C
f
2
R
1
1
R
R
V
V
−
•
•
π
•
+
+
•
=
For our example:
V
FB_VBAT(MIN) = 14.8mVP-P - good
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIM
TR,
starting from the actual static maximum, V
OUT_ST_POS, when
a load release occurs:
V
ERR
V
V
DC
OUT
POS
_
ST
_
OUT
+
=
For our example:
V
OUT_ST_POS = 1.222V
V
TOL
V
POSLIM
TR
OUT
TR
•
=
Where TOL
TR is the transient tolerance. For our
example:
POSLIM
TR = 1.296V
The minimum output capacitance is calculated as
follows:
()F
V
POSLIM
2
I
I
L
C
2
POS
_
ST
_
OUT
2
TR
2
)
MAX
(
VBAT
_
RIPPLE
OUT
)
MIN
(
OUT
−


+
•
=
F
f
2
R
1
Z
1
C
)
MIN
(
VBAT
_
SW
TOP
TOP
TOP
•
π
•


−
=



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