SC488 Semtech Corporation, SC488 Datasheet - Page 15

• Manufacturer Part Number: SC488
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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We will select an inductor value of 1.5μH to reduce the
ripple current, which can be calculated as follows:
and,
For our example:
From this we can calculate the minimum inductor current
rating for normal operation:
For our example:
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
(R
Where ERR
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
POWER MANAGEMENT
Application Information (Cont.)
© 2006 Semtech Corp.
I
I
RIPPLE_V
RIPPLE_V
ESR_TR(MAX)
I
I
RIPPLE_VBAT(MIN)
INDUCTOR
ST
= 36mV and, ERR
I
R
INDUCTOR(MIN)
BAT
BAT
ESR
):
ST
(
(
MIN
MAX
(
_
MIN
is the static output tolerance and ERR
ST
)
= 3.39A
)
(
)
MAX
= 12.2A
V
I
V
OUT
BAT
)
BAT
P-P
I
(
(
ESR_ST(MAX)
(
DC
MIN
RIPPLE
MAX
MAX
ERR
and I
(MIN)
= 18mV, therefore,
)
)
)
ST
V
V
RIPPLE_VBAT(MAX)
_
OUT
OUT
I
) and transient ESR
V
RIPPLE_VBA
ERR
BAT
DC
(
MAX
t
t
ON_
ON_
2
2
)
( T
VBAT
Ohms
VBAT
= 4.34A
MAX
L
L
(
(
MIN
MAX
)
A
(MIN)
)
P-P
)
A
A
P
P
DC
P
P
is
15
R
where ERR
case, I
For our example:
ERR
R
We will select a value of 6mΩ maximum for our design,
which would be achieved by using two 12mΩ output ca-
pacitors in parallel. Now that we know the output ESR we
can calculate the output ripple voltage:
and,
For our example:
V
Note that in order for the device to regulate in a controlled
manner, the ripple content at the feedback pin, V
be approximately 15mV
no smaller than 10mV
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
the FB point is less than 15mV
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.
RIPPLE_VBAT(MAX)
ESR_ST(MAX)
ESR_TR(MAX)
V
V
RIPPLE_VBA
RIPPLE_VBA
TR
R
ESR_
= 144mV and ERR
TRANS
= 8.3mΩ
= 17.6mΩ for a full 5A load transient.
TR
TR
is the load transient of 5A (10A - 5A).
( T
(
( T
= 20mV
MAX
is the transient output tolerance. For this
MAX
MIN
)
)
)
P-P
I
R
R
TRANS
ESR
ESR
P-P
P-P
and V
. Note that the voltage ripple at
DC
at minimum V
ERR
= 18mV, therefore,
I
I
RIPPLE_VBA
I
RIPPLE_VBA
RIPPLE_VBAT(MIN)
RIPPLE_VBA
TR
P-P
- whether internal or ex-
ERR
RIPPLE_VBAT(MIN)
2
DC
( T
( T
( T
BAT
MAX
MAX
MIN
= 26mV
, and worst case
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)
)
)
SC488
P V
P V
Ohms
as seen at
P
FB
P
P-P
, should