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SC1486AEVB 数据表(PDF) 15 Page - Semtech Corporation

部件名 SC1486AEVB
功能描述  Complete DDR1/2/3 Power Supply Controller
PDF  30 Pages
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制造商  SEMTECH [Semtech Corporation]
网页  http://www.semtech.com
标志 SEMTECH - Semtech Corporation

SC1486AEVB 数据表(HTML) 15 Page - Semtech Corporation

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 2006 Semtech Corp.
www.semtech.com
SC1486A
POWER MANAGEMENT
Application Information (Cont.)
Design Procedure (Cont.)
F
f
2
R
1
Z
1
C
)
MIN
(
VIN
_
SW
TOP
TOP
TOP
•
π
•


−
=
For our DDR2 VDDQ example we will use R
TOP = 45.3kΩ
and R
BOT = 17.4kΩ, therefore:
Z
TOP = 12.8kΩ and CTOP = 32pF
We will select a value of C
TOP = 27pF. Calculating the value
of V
FB based upon the selected CTOP:
P
P
TOP
)
MIN
(
VIN
_
SW
TOp
BOT
BOT
)
MIN
(
VIN
_
RIPPLE
)
MIN
(
VIN
_
FB
V
C
f
2
R
1
1
R
R
V
V
−
•
•
π
•
+
+
•
=
For our DDR2 VDDQ example:
V
FB_VIN(MIN) = 14.2mVP-P - good
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
TR, start-
ing 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 DDR2 VDDQ example:
V
OUT_ST_POS = 1.836V
V
TOL
V
POSLIM
TR
OUT
TR
•
=
Where TOL
TR is the transient tolerance. For our DDR2
VDDQ example:
POSLIM
TR = 1.944V
The minimum output capacitance is calculated as
follows:
()F
V
POSLIM
2
I
I
L
C
2
POS
_
ST
_
OUT
2
TR
2
)
MAX
(
VIN
_
RIPPLE
OUT
)
MIN
(
OUT
−


+
•
=
This calculation assumes the absolute worst case
condition of a full-load to no load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps my be
calculated by substituting the desired current for the I
OUT
term.
For our DDR2 VDDQ example:
C
OUT(MIN) = 760µF.
We will select 660µF, using two 330µF, 25m
Ω
capacitors in parallel.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
()
RMS
MIN
_
IN
OUT
OUT
)
MIN
(
IN
OUT
)
RMS
(
IN
A
V
I
V
V
V
I
•
−
•
=
For our DDR2 VDDQ example:
I
IN(RMS) = 4.27ARMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10µF,
1210 size, 25V ceramic capacitor can handle a little more
than 2A
RMS. Refer to manufacturer’s data sheets.
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOSFET
R
DS(ON) at VGS = 4.5V for purposes of this calculation:
A
2
I
I
I
)
MIN
(
VIN
_
RIPPLE
OUT
VALLEY
−
=
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.



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