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| 型號： | SC480 |
| 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)： | 16/25頁 |
| 文件大?。?/td> | 858K |
| 代理商： | SC480 |
16
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
Application Information (Cont.)
We will select an inductor value of 1.5
μ
H to reduce the
ripple current, which can be calculated as follows:
SC480
where ERR
is the transient output tolerance. For this
case, I
TRANS
is the load transient of 5A (10A - 5A).
For our example:
ERR
TR
= 144mV and ERR
DC
= 18mV, therefore,
R
ESR_TR(MAX)
= 17.6m
Ω
for a full 5A load transient.
We will select a value of 6m
Ω
maximum for our design,
which would be achieved by using two 12m
Ω
output ca-
pacitors in parallel. Now that we know the output ESR we
can calculate the output ripple voltage:
P
P
V
)
MIN
(
RIPPLE_VBA
I
ESR
R
)
MIN
(
RIPPLE_VBA
V
x
and,
P
P
V
)
MAX
(
RIPPLE_VBA
I
ESR
R
)
MAX
(
RIPPLE_VBA
V
x
For our example:
V
RIPPLE_VBAT(MAX)
= 20mV
P-P
and V
RIPPLE_VBAT(MIN)
= 26mV
P-P
Note that in order for the device to regulate in a controlled
manner, the ripple content at the feedback pin, V
, should
be approximately 15mV
P-P
at minimum V
, and worst case
no smaller than 10mV
. Note that the voltage ripple at
FB is smaller than the voltage ripple at the output capaci-
tor, due to the resistor divider. Also, when using internal
feedback (FB pin tied to 5V or GND), the FB resistor di-
vider is actually inside the IC. If V
as seen at
the FB point is less than 15mV
- whether internal or ex-
ternal FB is used - the above component values should be
revisited in order to improve this. For our example, since
the internal divider reduces the ripple signal by a factor of
(1.5V/1.8V), the internal FB ripple values are then 17mV
and 22mV, which is above the 15mV minimum.
When using external feedback, and with VDDQ greater
than 1.5V, a small capacitor, C
TOP
, can be used in parallel
with the top feedback resistor, R
TOP
, in order to ensure that
ripple at VFB is large enough. C
should not be greater
than 100pF. The value of C
can be calculated as fol-
lows, where R
is the bottom feedback resistor. Firstly
calculating the value of Z
TOP
required:
P
P
A
L
)
MIN
(
VBAT
ON_
t
OUT
V
)
MIN
(
BAT
V
)
MIN
(
BAT
RIPPLE_V
I
x
·
¨
§
and,
P
P
A
L
)
MAX
(
VBAT
ON_
t
OUT
V
)
MAX
(
BAT
V
)
MAX
(
BAT
RIPPLE_V
I
x
·
¨
§
For our example:
I
RIPPLE_VBAT(MIN)
= 3.39A
P-P
and I
RIPPLE_VBAT(MAX)
= 4.34A
P-P
From this we can calculate the minimum inductor current
rating for normal operation:
(MIN)
A
2
)
MAX
(
RIPPLE_VBA
I
)
MAX
(
OUT
I
)
MIN
(
INDUCTOR
I
For our example:
I
INDUCTOR(MIN)
= 12.2A
(MIN)
Next we will calculate the maximum output capacitor
equivalent series resistance (ESR). This is determined by
calculating the remaining static and transient tolerance
allowances. Then the maximum ESR is the smaller of the
calculated static ESR (R
ESR_ST(MAX)
) and transient ESR
(R
ESR_TR(MAX)
):
Ohms
)
MAX
(
BAT
V
_
RIPPLE
I
2
DC
ERR
ST
ERR
)
MAX
(
ST
_
ESR
R
x
Where ERR
is the static output tolerance and ERR
is
the DC error. The DC error will be 1% plus the tolerance
of the internal feedback. (Use 2% for external feedback,
which is 1% plus another 1% for the external resistors.)
For our example:
ERR
ST
= 36mV and, ERR
DC
= 18mV, therefore,
R
ESR_ST(MAX)
= 8.3m
Ω
Ohms
2
)
MAX
(
RIPPLE_VBA
I
TRANS
I
DC
ERR
TR
ERR
)
MAX
(
TR
ESR_
R
1
·
¨
¨
§
·
¨
§
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