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參數(shù)資料
| 型號(hào)： | SC480EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Complete DDR1/2/3 Memory Power Supply |
| 中文描述： | 完全DDR1/2/3內(nèi)存供電 |
| 文件頁數(shù)： | 17/25頁 |
| 文件大?。?/td> | 858K |
| 代理商： | SC480EVB |
17
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
Application Information (Cont.)
SC480
Ohms
0.015
V
0.015
R
Z
)
MIN
(
T
RIPPLE_VBA
BOT
TOP
x
Secondly calculating the value of C
TOP
required to achieve
this:
F
)
MIN
(
VBAT
_
SW
f
2
TOP
R
1
TOP
Z
x
1
TOP
C
x
1
·
¨
§
Since our example uses internal feedback ,this method
cannot be used, however the voltage seen at the internal
FB point is already greater than 15mV.
Next we need to calculate the minimum output capaci-
tance required to ensure that the output voltage does not
exceed the transient maximum limit, POSLIM
, starting
from the actual static maximum, V
OUT_ST_POS
, when a load
release occurs:
V
OUT_ST_POS
V
DC
ERR
OUT
V
For our example:
V
OUT_ST_POS
= 1.818V,
V
TR
TOL
OUT
V
TR
POSLIM
x
Where TOL
TR
is the transient tolerance. For our example:
POSLIM
TR
= 1.944V,
The minimum output capacitance is calculated as fol-
lows:
A
2
)
MAX
(
RIPPLE_VBA
I
)
MAX
(
OUT
I
init
I
and,
F
2
OUT_ST_POS
V
2
TR
POSLIM
2
Ifinal
2
Iinit
L
OUT(MIN)
C
x
This calculation assumes the condition of a full-load to no-
load step transient occurring when the inductor current is
at its highest. The capacitance required for smaller tran-
sient steps my be calculated by substituting the desired
current for the I
fi
nal
term. In this case I
fi
nal
is set for 5A.
For our example:
C
OUT(MIN)
= 392
μ
F.
We will select 440
μ
F, using two 220
μ
F, 12m
Ω
capacitors
in parallel.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
RMS
MIN
_
BAT
V
OUT
I
OUT
V
)
MIN
(
BAT
V
OUT
)
RMS
(
IN
A
V
I
x
x
For our example:
I
IN(RMS)
= 4.17A
RMS
Input capacitors should be selected with suf
fi
cient ripple
current rating for this RMS current, for example a 10
μ
F,
1210 size, 25V ceramic capacitor can handle approxi-
mately 3A
. Refer to manufacturer’s data sheets and
derate appropriately.
Finally, we calculate the current limit resistor value. As de-
scribed in the current limit section, the current limit looks
at the “valley current”, which is the average output cur-
rent minus half the ripple current.
A
2
)
MIN
(
RIPPLE_VBA
I
OUT
I
VALLEY
I
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under nor-
mal operating conditions.
Ohms
6
10
10
1.4
(ON)
x
DS
R
1.2
VALLEY
I
ILIM
R
x
x
x
For our example:
I
VALLEY
= 8.31A, R
DS(ON)
= 4m
Ω
, giving R
ILIM
= 5.62k
Ω
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