SC488 Semtech Corporation, SC488 Datasheet - Page 16

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

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When using external feedback, and with VDDQ greater
than 1.5V, a small capacitor, C
with the top feedback resistor, R
ripple at VFB is large enough. C
than 100pF. The value of C
lows, where R
calculating the value of Z
Secondly calculating the value of C
this:
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
from the actual static maximum, V
release occurs:
For our example:
Where TOL
POSLIM
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POWER MANAGEMENT
Application Information (Cont.)
Z
TOP
TR
= 1.944V,
0.015
R
TR
V
C
POSLIM
BOT
OUT_ST_POS
TOP
V
is the transient tolerance. For our example:
OUT_ST_POS
BOT
is the bottom feedback resistor. Firstly
2
TR
V
RIPPLE_VBA
= 1.818V
Z
TOP
f
1
SW
TOP
V
_
OUT
V
required:
TOP
VBAT
R
OUT
TOP
1
TOP
( T
,
can be calculated as fol-
TOP
TOP
(
, can be used in parallel
MIN
MIN
, in order to ensure that
TOL
TOP
should not be greater
OUT_ST_POS
ERR
)
)
required to achieve
TR
F
0.015
DC
V
V
, when a load
Ohms
TR
, starting
16
The minimum output capacitance is calculated as fol-
lows:
and,
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
For our example:
C
We will select 440μF, using two 220μF, 12mΩ capacitors
in parallel.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
For our example:
I
Input capacitors should be selected with suffi cient ripple
current rating for this RMS current, for example a 10μF,
1210 size, 25V ceramic capacitor can handle approxi-
mately 3A
derate appropriately.
IN(RMS)
OUT(MIN)
I
IN
C
(
RMS
I
OUT(MIN)
init
= 4.17A
= 392μF
)
RMS
. Refer to manufacturer’s data sheets and
I
OUT
V
fi nal
OUT
.
RMS
L
(
term. In this case I
MAX
POSLIM
V
)
BAT
(
Iinit
I
MIN
RIPPLE_VBA
TR
)
2
2
V
V
OUT
OUT_ST_POS
Ifinal
2
fi nal
( T
is set for 5A.
MAX
V
2
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BAT
I
OUT
SC488
_
)
2
MIN
A
F
A
RMS