SC486 Semtech Corporation, SC486 Datasheet - Page 17

• Manufacturer Part Number: SC486
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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Design Procedure (Cont.)
Secondly calculating the value of C
this:
For our DDR2 VDDQ example we will use R
and R
V
No additional capacitance is required, however a no-pop
space is recommended to allow for adjustment once the
design is complete, laid out and built.
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIM
starting from the actual static maximum, V
a load release occurs:
For our DDR2 VDDQ example:
V
Where TOL
VDDQ example:
POSLIM
The minimum output capacitance is calculated as
follows:
This calculation assumes the absolute worst case
condition of a full-load to no load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps my be
POSLIM
POWER MANAGEMENT
C
C
V
FB_VBAT(MIN)
OUT_ST_POS
OUT
OUT
2006 Semtech Corp.
TOP
_
(
MIN
ST
BOT
2
_
TR
)
POS
TR
= 23.2k , therefore
= 1.836V
= 1.900V
= 16.7mV
Z
L
TR
TOP
1
V
is the transient tolerance. For our DDR2
V
f
POSLIM
SW
OUT
OUT
I
_
R
OUT
VBAT
1
TOP
TOL
P-P
ERR
(
MIN
- good
I
TR
RIPPLE
)
TR
2
F
DC
V
V
_
V
VBAT
2
OUT
(
_
MAX
ST
TOP
_
)
POS
required to achieve
2
2
F
OUT_ST_POS
TOP
= 4.64k
, when
TR
,
17
calculated by substituting the desired current for the I
term.
For our DDR2 VDDQ example:
C
We will select 440µF, using two 220µF, 15m
capacitors in parallel, which will be good for load release
steps of up to 6.7A.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
For our DDR2 VDDQ example:
I
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
approximately 3A
sheets.
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOSFET
R
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
For our DDR2 VDDQ example R
I
We select the next lowest 1% resistor value: 13.0k
I
I
R
IN(RMS)
VALLEY
IN
VALLEY
OUT(MIN)
DS(ON)
ILIM
(
RMS
)
= 8.69A and R
= 4A
at V
I
= 839µF.
I
VALLEY
OUT
V
GS
RMS
OUT
= 4.5V for purposes of this calculation:
I
RIPPLE
1
2 .
V
BAT
RMS
_
2
VBAT
ILIM
R
(
MIN
. Refer to manufacturer’s data
10
DS
= 13.1k
(
)
MIN
(
ON
)
10
V
)
A
OUT
1
6
4 .
DS(ON)
Ohms
V
BAT
I
= 9m :
OUT
_
MIN
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A
SC486
RMS
OUT