sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 15

• Manufacturer Part Number: sc1486aitstr
• Description: Complete Ddr1/2/3 Power Supply Controller
• Manufacturer: Semtech Corporation

Design Procedure (Cont.)
For our DDR2 VDDQ example we will use R
and R
Z
We will select a value of C
of V
For our DDR2 VDDQ example:
V
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
ing from the actual static maximum, V
load release occurs:
For our DDR2 VDDQ example:
V
Where TOL
VDDQ example:
POSLIM
The minimum output capacitance is calculated as
follows:
POSLIM
POWER MANAGEMENT
Application Information (Cont.)
C
V
V
TOP
FB_VIN(MIN)
OUT_ST_POS
FB
OUT
2006 Semtech Corp.
TOP
_
VIN
FB
= 12.8k
_
(
MIN
ST
BOT
based upon the selected C
)
_
2
TR
POS
TR
= 17.4k , therefore:
= 14.2mV
Z
= 1.836V
V
= 1.944V
RIPPLE
TOP
TR
1
V
is the transient tolerance. For our DDR2
V
f
_
SW
and C
OUT
OUT
VIN
R
_
(
MIN
VIN
1
TOP
)
P-P
TOL
(
ERR
MIN
TOP
R
- good
)
BOT
= 32pF
F
TR
TOP
DC
V
= 27pF. Calculating the value
R
V
1
TOp
2
TOP
R
:
BOT
f
1
SW
OUT_ST_POS
_
VIN
TOP
(
MIN
)
= 45.3k
, when a
C
TR
TOP
, start-
V
P
P
15
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
calculated by substituting the desired current for the I
term.
For our DDR2 VDDQ example:
C
We will select 660µF, using two 330µF, 25m
capacitors in parallel.
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 a little more
than 2A
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.
I
I
C
IN(RMS)
IN
VALLEY
OUT(MIN)
DS(ON)
OUT
(
RMS
(
MIN
)
= 4.27A
at V
)
= 760µF.
RMS
I
OUT
V
GS
L
. Refer to manufacturer’s data sheets.
OUT
= 4.5V for purposes of this calculation:
I
POSLIM
RMS
RIPPLE
V
I
OUT
IN
_
2
(
MIN
VIN
(
)
MIN
TR
I
RIPPLE
2
)
V
A
OUT
V
_
2
VIN
OUT
(
MAX
_
V
ST
I
IN
OUT
)
_
_
POS
MIN
2
SC1486A
2
A
www.semtech.com
F
RMS
OUT