sc480imltrt Semtech Corporation, sc480imltrt Datasheet - Page 17

• Manufacturer Part Number: sc480imltrt
• Description: Sc480 Complete Ddr1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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:
V
Where TOL
POSLIM
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
POWER MANAGEMENT
Application Information (Cont.)
© 2006 Semtech Corp.
OUT_ST_POS
C
OUT(MIN)
Z
I
TOP
init
TR
V
= 1.818V,
= 1.944V,
TR
OUT_ST_POS
0.015
R
POSLIM
C
is the transient tolerance. For our example:
BOT
I
TOP
OUT
L
(
MAX
TR
POSLIM
2
V
RIPPLE_VBA
)
Z
Iinit
TOP
f
V
1
SW
TR
OUT
V
I
2
RIPPLE_VBA
_
OUT
2
VBAT
( T
R
V
TOP
MIN
1
OUT_ST_POS
TOL
TOP
Ifinal
OUT_ST_POS
(
)
MIN
ERR
required to achieve
TR
2
0.015
)
2
V
( T
DC
F
, when a load
MAX
2
V
Ohms
TR
, starting
)
F
A
17
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.
Finally, we calculate the current limit resistor value. As de-
scribed in the current limit section, the current limit looks
at the “valley current”, which is the average output cur-
rent minus half the ripple current.
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under nor-
mal operating conditions.
For our example:
I
IN(RMS)
VALLEY
OUT(MIN)
I
IN
R
(
= 8.31A, R
RMS
= 4.17A
ILIM
= 392μF.
)
I
RMS
VALLEY
. Refer to manufacturer’s data sheets and
I
RMS
VALLEY
V
fi nal
OUT
DS(ON)
term. In this case I
I
OUT
= 4mΩ, giving R
V
1.2
BAT
I
(
RIPPLE_VBA
MIN
R
)
DS
10
V
OUT
(ON)
2
10
ILIM
fi nal
( T
MIN
6
1.4
= 5.62kΩ
is set for 5A.
V
www.semtech.com
BAT
I
)
OUT
Ohms
SC480
A
_
MIN
A
RMS