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SC488 Datasheet(PDF) 16 Page - Semtech Corporation

Part # SC488
Description  Complete DDR1/2/3 Memory Power Supply
PDF  24 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC488 Datasheet(HTML) 16 Page - Semtech Corporation

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© 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC488
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
TOP should not be greater
than 100pF. The value of C
TOP can be calculated as fol-
lows, where R
BOT is the bottom feedback resistor. Firstly
calculating the value of Z
TOP required:
Ohms
0.015
V
0.015
R
Z
)
MIN
(
T
RIPPLE_VBA
BOT
TOP
Secondly calculating the value of C
TOP required to achieve
this:
F
)
MIN
(
VBAT
_
SW
f
2
TOP
R
1
TOP
Z
1
TOP
C
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
TR, starting
from the actual static maximum, V
OUT_ST_POS, when a load
release occurs:
V
DC
ERR
OUT
V
OUT_ST_POS
V
For our example:
V
OUT_ST_POS = 1.818V,
V
TR
TOL
OUT
V
TR
POSLIM
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
(
T
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
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
final term. In this case Ifinal is set for 5A.
For our example:
C
OUT(MIN) = 392μF.
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:
RMS
MIN
_
BAT
OUT
OUT
)
MIN
(
BAT
OUT
)
RMS
(
IN
A
V
I
V
V
V
I
For our example:
I
IN(RMS) = 4.17ARMS
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 approxi-
mately 3A
RMS. Refer to manufacturer’s data sheets and
derate appropriately.
Application Information (Cont.)



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